CELL DIVISION

TO: SUBASH CHANDER BY:ANIL KUMAR(2011A14BIV)

CELL

The cell (from Latin cella, meaning

"small room") is the basic structural,

functional, and biological unit of all

known living organisms

DISCOVERY

The cell was discovered by Robert Hooke in

1665.

His book Micrograhia

Thin slices of bottle cork.

Robert hook’s microscope Micrograph of cell

CELL THEORY

Cell theory has 3 basic points:

• All living things are made of cells.

• The cell is the smallest living thing that can

perform all the functions of life.

• All cells must come from pre-existing cells.

Cell division

Binary fission

Mitosis

Meiosis

BINARY FISSION

The cell divides into

two nearly equal sized

daughter cells.

The genetic material is

also equally split

MITOSIS

Mitosis is the process by

which chromosomes in a cell

nucleus are separated into two

identical sets of chromosomes, each

in its own nucleus

DISCOVERY

German zoologist Otto Bütschli

Term coined by Walther Flemming in 1882.

1843-1905

PHASES

o Interphase

o Prophase

o Metaphase

o Anaphase

o Telophase

o Cytokinesis

INTERPHASE

It is the longest phase.

G1

S

G2

G1 - first gapS - DNA synthesis

(replication)

G2 - second gap

M - mitosis

G1 (FIRST GAP)

The cell grows and functions normally.

A high amount of protein synthesis occurs .

The volume of cytoplasm increases.

Mitochondria and chloroplasts divide.

SYNTHESIS (S)

The cell duplicates its DNA .

This is also known as the

Swanson phase.

G2 (Gap 2)

The cell resumes its growth in

preparation for division.

G0 (Gap zero)

Some cells that do not divide often

or ever, enter a stage called G0 .

G0 phase semi-permanentally e.g.,

some liver, kidney, stomach

cells.

PROPHASE

• The chromatin becomes visible.

• The chromatin condenses into

double rod-shaped structures

called chromosomes.

• The centrosomes move to

opposite poles of the cell, forming

a bridge of spindle fibers.

• The nucleolus disperses.

ONION CELL

METAPHASE

Chromosomes

carrying genetic information

align in the equator of the cell.

Alignment is due to the

pulling powers generated by

the opposing kinetochore

microtubules.ONION CELL

micrograph

ANAPHASE

Chromosomes are split and the sister

chromatids move to opposite poles of

the cell.

ONION CELL

TELOPHASE

Two daughter nuclei form

in each daughter cell.

forming nuclear

envelopes around each

nucleus.

Onion cell

CYTOKINENSIS

It is the process in which

the cytoplasm of a single cell

is divided to form two daughter

cells.

Plant cells construct a cell

plate in the middle of the cell.

Onion cell

Onion (Allium) cells in different phases of the cell cycle

enlarged 800 diameters.

a. non-dividing cells

b. nuclei preparing for division .

c. dividing cells showing mitotic figures.

e. pair of daughter-cells shortly after division.

MEIOSIS

It is a specialized type of cell division which

reduces the chromosome number by half.

This process occurs in all sexually

reproducing eukaryotes (both single-celled

and multicellular) including animals, plants,

and fungi.

HISTORY

Meiosis was discovered and described

for the first time in sea urchin eggs in

1876 by the German biologist Oscar

Hertwig.

The term meiosis was introduced to

biology by J.B. Farmer and J.E.S.

Moore in 1905.

PHASES

Meiosis I Prophase I

• Leptotene

• Zygotene

• Pachytene

• Diplotene

• Diakinesis

Metaphase I

Anaphase I

Telophase I

Meiosis II

PROPHASE I

Leptotene :

Individual chromosomes—each

consisting of two sister

chromatids—condense from the

diffuse interphase conformation

into visible strands within the

nucleus.

ZYGOTENE

At this stage, the synapsis

(pairing/coming together) of

homologous chromosomes takes

place.

Synaptonemal complex

PCHYTENE

It is the stage when chromosomal

crossover (crossing over) occurs.

Nonsister chromatids of homologous

chromosomes may exchange

segments over regions of homology.

At the sites where exchange

happens, chiasmata form

DEPLOTENE

The synaptonemal complex degrades

and homologous chromosomes separate

.

The chiasmata remain on the

chromosomes until they are severed in

anaphase I om one another a little.

DIAKINENSIS

This is the first point in meiosis where the

four parts of the tetrads are actually

visible.

The nucleoli disappear, the nuclear

membrane disintegrates into vesicles, and

the meiotic spindle begins to form.

METAPHASE I

Homologous pairs move

together along the

metaphase plate.

As kinetochore

microtubules from both

centrioles attach to their

respective kinetochores.

Lily ovulary cell

ANAPHASE I

Kinetochore microtubules shorten.

Pulling homologous

chromosomes to opposite poles.

This allows the sister chromatids

to remain together while

homologs are segregated.

Lilium anther

TELOPHASE I

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.Lily pollen

MEIOSIS II

Prophase II

Metaphase II

Anaphase II

Telophase II

PROPHASE II

The disappearance of the

nucleoli

and the nuclear

envelope again.

Centrioles move to the

polar regions .

Lilium anther

METAPHASE II

Single chromosomes align on the

metaphase plate .

Lily pollen

ANAPHASE IIThe centromeres separate.

The two chromatids of

each chromosome move to

opposite poles on the

spindle.

The separated chromatids

are now called

chromosomes in their own

right. Lily pollen

TELOPHASE II

A nuclear envelope forms

around each set of

chromosomes.

LILY POLLEN

FOUR HAPLOID CELL

FORM

A nuclear envelope forms around

each set of chromosomes.

THANKS