1. Meiosis
Submitted by:
Karla Cunanan
Kristine Chua

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

4. Stages
 Meiosis 1 (reductional division)
 Meiosis 2 (equational division)

5. Definition
 Meiosis I separates homologous chromosomes, producing two
haploid cells (N chromosomes, 23 in humans), so meiosis I is
referred to as a reductional division.
 Meiosis II is the second part of the meiotic process.
Mechanically, the process is similar to mitosis, though its genetic
results are fundamentally different. The end result is production
of four haploid cells (23 chromosomes, N in humans) from the
two haploid cells (23 chromosomes, N * each of the
chromosomes consisting of two sister chromatids) produced in
meiosis I.

6. Meiosis I Sub stages
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I

8.  Prophase 1 - DNA is exchanged between homologous chromosomes in
a process called homologous recombination.
 Metaphase 1 - Homologous pairs move together along the metaphase
plate: As kinetochore microtubules from both centrioles attach to their
respective kinetochores, the homologous chromosomes align along an
equatorial plane that bisects the spindle, due to continuous
counterbalancing forces exerted on the bivalents by the microtubules
emanating from the two kinetochores of homologous chromosomes.
 Anaphase 1 - kinetochores (bipolar spindles) microtubules shorten,
severing the recombination nodules and pulling homologous
chromosomes apart.
 Telophase 1 -The last meiotic division effectively ends when the
chromosomes arrive at the poles. Each daughter cell now has half the
number of chromosomes but each chromosome consists of a pair of
chromatids.

10. Sub stages of prophase 1
 Leptotene
 Zygotene
 Pachytene
 Diplotene

11.  Leptotene - The first stage of prophase I is the leptotene stage, also known as
leptonema, from Greek words meaning "thin threads".In this stage of prophase I,
individual chromosomes—each consisting of two sister chromatids—change from the
diffuse state they exist in during the cell's period of growth and gene expression, and
condense into visible strands within the nucleus.
 Zygotene - The zygotene stage, also known as zygonema, from Greek words meaning
"paired threads, occurs as the chromosomes approximately line up with each other into
homologous chromosome pairs.
 Pachytene - The pachytene stage, also known as pachynema, from Greek words
meaning "thick threads,is the stage when chromosomal crossover (crossing over)
occurs. Nonsister chromatids of homologous chromosomes may exchange segments
over regions of homology.
 Diplotene - During the diplotene stage, also known as diplonema, from Greek words
meaning "two threads”, the synaptonemal complex degrades and homologous
chromosomes separate from one another a little.

12. Meiosis II Sub stages
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II

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

16. Why is Meiosis Important
Meiosis is a phase in sexually
reproductive organisms, in which
cell-division takes place. It is of
great importance, because it
creates genetic diversity in the
population. .

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

18. END