Cell division- a process of perpetuity. Mitosis for somatic cells and meiosis for germ cells.

1. Chromosomal theory of inheritance
Bipasha Datta
Genetics and Plant Breeding

2. A brief description
First proposed by – Sutton & Boveri in 1903.
Popularly known as Sutton-Boveri hypothesis.
This theory states that the inheritance of
characters from parents to their offspring is
governed by chromosomes.
or
Chromosomes are considered as physical basis of
inheritance.

3. They concluded that chromosomes contain
genes which govern the transmission of
characters from parents to their offspring.
This concept was developed working with
grasshoppers.

4. Concepts which support Sutton-Bovery
Hypothesis
 Sex chromosomes
 Sex linkages
 Structural chromosomal changes
 Monosomic & nullisomic analysis
 Bar locus
 Crossing over

5. Cell Division

6.  The process of reproduction or formation of new
cells from the pre-existing cells is called as Cell
division.
 The cell which undergoes division is called as
mother cell.
 The new cells which are formed by the process of
cell division are termed as daughter cell.

7.  In bacteria, cell division takes place by fission
(cleavage) of pre-existing cell.

8.  In Higher organisms (Eukaryotes) – there are two
types of cell division
I. Mitosis
II. Meiosis

9. Cell Cycle

10. The period in which one cycle of cell division is
completed is called cell cycle.
A cell cycle consists of two phases
A. Interphase
B. Mitotic phase (Somatic cell)
or
Meiotic phase (Gametic cell)

12. A. Interphase

13. It consists of following sub-phases,
viz.,
a). G1– the resting phase
b). S – the period of DNA replication
c). G2– the resting phase

14. a). G1 :
 It is a pre-DNA replication
phase.
 Thus this a phase between
telophase and S phase.
 This is the longest phase
which takes 12 hours in Vicia
faba.
 Protein and RNA syntheses
take place during this place.

15. b). S:
 This phase comes after
G1.
The chromosome and
DNA replications take
place during this phase.
This phase takes lesser
time than G1.
In Vicia faba, it takes six
hours.

16. c). G2 :
 This is the post-DNA
replication phase.
 It is the last stage of
interphase.
 Protein and RNA
syntheses occur during
this stage.
 This phase also takes
12 hours in Vicia faba.

17. B. Mitotic phase

18. • Separation of replicated DNA into two daughter
nuclei without recombination.
• Thus the nuclei have the same chromosome as
that of parent nucleus.
The M phase consists of four stages
ⅰ. Prophase
ⅱ. Metaphase
ⅲ. Anaphase
ⅳ. Telophase

20. Significance of mitosis
• Mitosis plays an important role in the life of living organisms in various way such as-
• After fusion of male and female gamate zygote is formed. Mitosis is responsible for
development of a zygote into adult organism.
• Mitosis is essential for normal growth and development of living organisms. It gives a
definite shape to a specific organsm.
• In plants, mitosis leads to formation of new parts, viz., roots, leaves, stems and branches.
It also helps in repairing of damaged parts.
• In case of vegetatively propagated crops like sugarcane, sweet potato, potato etc., mitosis
helps in asexual propagation. Mitosis leads to production of identical progeny in such
crops.
• Mitosis is useful in maintaining the purity of types because it leads to production of
identical daughter cells and does not allow segregation and recombination to occur.
• In animals, it helps in continuous replacement of old tissues with new ones, such as gut
epithelium and blood cells.

21. B. Meiotic phase

22. The process of meiosis consists of two
types of division
• 1). First meiotic division
• 2). Second meiotic division

23. 1). First meiotic division
• It results in reduction of chromosome number in
each new cell to just half of the mother cell.
• Thus it is referred to as reductional division.
• It consists of four different phases, viz.,
ⅰ. Prophase Ⅰ
ⅱ. Metaphase Ⅰ
ⅲ. Anaphase Ⅰ
ⅳ. Telophase Ⅰ

24. 2). Second meiotic division
• It is required to reduce the number of chromatids
per chromosome.
• It consists of four stages i.e.,
ⅰ. Prophase Ⅱ
ⅱ. Metaphase Ⅱ
ⅲ. Anaphase Ⅱ
ⅳ. Telophase Ⅱ
This stages are similar to mitosis stages.

26. Meiosis Ⅱ differs from mitosis in the
following main aspects:
• the interphase prior to meiosis Ⅱ is very short. It
does not have S period because each chromosome
already contains two chromatids.
• The two chromatids in each chromosome are not
sisters throughout. i.e., some chromatids have alternate
segments of non-sister chromatids due to
recombination.
• The meiosis (meiotic mitosis) Ⅱ deals haploid
chromosome number, whereas normal mitosis deals
with diploid chromosome number.

27. Cytokinesis
• The division of cytoplasm takes place either by
cell plate method (in plants) or by furrow method
(in animals).
• The cytokinesis may take place after meiosis Ⅰ
and meiosis Ⅱ separately or sometimes it may
take place at the end of meiosis Ⅱ only.
• In maize, it occurs after meiosis Ⅰ and meiosis
Ⅱ.
• However, in Trillium cytokinesis occurs only at
the end of Meiosis Ⅱ.

28. Significance of meiosis
• It helps in maintaining the chromosome number constant in a species.
• Meiosis facilitates segregation and independent assortment of
chromosomes and genes.
• The recombination of genes also takes place during meiosis.
Recombination of genes results in generation of variabilty in a
biological population which is important from evolution points of
view.
• In sexually reroducing species, meiosis is essential for the continuity
of generation. Because meiosis results in the formation of male and
female gametes and union of such gametes leads to the development
of zygote and thereby new individual.

29. Comparison of MITOSIS and MEIOSIS
MITOSIS
• It occurs in somatic tissues.
• Consists of one nuclear division.
• One cell cycle results in
production of two daughter cells.
• Total DNA of nucleus replicates
during S phase.
• There is no pairing between
homologous chromosomes.
MEIOSIS
• It occurs in reproductive tissues.
• Consists of two nuclear divis ions.
• One cell cycle results in the
formation of four daughter cells.
• 99.7% DNA replicates during S phase
and remaining 0.3% during zygotene
stage.
• Homologous chromosomes pair
during during pachytene.

30. • Daughter cells are identical
with mother cell in
structure and chromosome
composition.
• Segregation and
recombinatio do not occur.
• Chromosomes are in the
form of dyad at metaphase.
• At metaphase, centromere
of each bivalent divides
longitudinally.
• Daughter cells are different
from mother cell in
chromosome number and
composition.
• Crossing over takes place
during pachytene.
• Chromosomes are in the
form of tetrad at
metaphase.
• The centromere does not
divide at metaphase.
MITOSIS MEIOSIS

31. • The chromosome number
of daughter cells is the
same as that of mother cell
(2n).
• Maintains purity due to
lack of segregation and
recombination.
• One member of sister
chromatids move to
opposite pole during
anaphase.
• Daughter cells contain half
the chromosome number
of mother cell (n).
• Generates variability due to
segregation and
recombination.
• One member of
homologous chromosomes
moves to opposite pole
during anaphase Ⅰ.
MEIOSISMITOSIS