Mitosis and Meiosis, Biology tutorial

Mitosis:

Mitosis is a reproduction of stomach, skin, heart, hair, cheek etc. cells. These cells are Autosomal cells. This is also the form of asexual reproduction, where one organism or cell reproduces itself. Some organisms which reproduce asexually are hydra, bacteria, and single celled organisms. Mitosis generates two daughter cells which are identical to parent cell. If parent cell is haploid (N), then daughter cells will be haploid. If parent cell is diploid, the daughter cells will also be diploid.

Function of Mitosis:

Mitosis follows up G2-phase of the interphase in a cell cycle. DNA is already duplicated and controlled by then. At the starting of mitosis nuclear envelop vanishes and chromosomes condense strongly by folding in the spiral-like way around protein molecules. Being compact chromosomes can be more simply divided over the daughter nuclei than if they would be unfolded. Condensed single chromosomes can be well visualized under a light microscope.

Schematically: for diploid mothercells (for example cells of a leaf)

2n →2n + 2n

or if the mothercell was haploid (e.g. the thallus in liverworts)

n →n + n

Mitotic Division:

i) Interphase (inter = in between): interphase is the long lasting phase of cell cycle. Interphase is regarded as the phase during which cells perform their normal cellular functions, that is take up nutrients, grow, read DNA and generate proteins, and arrange themselves for mitosis, in particular by replicating their DNA.

ii) Prophase (pro = before): preparative phase. This is the first phase of mitosis. Chromosomes start to spiralize and nuclear membrane and nuclear bodies (nucleoli = place in nucleus active in synthesis of ribosomes) vanish. Centrosomes (comprising each of a pair of centrioles - only in animal cells) which have been duplicated during G2-phase separate now. All chromosomes become observable and comprise now of two chromatids which are attached together at centromere.

iii) Metaphase (meta = mid): middle phase. At metaphase every chromosomes are entirely spiralized and move to the middle of the cell. The spindle of microtubules (thread-like structures made of tubulin polymers) is prepared from pole to pole (from centrioles in animal cells) and from pole to centromere.

iv) Anaphase (ana = upper): the separation phase. Two chromatids of each chromosome are pulled apart by microtubules joined to kinetochore (= a specialized area of centromere) in direction of opposite poles. Every single chromatid can be considered as new chromosome from now on.

v) Telophase (telos = end): the final phase. This is the termination of nuclear division. Spindle vanishes, the chromosomes despiralize, a new nuclear envelop is formed and new nucleoli are observable.

vi) Cytokinesis (kytos = hollow vessel = cell and kinesis = movement): two daughter cells become independent. During cytokinesis which follows up actual mitosis, cytoplasm of daughter cells is divided by the cell membrane (and in plants also a cell wall) in two single compartments. In animal cells separation of the new cells engages a cleavage furrow which pinches cell membrane. In plants, this procedure is classified by formation and growth of the cell plate which expands from space between two daughter nuclei towards cell periphery. At times remnants of spindle (phragmoplast) are involved in the attachment of new wall.

Meiosis:

Meiosis is a sperm and egg production of cells. Such cells are Gamete or Sex cells. Every cell has to undergo division procedure twice for the cell to end up with half the number of chromosomes. Cells pass on genetic information to offspring. This is the form of Sexual reproduction, where one organism or cells reproduces by crossing with another organism or cell. Kinds of organisms which reproduce sexually are; plants, animals, and insects. Meiosis generates daughter cells which have one half the number of chromosomes as parent cell.

Function of Meiosis:

1) Meiosis functions to decrease the number of chromosomes to one half. Every daughter cell which is produced will have one half as many chromosomes as parent cell. Meiosis is part of sexual procedure as gametes (sperm, eggs) have one half the chromosomes as diploid (2N) individuals. In animals, meiosis takes place only when gametes (sperm, eggs) are formed.

2) Meiosis allows organisms to reproduce sexually. Gametes (sperm and eggs) are haploid. It engages two divisions producing the total of four daughter cells. The cell going through meiosis will split two times; First division is meiosis 1 and second is meiosis 2. Phases have the same names as those of mitosis. A number points out the division number (1st or 2nd).

meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1

meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2

In the first meiotic division, number of cells is doubled but number of chromosomes is not. This results in 1/2 as many chromosomes per cell. Second meiotic division is like mitosis; number of chromosomes doesn't get decreased.

Meiotic Division:

i) Prophase I: Homologous chromosomes become paired. Crossing-over takes place between homologous chromosomes.

ii) Metaphase I: Homologous pairs become aligned in the center of the cell. The random alignment pattern is called independent assortment.

For example, a cell with 2N = 6 chromosomes could have any of the alignment patterns shown at the left.

iii) Anaphase I: Anaphase I is when tetrads separate, and are drawn to opposite poles by spindle fibers. Centromeres in Anaphase I remain intact.

iv) Telophase I: Telophase I is like Telophase of mitosis, except that only one set of (replicated) chromosomes is in each cell. Depending on species, new nuclear envelopes may or may not create.

i) Prophase II: During Prophase II nuclear envelopes (if they formed in Telophase I) dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. Indeed Meiosis II very resembles mitosis.

ii) Metaphase II: Metaphase II is like mitosis, with spindles moving chromosomes in equatorial area and joining to opposite sides of centromeres in kinetochore region.

iii) Anaphase II: During Anaphase II, centromeres divide and former chromatids (now chromosomes) are separated in opposite sides of cell.

iv) Telophase II: Telophase II is just like Telophase of mitosis. Cytokinesis divides the cells.

Mitosis or Meiosis? Sperm or Egg?:

In several species, germ cells migrating in gonad are bipotential and can distinguish in either sperm or ova, depending on their gonadal environment. When ovaries of salamanders are experimentally transformed in testes, resident germ cells finish their oogenic differentiation and start developing as sperm. Likewise, in housefly and mouse, the gonad is capable to direct the differentiation of germ cell Therefore, in most organisms, sex of the gonad and of its germ cells is same. There are two decisions which presumptive germ cells have to make. First is whether to go in meiosis or to remain mitotically dividing stem cell. Second is whether to become the egg or a sperm. Mitotic/meiotic decision is handled by the single non-dividing cell at the end of each gonad, the distal tip cell. Germ cell precursors near this cell divide mitotically, forming pool of germ cells; but as these cells get farther away from distal tip cell, they go into meiosis. If distal tip cell is destroyed by the focused laser beam, all the germ cells enter meiosis, and if the distal tip cell is placed in the different location in gonad, germ line stem cells are produced near its new position. Distal tip cell extends long filaments which touch distal germ cells. The extensions contain in their cell membranes the Lag-2 protein, a C. elegans homologue of Delta.

All the germ cell precursors of nematodes homozygous for recessive mutation glp-1 start meiosis, leaving no mitotic population. When genetic chimeras are made in which wild-type germ cell precursors are found inside the mutant larva, wild-type cells are able to react to distal tip cells and endure mitosis. Though, when mutant germ cell precursors are found inside wild-type larvae, they all go into meiosis.

After the germ cells start their meiotic divisions, they still should become either sperm or ova. Usually, in each ovotestis, most proximal germ cells generate sperm, whereas the most distal (near the tip) become eggs. This means that germ cells entering meiosis early turn into sperm, whereas those entering meiosis later turn into eggs. Genetics of this switch are presently being examined.

During early development, fem genes, particularly fem-3, are vital for specification of sperm cells. Loss-of-function mutations of these genes convert XX nematodes in females (that is, spermless hermaphrodites). Fog gene products activate genes involved in transforming germ cell in sperm and also inhibit those genes which would otherwise direct germ cells to start oogenesis. Oogenesis can start only when fem activity is suppressed. This suppression seems to act at level of RNA translation. 3´ untranslated region (3´ UTR) of fem-3 mRNA has a sequence which binds the repressor protein during normal development. If this region is mutated such that repressor can't bind, fem-3 mRNA remains translatable, and oogenesis never takes place. Result is hermaphrodite body which generates only sperm. Trans-acting repressor of fem-3 message is the combination of Nanos and Pumilio proteins (the same combination which represses hunchback message translation in Drosophila). Up-regulation of Pumilio expression may be vital in regulating germ line switch from spermatogenesis to oogenesis, as Nanos is made constitutively. Nanos seems to be essential in C. elegans (as it is in Drosophila) for survival of all germ line cells.

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