2 cell & cell cycle

CELL CYCLE
Described by Howard and Pele in 1953
Cell Cycle is defined as “The sequence of
events involving growth and division, a cell
undergoes from the time of its formation by
division of parent cell to its own division into
daughter cells .”

Five Phases of the Cell Cycle
 Interphase
It is divided into three phases-:
G1 - primary growth phase
S – synthesis; DNA replicated
G2 - secondary growth phase
collectively these 3 stages are
called interphase
M - Mitosis
C – Cytokinesis 3

CONCEPT OF G0 PHASE-
 Nonproliferative cells in eukaryotes generally enter
the quiescent G0 state from G1 and remain quiescent
for long periods of time, possibly indefinitely , e.g.
neurons
Cellular senescence occurs in response to DNA
damage or degradation that would make a cell's
progeny nonviable.
 Some cells enter the G0 phase for a short period of
time e.g., liver and kidney cells.
Many cells do not enter G0 and continue to divide
throughout an organism's life, e.g. epithelial cells

Interphase
(a) G1 Stage
1st growth stage after cell
division
Cells mature by making more
cytoplasm & organelles
Cell carries on its normal
metabolic activities
7

Interphase –> S Stage
Synthesis stage
DNA is copied or replicated
8
Two identical
copies of
DNA
Original DNA

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
9

Interphase
Animal Cell Plant Cell
Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

What Happens in Interphase G2………
11
What the cell looks like
Animal Cell
What’s occurring

Cell Cycle
12
ughter Cells
DNA Copied
Cells
Mature Cells prepare for Division
Cell Divides into Identical cells

Chromosomes
 All eukaryotic cells store genetic information
in chromosomes.
 Human cells have 46 chromosomes.
 23 nearly-identical pairs

Structure of Chromosomes
 Chromosomes are composed of a complex of DNA
and protein called chromatin that condenses
during cell division
 DNA exists as a single, long, double-stranded fiber
extending chromosome’s entire length.
 Each unduplicated chromosome contains one DNA
molecule, which may be several inches long

 After every 200 nucleotide pairs, the DNA wraps twice around a
group of 8 histone proteins to form a nucleosome.
 Higher order coiling and supercoiling also helps in condensing
and packaging the chromatin inside the nucleus.

Chromosomes
 Non-homologous chromosomes
 Look different
 Control different traits
 Sex chromosomes
 Are distinct from each other in their
characteristics
 Are represented as X andY
 Determine the sex of the individual, XX being
female, XY being male
 In a diploid cell, the chromosomes occur in pairs.
The 2 members of each pair are called
homologous chromosomes or homologues.

Chromosomes
 A diploid cell has two sets of each of its chromosomes
 In human being 46 chromosomes are present(2n = 46)
 In a cell in which DNA synthesis has occurred all the chromosomes are
duplicated and thus each chromosome consists of two identical sister
chromatids
Maternal set of
chromosomes
Paternal set of
chromosomes
2n = 6
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere

Homologues Chromosomes
 Homologous chromosomes:
• Look the same
• Control the same traits
• May code for different forms of each trait
• Independent origin - each one was inherited from a
different parent

Chromosome Duplication
0.5 µm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Sister
chromatids
Centrometers
Sister chromatids
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
 In preparation for cell division, DNA is replicated and the chromosomes condense

 Because of duplication, each condensed chromosome consists
of 2 identical chromatids joined by a centromere.
 Each duplicated chromosome contains 2 identical DNA
molecules (unless a mutation occurred), one in each chromatid:
Chromosome Duplication
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Two unduplicated
chromosomes
Centromere
Sister
chromatids
Sister
chromatids
Duplication
Non-sister
chromatids
Two duplicated chromosomes

 The centromere is a constricted region of the chromosome containing a
specific DNA sequence, to which is bound 2 discs of protein called
kinetochores.
 Kinetochores serve as points of attachment for microtubules that move the
chromosomes during cell division:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Metaphase chromosome
Kinetochore
Kinetochore
microtubules
Centromere
region of
chromosome
Sister Chromatids

 Diploid - A cell possessing two copies of each chromosome
(human body cells).
 Homologous chromosomes are made up of sister chromatids
joined at the centromere.
 Haploid - A cell possessing a single copy of each chromosome
(human sex cells).

 KARYOTYPE it is a
chromosome complement
of a cell or organism
depicting the number , size
and form of the
chromosome as seen in
metaphase of mitosis.
 Chromosomes are
assembled as homologous
pairs in decreasing order of
length.

CELL DIVISION
 Cell division involves a single cell (called a mother
cell) dividing into two daughter cells.This leads to
growth in multicellular organisms (the growth of
tissue) and to procreation (vegetative
reproduction) in unicellular organisms.
 Prokaryotic cells divide by binary fission.
 Eukaryotic cells usually undergo a process of
nuclear division, called mitosis, followed by
division of the cell, called cytokinesis.
 A diploid cell may also undergo meiosis to
produce haploid cells, usually four. Haploid cells
serve as gametes in multicellular organisms, fusing

 DNA replication, or the process of duplicating
a cell's genome, is required every time a cell
divides. Replication, like all cellular activities,
requires specialized proteins for carrying out
the job.

Protein synthesis
 Cells are capable of synthesizing new
proteins, which are essential for the
modulation and maintenance of cellular
activities.
 This process involves the formation of new
protein molecules from amino acid building
blocks based on information encoded in
DNA/RNA.
 Protein synthesis generally consists of two
major steps: transcription and translation.

Transcription
 Process by which genetic information encoded in DNA is
copied onto messenger RNA
 Occurs in the nucleus
 DNA  mRNA
Translation
 Process by which information encoded in mRNA is used
to assemble a protein at a ribosome
 Occurs on a Ribosome
 mRNA  protein

CELL DIVISION OCCURS IN 3 WAYS-
AMITOSIS
MITOSIS
MEIOSIS

AMITOSIS
 Amitosis is very simple, it occurs without the
formation of spindle and appearance of
chromosome
 Nuclear envelope remains intact.
 E.g – Macromolecules of ciliates such as
paramecium.
 In each case division of nucleus called
KARYOGENESIS, occurs before the division of
cytoplasm termed CYTOKINESIS

MITOSIS
 MITOSIS was 1ST described by GERMAN BIOLOGIST
EDUARD STRASBURGER in plant cell in1875
and later by another GERMAN BIOLOGISTWALTHER
FLEMMING in animal cell 1879.
 TERMED MITOSIS byWALTHER FLEMMING in 1882.
DEFINITION
 It is defined as" the division of a parent cell into 2
identical daughter cells each with a nucleus having
the same amount of DNA, the same number and
kind of chromosomes and same hereditary
instructions as parent cell.”
 Hence also known as EQUATIONAL DIVISION.

INTERPHASE
 Period between two successive division.
 Greater part of interphase is called G1 stage.
 S Stage: - Synthesis of DNA takes place.
 G2: - Protein synthesis takes placed required
for cell division.

MECHANISM OF MITOSIS
 It involve a series of changes in nucleus as
well as cytoplasm
Therefore often called as indirect division
Two main events in mitosis -:
 Karyokinesis
 Cytokinesis

Karyokinesis may be divided into four stages 
 PROPHASE
 METAPHASE
 ANAPHASE
 TELOPHASE

STAGES OF MITOSIS
EARLY PROPHASE
 Early in the prophase stage the chromatin fibers shorten into
chromosomes
 Each prophase chromosome consists of a pair of identical double-
stranded chromatids.

LATE PROPHASE
 Nucleolus disappears, the nuclear envelope breaks down,
and the two centrosomes begin to form the mitotic spindle
(which is an assembly of microtubules).
 As the microtubules extend in length between the
centrosomes, the centrosomes are pushed to opposite "poles"
(extremes) of the cell.
 Eventually, the spindle extends between two opposite poles
of the cell.

Review of Prophase
38
What the cell looks like

METAPHASE
 CHROMOSOME CONDENSATIONCONTINUES INTO
METAPHASE.
 MOST STRIKING FEATURE-CHROMOSOMES BECOME
ALIGNEDWITHTHERECENTOMERES IN A SINGLE
TRANSVERSE PLANE.
 THIS PLANE LIES PERPENDICULARTOTHE LONG AXIS
OF SPINDLE AND IS KNOWN AS EQUTORIAL PLATE.
 OFTEN KNOWN AS METAPHASE PLATE.

Metaphase
40
Poles
Equator of Cell

ANAPHASE
 The centromeres split separating the two members of
each chromatid pair - which then move to the
opposite poles of the cell.
 When they are separated , the chromatids are
now called as chromosomes.
 As the chromosomes are pulled by the microtubules
during anaphase, they appear to be "V"-shaped
because the centromeres lead the way, dragging the
trailing arms of the chromosomes towards the poles

TELOPHASE
Telophase begins after the chromosomal movement stops.
 The identical sets of chromosomes - which are by this stage at
opposite poles of the cell, uncoil and revert to the long, thin,
thread-like chromatin form.
 A new nuclear envelope forms around each chromatin mass.
 Nucleoli appear.
 Eventually the miotic spindle breaks-up.

Cytokinesis
• Cleavage of cell into two
halves
– Animal cells
 Constriction belt of
actin filaments
– Plant cells
 Cell plate
– Fungi and protozoa
 Mitosis occurs
within the nucleus

Cytokinesis In Animal And Plant
Cells
Daughter cells
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
100 µm
1 µmVesicles
forming
cell plate
Wall of
patent cell Cell plate
New cell wall
(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)

Meiosis
Specialized type of cell division that produces
germ cells.ova and spermatozoa.
This process has 2 crucial results:-
 Reduction in number of chromosomes from
diploid (2n) to haploid (1n).
 Recombination of genes.
It is divided in 2 separate divisions-
 Meiosis 1/reductional division.
 Meiosis 2/equatorial division.

MEIOSIS I
 Amount of DNA is doubled to
4n and the chromosome
number is also doubled to 4n
during S phase.

Prophase 1 -:
PROPHASE 1 is subdivided into following 5
phases -:
1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene
5. Diakinesis

1. LEPTOTENE
Chromosomes condense forming
long strands.
 Chromosome become visible each
chromosomes consists of two
chromatids.
 At first the chromosomes are seen as
threads bearing bead thickening
(chromosome) along their length.

2.ZYGOTENE -:
Pairing of chromosome also referred as
synapsis, takes place.
Synaptonemal complex is formed by a pair of
synapsed homologous chromosomes i called a
bivalent or a tetrad.

3. PACHYTENE
Chiasmata (crossing over sites) start
forming
 2 chromatid of each chromosomes
become distinct.The bivalent now has
formed chromatids in it called aTetrad
which is clearly visible at this stage.
 Two central chromatids becomes
coiled over each other so that they
cross at number of points called
crossing over.

4.DIPLOTENE
 Dissolution of the synaptonemal complex
 Chromosome continue to condense and then begin
to separate, revealing X shaped structures, called
chiasmata.
 2 chromosomes of bivalent now try to move apart
as they do so chromatid break at the point of
crossing.

DIAKINESIS
 The final stage of meiotic prophase I.
 Marked by terminalisation of chiasmata
 Nucleolus and nuclear envelop disappear, freeing
the chromosomes into the cytoplasm.
 Diakinesis represents transition to metaphase- 1

 METAPHASE 1-
Homologus chromosomes align
as pairs on the equatorial plate of
spindle apparatus.
Spindle Equatorial

ANAPHASE 1-
 Homologous chromosomes migrate away from each
other going to opposite poles.
 Sister chromatids still remain associated at their
centromeres

TELOPHASE 1-
 Nuclear membrane and
nucleolus reappear .
 Nuclei are formed
 Cytokinesis occur giving rise
to 2 daughter cells.

 The stage between the two meiotic divisions
is called interkinesis and is generally short
lived.
 Interkinesis is followed by prophase II, a
much simpler prophase than prophase I.

Figure 13.7 The stages of meiotic cell division: Meiosis II

Meiosis II : Separates sister
chromatids
 Proceeds similar to mitosis
 THERE IS NO INTERPHASE II !

Prophase II
 A spindle apparatus starts forming
 The nuclear membrane disappears by the
end of prophase II .
 The chromosomes again become
compact.

Metaphase II
 The chromosomes are positioned on the
metaphase plate in a mitosis-like fashion
 Kinetochores of sister chromatids of each
chromosome pointing toward opposite poles

Anaphase II
 The centromeres of sister chromatids finally
separate
 The sister chromatids of each pair move toward
opposite poles
(Now individual chromosomes)

Telophase II and Cytokinesis
 Nuclear envelop forms around two groups of
chromosomes once again .
 Cytokinesis follows resulting into four haploid
daughter cells.
Haploid cells (n)

Cell cycle checkpoints
They are used by the cell to monitor and regulate the progress of the cell cycle.
 Checkpoints prevent cell cycle progression at specific points, allowing verification of
necessary phase processes and repair of DNA damage.
The cell cannot proceed to the next phase until checkpoint requirements have been
met.
Several checkpoints are designed to ensure that damaged or incomplete DNA is not
passed on to daughter cells.
 Three main checkpoints exist:
1. G1/S checkpoint . G1/S transition is a rate-limiting step in the cell cycle and is
also known as restriction point.
2. G2/M checkpoint.
p53 plays an important role in triggering the control mechanisms at both G1/S and
G2/M checkpoints.

3. Spindle assembly checkpoint, between
metaphase and anaphase
Cell cycle can be arrested if spindle fibers are
not attached properly to chromatids

Regulation of the Cell Cycle
 Cyclin: major control switch for the cell cycle
 Cdk (Cyclin-dependent kinase (phospohorylation)): major control switch; activated by cyclin;
causesG1  S or G2  M.
 Cyclins form the regulatory subunit and Cdks the catalytic subunit
 Checkpoints
 DNA damage checkpoints, including tumor suppressor genes
 p53: protein that blocks the cell cycle if DNA is damaged and can cause apoptosis.A p53 mutation is the most
frequent mutation leading to cancer.
 Spindle checkpoints

Two families of proteins are involved in involved in the regulation
process—
i) Cyclin-dependent protein kinases (Cdks)
ii) Cyclins.
a)Cyclin-Dependent Protein Kinase (Cdks)
A Cdks is an enzyme that adds negatively charged phosphate groups to other molecules in a
process called phosphorylation.
Through phosphorylation, Cdks signal the cell that it is ready to pass into the next stage of the cell
cycle.
Cyclin-Dependent Protein Kinases are dependent on cyclins & Cyclins bind to Cdks, activating the
Cdks to phosphorylate other molecules.
b)Cyclins
Cyclins are named such because they undergo a constant cycle of synthesis and degradation
during cell division.
When cyclins are synthesized, they act as an activating protein and bind to Cdks forming a cyclin-
Cdk complex.
This complex then acts as a signal to the cell to pass to the next cell cycle phase.
Eventually, the cyclin degrades, deactivating the Cdk, thus signaling exit from a particular phase.

2 cell & cell cycle

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