DNA- Deoxyribonucleic Acid
Genome- all the genes an organism has
Chromosomes- structures made of DNA
Chromatin- DNA/protein complex that
makes up chromosomes
Synthesis – growth
Mitosis - division
3. Cell cycle
is the sequence of events that result into
◦ growth and division.
cell cycle has two phases:
◦ Interphase longest phase
◦ Mitosis short phase
generally lasts at least 12 to 24 hours in
during this period, the cell is constantly
◦ synthesizing RNA,
◦ producing protein
◦ growing in size.
interphase can be divided into 3 steps:
G1 (the time gap between mitosis and DNA
S (the period of DNA synthesis), and
G2 (the gap between DNA duplication and the next
S -DNA replication
G2-Proteins needed for mitosis are
A 4th phase is described G0
DNA stays loosely packed in the nucleus
Gap 0 (G0):
There are times when a cell will leave the
cycle and quit dividing.
This may be a temporary resting period
or more permanent.
An example of the latter is a cell that has
reached an end stage of development and
will no longer divide (neuron, muscle).
Temporary resting cells example liver
Gap 1 (G1)
longest and most variable phase
Cells increase in size
produce RNA and synthesize protein.
An important cell cycle control mechanism
activated during this period (G1
◦ ensures that everything is ready for DNA
To produce two similar daughter cells, the
complete DNA instructions in the cell must
DNA replication occurs during this S
Gap 2 (G2)
cell continue to grow and produce new
At the end of this gap is another control
checkpoint (G2 Checkpoint)
◦ to determine if the cell can now proceed to
enter M (mitosis) and divide.
Mitosis or M Phase:
Cell growth and protein production stop at
this stage in the cell cycle.
All of the cell's energy is focused on the
complex and orderly division into two similar
Mitosis is much shorter than interphase,
lasting perhaps only one to two hours.
As in both G1 and G2, there is a Checkpoint
in the middle of mitosis (Metaphase
◦ ensures the cell is ready to complete cell division
Rates of cell division varies from tissue
Epithelial tissue subject to loss of cell
division is rapid
In healing of wounds division is also rapid
Progression through the cell cycle is
halted by a variety of adverse conditions
◦ inadequate nutrition (nutrient stress)
◦ inappropriate cellular microenvironments
◦ DNA damage
Damage here can arrest the cell cycle not
only at the G1 restriction point but also
during S or at a checkpoint in G2
G1 arrest may permit repair of the
damage before the cell enters S phase,
so that the damaged DNA does not
reproduce gene defects during
If the problem encountered at any
checkpoint cannot be corrected fairly
quickly while cycling is halted,
proteins encoded by tumor suppressor
genes are activated and that cell’s activity
is redirected toward cell suicide or
17. Purpose of cell division
Why cell division?
◦ Growth of an organism
◦ Repletion of cells lost
Occurs in Somatic cells
Identical cells produced with same
In adult specialized cells
embryonic division may lead to
Mitosis produces two daughter cells that
are identical to the parent cell.
If the parent cell is haploid (N), then the
daughter cells will be haploid.
If the parent cell is diploid, the daughter
cells will also be diploid.
cell division allows multicellular organisms
to grow and repair damaged tissue
The chromosomes coil (condense).
The nuclear membrane disintegrates.
Spindle fibers (microtubules) form,
Chromosomes further condense
Attach to mitotic spindles at centromere
become aligned at the equator
happens at the end of telophase
A belt-like contractile ring of actin filaments
and myosins develops in the peripheral
constriction produces a cleavage furrow
and progresses until the cytoplasm and its
organelles are divided into two daughter
cells, each with one nucleus.
Most tissues undergo cell turnover
with slow cell division and cell death.
Nerve tissue and cardiac muscle are
exceptions because their differentiated
cells cannot undergo mitosis.
process by which two successive cell
divisions produce cells called gametes
containing half the number of
chromosomes found in somatic cells.
fate of meiosis
Production of gametes
Chromosomes are halved
Reassortment of genes
There are 22 pairs of matching chromosomes, the
autosomes, and one pair of sex chromosomes.
If the sex pair is XX, the individual is genetically
if the pair is XY, the individual is genetically male
One chromosome of each pair is derived from the
maternal and paternal gametes.
Thus each gamete contains a haploid number of
the union of the gametes at fertilization restores
the diploid number of 46.
cell division that takes place in the germ
also known as reductional division.
One diploid cell produces 4 haploid cells
Has 2 process:-
◦ Meiosis I
◦ Meiosis II
33. Meiosis I
Meiosis results in the formation of gametes
In meiosis I:
◦ Homologous chromosomes join together=
◦ homologous chromosomes are separated
◦ crossing over = genetic material exchange
(genetic recombination) occurs.
In meiosis II:
◦ sister chromatids are separated
◦ sequence of phases resembles mitosis.
Parent cell – chromosome
1st division - pairs split
2nd division – produces 4
gamete cells with ½ the
original no. of
37. Meiotic Prophase I
a long and complex phase that differs
considerably from mitotic prophase
chromosomes in the parent cell form
◦ Pair of homologous chromosomes
◦ occurs between the maternal and paternal
each paired chromosomes has two
Or tetrads four copies of each genetic
During synapsis recombination or
crossing over , four chromatids mixes up
the genes inherited from each parent
yields a new and different set of genes to
be passed on to the next generation.
Has a short duration,
chromosomes move towards the
equator of the cell and come to lie in
two parallel metaphase plates.
The 2 parallel plates are formed by
one set each of the homologous
41. Anaphase 1
similar to anaphase in mitosis except
that each chromosome consists of 2
chromatids that remain held together.
2 Chromosomes migrate to the poles.
actual reduction in the chromosome
number (from 2n to n) occurs
However, each chromosome found at
the poles consists of two chromatids.
chromosomes at each pole uncoil and
elongate to form the chromatin.
nucleolus reappears at each pole.
Spindle fibres and asters disappear
and centrioles split.
nuclear membrane is formed at each
pole resulting in the formation of two
44. Cytokinesis - I
Simultaneously with the formation of 2
a cleavage furrow appears in the
middle of the cell.
furrows gradually deepen and divide
the cell into 2 daughter cells.
46. Meiosis Prophase II
Resembles the prophase stage of
In each of the two new cells:
◦ nuclear membrane breaks down
◦ chromosomes collect together.
Crossing over does not occur in this
47. Metaphase II
The double-stranded chromosomes
form a single line in the middle of the
Spindle fibers extend from the
centrioles at the poles to the
centromere of each double-stranded
48. Anaphase II
The sister chromatids of each double-
stranded chromosome are pulled
apart at the centromere.
Each chromatid (single strand) is
pulled to the opposite pole of the cell.
50. Telophase II and Cytokinesis
The single-stranded chromosomes arrive at
opposite ends of the cell.
A cleavage furrow forms
Cytoplasm in both cells divides
Produces a total of four haploid daughter
These daughter cells mature:
◦ sperm in males
◦ oocytes in females.
52. Errors in Meiosis
It is estimated that from 10–25% of all
human fertilized eggs contain
these are the most common cause of
pregnancy failure (35% of the cases).
These chromosome abnormalities arise
from errors in meiosis,
usually meiosis I; occur more often
(90%) during egg formation than during
sperm formation; become more frequent
as a woman ages.
Euploid – any exact multiple of n (23)
Aneuploid— the gain or loss of whole
Chromosomal abnormalities account for 7%
of major birth defects
gene mutations account for an additional
most common chromosomal abnormality
caused by nondisjunction,
◦ the failure of chromosomes to correctly separate
Zygotes missing one chromosome
("monosomy") cannot develop to birth
(except for females with a single X
An extra chromosome ("trisomy") is also
lethal except for chromosomes 13, 18,
trisomy 21 is the cause of Down
Describe the cell cycle
Describe the 2 processes of cell
State the importance of cell division
Explain the fate of errors in meiosis