Meiosis is a double division which occurs in the diploid cells and give rise to four haploid cells ,each having half the number of chromosomes as compared to the parent cell. Meiosis is the process in which a single cell divides twice to form four haploid daughter cells. These cells are the gametes – sperms in males and egg in females. For more details, visit @biOlOgy BINGE-insight learning

1. THE REDUCTIONAL DIVISION
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2.  Meiosis is a double division which occurs in the diploid cells and give rise to four haploid cells ,each having
half the number of chromosomes as compared to the parent cell.
 Meiosis is the process in which a single cell divides twice to form four haploid daughter cells.
 These cells are the gametes – sperms in males and egg in females.
 Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms used to produce
the gametes, such as sperm or egg cells.
 It involves two rounds of division that ultimately result in four cells with only one copy of each paternal and
maternal chromosome (haploid).
 Additionally, prior to the division, genetic material from the paternal and maternal copies of each
chromosome is crossed over, creating new combinations of code on each chromosome.
 Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to
create a cell with two copies of each chromosome again, the zygote.

4.  Mitosis is used for almost all of your body’s cell division needs. It adds new cells during development and
replaces old and worn-out cells throughout your life.
 The goal of mitosis is to produce daughter cells that are genetically identical to their mothers, with not a
single chromosome more or less.
 Meiosis, on the other hand, is used for just one purpose in the human body: the production of gametes—
sex cells, or sperm and eggs.
 Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell.
 To put that another way, meiosis in humans is a division process that takes us from a diploid cell—one with
two sets of chromosomes—to haploid cells—ones with a single set of chromosomes.
 In humans, the haploid cells made in meiosis are sperm and eggs. When a sperm and an egg join in
fertilization, the two haploid sets of chromosomes form a complete diploid set: a new genome.

5.  “Meiosis is the type of cell division that
results in four daughter cells, each with half
the number of chromosomes of the parent
cell.”

6. THE PROCESS OF MEIOSIS IS DIVIDED INTO 2 STAGES. EACH STAGE
IS SUBDIVIDED INTO SEVERAL PHASES.
 Meiosis I:
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I
 Cytokinesis I
 Meiosis II:
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II
 Cytokinesis II

7.  In many ways, meiosis is a lot like mitosis.
 The cell goes through similar stages and uses similar strategies to organize and separate
chromosomes.
 In meiosis, however, the cell has a more complex task. It still needs to separate sister
chromatids (the two halves of a duplicated chromosome), as in mitosis.
 But it must also separate homologous chromosomes, the similar but nonidentical
chromosome pairs an organism receives from its two parents.
 These goals are accomplished in meiosis using a two-step division process.
 Homologue pairs separate during a first round of cell division, called meiosis I.
 Sister chromatids separate during a second round, called meiosis II.
 Since cell division occurs twice during meiosis, one starting cell can produce four gametes
(eggs or sperm).

8.  A couple of homologous chromosomes,
or homologs, are a set of one maternal and
one paternal chromosome that pair up with
each other inside a cell during fertilization.
 Homologs have the same genes in the
same loci where they provide points along
each chromosome which enable a pair of
chromosomes to align correctly with each
other before separating during meiosis.

9.  It is called the heterotypic or reduction division.
 It has 4 stages:
 Prophase1
 Metaphase1
 Anaphase 1
 Telophase1

10.  Prophase I is by far the longest phase of meiosis . During prophase I, homologous
maternal and paternal chromosomes pair, synapse, and exchange genetic
information (by homologous recombination), forming at least one crossover per
chromosome.These crossovers become visible as chiasmata (plural;
singular chiasma).
 Prophase is divided into 5 stages:
 Leptotene
 Zygotene
 Pachytene
 Diplotene
 Diakinesis

11.  The first stage of prophase I is
the leptotene stage, also known as leptonema.
 In this stage of prophase I, individual
chromosomes—each consisting of two
replicated sister chromatids—become
"individualized" to form visible strands within
the nucleus.

12.  Leptotene is followed by the zygotene stage, also
known as zygonema.
 It is also called the bouquet stage because of the way
the telomeres cluster at one end of the nucleus.
 The two homologous chromosomes get attached to
each other laterally.
 The process of attachment is called synapsis.
 Chromosomes form pairs called bivalents.

14.  The paired chromosomes or bivalents shorten.
 Size of nucleolus reaches the maximum.
 Each bivalent or chromosome pair is made up of 4 chromatids,two of each
chromosome.
 The chromatids belonging to same chromosomes are called sister chromatids.
 Chromatids belonging to two different chromosomes of a homologous pair are
called non-sister chromatids.
 In pachytene stage,there occurs exchange of genetic material between non-
sister chromatids of two homologous chromosomes.
 This is called crossing over.
 After crossing over,the chromatids of same chromosomes become different.

16.  Diplotene is the stage when the synaptonemal
complex disassembles and homologous chromosomes separate
from one another a little.
 However, the homologous chromosomes of each bivalent remain
tightly bound at chiasmata, the regions where crossing-over
occurred.
 Therefore the homologous chromosomes undergo desynapsis
except in the region of crossing over.
 The chromatids also separate and become distinguishable –tetrad
stage.

18.  Chromosomes condense further during the diakinesis stage.
 This is the first point in meiosis where the four parts of the tetrads are actually
visible.
 Sites of crossing over entangle together, effectively overlapping, making
chiasmata clearly visible.
 Simultaneously,nuclear envelope disintegrates.
 In animal cells,the two centrioles move to the opposite poles of the nucleus.
 They develop a number of radiating microtubules from their surface called astral
rays.
 Centrioles with astral rays is called aster.

20.  Homologous pairs move together along the
metaphase plate: As kinetochore
microtubules from both spindle poles attach
to their respective kinetochores, the paired
homologous chromosomes align along an
equatorial plane that bisects the spindle.
 The centromeres slightly project towards
the pole.
 Since there are 2 centromeres in each
bivalent ,the centromeres of all bivalents
produce a double metaphasic plate.

21.  Kinetochore microtubules shorten, pulling homologous
chromosomes (which each consist of a pair of sister chromatids) to
opposite poles.
 Nonkinetochore microtubules lengthen, pushing the centrosomes
farther apart.
 The cell elongates in preparation for division down the center.
 The homologous chromosomes break their connections and
separate out .This is called disjunction.

23.  The first 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.
 The microtubules that make up the spindle network disappear,
and a new nuclear membrane surrounds each haploid set.
 The chromosomes uncoil back into chromatin. Sister chromatids
remain attached during telophase I.

25.  Cells may enter a period of rest known as interkinesis or
interphase II.
 It is transitory stage of short duration between meiosis
1 and meiosis 2.
 RNA , proteins and other biomolecules are synthesized
during this phase.
 DNA replication is absent.
 Centriole pair also replicates.

26.  It is called homotypic or equational division.
 It is divided into 4 phases:
 Prophase
 Metaphase
 Anaphase
 Telophase

28.  Cytokinesis can be of 2 types:
 successive
 simultaneous

29.  Absence of long interphase and presence of interkinesis.
 No chromosome replication in S-phase.
 2 chromatids of a chromosome are genetically different.
 Simpler and brief.
 Daughter cells do not resemble each other .
 Neither they resemble to their mother cell.

30.  Sexual reproduction
 Maintenance of chromosome number
 Crossing over
 Recombinations