molecular mechanisms in cell division

Overview of Cell Division
Phases of Cell Division
Molecular mechanisms in Cell Division
Important structures and key components in DNA
synthesis
DNA polymerases and the process of DNA replication
Proof reading and repair
Regulation of Cell Cycle

Most eukaryotic cells will
pass through an ordered
series of events in which the
cell duplicates its contents
and then divides into two
cells
This process of cell division
in multicellular organisms
must be highly ordered and
tightly regulated

Mitosis is the process by which a eukaryotic cell
duplicates its DNA and then divides into two
daughter cells, each of which contains the exact
genetic material as the mother cell and gets
roughly an equal share of other cellular
components
If the DNA of a human cell were uncoiled and
stretched, it would extend approximately 2
meters!

Meio: to reduce
A form of nuclear division in which the
chromosome number is halved from the diploid
number (2n) to haploid number (n)
It is preceded by DNA replication during
interphase in the parent cell. This is followed by
2 cycles of nuclear division and cell divisions-
Meiosis I and Meiosis II

Mitosis generates two
genetically identical diploid
daughter cells
Meiosis generates four haploid
daughter cells, none of which are
genetically identical

Starting from a single-celled zygote… An adult
human being has approximately 100,000 billion
cells
Cell division does not stop with formation of
mature organism, but continues throughout its
life
Tens of millions of cells undergo division at any
given moment in an adult human. This amount of
division is needed to replace cells that have aged
or died

Two major cell cycle phases - based on cell
activities readily visible under light microscope:
Interphase - occupies bulk of cycle; divided into G1
(first gap), S (synthesis) & G2 (second gap)
M phase – M for "mitotic"; this stage includes mitosis
(duplicated chromosomes are separated into 2 nuclei)
& cytokinesis (entire cell & its cytoplasm divide into 2
daughter cells)

G1 - growth phase 1
S - DNA synthesis
G2 - further growth
M - cell division
Mitosis:
– prophase, prometaphase,
metaphase, anaphase
and telophase
Cytokinesis
InterphaseMitoticphase

G1 (G=gap) is the interval between the
completion of mitosis and the beginning of DNA
synthesis
During G1, the cell monitors its own
environment and size before it commits itself to
DNA replication. Cells in G1 (if not committed to
DNA replication) can pause their progress and
enter a specialized resting state G0
S phase - replication of nuclear DNA
Interphase Mitotic phase
G1 S G2 Pro Prometa Meta Ana Telo

G2 is the second “Gap” phase:
Nucleus well defined and bound by nuclear
envelope
Outside nucleus are two pairs of centrioles
formed during early interphase
Microtubules extend from centrioles in a radial
array called asters

Prophase
Changes occurs in both nucleus and cytoplasm
Nucleus: Chromatin fibres become more tightly
coiled and condense into discrete chromosomes.
The duplicated chromosome appears as 2
identical sister chromatids joined by centromere
Cytoplasm: formation of mitotic spindle begins

Prometaphase
Nuclear envelope develop fragments.
Microtubules can now invade the nucleus and
interact with the chromosomes
Microtubule attach to kinetochore on each
chromosomes centromere
Asters, radiate from centrioles and anchor
themselves to membrane plasma

Metaphase
Centrioles at opposite poles of the cell
Chromosome convene on the metaphase plate
(imaginary plane of equal distant between
spindles of two poles)

Anaphase
Begins when paired centromeres of each
chromosome separate, liberating each sister
chromosome from one another (each chromatid
is considered one full fledged chromosome)
Chromosomes begin moving along microtubule
toward opposite poles of the cell

Telophase
Nucleolus begins to form at the two poles of the
cells. Nuclear envelopes are formed
Chromatin fibre of each chromosome become
less tightly coiled
Mitosis ends and cytokinesis begins

Cytokinesis
Occurs by a process called cleavage: begins with
a cleavage furrow, a shallow grove near the
metaphase plate
In cytoplasmic side of the furrow, are contractile
actin proteins. As the actin microfilament
contract, its diameter shrinks, cleavage furrow
deepens until cell pinched into two

DNA replication begins at specific locations in
the genome, called "origins“
Unwinding of DNA at the origin, and synthesis of
new strands, forms a replication fork. In addition
to DNA polymerase, the enzyme that synthesizes
the new DNA by adding nucleotides matched to
the template strand, a number of other proteins
are associated with the fork and assist in the
initiation and continuation of DNA synthesis

The replication fork is a structure that forms
within the nucleus during DNA replication. It is
created by helicases, which break the hydrogen
bonds holding the two DNA strands together
The resulting structure has two branching
"prongs", each one made up of a single strand of
DNA

DNA polymerases are a family of enzymes that
carry out all forms of DNA replication
To begin synthesis, a short fragment of DNA or
RNA, called a primer, must be created and paired
with the template DNA strand
DNA polymerase then synthesizes a new strand
of DNA by extending the 3' end of an existing
nucleotide chain, adding new nucleotides
matched to the template strand one at a time via
the creation of phosphodiester bonds

The PCR does in the test tube what every
bacterium does in its tube of media or on an
agar-plate and each of us do every day: we all
produce billions of exact copies of our own DNA;
AMPLIFYING our DNA millions of time
The enzyme DNA polymerase was discovered in
the 1950s and our knowledge of the process has
been increasing ever since

 TARGET DNA to be copied. In theory only a single
molecule is needed
 A set of short (15 to 40 bases) single stranded PRIMERS
of DNA, that will bind to complementary regions of the
opposing stands of the target DNA molecule
 An excess of the 4 nucleotide triphosphates, ATP, GTP,
CTP, TTP
 The enzyme, DNA polymerase
 Various buffers and cofactors like magnesium ions
required by DNA polymerase

Double helix target DNA strands are separated so
the primers could bind and the DNA polymerase
could function
Heat separates DNA strands and that
complementary strands then rejoin through base
pairing when the temperature is subsequently
lowered

 Lowering the temperature enough to allow the
primers, which were small and in vast excess, to
bind (ANNEAL) to their respective
complementary target DNA sequence
 DNA polymerase allows polymerization reaction
with the triphosphate nucleotides to occur

5′
3′
3′
5′
5′
3′
3′
5′
5′
3′
3′
5′
5′
5′3′
3′
1
2
3
DNA

5′
3′
3′
5′
5′
5′3′
3′
5′
3′
3′
5′
5′
5′3′
3′
4
5

 The DNA polymerase fills in the missing portion
of each strand making two new double stranded
regions of DNA
 The whole process is repeated several times thus
yielding exponential amount of DNA strands
2 4 8 16 32 After 12 cycles… 8192
After 20 cycles… 2097152

Beginning with a single piece of DNA, PCR can generate
100 billion identical copies of a specific DNA sequence !!!

PCR takes place in a tube which is kept in a
machine called “thermal cycler”

For all living eukaryotic organisms it is essential
that the different phases of the cell cycle are
precisely coordinated
Errors in this coordination may lead to
chromosomal alterations. Chromosomes or parts
of chromosomes may be lost, rearranged or
distributed unequally between the two daughter
cells

Nutrients
Growth factors
Cell size Regulatory proteins
& Protein kinases
Cell-cell contact

Much of the control of the progression through the
phases of a cell cycle are exerted at checkpoints
There are many such checkpoints but the three
most critical are those that occur near the end of
G1 prior to S-phase entry, near the end of G2 prior
to mitosis, and at metaphase…

G0
G2 Checkpoint
G1 Checkpoint
Metaphase
Checkpoint
Is cell big enough?
Is environment favourable?
Is all DNA replicated?
Is cell big enough?
Is environment favourable?
Are all chromosomes
aligned on spindle?

molecular mechanisms in cell division

molecular mechanisms in cell division

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molecular mechanisms in cell division