This document provides an overview of cell division, chromosomes, and DNA structure. It discusses the stages of mitosis and meiosis, the differences between them, and their significance. Mitosis produces two identical daughter cells and occurs in body cells, while meiosis produces four non-identical gametes through two cell divisions and occurs in reproductive cells to halve the chromosome number. The key events of the cell cycle and each phase of mitosis and meiosis are described in detail.

1. BIOLOGY- CLASS X
STRUCTURE OF CHROMOSOMES,
CELL CYCLE AND CELL DIVISION

2. LEARNING OUTCOME
The concept of cell division will be
understood
The two types of division- mitosis and
meiosis will be focussed upon
A basic understanding of meiosis as a
reduction division will be explained
Differences between mitosis and meiosis will
be understood
Structure of chromosome and its
components will be explained

3. NEED FOR CELL DIVISION
Cell division helps living organisms to perpetuate
generation after generation. The characters from
parental cell are passed on to the daughter cells
through chromosomes which ensure normal
functioning of the cells hence the normal working
of the organism takes place.

4. WHAT ARE CHROMOSOMES?
Chromosome comes from the word " chroma"which
means colour and "soma "which means body.
They are highly coiled and condensed chromatin
fibre.
Usually the nucleus of a cell is not prominent under
a microscope but if the same cell is stained with
a suitable dye ,the structures inside the nucleus
become prominent.
The nucleus contains the DNA of the cell which
is organized into compact structures known as
chromosomes.
Each chromosome is made up of one long DNA
molecule associated with proteins.
The complex of DNA and protein is called
chromatin. Chromatin appears in the form of of
long thin and darkly stained fibre.
During cell division these fibres coil and
condense becoming thick chromosomes.

5. DISCOVERY OF CHROMOSOMES
It was first studied in 1882 by a
German scientist, Walther
Flemming. He observed rapidly
dividing cells of larvae of
salamander. He felt that the threads
appeared to be dividing lengthwise
hence he named the division
mitosis which means thread.
CHROMATIN
It is formed of two substances
1) DNA (deoxyribonucleic acid)-40%
2) Histones (a type of protein)- 60%

6. ROSALIND FRANKLIN
WATSON & CRICK

7. MOLECULAR STRUCTURE OF DNA
 The shape was first studied by
Rosalind Franklin in 1953. The
structure was worked out by Watson
and Crick in the same year for which
they were awarded the Nobel prize in
1962.
1) A single DNA molecule is very
large hence it is termed as a
macromolecule.
2) It is composed of two
complementary strands wound
around each other in a double helix.

8. 3) Each DNA strand is composed of
nucleotides.
4) Nucleotides are made up of
phosphate, sugar (pentose-
deoxyribose) and a nitrogenous base
5) One nitrogenous base is attached
to another nitrogenous base of the
complementary strand with the help
of hydrogen bonds giving a ladder
like arrangement.
6) The bases are Adenine (A),
Guanine (G), Cytosine (C) and
Thymine (T).
7) Adenine pairs with Thymine with
two hydrogen bonds and Guanine
pairs with Cytosine with three
hydrogen bonds. This is known as
the base pairing rule.

9. HISTONE PROTEINS AND NUCLEOSOME
These are proteins that help in the
coiling and packaging of DNA into units
known as nucleosome.
Each nucleosome contains a core of 8
histone proteins which is surrounded by
a DNA strand. Another histone protein
acts as a linker DNA linking two
nucleosomes. A single human
chromosome may have about a million
nucleosomes.
The nucleus of a human cell is about 6
micrometres in diameter and it needs to
accommodate approximately 2 m of
DNA, therefore, the entire chromatin
fiber is coiled and supercoiled like that
of a telephone cord to eventually form
chromosomes.

10. RNA- Ribonucleic acid
Another kind of genetic
material is present in the
living organisms known as
RNA.
In RNA instead of Thymine,
Uracil is found, and the
pentose sugar found is
ribose.
RNA is usually single
stranded. It is the genetic
material in some viruses.

11. STRUCTURE OF CHROMOSOMES
In a condensed form a chromosome is seen during
the beginning of cell division.
It consists of two chromatids joined at a point of
attachment known as centromere which appears as a
small constricted region.
FUNCTIONS OF CENTROMERE
1) Attachment of two sister chromatids
2) Attachment of spindle fibres during cell division
The ends of a chromosome is known as telomere.
The centromere lies in in the primary constriction
region of the chromosome. Sometimes a secondary
constriction region might also be there in the
chromosome where the nucleolar organiser region
(NOR) lies. This region is responsible for formation of
of nucleolus which lies within the nucleus of a cell.
After cell division is completed the chromosomes
decondense into long and fine threads like
chromatin fibres.

12. TELOCENTRIC- The centromere is present at the tip of the
chromosome giving one single arm.
ACROCENTRIC- The centromere is present near the tip of the
chromosome giving one long and one very short arm.
SUBMETACENTRIC- The centromere is present near the centre of the
chromosome giving one long and one short arm.
METACENTRIC- The centromere is present at the centre of the
chromosome giving two equal arms.

13. WHAT ARE GENES?
Genes are specific
sequence of nucleotides on
a chromosome that encode
particular proteins which
express in the form of some
particular feature of the
body. These are segments
of DNA which act as units of
heredity which are
transferred from parents to
offsprings and are
responsible for governing
specific characteristics of
the offspring.

14. NEED FOR NEW CELLS
 1) FOR GROWTH- Cell division leads to
formation of a cluster of cells which perform
different functions in the body.

2) FOR REPLACEMENT- Time to time our cells
get destroyed due to wear and tear hence they
need to be replaced so that the older cells can
be replaced by new and more efficient cells.

3) FOR REPAIR-Sometimes, due to injury and
fracture, part of our tissues get ruptured.
Repair of such injuries take place by cell
division.

4) FOR REPRODUCTION- A special kind of cell
division known as meiosis leads to the
formation of sperms and eggs which have half
the number of chromosomes as their parent
cell. After fertilization of both gametes the
original chromosome number is restored.

15. CELL CYCLE
The process of cell division takes
place through a series of events which
is known as cell cycle.
A cell cycle consists of two phases , a
non dividing phase called interphase
and a dividing phase called M phase or
simply mitosis.
INTERPHASE
It is subdivided into three stages
1 1) FIRST GROWTH PHASE (G1)-
a) RNA and proteins are synthesized
b) Volume of cytoplasm increases
c) Mitochondria and chloroplast
divide as they have their own DNA
At the end of this phase the cells can either
withdraw from the cell cycle and enter into a
resting phase (R) or they can enter into the
synthesis phase(S).

16.  2) SYNTHESIS PHASE (S)-
 a) More DNA is synthesized
b) The chromosomes are
duplicated

3) SECOND GROWTH PHASE
(G2)-
It is a shorter growth phase
in which RNA and proteins
necessary for cell division
continue to be synthesized.

17. FORMATION OF NEW DNA
During S phase of interphase
duplication of DNA takes
place where it forms a copy
of its own. This process is
known as replication.
During this process the DNA
double helix opens at one
end making the two strands
free which are known as the
parental strand and two new
strands complementary to
the parental strand is
produced for the whole
length of DNA.

18. EXCEPTIONS IN CELL CYCLE

1) The nerve cells do not divide once they are
formed in the embryo hence they cannot be
replaced.

2) Liver cells divide only once every year to two
years to replace damaged or destroyed cells.

3) Surface cells lying on the skin are continuously
lost and replaced as these are prone to maximum
wear and tear.

4) Special germinal cells in the ovary and testis
undergo a special type of cell division known as
meiosis which leads to formation of gametes.

5) Uncontrolled cell divisions lead to formation of
tumours which might lead to cancer.


19. CELL DIVISION
It is divided into two types
a) Mitosis- Cell division leading to production of diploid cells for
growth and development.
b) Meiosis- Cell division leading to the production of haploid cells or
gametes i.e. sperms or eggs.

20. MITOSIS
Mitosis is the cell division in which
one parent cell divides into two
identical daughter cells. The same
chromosome number is maintained at
each cell division.
Mitosis is divided into 2 steps i.e.
karyokinesis and cytokinesis.
Karyokinesis or division of nucleus.
It is divided into four stages-
prophase, metaphase , anaphase and
telophase.
Cytokinesis is the division of
cytoplasm.

21. PROPHASE

1) The chromosomes become short and
thick and are clearly visible inside the
nucleus.
2) Each chromosome has duplicated to
form two chromatids.
3) The two sister chromatids remain
attached to each other at a region known
as centromere.
4) The centrosome in animal cells split to
form 2 centrioles and each centriole
starts moving towards two opposite
poles. The centrioles are surrounded by
radiating rays known as asters.
5) Spindle fibres appear between the two
daughter centrioles.
6) The nuclear membrane and nucleolus
disappear.
7) The duplicated chromosomes start
moving towards the equatorial region of
the cell

22. METAPHASE
1) The duplicated
chromosomes are arranged
on the equatorial plane.
2) Each chromosome gets
attached to a spindle fibre
by its centromere.

23. ANAPHASE

1) The centromere
attaching the two
chromatids divide and
the two sister
chromatids of each
chromosome split and
they are pulled towards
opposite poles.

2) This happens due to
contraction of spindle
fibres.

24. TELOPHASE

1) Each chromatid or daughter
chromosomes lengthen, become
thinner and decondense into a
network of chromatin threads.

2) Nuclear membrane reappears.

3) Nucleolus reappears in each
daughter nucleus.

4) The contents of the cytoplasm
including the mitochondria and
chloroplast in plant cells are
randomly distributed in the daughter
cells.

25. CYTOKINESIS
1)Division of cytoplasm
at the end of telophase.
2) A furrow appears in
the cell membrane in the
periphery and proceeds
towards the centre of the
cell which deepens and
finally splits the
cytoplasm into two
giving rise to two new
cells.

26. STAGES OF MITOSIS IN A PLANT CELL

27. ANIMAL MITOSIS PLANT MITOSIS
Asters are formed Asters are not formed
Cytokinesis by furrowing of
cytoplasm
Cytokinesis by cell plate
formation
Occurs in most tissues
throughout the body for
growth and replacement
Occurs mainly at the growing
tips for lengthening and at
sides for increase in girth

28. SIGNIFICANCE OF MITOSIS
1) Growth or increase in the body size due to
formation of new cells in the tissues.
2) Repair of damaged cells and wounded tissues by
renewal of lost cells.
3) Replacement of the old and dead cell such as the
replacement of the blood cells and the epidermal cells
of the skin.
4) Asexual reproduction in which the unicellular
organisms such as Amoeba or yeast cells divide into
two.
5) Maintains same chromosome number in daughter
cells.

29. MEIOSIS
Meiosis is a reduction division producing gametes with
half the number of chromosomes than that of the parent
cell.
‘Meion’ refers to lessen which means reduction in
chromosome number. It takes place in reproductive
organs which lead to formation of gametes like sperms
and ova in humans and pollen grains and ovules in
plants.
There are two division processes under meiosis - meiosis
1 and meiosis 2 which ensure that the gametes produced
will have half the number of chromosomes than that of
the parent cell. The original chromosome number of the
cell is restored after fertilization.

30. STAGES OF MEIOSIS

31. SIGNIFICANCE OF MEIOSIS
1) Chromosome number is halved in gametes so
that on fertilization the normal number 2n is
restored.
2) It provides for mixing up of genes which occur
in two ways-
a) The maternal and paternal chromosomes get
mixed up during the first reduction division as
they separate from the homologous pairs.
b) The maternal and paternal chromosomes
exchange genetic material between two members
of a homologous pair by a process known as
crossing over which leads to genetic
recombination.
This mixing up of genes during meiotic division
leads to variation in the progeny which ensures
that children of same parents are different from
each other in certain respects.
CROSSING OVER

32. MITOSIS MEIOSIS
Mitosis takes place in the somatic or
body cells
Meiosis takes place in reproductive
cells
It provides for growth and replacement It leads to gamete formation
It takes place throughout the lifetime It occurs only in reproductively active
age
It produces two daughter cells It produces 4 daughter cells
It has full set of chromosomes which it
passes to each daughter cell hence the
chromosome number of the daughter
cells is diploid i.e. 2n
Only half the number of chromosomes,
one from each pair is passed into each
daughter cell hence the daughter cells
have haploid number of chromosomes
Mitosis comprises of one single nuclear
division after chromosome duplication
Meiosis comprises of two nuclear
divisions after chromosome duplication
All daughter cells are identical The gametes produced are having
randomly assorted chromosomes
leading to genetic variation.

33. CHECK YOUR PROGRESS
1) Distinguish
a) Animal mitosis and plant mitosis (direction of
cytokinesis)
b) Mitosis and meiosis (number of daughter
cells formed)
c) G1 phase and S phase (events taking place)

35. 3

36. USEFUL LINKS
 https://youtu.be/xsrH050wnIA
 https://youtu.be/DwAFZb8juMQ
 https://youtu.be/c5hA0WCv1lg
 https://youtu.be/TJfPbtXmngs
 https://youtu.be/C1CRrtkWwu0