MeiosisMeiosis

Submitted by:Submitted by: Karla CunananKarla Cunanan Kristine Chua Kristine Chua

Meiosis- Meiosis- is a special type is a special type of cell division of cell division that produces that produces gametes with gametes with half as many half as many chromosomes. chromosomes.

StagesStages

Meiosis 1 (reductional Meiosis 1 (reductional division)division)

Meiosis 2 (equational Meiosis 2 (equational division)division)

DefinitionDefinition

Meiosis I separates homologous chromosomes, Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reductional humans), so meiosis I is referred to as a reductional division.division.

Meiosis II is the second part of the meiotic process. Meiosis II is the second part of the meiotic process. Mechanically, the process is similar to mitosis, Mechanically, the process is similar to mitosis, though its genetic results are fundamentally though its genetic results are fundamentally different. The end result is production of four haploid different. The end result is production of four haploid cells (23 chromosomes, N in humans) from the two cells (23 chromosomes, N in humans) from the two haploid cells (23 chromosomes, N * each of the haploid cells (23 chromosomes, N * each of the chromosomes consisting of two sister chromatids) chromosomes consisting of two sister chromatids) produced inproduced in

meiosis I. meiosis I.

Meiosis I Sub stagesMeiosis I Sub stages

Prophase IProphase I Metaphase IMetaphase I Anaphase IAnaphase I Telophase ITelophase I

Prophase 1 - DNA is exchanged between homologous Prophase 1 - DNA is exchanged between homologous chromosomes in a process called homologous chromosomes in a process called homologous recombination.recombination.

Metaphase 1 - Homologous pairs move together along the Metaphase 1 - Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of microtubules emanating from the two kinetochores of homologous chromosomes.homologous chromosomes.

Anaphase 1 - kinetochores (bipolar spindles) microtubules Anaphase 1 - kinetochores (bipolar spindles) microtubules shorten, severing the recombination nodules and pulling shorten, severing the recombination nodules and pulling

homologous chromosomes apart.homologous chromosomes apart. Telophase 1 -The last meiotic division effectively ends Telophase 1 -The last meiotic division effectively ends

when the chromosomes arrive at the poles. Each daughter when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each cell now has half the number of chromosomes but each

chromosome consists of a pair of chromatids.chromosome consists of a pair of chromatids.

Sub stages of prophase 1Sub stages of prophase 1

LeptoteneLeptotene ZygoteneZygotene PachytenePachytene DiploteneDiplotene

Leptotene - The first stage of prophase I is the leptotene stage, Leptotene - The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin also known as leptonema, from Greek words meaning "thin threads".In this stage of prophase I, individual chromosomes—threads".In this stage of prophase I, individual chromosomes—each consisting of two sister chromatids—change from the diffuse each consisting of two sister chromatids—change from the diffuse state they exist in during the cell's period of growth and gene state they exist in during the cell's period of growth and gene expression, and condense into visible strands within the nucleus.expression, and condense into visible strands within the nucleus.

Zygotene - The zygotene stage, also known as zygonema, from Zygotene - The zygotene stage, also known as zygonema, from Greek words meaning "paired threads, occurs as the Greek words meaning "paired threads, occurs as the chromosomes approximately line up with each other into chromosomes approximately line up with each other into homologous chromosome pairs.homologous chromosome pairs.

Pachytene - The pachytene stage, also known as pachynema, from Pachytene - The pachytene stage, also known as pachynema, from Greek words meaning "thick threads,is the stage when Greek words meaning "thick threads,is the stage when chromosomal crossover (crossing over) occurs. Nonsister chromosomal crossover (crossing over) occurs. Nonsister chromatids of homologous chromosomes may exchange segments chromatids of homologous chromosomes may exchange segments over regions of homology.over regions of homology.

Diplotene - During the diplotene stage, also known as diplonema, Diplotene - During the diplotene stage, also known as diplonema, from Greek words meaning "two threads”, the synaptonemal from Greek words meaning "two threads”, the synaptonemal complex degrades and homologous chromosomes separate from complex degrades and homologous chromosomes separate from one another a little.one another a little.

Meiosis II Sub stagesMeiosis II Sub stages

Prophase IIProphase II Metaphase IIMetaphase II Anaphase IIAnaphase II Telophase IITelophase II

Prophase II we see the disappearance of the nucleoli Prophase II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second polar regions and arrange spindle fibers for the second meiotic divisionmeiotic divisionMetaphase II, the centromeres contain two kinetochores Metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.I, perpendicular to the previous plate.anaphase II, where the centromeres are cleaved, anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they convention are now called sister chromosomes as they move toward opposing poles.move toward opposing poles.telophase II, which is similar to telophase I, and is telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells.now complete and ends up with four new daughter cells.

Why is Meiosis ImportantWhy is Meiosis ImportantMeiosis is a phase in Meiosis is a phase in sexually reproductive sexually reproductive

organisms, in which cell-organisms, in which cell-division takes place. It is division takes place. It is

of great importance, of great importance, because it creates genetic because it creates genetic

diversity in the diversity in the population. .population. .

ImportanceImportance

Meiosis is a process of gamete Meiosis is a process of gamete formation in which diploid germ-line formation in which diploid germ-line cells, that is, the cells that are set cells, that is, the cells that are set aside early in animal development aside early in animal development for sexual reproduction, yield four for sexual reproduction, yield four genetically different haploid cells. It genetically different haploid cells. It occurs only in sex cells, which are occurs only in sex cells, which are eggs and sperms .eggs and sperms .

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