Lecture 6
The Cell Cycle
1. Overview
2. Mitosis
3. Cytokinesis
4. Binary Fission
Prepared by Mayssa Ghannoum
Overview
The ability of organisms to reproduce their own
kind is the one characteristic that best
distinguishes living things from nonliving matter.
 The unique capacity to procreate has a cellular
basis:
“Where a cell exists, there must have been a
preexisting cell”.
 The continuity of life is based on the reproduction
of cells, or cell division.

The Key Roles of Cell division

Cell division plays several important roles in the life
of an organism.
a.
It enables sexually reproducing organisms to
develop from a single cell -- the fertilized egg, or
zygote.
b.
After an organism is fully grown, cell division
function in renewal and repair.
The Key Roles of Cell division
 The
Cell division process is a part of the cell
cycle.
 Cell
cycle: the life of a cell from the time it is
first formed from a dividing parent cell until its
own division into two cells.

Passing identical material to cellular offspring
is a crucial function of cell division.
The Functions of Cell Division
 Most
Cell division involves the distribution of
identical genetic material –DNA – to two
daughter cells.
 DNA is passed from one generation of cells
to the next.
 A dividing cell duplicates its DNA, allocates
(distributes) the two copies to opposite ends
of the cell, and only then splits into daughter
cells.
Cellular Organization of the Genetic Material
 A cell’s
composition of DNA, its genetic
information, is called genome.
 The
overall length of DNA in a eukaryotic cell is
enormous compared to the diameter of the cell
itself but the replication and distribution of DNA
is manageable because the DNA molecules are
packaged into chromosomes.
Cellular Organization of the Genetic Material
 Eukaryotic chromosomes are
made of chromatin,
a complex of DNA associated with proteins
molecules.
 Each
single chromosome contains one very long,
linear DNA molecule that carries several hundred
genes.
are the units that specify an organism’s
inherited traits.
 Genes:
 Every
eukaryotic species has a characteristic number
of chromosome in each cell nucleus.
 Somatic
cells contain 46 chromosomes made up of
two sets of 23, one set is inherited from each parent.
 Gametes
( sperm and egg) have half number of
chromosomes as somatic cells, or one set of 23
chromosomes.
 Somatic
cells: all body cells except the reproductive
cells.
 Gametes:
Reproductive cells
Distribution of chromosomes During
Eukaryotic Cell Division
 In
a non dividing cell, each chromosome is in the
form of a long, thin chromatin fiber.
 After
DNA duplication, the chromosomes
condense:
Each chromatin fiber becomes densely coiled and
folded; the chromosomes become much shorter
and so thick.
Distribution of chromosomes During
Eukaryotic Cell Division
 Each
duplicated chromosome has two sister
chromatids.
 The
two chromatids, each containing an identical DNA
molecule, are initially attached all along their lengths by
adhesive protein complexes called cohesions.
 The
duplicated chromosomes has a narrow waist called
centromere, where the 2 chromatids are closely
attached.
The Cell cycle
 Mitosis:
is the division of the nucleus.
 Mitosis is usually immediately followed by
cytokinesis.
 Cytokinesis is the division of the cytoplasm.
One parent cell is divided into
two daughter cells genetically identical
to their parent cell.
Phases of the Cell Cycle
 Mitosis
is just one part of the cell cycle.
 In
fact, the Mitotic (M) phase, which includes both:
Mitosis and Cytokinesis, is the shortest part of the cell
cycle.
 The
longest stage of the cycle is called Interphase.
 Interphase
 During
accounts for about 90% of the cell cycle.
interphase, the cell grows and copies its
chromosomes in preparation for cell division.
 Interphase can be divided into
three sub phases:
1)
The G1 phase (first Gap): Cell
growth
2)
The S phase (synthesis): Cell
copies its chromosomes
3)
The G2 phase (second Gap):
Cell grow more as it completes
preparation for cell division.
M
phase: the cell divides
 Daughter
the cycle.
cells may then repeat
Mitosis
 Mitosis
is conventionally broken down into five stages:
1- Prophase
2- Prometaphase
3- Metaphase
4- Anaphase
5- Telophase
 Cytokinesis
completes then the Mitotic phase.
G2 of Interphase

A nuclear envelope bounds the nucleus.

The nucleolus is visible.

Two centrosomes have formed by
replication of a single centrosome.

Each centrosome features two centrioles.

Chromosomes, duplicated during S phase
cannot be seen individually because they
have not yet condensed.
Prophase

The chromatin fibers become more tightly
coiled, condensing into chromosomes .

The nucleolus disappear.

Each duplicated chromosome appears as
two identical sister chromatids joined at
centromeres.

The mitotic spindle begins to form.
It’s composed of the centrosomes and
microtubules that extend from them.

Centrosomes move away from each other
propelled by the lengthening microtubules
between them.
Prometaphase
The nuclear envelope breaks into
fragments.
 The microtubules extending from each
centrosome invade the nuclear area.
 Chromosomes become even more
condensed.
 Each of the two chromatids has a
kinetochore, a specialized protein located
at the centromere.
 Some microtubules attach to the
kinetochore becoming "Kinetochore
microtubules”.

Metaphase
 It
is the longest stage of Mitosis
 Centrosomes
are at opposite poles of
the cell.
 The
chromosomes are aligned on the
metaphase plate, an imaginary plane
that is equidistant between the spindle’s
two poles.
 The
Kinetochores of the sister
chromatids are attached to microtubules
coming form opposite poles.
Anaphase

It is the shortest stage of mitosis.

Anaphase begins with the cohesion proteins
are cleaved, which allows the 2 sister
chromatids of each pair to part suddenly.

The two liberated daughter chromosomes
begin moving toward opposite ends of the cell
as their kinetochore microtubules shorten.

The cell elongates.

By the end of anaphase, the two ends of the
cell have equivalent and complete collections
of chromosomes.
Telophase
 Two
daughter nuclei form in the cell.
 Nuclear
envelopes arise from the
fragments of the parent cell’s nuclear
envelope.
 Nucleolus
reappear.
 The
chromosomes become less
condensed.
 Mitosis,
the division of one nucleus into
two genetically identical nuclei, is now
complete.
Cytokinesis
Cytokinesis which is the division of
the cytoplasm is usually well under
way by late telophase, so the two
daughter cells appear shortly after the
end of mitosis.
 Cytokinesis differ between animal and
plant cells.

Cytokinesis
 In
animal cells, cytokinesis occurs
by a process known as cleavage.
 The
first sign of cleavage is the
appearance of a cleavage furrow, a
shallow groove in the cell surface.
 The
cleavage furrow deepens until
the parent cell is pinched in two,
producing two completely
separated cells, each with its own
nucleus and share of cytosol, and
organelles.
Cytokinesis
In plant cells, which have cell walls;
cytokinesis occurs by a process called
formation of cell plate.
 Cell wall materials collect in the cell
plate as it grows.
 The cell plate enlarges until its
surrounding membrane fuses with the
plasma membrane along the perimeter of
the cell.
 Two daughter cells result, each with its
own plasma membrane.
 A new cell wall arising from the contents
of the cell plate is formed between the
daughter cells.

Mitosis in a plant cell
Binary Fission
 The
asexual reproduction of single-celled
eukaryotes, such as amoeba, includes mitosis
and occurs by a type of cell division called
binary fission.
 Binary fission means division in half.
 Prokaryotes ( bacteria and archaea) also
reproduce by binary fission, but the prokaryotic
process does not involve mitosis.
Binary Fission

In bacteria, most genes are carried on a
single bacterial chromosome that consists of a
circular DNA and associated proteins.

The process of cell division is initiated when
the DNA of the bacterial chromosome begins
to replicate at specific place called the origin
of replication, producing two origins.
Binary Fission
 As
the chromosome continue to replicate, one
origin moves rapidly toward the opposite pole of
the cell.
 Meanwhile,
the cell elongates.
 When
replication is complete and the bacterium
reach about twice its initial size, its plasma
membrane grows inward, dividing the parent
bacteria cell into two daughter cells.
 Each
cell inherits a complete genome.
Bacterial cell division by Binary Fission