Mitosis and meiosis

MITOSIS and

MEIOSIS Prepared by: HERNAEZ, HERSHEY ANNE C. MARILIM, ERIKA JAZMINE O.

CELL CYCLE

The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication (replication). In cells without a nucleus (prokaryotic), the cell cycle occurs via a process termed binary fission. In cells with a nucleus (eukaryotes), the cell cycle can be divided in two periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA—and themitosis (M) phase, during which the cell splits itself into two distinct cells, often called "daughter cells" and the final phase, cytokinesis, where the new cell is completely divided. The cell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed.

So, the cell cycle consists of:

*Interphase*Mitosis

• Chromosomes are not visible because they are uncoiled.

INTERPHASEInterphase is the phase of the cell cycle in which the cell

spends the majority of its time and performs the majority of its purposes including preparation for cell division. The stages of

interphase are:

• G1 (Gap 1), in which the cell grows and functions normally. During this time, much protein synthesis occurs and the cell grows (to about double its original size) - more organelles are produced, increasing the volume of the cytoplasm. If the cell is not to divide again, it will remain in this phase.

• Synthesis (S), in which the cell duplicates its DNA (via semiconservative replication). This is also known as the Swanson phase.

• G2 (Gap 2), in which the cell resumes its growth in preparation for mitosis.

MITOSIS Mitosis is the process by which a eukaryotic

 cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle—the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell.

PROPHASE

• The chromosomes coil.• The nuclear membrane disintegrates.

• Spindle fibers form.

METAPHASE

• The nuclear membrane is completely gone.• The chromosomes become aligned.

ANAPHASE

• The chromatids separate; the number of chromosomes doubles.

• Pushed and pulled toward opposite poles by the spindle fibers.

TELOPHASE

• The cell divides into two.• The chromosomes uncoil.• The nucleus reforms.

• The spindle apparatus disassembles.

MEIOSIS

Meiosis is a special type of cell division necessary for sexual reproduction in eukaryotes. The cells produced by meiosis are gametes or spores. In many organisms, including all animals and land plants, gametes are called sperm and egg cells.

Meiosis I : Reductional Division

Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromatids, it is only considered as being N, with 23 chromosomes. This is because later, in Anaphase I, the sister chromatids will remain together as the spindle fibres pull the pair toward the pole of the new cell. In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) - two from each daughter cell from the first division.

Prophase IDivided into 5 substages:

LeptonemaDuring this stage, the chromosomes begin to condense and become visible. Researchers also believe that homologous pair searching begins also at this stage.

ZygonemaThe chromosomes continue to become denser. The homologous pairs have also found each other and begin to initially align with one another, referred to as 'rough pairing'. Lateral elements also form between the two homologous pairs, forming a synaptonemal complex.

PachynemaCoiling and shortening continues as the chromosomes become more condense. A synapsis forms between the pairs, forming a tetrad.

DiplonemaThe sister chromatids begin to separate slightly, revealing points of the chiasma. This is where genetic exchange occurs between two non-sister chromatids, a process known as crossing over.

Diakinesis

The chromosomes continue to pull apart, but non-sister chromatids are still loosely associated via the chiasma. The chiasma begin to move toward the ends of the tetrad as separation continues. This process is known as terminalization. Also during diakinesis, the nuclear envelope breaks down and the spindle fibers begin to interact with the tetrad.

METAPHASE I

• The centrioles are at opposite poles of the cell.• The pairs of homologous chromosomes (the bivalents), now as

tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the

metaphase plate.• Spindle fibers from one pole of the cell attach to one chromosome

of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again,

seen as sister chromatids).

ANAPHASE I

• Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite

poles of the cell as a result of the action of the spindle.• Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is

that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate.

TELOPHASE I

• The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still

having two chromatids.• A nuclear envelope reforms around each chromosome set, the

spindle disappears, and cytokinesis follows. In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of

the two progeny cells has a nucleus with a haploid set of replicated chromosomes.

• Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Other cells do exhibit chromosome decondensation at this time; the chromosomes

recondense in prophase II.

PROPHASE II

• While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II.

• The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a

daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes.

• The nuclear envelope breaks down, and the spindle apparatus forms.

METAPHASE II

• Each of the daughter cells completes the formation of a spindle apparatus.

• Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis. This is in contrast to

metaphase I, in which homologous pairs of chromosomes align on the metaphase plate.

• For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to

a kinetochore microtubule coming from that pole.

ANAPHASE II

• The centromeres separate, and the two chromatids of each chromosome move to

opposite poles on the spindle. The separated

chromatids are now called chromosomes in their own right.

TELOPHASE II

• A nuclear envelope forms around each set of chromosomes.• Cytokinesis takes place, producing four daughter cells

(gametes, in animals), each with a haploid set of chromosomes.

• Because of crossing-over, some chromosomes are seen to have recombined segments of the original parental

chromosomes.