q110/random_k4/random_4.md

Lecture – 3
Cell division : Mitosis
& Meiosis
by

Dr. Praveen Kumar

Asst. Prof. GPB

MSSSoA, CUTM, Odisha

Cell Division

All cells are derived from

pre-existing cells

New cells are produced for

growth and to replace
damaged or old cells
Differs in prokaryotes

(bacteria) and eukaryotes
(protists, fungi, plants, &
animals)

2

Original DNA
strand

DNA Replication
DNA must
be copied
or
replicated
before cell
division
Each new
cell will
then have
an
identical
copy of

Two new,
identical DNA
strands

3

Cell Cycle

DNA Copied

Cells prepare for Division

Cells
Mature

4

Daughter Cells

Cell Divides into Identical cells

Interphase - G1 Stage
1st growth stage after cell

division

Cells mature by making

more cytoplasm &
organelles

Cell carries on its normal

metabolic activities

5

Interphase – S Stage

Synthesis stage
DNA is copied or replicated

Two identical
copies of
DNA

Original DNA

6

Interphase – G2 Stage

2nd Growth Stage
Occurs after DNA has been copied
All cell structures needed for

division are made (e.g. centrioles)

Both organelles & proteins are

synthesized

7

What’s Happening in Interphase?

What the cell looks like

Animal Cell

What’s occurring

8

Mitosis

Division of the

nucleus

Also called
karyokinesis
Only occurs in

eukaryotes

Has four stages
Doesn’t occur in
some cells such
as brain cells

9

Four Mitotic Stages

Prophase
Metaphase
Anaphase
Telophase

10

Early Prophase
Chromatin in nucleus

condenses to form visible
chromosomes

Mitotic spindle forms from
fibers in cytoskeleton or
centrioles (animal)

Cytoplasm

Nuclear Membrane

Nucleolus

Chromosomes

Late Prophase

Nuclear membrane & nucleolus

are broken down

Chromosomes continue

condensing & are clearly visible

Spindle fibers called

kinetochores attach to the
centromere of each
chromosome

Spindle finishes forming

12

between the poles of the cell

Metaphase

Chromosomes, attached to the

kinetochore fibers, move to the center
of the cell

Chromosomes are now lined up at the

Equator of Cell

equator

Pole of
the Cell

13

Anaphase

Occurs rapidly
Sister

chromatids are
pulled apart to
opposite poles
of the cell by
kinetochore
fibers

14

Telophase

Sister chromatids at opposite

poles

Spindle disassembles
Nuclear envelope forms around
each set of sister chromatids

Nucleolus reappears
CYTOKINESIS occurs
Chromosomes reappear as

chromatin

15

Cytokinesis

Means division of the cytoplasm
Division of cell into two, identical

halves called daughter cells

In plant cells, cell plate forms at

the equator to divide cell

In animal cells, cleavage furrow

forms to split cell

16

Daughter Cells of Mitosis

Have the same number of

chromosomes as each other and
as the parent cell from which
they were formed

Identical to each other, but

smaller than parent cell

Must grow in size to become

mature cells (G1 of Interphase)

17

Meiosis
Formation of Gametes
(Eggs & Sperm)

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Facts About Meiosis
Preceded by interphase which
includes chromosome replication
Two meiotic divisions --- Meiosis

I and Meiosis II

Called Reduction- division
Original cell is diploid (2n)
Four daughter cells produced that

are monoploid (1n)

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Facts About Meiosis

Daughter cells contain half  the
number of chromosomes as the
original cell

Produces gametes (eggs & sperm)
Occurs in the testes in males

(Spermatogenesis)

Occurs in the ovaries in females

(Oogenesis)

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Why Do we Need Meiosis?
It is the fundamental basis

of sexual reproduction

Two haploid (1n) gametes are

brought together through
fertilization to form a diploid
(2n) zygote

21

Replication of Chromosomes

Occurs in
Interphase

Replication is the

process of duplicating
a chromosome
Occurs prior to

division

Replicated copies are

called sister
chromatids

Held together at

centromere

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Meiosis: Two Part Cell
Division

Sister
chromatids
separate

Homologs
separate

Meiosis
I

Meiosis
II

Diploid

Diploid

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Haploid

Meiosis I: Reduction Division

Nucleus

Early Prophase I
(Chromosome
number doubled)

Spindle
fibers

Nuclear
envelope

Late Prophase
I

Metaphase I

Anaphase I

Telophase I (diploid)

24

Prophase I

Early prophase
Homologs pair.
Crossing over
occurs.

25

Late prophase
Chromosomes condense.
Spindle forms.
Nuclear envelope
fragments.

Tetrads Form in Prophase
I

Homologous chromosomes
(each with sister chromatids)

Join to form a TETRAD

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Called Synapsis

Crossing-Over

Homologous

chromosomes in a
tetrad cross over
each other
Pieces of

chromosomes or
genes are
exchanged

Produces Genetic
recombination in
the offspring

27

Metaphase I

Homologous pairs
of chromosomes
align along the
equator of the cell

28

Anaphase I

Homologs separate and
move to opposite poles.

Sister chromatids remain
attached at their centromeres.

29

Telophase I

Nuclear envelopes
reassemble.

Spindle disappears.

Cytokinesis divides cell into
two.

30

Meiosis II

Gene  X

Only one homolog of each

chromosome is present in
the cell.

Sister chromatids carry
identical genetic
information.

Meiosis II produces gametes with
one copy of each chromosome and
thus one copy of each gene.

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Meiosis II: Reducing
Chromosome Number

Prophase II

Metaphase II

32

Telophase II

Anaphase II

4 Identical haploid
cells

Prophase II

Nuclear envelope
fragments.

Spindle forms.

33

Metaphase II

Chromosomes align
along equator of cell.

34

Anaphase II

Equator

Pole

Sister chromatids
separate and move
to opposite poles.

35

Telophase II

Nuclear envelope
assembles.

Chromosomes
decondense.

Spindle disappears.

Cytokinesis divides cell
into two.

36

Results of Meiosis
Gametes (egg & sperm)
form

Four haploid cells with one
copy of each chromosome

One allele of each gene

Different combinations of
alleles for different genes
along the chromosome

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Comparison of Divisions
Meiosis
Mitosis

Number of
divisions

Number of
daughter cells

Genetically
identical?

1

2

Yes

2

4

No

Chromosome # Same as parent

Half of parent

Where

Somatic cells

Germ cells

When Throughout life

At sexual maturity

Role
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Growth and
repair

Sexual reproduction