XI Biology - Topic Cell division-Meiosis, Notes for NEET,CET. It includes detailed description of meiosis with images.

1. Cell Division-
Meiosis
Tarini Joshi
M.Sc., M.Ed., NET, SET.

3. Meiosis:
a) The term meiosis was coined by J. B. Farmer in 1905.
b) It takes place only in reproductive cells during the formation of gametes.
By this division, the number of chromosomes is reduced to half, hence it is
also called reductional division.
c) The cells in which meiosis take place are termed as meiocytes.
d) Meiosis produces four haploid daughter cells from a diploid parent cell.
Meiosis is of two subtypes:
A. First meiotic division or Heterotypic division – (Meiosis I)
B. Second meiotic division or Homotypic division (Meiosis II)

4. Genetics Terminology: Ploidy
Refers to the number of sets of chromosomes in cells.
● Haploid – one copy of each chromosome
– designated as “n”, the number of
chromosomes in one “set” in gametes
● Diploid – two sets of chromosomes (two of
each chromosome)
– designated as “2n” in somatic cells
Diploid organisms receive one of each type of chromosome from female
parent (maternal chromosomes) and one of each type of chromosome from
male parent (paternal chromosomes)
DO YOU KNOW?

5. Chromosomes exist in homologous pairs in diploid (2n) cells.
Genetics Terminology: Homologues
Exception: Sex chromosomes (X, Y).
All other chromosomes (autosomes) have homologues.

7. Meiosis I
First meiotic division or Heterotypic division 1) During 1st meiotic division,
diploid cell is divided into two haploid cells.
2) The daughter cells resulting from this division are different from the parent
cell in chromosome number. Hence this division is also called heterotypic
division.
3) It consists of the phases like –
a. prophase-I
(Leptotene ,Zygotne,Pachytene,Diplotene,Diakinesis)
b. metaphase-I
c. anaphase-I
d. telophase-I
e. cytokinesis-I

8. Leptotene: (leptos –thin,nema
thread)
1) The volume of nucleus increases.
2) The chromosomes become distinct,
long thread-like and coiled.
3) They take up a specific orientation-
the 'bouquet stage' inside the
nucleus.
4) This is characterised with the ends
of chromosomes converged towards
that side of the nucleus where the
centrosome lies.
5) The centriole divides into two and
migrate to opposite poles.

9. Zygotene: (zygo-Pair,nema –
thread)
1) Intimate pairing of non-sister
chromatids of homologous
chromosomes takes place by
formation of synaptonemal
complex. This pairing is called
synapsis.
2) Each pair consists of a
maternal chromosome and a
paternal chromosome.
Chromosomal pairs are called
bivalents or tetrads.

10. Pachytene: Pachus- thin,nema-thread)
1) At this stage, tetrads become clearer in
appearance because of presence of four visible
chromatids.
3) The homologous chromosomes of each pair
begin to separate from each other. However,
they do not completely separate but remain
attached together at one or more points.
4) These points appear like a cross (X)known as
chiasmata.
5) Chromatids break at these points and broken
segments are exchanged between non-sister
chromatids of homologous chromosomes. This
is called as crossing- over or recombination.

11. Diplotene: (diplo-two)
1) Though chiasmata are
formed in pachytene, they
become clearly visible in
diplotene
2) the beginning of repulsion
between synapsed
homologous chromosomes.
This is called desynapsis.
3) It involves disappearance of
synaptonemal complex.

12. Diakinesis:
1) In this phase, the chiasmata
move along the length of
chromosomes from the
centromere towards the ends of
chromosomes.
2) The displacement of
chiasmata is termed as
terminalization. The terminal
chiasmata exist till the
metaphase.
3) The nucleolus and the
nuclear membrane disappears.

13. b. Metaphase-I:
1. The spindle fibers become well-developed.
2. The tetrads move towards the equator and they orient themselves on the equator in
such a way that centromeres of homologous tetrads lie towards the poles and arms
towards the equator.
3. Due to increasing repulsive forces between homologous chromosomes.

15. Anaphase-I:
1. In this phase, homologous chromosomes are pulled away from each other and carried
towards opposite poles by spindle apparatus. This is called as disjunction.
2. The two sister chromatids of each chromosome do not separate in meiosis-I.
Chromosomes, they are ready to separate from each other.
3. This is reductional division.
4. The sister chromatids of each chromosome are connected by a common centromere.
5. Both sister chromatids of each chromosome are now different in terms of genetic content
as one of them has undergone the recombination.

16. d. Telophase-I:
1) The haploid number of chromosomes after reaching their respective poles, become
uncoiled and elongated.
2) The nuclear membrane and the nucleolus reappear and thus two daughter nuclei are
formed.

17. Cytokinesis-I:
1. After the karyokinesis, cytokinesis
occurs and two haploid cells are
formed.
2. In many cases, these daughter cells
pass through a short resting phase or
interphase/ interkinesis.
3. In some cases, the changes of the
telophase may not occur.
4. The anaphase directly leads to the
prophase of meiosis II.

18. Meiosis II
Second meiotic division or Homotypic Division
1. During this division, two haploid cells formed during first
meiotic division divide further into four haploid cells.
2. This division is similar to mitosis.
3. The daughter cells formed in second meiotic division are
similar to their parent cells with respect to the chromosome
number formed in meiosis-I. Hence this division is called
homotypic division.
It consists of the following phases: - prophase-II
metaphase-II anaphase-II telophase-II cytokinesis-II

19. Prophase-II:
1. The chromosomes are
distinct with two chromatids.
2. Each centriole divides into
two resulting in the formation
of two centrioles which
migrate to opposite poles and
form asters.
3. Spindle fibers are formed
between the centrioles.
4. The nuclear membrane and
nucleolus disappear.

20. Metaphase-II:
1. Chromosomes gets arranged at
the equator.
2. Some spindle fibers are
attached to the centromeres and
some are arranged end to end
between two opposite centrioles

21. Anaphase-II:
1. The two chromatids of each
chromosome are separated by
the division of the centromere
2. The separated chromatids
become daughter
chromosomes and move to
opposite poles due to the
contraction of the spindle
fibers attached to
centromeres.

22. Telophase-II:
1. During this stage the
daughter chromosomes
uncoil.
2. The nuclear membrane
reappear and nucleolus
reappear

23. Cytokinesis-II:
1. Cytokinesis occurs after nuclear
division.
2. Two haploid cells are formed from
each haploid cell. Thus, in all, four
haploid daughter cells are formed.
3. These cells undergo further
changes to develop into gametes.

24. Significance of Meiosis:
1. Meiotic division produces gametes. If it is absent, the number of
chromosomes would double or quadruple resulting in the formation of
monstrosities (abnormal forms).
2. The constant number of chromosomes in a given species across
generations is maintained by meiosis.
3.Because of crossing over, exchange of genetic material takes place
leading to genetic variations, which are the raw materials for evolution.

25. Genetic shuffling of Meiosis I
In addition to a new combination of chromosomes resulting from
fertilization, there are also events in Meiosis I that shuffle the genes.
1. Crossing over in Prophase I.-
Homologues break at identical locations,
then rejoin opposite partners chromosome.
2. Independent assortment in Metaphase I.
From the Virtual Cell Biology Classroom on ScienceProfOnline.com

26. Genetic shuffling of Meiosis I
From the Virtual Cell Biology Classroom on ScienceProfOnline.com

27. Independent assortment of chromosomes
meiosis introduces genetic variation
gametes of offspring do not have same combination of genes as
gametes from parents
random assortment in humans produces
223 (8,388,608) different combinations in gametes
Variation from genetic recombination
from Dad
from Mom offspring
new gametes
made by offspring

29. Test Your Knowledge..

30. Q1. In terms of Independent Assortment -how many different combinations of
sperm could a human male produce?
Q2. What are the three sources of
genetic variation?
Q3. A cell containing 20 chromosomes (diploid) at the beginning of meiosis would,
at its completion, produce cells containing how many chromosomes?
Q4. A cell containing 40 chromatids at the beginning of meiosis would, at its
completion, produce cells containing how many chromosomes?

31. Ans 1. Formula: 2n
Human chromosomes: 2n = 46
n = 23
223 = ~8 million combinations
Ans2.
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Ans 3. 10 chromosomes (haploid)
Ans 4. 10 chromosomes

32. References –
NCERT Textbook of Biology
H.Sc. Textbook of Biology
https://www.google.com/images