one cell splits into two during cell division - video

Meat, Milk From Cloned Animals’ Progeny May Have Entered Food In US

Washington: Food and milk from the offspring of cloned animals may have entered the US food supply, the US government said on Tuesday, but its impossible to know because there is no difference between cloned and conventional products.
The US Food and Drug Administration said in January meat and milk from cloned cattle, swine and goats and their offspring were as safe as products from traditional animals. Before then, farmers and ranchers had followed a voluntary moratorium on the sale of clones and their offspring.
While the FDA evaluated the safety of food from clones and their offspring, the agriculture department was in charge of managing the transition of these animals into the food supply.
“It is theoretically possible” offspring from clones are in the food supply, said Siobhan De-Lancey, an FDA spokeswoman.
There are an estimated 600 cloned animals in the US. Proponents say that cloning is a way to create more disease-resistant animals that produce more milk and better meat.
The cloning industry and the FDA say cloned animals and their offspring are as safe as their traditional counterparts.
Critics contend not enough is known about the technology to ensure it is safe, and say the FDA needs to address concerns over animal cruelty and ethical issues. “It worries me that this technology is out of control in so many ways,” said Charles Margulis, a spokesman with the Center for Environmental Health.
FDA and USDA said it is impossible to differentiate between cloned animals, their offspring and conventionally bred animals, making it difficult to know if offspring are in the food supply.
“But they would be a very limited number because of the very few number of clones that are out there and relatively few of those clones are at an age where they would be parenting,” said Bruce Knight, USDA’s undersecretary for marketing and regulatory programs.
Major food companies including Tyson Foods Inc, the largest US meat company, and Smithfield Foods Inc have said that they would avoid using cloned animals because of safety concerns. REUTERS

Wellington: Scientists have discovered the crucial ovulationtriggering role played by a small protein molecule in the brain, a finding that could hold the key to new therapies for infertility.
Dubbed kisspeptin, the protein is known to play a vital role in kick-starting puberty.
Now, a group from the University of Otago led by Professor Allan Herbison, in collaboration with Cambridge University researchers, has published the first evidence that kisspeptin signalling in the brain is also essential for ovulation to occur in adults.
Studying female mice, the researchers found that signalling between kisspeptin and its cell receptor GPR54 was essential to activate gonadotrophin-releasing hormone (Gn-RH) neurons, the nerve cells known to initiate ovulation.
The research appears in the latest issue of the Journal of Neuroscience. “This is an exciting finding, as people have been trying to find out precisely how the brain controls ovulation for more than 30 years. This work now reveals a crucial link in the brain circuitry responsible,” Herbison said in a statement. REUTERS

An electrophoretic mobility shift assay (EMSA), also referred as a gel shift assay, gel mobility shift assay, band shift assay, or gel retardation assay, is a common technique used to study protein-DNA or protein-RNA interactions. This procedure can determine if a protein or mixture of proteins is capable of binding to a given DNA or RNA sequence, and can sometimes indicate if more than one protein molecule is involved in the binding complex. Gel shift assays are often performed in vitro concurrently with DNase footprinting, primer extension, and promoter-probe experiments when studying transcription initiation, DNA replication, DNA repair or RNA processing and maturation. Although precursors can be found in earlier literature, most current assays are based on methods described by Garner and Revzin [1] and Fried and Crothers [2].

Principle

A mobility shift assay generally involves electrophoretic separation of a protein-DNA or protein-RNA mixture on a polyacrylamide or agarose gel for a short period (about 1.5-2 hr for a 15- to 20-cm gel). [3] The speed at which different molecules (and combinations thereof) move through the gel is determined by their size and charge, and to a lesser extent, their shape (see gel electrophoresis). The control lane (DNA probe without protein present) will contain a single band corresponding to the unbound DNA or RNA fragment. However, assuming that the protein is capable of binding to the fragment, the lane with protein present will contain another band that represents the larger, less mobile complex of nucleic acid probe bound to protein which is 'shifted' up on the gel (since it has moved more slowly).

Under the correct experimental conditions, the interaction between the DNA and protein is stabilized and the ratio of bound to unbound nucleic acid on the gel reflects the fraction of free and bound probe molecules as the binding reaction enters the gel. This stability is in part due to the low ionic strength of the buffer, but also due to a "caging effect", in that the protein, surrounded by the gel matrix, is unable to diffuse away from the probe before they recombine. If the starting concentrations of protein and probe are known, the affinity of the protein for the nucleic acid sequence may be determined. If the protein concentration is not known, it can be determined by increasing the concentration of DNA probe until further increments do not increase the fraction of protein bound. By comparison with a set of standard dilutions of free probe run on the same gel, the number of moles of protein can be calculated. [3]

An antibody that recognizes the protein can be added to this mixture to create an even larger complex with a greater shift. This method is referred to as a supershift assay, and is used to unambiguously identify a protein present in the protein-nucleic acid complex.

Often, an extra lane is run with a competitor oligonucleotide to determine the most favorable binding sequence for the binding protein. The use of different oligonucleotides of defined sequence allows the identification of the precise binding site by competition (not shown in diagram). Variants of the competition assay are useful for measuring the specificity of binding and for measurement of association and dissociation kinetics.

For visualization purposes, the nucleic acid fragment is usually labeled with a radioactive, fluorescent or biotin label. Standard ethidium bromide staining is less sensitive than these methods and can lack the sensitivity to detect the nucleic acid if small amounts are used in these experiments. When using a biotin label, streptavidin conjugated to an enzyme such as horseradish peroxidase is used to detect the DNA fragment (Non-radioactive EMSA review).

Cell membrane

From Wikipedia, the free encyclopedia

The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer") is a selectively permeablelipid bilayer found in all cells.^[1] It contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes such as cell adhesion, ion channel conductance and cell signaling. The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton and, if present, the cell wall.

Function

The cell membrane surrounds the cytoplasm of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment, thereby serving a function similar to that of skin. In fungi, some bacteria, and plants, an additional cell wall forms the outermost boundary; however, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix to help group cells together in the formation of tissues.

The barrier is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.

Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to communicate with each other. Protein receptors are found ubiquitously and function to receive signals from both the environment and other cells. These signals are transduced into a form that the cell can use to directly effect a response. Other proteins on the surface of the cell membrane serve as "markers" that identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the immune system.

Structure

Lipid bilayer

The cell membrane consists primarily of a thin layer of amphipathicphospholipids which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer approximately 7 nm thick, barely discernible with a transmission electron microscope.^[1]

The arrangement of hydrophilic and hydrophobic heads of the lipid bilayer prevent polar solutes (e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores and gates.

Flippases and Scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Along with NANA, this creates an extra barrier to charged moities moving through the membrane.

Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis.

Integral membrane proteins

The cell membrane contains many integral membrane proteins, which pepper the entire surface. These structures, which can be visualized by electron microscopy or fluorescence microscopy, can be found on the inside of the membrane, the outside, or membrane spanning. These may include integrins, cadherins, desmosomes, clathrin-coated pits, caveolaes, and different structures involved in cell adhesion.

Membrane skeleton

The cytoskeleton is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles that extend from the cell. Anchoring proteins restricts them to a particular cell surface — for example, the apical surface of epithelial cells that line the vertebrategut — and limits how far they may diffuse within the bilayer. The cytoskeleton is able to form appendage-like organelles, such as cilia, which are microtubule-based extensions covered by the cell membrane, and filopodia, which are actin-based extensions. These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and/or make contact with the substrate or other cells. The apical surfaces of epithelial cells are dense with actin-based finger-like projections known as microvilli, which increase cell surface area and thereby increase the absorption rate of nutrients. Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb.

Structure and the Fluid mosaic model

Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (June 2008)

According to the fluid mosaic model of S. J. Singer and Garth Nicolson, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less freely^[2]. This picture may be valid in the space scale of 10 nm. However, the plasma membranes contain different structures or domains that can be classified as (a) protein-protein complexes; (b) lipid rafts, (c) pickets and fences formed by the actin-based cytoskeleton; and (d) large stable structures, such as synapses or desmosomes.

The fluid mosaic model can be seen when the membrane proteins of two cells (e.g., a human cell and a mouse cell) are tagged with different-coloured fluorescent labels. When the two cells are fused, the two colours intermix, indicating that the proteins are free to move in the 2D plane.

Composition

Cell membranes contain a variety of biological molecules, notable lipids and proteins. Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:

• Fusion of intracellular vesicles with the membrane (exocytosis) not only excretes the contents of the vesicle but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs around extracellular material that pinch off to become vesicles (endocytosis).
• If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
• Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), exchange of molecules with this small reservoir is possible.

In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called homeostasis, area regulation or tension regulation.

Lipids

The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and steroids. The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant.^[3] In RBC studies, 30% of the plasma membrane is lipid.

The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of fatty acids chains have a profound effect on membranes fluidity^[4] as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting point (increasing the fluidity) of the membrane. The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation.

The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Phospholipid molecules in the cell membrane are "fluid" in the sense that they are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, movement of phospholipid molecules between layers is not energetically favourable and does not occur to an appreciable extent. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane.

In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane.^[1]

Carbohydrates

About 5% of the plasma membrane weight is carbohydrate, predominantly glycoprotein, but with some lipoprotein (cerebrosides and gangliosides). For the most part, no glycosylation occurs on other unit membranes, and only ever occurs on the extracellular surface of cell membranes.

The glycocalyx is an important feature in all cells, especially epithelia with microvilli. Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing, and many others.

The penultimate sugar is galactose and the terminal sugar is sialic acid, as the sugar backbone is modified in the golgi apparatus. Sialic acid carries a negative charge, providing an external barrier to charged particles.

Proteins

Type	Description	Examples
Integral proteins or transmembrane proteins	Span the membrane and have a hydrophilic cytosolicdomain, which interacts with internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of α-helices and β sheet protein motifs.	Ion channels, proton pumps, G protein-coupled receptor
Lipid anchored proteins	Covalently-bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane.	G proteins
Peripheral proteins	Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and, once reacted the molecule, dissociates to carry on its work in the cytoplasm.	Some enzymes, some hormones

The cell membrane plays host to a large amount of protein that is responsible for its various activities. The amount of protein differs between species and according to function, however the typical amount in a cell membrane is 50%.^[4] These proteins are undoubtedly important to a cell: Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms.^[3]

The cell membrane, being exposed to the outside environment, is an important site of cell-cell communication. As such, a large variety of protein receptors and identification proteins, such as antigens, are present on the surface of the membrane. Functions of membrane proteins can also include cell-cell contact, surface recognition, cytoskeleton contact, signalling, enzymic activity, or transporting substances across the membrane.

Most membrane proteins must be inserted in some way into the membrane. For this to occur, an N-terminus "signal sequence" of amino acids directs proteins to the endoplasmic reticulum, which inserts the proteins into a lipid bilayer. Once inserted, the proteins is then transported to its final destination in vesicles, where the vesicle fuses with the target membrane.

Variation

The cell membrane has slightly different composition in different cell types and has therefore different denominations in different cell types:

• Sarcolemma in myocytes
• Oolemma in oocytes.

 Permeability

The permeability of membranes is the ease of molecules to pass it. This depends mainly on electric charge and, to a slightly lesser extent, on the molar mass of the molecule. Electrically-neutral and small molecules pass the membrane easier than charged, large ones.

The electric charge phenomenon results in pH parturition of substances throughout the fluid compartments of the body.

DNA Replication   How DNA Makes Copies of Itself

Figure 1 Before a cell divides, its DNA is replicated (duplicated.) Because the two strands of a DNA molecule have complementary base pairs, the nucleotide sequence of each strand automatically supplies the information needed to produce its partner.  If the two strands of a DNA molecule are separated, each can be used as a pattern or template to produce a complementary strand.  Each template and its new complement together then form a new DNA double helix, identical to the original.  Before replication can occur, the length of the DNA double helix about to be copied must be unwound.  In addition, the two strands must be separated, much like the two sides of a zipper, by breaking the weak hydrogen bonds that link the paired bases.   Once the DNA strands have been unwound, they must be held apartto expose the bases so that new nucleotide partners can hydrogen-bond to them.    The enzyme DNA polymerasethen moves along the exposed DNA strand, joining newly arrived nucleotides into a new DNA strand that is  complementary to the template.  Each cell contains a family of more than thirty enzymes to insure the accurate replication of DNA.

Primers Though DNA polymerase can elongate a polynucleotide strand by adding new nucleotides, it cannot start a strand from scratch because it can only bond new nucleotides to a free sugar (3') end of a nucleotide chain. DNA polymerase requires the assistance of a primer, a previously existing short strand of DNA (or RNA) that is complementary to the first part of the DNA segment being copied.  This small strand of nucleotides anneals (binds) by complementary base pairing to the beginning of the area being copied.  With the primer in place, DNA polymerase is then able to continue adding the rest of the pairs of the segment until a new double strand of DNA is completed.  Primers are formed from free nucleotides in the cell by enzymes called DNA primases.   Replication occurs differently on antiparallel strands of DNA. That nucleotides can be added only to the sugar or 3' end of the growing complementary chain presents no problem for the side of the DNA chain opening at its phosphate or 5' end.  The primer that binds to the first few exposed bases will end with a sugar (3') where the phosphate of a new nucleotide can be attached.  From there on, DNA polymerase can continuously synthesize the growing complementary strand.  This strand of DNA is called the leading strand.  A nice little animation of DNA synthesis on the leading strand can be seen at the Nobel Prize e-museum site at http://www.nobel.se/medicine/educational/dna/a/replication/replication_ani.html. A different challenge faces DNA polymerase when the complementary side of the DNA molecule begins unzipping from its sugar (3') toward its phosphate (5') end.  A primer of complementary molecules attaching to the opening end of this chain would have a phosphate not a sugar at its exposed end so that new nucleotides could not be joined. To get around this problem, this strand is synthesized in small pieces backward from the overall direction of replication.  This strand is called the lagging strand.  The short segments of newly assembled DNA from which the lagging strand is built are calledOkazaki fragments. As replication proceeds and nucleotides are added to the 3' end of the Okazaki fragments, they come to meet each other.  The primer fragments are then booted out by enzymes and replaced by appropriate DNA nucleotides.  The whole thing is then stitched together by another enzyme called DNA ligase.  The Nobel e-museum also has an animation of this process at http://www.nobel.se/medicine/educational/dna/a/replication/lagging_ani.html . Figure 2  Replication occurs simultaneously at multiple places along a DNA strand. Because human DNA is so very long (with up to 80 million base pairs in a chromosome) it unzips at multiple places along its length so that the replication process is going on simultaneously at hundreds of places along the length of the chain.  Eventually these areas run together to form a complete chain.  In humans, DNA is copied at about50base pairs per second. The process would take a month (rather than the hour it actually does) without these multiple places on the chromosome where replication can begin.  DNA replication is extraordinarily accurate. DNA polymerase makes very few errors, and most of those that are made are quickly corrected by DNA polymerase and other enzymes that "proofread" the nucleotides added into the new DNA strand.  If a newly added nucleotide is not complementary to the one on the template strand, these enzymes remove the nucleotide and replace it with the correct one.  With this system, a cell's DNA iscopied with less than one mistake in a billion nucleotides.  This is equal to a person copying 100 large (1000 page) dictionaries word for word, and symbol for symbol, with only one error for the whole process!

Meiosis     How Chromosomes are passed from parent to offspring
	Each human cell (aside from red blood cells and gametes) contains a full set of 46 chromosomes. Clearly havoc would result if aspermand egg cell each containing 46 human chromosomes were to fuse!  Not only would the resulting offspring have 98 chromosomes in each cell but the number would keep on doubling with each successive generation.  For this reason a process other than mitosiswhich produces cells with adiploidnumber of chromosomes is necessary to produce the sperm and egg cells.     The process by which the chromosome number is halved and chromosomes are sorted and packaged to be passed on to an organism’s offspring is called meiosis.  Each of the resulting reproductive cells, or gametes(sperm and egg), has only a single set of 22 autosomes plus a single sex chromosome, either an X or a Y.  A cell with a single chromosome set is called a haploid cell.  By means of sexual intercourse, a sperm cell carrying one 23 chromosome set from the father reaches and fuses with an egg cell carrying a corresponding set of 23 chromosomes from the mother.   The resulting fertilized egg, or zygote, contains the two haploid sets of chromosomes bearing genes originating in both the maternal and paternal family lines.      As a human develops from a zygote to a sexually mature adult, the new combination of genes in the zygote are passed on with precision to all somatic cells of the body by the process ofmitosis.
The process of meiosis has many similarities to the process of mitosis--chromosomes replicate before the process begins and shorten and thicken to look like the chromosomes at the beginning of mitosis.  A spindle with fibers appears and the nuclear membrane dissolves.    However, as meiosis begins, each chromosome is mysteriously attracted to its special homologous partner.  The two #1 chromosomes--one from the paternal set and one from the maternal set--wrap tightly about each other in a process called synapsis.  Since each #1 chromosome is already doubled, a tetradof four chromosomes is created.  The same thing happens with the other chromosomes.  Each homologous pair forms its own tetrad.  All the tetrads arrange themselves on the spindle.  Two quick divisions follow and the chromosomes are pulled apart.  The four chromosomes of each tetrad are first separated into twos and then into ones.
Before meiosis begins each chromosome in the pair has already doubled.  The double maternal chromosome (black) and the double paternal chromosome (white) attach to the spindle. As the cell divides, the double maternal chromosome and the double paternal chromosome move toward opposite poles.  The pair in each daughter cell starts to pull apart in the direction of the arrows.  Another cell division occurs. The final result is four cells, each containing the haploid number of chromosomes.  Two of the cells contain a chromosome from the maternal set and two contain a chromosome from the paternal set.
The diagram above shows only a single chromosome pair. Actually everything that is happening to this tetrad is happening simultaneously to the other 22 tetrads.  The result is always four cells, each having a single #1 chromosome, and one #2 and one #3 and so on up to one each of 23 chromosomes.  Thus each cell has one complete set of chromosomes and is ready to become either a sperm or egg cell.  The chromosomes in each sperm or egg are a random mix from the maternal set and paternal set of the original cell.  Because each of the 23 pairs of chromosomes can line up in two different ways one person can produce more than 8 million (223) different kinds of eggs or sperm.  When fertilization occurs, 223 x 223  or 70 trillion different zygotes are possible! For the genealogist, this means that any one of his or her chromosomes could have come from any one of eight great-grandparents or, going back say twenty generations, from any one of over a million potential ancestors (in reality, many of these potential ancestors may be the same individuals as lines cross and intermarry over generations.)  In the same light, after so many generations of shuffling and dividing, contributions from some ancestors may have been lost completely.  With only one exception, it is impossible to trace the path of individual chromosomes in the nucleus from generation to generation.
Cross Over Animation And, to complicate matters even further, the chromatids in a tetrad pair so tightly at the beginning of meiosis that non-sister chromatids from homologous chromosomes actually exchange genetic material in a process known ascrossing over. This further shuffles the ancestral genes so that a single chromosome in a gamete may contain genes from both maternal and paternal ancestors.Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.  It is estimated that during meiosis in humans, there is an average of two to three crossovers for each pair of homologous chromosomes.
Differences Between Sperm and Egg Formation What they mean to the molecular genealogist         The process of meiosis and gamete formation is fundamentally the same in males and females.  However, whereas gametogenesis (formations of gametes) results in four functional sperm cells for each meiotic division in males, the same process in females gives rise to only a single functional egg capable of being fertilized and developing into an embryo.  A mature sperm has a head, which contains a nucleus with its haploid set of chromosomes, a long tail which propels the sperm through its fluid surroundings, and between these a midpiece containing several hundred mitochondria which supply the energy necessary for the  long journey in search of the egg.  On the other hand, the development of the egg cell involves an unequal cell division.  It produces one relatively large primary egg cell which receives all of the cell parts--including thousands of mitochondria--from the starting cell while the other three cells that result from the meiotic process form small polar bodies that, at first, attach themselves to the surface of the primary egg but eventually deteriorate.  At fertilization, the entire sperm does not enter the egg.  Practically the only contribution that the sperm cell makes to the zygote is its haploid nucleus with its set of 23 chromosomes.  Importantly to the genealogist, the tail and midsection of the sperm drop off outside the egg meaning that virtually no mitochondria from the male parent enter the zygote.  All of the mitochondria (with its own DNA you will recall) in the developing embryo come from the mother. The diagram below illustrates the differences between the formation of sperm and egg cells.
The cross-over animation above is a modification of an animation by Hironao NUMABE, M.D, Tokyo Medical University, Department of Pediatrics Genetic Study Group  http://www.tokyo-med.ac.jp/genet/mfi-e.htm

 Mitosis  

 The Dance of the Chromosomes  

When a human cell divides, its 46 chromosomes must be copied, or replicated, and each of the two new cells must receive only one copy of each chromosome.  Mitosis (from the Greek mitos = thread) is the process that sorts the genetic material into two new nuclei and ensures that both contain exactly the same genetic information.

Embryos, babies and children grow using mitosis, and mitosis occurs all the time in our adult bodies, as new cells replace old ones--such as worn-out blood cells or skin cells injured by cuts or burns. 

Though mitosis is a smooth continuous process, biologists have divided it into several stages. 

Interphase Interphase is the cell growth phase in which a cell increases in size and carries out activities that support the organism.  It is technically not a part of mitosis.  Near the end of this phase, the chromosomes of the cell duplicate in preparation for cell division.  By the time a cell is ready to divide, there are two copies of each chromosome (the sister chromatids.)

Prophase   The chromosomes coil, becoming short and thick. The nuclear membrane appears to dissolve and the chromosomes float in the cytoplasm.  The spindle, a football-shaped, cage like structure consisting of thin fibers forms in the cytoplasm.  The spindle fibers attach to the centromeres of the chromosomes and to both ends of the cell.

Metaphase All of the chromosomes line up across the center of the cell.

Anaphase The chromosomes separate.  One copy of each chromosome is pulled to each end of the cell by the spindle fibers

Telophase The cell membrane begins to pinch the cell in two to divide the cytoplasm.  A new nuclear membrane forms in each daughter cell.  The daughter cells contain the same genetic information as was found in the original cell and as each other because the chromosomes in each cell are the same.

Interphase   for the two new cells begins The chromosomes uncoil and the cells begin to grow. Images from http://www.biodidac.bio.uottawa.ca

Chromosomes

Just before the cell divides, the DNA of each chromosomereplicates(makes a copy of itself.)  Following this, the chromosomes coil up tightly which allows them to be sorted and moved to the new cells without tangling and breaking apart.  This sorting and moving process occurs before the cell divides and is calledmitosis.  For a brief discussion of mitosis, click here.   During this period of cell division, chromosomes appear as dense, bulky objects when the cell is viewed through a microscope. At very high  magnifications, they have the shape of a fuzzy, bulky X.  Each half of the X comprises one chromatid--an exact copy of the original chromosome.  The two chromatids (often referred to as "sister chromatids") are joined  together at a specific small region called thecentromere.     Using a microscope, it is possible to count and characterize the individual chromosomes during the time they are coiled and condensed.  A photograph of the entire set of chromosomes can  be made. Then the images of the individual chromosome can be cut out and arranged by shape and size in an orderly arrangement called akaryotype(see below).  (This is a nightmare project to the untrained eye, a simulation of which is often assigned to 10th grade students as a learning exercise!)

In most higher plants and animals, including humans, chromosomes from the body cells can be matched up in pairs.  The two chromosomes of a pair are calledhomologous chromosomes.  The members of most homologous pairsof chromosomes look alike.  They are the same length, their centromeres are in the same position, they show the same pattern of light and dark bands when stained, and they carry genes for the same inherited characteristics, line up on the chromosome in the same order.

A Human Male Karyotype A Human Female Karyotype

The occurrence of pairs of chromosomes in our karyotype is a result of our sexual origins.   We inherit one member of each chromosome pair from each parent.  So the 46 chromosomes in our somatic cells are actuallytwo sets of 23 chromosomes—a maternal set (from our mother) and a paternal set(from our father.)A cell with two of each kind of chromosome is called adiploid celland is said to contain a diploid, or 2n, number of chromosomes.      In humans, the homologous pairsare defined and numbered andcarry the genes for the same trait in each person. For example, human chromosome #1 contains, along with many others, the genes for the Rh blood protein and for a starch-digesting enzyme in the saliva.  However, the corresponding genes on the two homologous chromosomes are not necessarily identical. For instance, some chromosomes have a gene for the protein that makes a person Rh-positive, and some have a gene coding for a different version of this protein (Rh-negative) at the Rh location.  Different versions of the same gene are referred to as alleles.  An individual with two genes the same for a trait is said to behomozygousfor that trait.  A person with two different alleles for the same trait is heterozygousfor that trait. In human males, the partners of 22 of the pairs of chromosomes look similar, but the twenty-third pair is mismatched with two unlike chromosomes, called X and Y (see the far right chromosome pair in the bottom row of the male karyotype above.)  In the cells of a female, both members of homologous pair #23 are X chromosomes (far right pair of chromosomes in the bottom row in the female karyotype.) The X and Y chromosomes are called thesex chromosomes,because they differ between the sexes and because they carry the genes that determine the sex of the individual.  The other 22 chromosomes are calledautosomal chromosmesor simplyautosomes. It is the Y chromosome that is of major interest to the genealogist because, as it is handed from father to son, virtually unchanged, it becomes a signature or fingerprint for the surname which is passed down in the same way in many cultures. For a chart demonstrating how the Y chromosome is passed through a family,click here.

 

Eukaryote

From Wikipedia, the free encyclopedia

Eukaryotes Fossil range: Proterozoic - Recent
Ostreococcus is the smallest known free living eukaryote with an average size of 0.8 µm.

Animals, plants, fungi, and protists are eukaryotes (IPA: /juːˈkærɪɒt/ or IPA: /-oʊt/), organisms whose cells are organized into complex structures enclosed within membranes. The defining membrane-bound structure which differentiates eukaryotic cells from prokaryotic cells is the nucleus. The presence of a nucleus gives these organisms their name, which comes from the Greek ευ (eu), meaning "good/true", and κάρυον (karyon), "nut". Many eukaryotic cells contain other membrane-bound organelles such as mitochondria, chloroplasts and Golgi bodies.

Cell division in eukaryotes is different from organisms without a nucleus (prokaryotes). It involves separating the duplicated chromosomes, through movements directed by microtubules. There are two types of division processes. In mitosis, one cell divides to produce two genetically-identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.

Eukaryotes appear to be monophyletic, and so make up one of the three domains of life. The two other domains, bacteria and archaea, are prokaryotes, and have none of the above features. But eukaryotes do share some aspects of their biochemistry with archaea, and so are grouped with archaea in the cladeNeomura.

Cell features

Eukaryotic cells are typically much larger than prokaryotes. They have a variety of internal membranes and structures, called organelles, and a cytoskeleton composed of microtubules, microfilaments, and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division.

[edit]Internal membrane

Eukaryotic cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles or vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle. It is probable that most other membrane-bound organelles are ultimately derived from such vesicles.

The nucleus is surrounded by a double membrane (commonly referred to as a nuclear envelope), with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form what is called the endoplasmic reticulum or ER, which is involved in protein transport and maturation. It includes the Rough ER where ribosomes are attached, and the proteins they synthesize enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the Smooth ER. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, called Golgi bodies or dictyosomes.

Vesicles may be specialized for various purposes. For instance, lysosomes contain enzymes that break down the contents of food vacuoles, and peroxisomes are used to break down peroxide, which is toxic otherwise. Many protozoa have contractile vacuoles, which collect and expel excess water, and extrusomes, which expel material used to deflect predators or capture prey. In multicellular organisms, hormones are often produced in vesicles. In higher plants, most of a cell's volume is taken up by a central vacuole, which primarily maintains its osmotic pressure.

[edit]Mitochondria and plastids

Mitochondria are organelles found in nearly all eukaryotes. They are surrounded by double membranes (known as the phospholipid bi-layer), the inner of which is folded into invaginations called cristae, where aerobic respiration takes place. They contain their own DNA and ribosomes and are only formed by the fission of other mitochondria. They are now generally held to have developed from endosymbiotic prokaryotes, probably proteobacteria. The few protozoa that lack mitochondria have been found to contain mitochondrion-derived organelles, such as hydrogenosomes and mitosomes.

Plants and various groups of algae also have plastids. Again, these have their own DNA and developed from endosymbiotes, in this case cyanobacteria. They usually take the form of chloroplasts, which like cyanobacteria contain chlorophyll and produce energy through photosynthesis. Others are involved in storing food. Although plastids likely had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion.

Endosymbiotic origins have also been proposed for the nucleus, for which see below, and for eukaryotic flagella, supposed to have developed from spirochaetes. This is not generally accepted, both from a lack of cytological evidence and difficulty in reconciling this with cellular reproduction.

[edit]Cytoskeletal structures

Many eukaryotes have long slender motile cytoplasmic projections, called flagella. These are composed mainly of tubulin and shorter cilia, both of which are variously involved in movement, feeding, and sensation. These are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a basal body, also called a kinetosome or centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella also may have hairs, or mastigonemes, and scales connecting membranes and internal rods. Their interior is continuous with the cell's cytoplasm. Microfilamental structures composed by actin and actin binding proteins, e.g., α-actinin, fimbrin, filamin are present in submembraneous cortical layers and bundles, as well. Motor proteins of microtubules, e.g., dynein or kinesin and actin, e.g., myosins provide dynamic character of the network.

Centrioles are often present even in cells and groups that do not have flagella. They generally occur in groups of one or two, called kinetids, that give rise to various microtubular roots. These form a primary component of the cytoskeletal structure, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles may also be associated in the formation of a spindle during nuclear division.

Significance of cytoskeletal structures is underlined in determination of shape of the cells, as well as their being essential components of migratory responses like chemotaxis and chemokinesis. Some protists have various other microtubule-supported organelles. These include the radiolaria and heliozoa, which produce axopodia used in flotation or to capture prey, and the haptophytes, which have a peculiar flagellum-like organelle called the haptonema.

[edit]Plant cell wall

Further information: Cell wall

Plant cells have a cell wall, a fairly rigid layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major carbohydrates making up the primary cell wall are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.

[edit]Differences between eukaryotic cells

There are many different types of eukaryotic cells, though animals and plants are the most familiar eukaryotes, and thus provide an excellent starting point for understanding eukaryotic structure. Fungi and many protists have some substantial differences, however.

[edit]Animal cell

An animal cell is a form of eukaryotic cell that makes up many tissues in animals. The animal cell is distinct from other eukaryotes, most notably plant cells, as they lack cell walls and chloroplasts, and they have smaller vacuoles. Due to the lack of a rigid cell wall, animal cells can adopt a variety of shapes, and a phagocytic cell can even engulf other structures.

There are many different cell types. For instance, there are approximately 210 distinct cell types in the adult human body.

[edit]Plant cell

Further information: Plant cell

Plant cells are quite different from the cells of the other eukaryotic organisms. Their distinctive features are:

• A large central vacuole (enclosed by a membrane, the tonoplast), which maintains the cell's turgor and controls movement of molecules between the cytosol and sap
• A primary cell wall containing cellulose, hemicellulose and pectin, deposited by the protoplast on the outside of the cell membrane; this contrasts with the cell walls of fungi, which contain chitin, and the cell envelopes of prokaryotes, in which peptidoglycans are the main structural molecules
• The plasmodesmata, linking pores in the cell wall that allow each plant cell to communicate with other adjacent cells; this is different from the functionally analogous system of gap junctions between animal cells.
• Plastids, especially chloroplasts that contain chlorophyll, the pigment that gives plants their green color and allows them to perform photosynthesis
• Higher plants, including conifers and flowering plants (Angiospermae) lack the flagellae and centrioles that are present in animal cells.

[edit]Fungal cell

Fungal cells are most similar to animal cells, with the following exceptions:

• A cell wall containing chitin
• Less definition between cells; the hyphae of higher fungi have porous partitions called septa, which allow the passage of cytoplasm, organelles, and, sometimes, nuclei. Primitive fungi have few or no septa, so each organism is essentially a giant multinucleate supercell; these fungi are described as coenocytic.
• Only the most primitive fungi, chytrids, have flagella.

[edit]Other eukaryotic cells

Eukaryotes are a very diverse group, and their cell structures are equally diverse. Many have cell walls; many do not. Many have chloroplasts, derived from primary, secondary, or even tertiary endosymbiosis; and many do not. Some groups have unique structures, such as the cyanelles of the glaucophytes, the haptonema of the haptophytes, or the ejectisomes of the cryptomonads. Other structures, such as pseudopods, are found in various eukaryote groups in different forms, such as the lobose amoebozoans or the reticulose foraminiferans.

[edit]Reproduction

Nuclear division is often coordinated with cell division. This generally takes place by mitosis, a process that allows each daughter nucleus to receive one copy of each chromosome. In most eukaryotes, there is also a process of sexual reproduction, typically involving an alternation between haploid generations, wherein only one copy of each chromosome is present, and diploid generations, wherein two are present, occurring through nuclear fusion (syngamy) and meiosis. There is considerable variation in this pattern, however.

Eukaryotes have a smaller surface to volume area ratio than prokaryotes, and thus have lower metabolic rates and longer generation times. In some multicellular organisms, cells specialized for metabolism will have enlarged surface areas, such as intestinal vili.

[edit]Origin and evolution

The origin of the eukaryotic cell was a milestone in the evolution of life, since they include all complex cells and almost all multi-cellular organisms. The timing of this series of events is hard to determine; Knoll (2006) suggests they developed approximately 1.6 - 2.1 billion years ago. Some acritarchs are known from at least 1650 million years ago, and the possible alga Grypania has been found as far back as 2100 million years ago. ^[2] Fossils that are clearly related to modern groups start appearing around 1.2 billion years ago, in the form of a red alga.

Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australianshales indicates that eukaryotes were present 2.7 billion years ago.^[3][4]

rRNA trees constructed during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group (not technically a true crown), which was usually divided by the form of the mitochondrial cristae. The few groups that lack mitochondria branched separately, and so the absence was believed to be primitive; but this is now considered an artifact of long-branch attraction, and they are known to have lost them secondarily.^[5][6]

Trees based on actin and other molecules have painted a different and more complete picture. Most eukaryotes are now included in one of the following supergroups, although the relationship between these groups, and the monophyly of each group, is not yet clear:^[7][8]

Opisthokonts	Animals, fungi, choanoflagellates, etc.
Amoebozoa	Most lobose amoebae and slime moulds
Rhizaria	Foraminifera, Radiolaria, and various other amoeboid protozoa
Excavates	Various flagellate protozoa
Archaeplastida (or Primoplantae)	Land plants, green algae, red algae, and glaucophytes
Chromalveolates	Heterokonts, Haptophytes, Cryptomonads, and Alveolates.

Several authorities recognize two larger clades, the unikonts and the bikonts, that derive from an ancestral uniflagellar organism and a biflagellate respectively. In this system, the opisthokonts and amoebozoans are considered unikonts, and the rest bikonts. The chromalveolates were originally thought to be two separate groups, the chromists and the alveolates, but the former was proved to be paraphyletic to the latter, and the two groups combined. Some small protist groups have not been related to any of these supergroups, in particular the centrohelids.

Eukaryotes are closely related to Archaea, at least in terms of nuclear DNA and genetic machinery, and some place them with Archaea in the clade Neomura. In other respects, such as membrane composition, they are similar to eubacteria. Three main explanations for this have been proposed:

• Eukaryotes resulted from the complete fusion of two or more cells, where the cytoplasm formed from a eubacterium, and the nucleus from an archaeon,^[9] or from a virus.^[10][11]
• Eukaryotes developed from Archaea, and acquired their eubacterial characteristics from the proto-mitochondrion.
• Eukaryotes and Archaea developed separately from a modified eubacterium.

Eukarya Bikonta Apusozoa Archaeplastida Chromalveolata Rhizaria Excavata Unikonta Amoebozoa Opisthokonta Metazoa Choanozoa Eumycota Likely cladogram of Eukarya

The origins of the endomembrane system and mitochondria are also unclear.^[12] The phagotrophic hypothesis proposes that eukaryotic-type membranes lacking a cell wall originated first, with the development of endocytosis, while mitochondria were acquired by ingestion as endosymbionts.^[13] The syntrophic hypothesis proposes that the proto-eukaryote relied on the proto-mitochondrion for food, and so ultimately grew to surround it. Here the membranes originated after the engulfment of the mitochondrion, in part thanks to mitochondrial genes (the hydrogen hypothesis is one particular version).^[14]

In a study using genomes to construct supertrees, Pisani et al (2007) suggest that, along with evidence that there was never a mitochondrion-less eukaryote, eukaryotes evolved from a syntrophy between an archaea closely related to Thermoplasmatales and an α-proteobacterium, likely a symbiosis driven by sulfur or hydrogen. The mitochondrion and its genome is a remnant of the α-proteobacterial endosymbiont.^[15]

[edit]See also

• List of sequenced eukaryotic genomes

[edit]References