Cell cycle & cell division

• BSC 401
• HOT COURSE
•
• GENETICS AND PLANT BREEDING
• TOPIC: CELL CYCLE AND CELL DIVISION
• Debasish Prusty
• ALM 4016
• IV Bsc(Ag)

Cell division
• What is cell division?
• Why is it important for us to know?
• Why do cell do it?
• An adult human being estimated about
hundred trillion cells.
• Yet we start life as single cell.

Cell division
• Cell Division Also known as Mitosis
• Takes place in Regular Body Cells
• Keeps Cells Living and Growing
• Regenerate missing body parts

Types
• Asexual reproduction involves a single cell
dividing to make 2 new, identical daughter
cells.
• Mitosis & binary fission are examples of
asexual reproduction.
• Sexual reproduction involves two cells (egg &
sperm) joining to make a new cell (zygote)
that is NOT identical to the original cells
Meiosis is an example.

Types of cell divison in bacteria
Binary fission
Bacterial
conjugation
Bacterial
transformation
Bacterial
transduction

Binary fission
• This type of cell division takes place in lower
organisms(prokaryotes) like Archae and Bacteria
and primitive organelles of eukaryotes.
• It results in the reproduction of a living
prokaryotic cell (or organelle) by dividing the cell
in to two parts.
• No or less variation occurs.
• Create large number of offspring in a short time.

Question?
If the bacterium can fill upto
1000ml in 36hours, and its
dividing time is 36 minutes,
then after how many
minutes can the bacteria fill
upto 125ml level.
Assume that bacteria is
provided sufficient nutrient,
and also the wastes are
removed in intervals.

Bacterial conjugation
• Conjugate means to come together
• It happens in only bacteria having plasmid.
• It create genetic diversity
• Bacteria transfer a copy of plasmid to another.

Bacterial transformation
• Bacteria can pick up a free plasmid from
environment, thus becoming transformed.
• This too create genetic diversity
• Which famous scientist was linked with this
experiment?

Bacterial Transduction
• Trans=across, duction=to move
• The movement of bacterial dna from one
bacterium to another via a viral replication
• Viral dna insert into host dna for latent stage
• Viral dna excises itself to enter lytic cycle &
pick bacterial dna.

Multiple fission
• Fission of protists.
• Multiple fission at the cellular level occurs in
many protists, e.g. sporozoans and algae.
• nucleus of the parent cell divides several times
by amitosis, producing several nuclei.

Multiple fission
• Fission of apicomplexans
• In the apicomplexans, a phylum of parasitic
protists, multiple fission, or schizogony, is
occurs as merogony, sporogony or
gametogony.
• Merogony results in merozoites, which are
multiple daughter cells, sporogony results in
sporozoites, and gametogony results in
microgametes.

Plasmotomy
• Another mechanism of fission occur in some
protozoas like opalina and pelomyxa.
• a multinucleate adult parent undergoes
cytokinesis to form two multinucleate( or
coenocytic) daughter cells.
• The daughter cells so produced undergo
karyokinesis, further.

Fragmentation
• It is a form of asexual reproduction or cloning
where an organism is split into fragments.
• Each of these fragments develop into mature,
fully grown individuals that are clones of the
original organism.
• In echinoderms, this method of reproduction
is usually known as fissiparity.

Phases of the Cell Cycle
1. G1 (gap phase 1)
– Primary growth phase, longest phase
2. S (synthesis)
– Replication of DNA
3. G2 (gap phase 2)
– Organelles replicate, microtubules organize
4. M (mitosis)
– Subdivided into 5 phases
5. C (cytokinesis)
– Separation of 2 new cells

Does mitosis occur before or after
interphase? Is this merely an issue of
“point of view”?
• Mitosis must be considered subsequent to
interphase, since interphase is carried out in
preparation step for mitosis. Therefore, this is not
merely a point of view issue

length of the cell cycle
• The time it takes a cell to complete the cell cycle
depends on the type of cell that is dividing.
• The cell cycle for some eukaryotic cells might only
take eight minutes, but others can take up to a
year.
• Most of the cells in the human body can
complete the cell cycle in about 24 hours.
• The cells of some organisms divide very quickly.
For example, the fertilized egg of the zebra fish
divides into 256 cells in 2.5 hours.

Interphase
• The longest stage of a Cell’s life
• A new cell begins interphase with a period of
rapid growth.
• Cellular activities, such as making proteins,
follow.
• Each cell that is actively dividing eventually
copies its DNA and prepares for cell division.

Interphase
• A cell’s DNA is called chromatin during
interphase.
• Chromatin is long, thin strands of DNA in the
nucleus.
• If scientists add dye to a cell in interphase, the
nucleus looks like a plate of spaghetti.
• This is because the nucleus contains strands of
chromatin tangled together.

Interphase - G1 Stage
• 1st growth stage
• This is a period of rapid growth.
• G1 is the longest stage of the cell cycle
• Cells mature by making more cytoplasm &
organelles
• Cell carries on its normal metabolic activities
• Most cells continue the cell cycle.
• Some cells stop the cell cycle at the G1 stage.
• Mature nerve cells in your brain remain in G1 and
do not divide again.

G₀ stage
Some cells that do not
divide further exit G1.
They enter an inactive
stage called quiescent
stage G₀.
Cells in this stage
remain metabolically
active but no longer
proliferate.

Interphase – S Stage
• Synthesis stage
• DNA is replicated

• During this time the amount of DNA per cell
doubles
• If the initial amount of DNA is denoted as 2C
then it increases to 4C
• No increase in the chromosome number; if
the cell had 2n number of chromosomes at
G1, even after S phase the number of
chromosomes remains the same, i.e., 2n
• DNA is synthesised but remain uncondensed.

Uncondensed DNA

Interphase– G2 Stage
• 2nd Growth Stage
• Occurs after DNA has been copied
• All cell structures needed for division like
centrioles are duplicated

Interphase– G2 Stage
• Both organelles & proteins are synthesized
• During G2, the cell regain energy that was
spent during S-phase.
• The cell also stores energy that will be used
during the mitotic phase of the cell cycle.

Organelle replication
• Before a cell divides, it makes a copy of each
organelle and they are distributed between
the two new cells.
• This way, the two new cells can function
properly.
• Some organelles, such as the mitochondria
and chloroplasts, have their own DNA, so
these organelles can make copies of
themselves on their own.

Organelle replication
• A cell produces other organelles from
materials such as proteins and lipids.
• A cell makes these materials using the
information in the DNA inside the nucleus.
• Organelles are copied during all stages of
interphase

Stages of mitosis
prophase metaphase

Stages of mitosis
anaphase telophase

What are some examples of organs and tissues
in which mitosis is more frequent, less frequent
or practically absent?
• Tissue like epithelial tissues and bone marrow in
animals and meristem tissue in plants exhibit frequent
mitosis.
• Mitosis is less frequent in tissues that are renewed
slowly, such as bones in adults and connective tissue.
• Mitosis is almost absent in the nervous tissue and
striated muscle tissue (skeletal and cardiac).
• Nervous tissue develops through stimulus via the
development of new electrical networks between cells,
and striated muscle tissue grows via cellular
hypertrophy.

Early prophase
• Chromatin in nucleus condenses to form
visible chromosomes.
• Mitotic spindle from fibers in cytoskeleton.
• Disintegration of nuclear membrane initiates.

Late Prophase
• Nuclear membrane & nucleolus are broken
down
• Chromosomes continue condensing & are
clearly visible
• Spindle fibers called kinetochores attach to
the centromere of each chromosome
• Spindle finishes forming between the poles of
the cell

Why is it important for chromosomes to be condensed
during mitosis and decondensed during interphase?
• During mitosis, the main problem is the correct
separation of chromosome sets between daughter
cells. If chromosomes were decondensed, long tiny
fibers of DNA would be dispersed in cytoplasm after
the karyotheca is broken and chromosomes could not
be easily organized and moved by the spindle fibers.
• During interphase, the function of chromosomes, or
rather, of DNA molecules, is the synthesis of RNA and
therefore proteins. For this task, it is necessary for
functional molecular regions to be decondensed (these
regions form the euchromatin). Furthermore, during
interphase, DNA replication occurs to prepare for cell
division. In this process it is necessary for DNA
molecules to serve as templates for the new DNA
chains being produced

What is the mitotic apparatus?
• The mitotic apparatus is a set of aster fibers and spindle
fibres. Aster fibers are radial structures around each
centriole pair. Spindle fibers are fibers that extend
across the cell between the two centriole pairs located
at opposite cell poles. The mitotic apparatus appears in
prophase and plays an important role in the orientation
and holding of chromosomes and other cellular
elements, so as to cause them to separate and migrate
to opposite cell poles.
• Substances that prevent the formation of the mitotic
apparatus, such as colchicine, a molecule that binds to
tubulin molecules and prevents the synthesis of
microtubules, interrupt cell division. Colchicine is used
to study chromosomes, since it paralyzes mitosis when
chromosomes are condensed, making them easier to
view under a microscope.

Metaphase
• Chromosomes, attached to the kinetochore
fibers, move to the center , of the cell of the
cell.
• Chromosomes are now lined up at the
equator.

What is the name for the disease
caused by uncontrolled mitosis in
pluricellular organisms?
• Uncontrolled mitotic cell division is called neoplasia.
(the formation of new strange tissues)
• Cancers are malignant neoplasias.
• Neoplasias whose cells cannot disseminate to
distant sites are called benign neoplasias.

Question?
• A fruit fly population grows by half every 16th
minute. A starting population of 16 fruit fly
will grow into how many bacteria after 16
hours?

Anaphase
• Occurs rapidly
• Sister chromatids are pulled apart to opposite
poles of the cell by kinetochore fibers
• As the single-stranded chromosomes move to
opposite sides of the cell, the cell begins to
get longer.
• Each pole now has identical sets of gene.

Question?
• A bacteria culture deprived of nutrients goes
into remission by halving everyday. If there is
starting of 1024 bacteria, how many bacteria
would be there after 5 days?

Telophase
• During telophase, the spindle fibers that
helped divide chromosomes begin to
disappear.
• The nuclear membrane and nucleoli (nucleus)
reform.
• The chromosomes begin to uncoil.
• A nuclear membrane grows around each set
of chromosomes at either end of the cell.
• Two new identical nuclei form.

Do You Know?
• Centrioles are structures also present in most
protists and in some primitive fungi.

Telophase
• CYTOKINESIS occurs
• Chromosomes reappear as chromatin
• Now the chromatid can be called as future
chromosome.
• Chromosomes cluster at opposite spindle poles and
their identity is lost as discrete elements.
• Nuclear envelope assembles around the
chromosome clusters.
• Nucleolus, golgi complex and ER reform.

Question?
• A certain strain of bacteria is counted at about
19.5 million. If the bacteria quintuple every
fourth day, what is the best estimation of
starting population 40 days ago?
• 50 bacterium

Can mitosis occur in haploid (n) cells?
What about in triploid cells?
• Mitotic cell division can occur in haploid (n)
cells, diploid (2n) cells, triploid (3n) cells, etc.
Mitosis is a copying process that does not
interfere with cell ploidy.

Cytokinesis
• Means division of the cytoplasm
• Division of cell into two, identical halves called
daughter cells
• In plant cells, cell plate forms at the equator
to divide cell
• In animal cells, cleavage furrow forms to split
cell

The two new cells each exactly like the
other are called Daughter Cells.

Regulator
• Cyclin dependent kinase(cdk)
• G1 cyclin
• Mitotic cyclin

Cyclin-Dependent 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.
• This protein is always present but remain
inactivated, unless bound to cyclin.
• Cyclins bind to Cdks, activating the Cdks to
phosphorylate other molecules.

G1 cyclins
• G1 cyclins bind to Cdk proteins during G1.
• It bound and activated, the Cdk signals the
cell's exit from G1 and entry into S phase.
• When the cell reaches an appropriate size
and the cellular environment is correct for
DNA replication, the cyclins begin to degrade.
• G1 cyclin degradation deactivates the Cdk and
leads to entry into S phase.

Mitotic cyclin
• Mitotic cyclins accumulate gradually during
G2.
• Once they reach a high enough concentration,
they can bind to Cdks.
• When mitotic cyclins bind to Cdks in G2, the
resulting complex is known as Mitosis-
promoting factor (MPF).
• This complex acts as the signal for the G2 cell
to enter mitosis.

Mitotic cyclin
• Once the mitotic cyclin degrades, MPF is
inactivated and the cell exits mitosis by
dividing and re- entering G1.
• The cellular signals that we described earlier
(cell size, completion of DNA replication, and
cellular environment) provide the signals that
regulate the synthesis and degradation of
cyclins.

Question?
• What is the name of the complex formed
when mitotic cyclins bind to Cdks and what is
the role of this complex?
• Ans.
• Activation of M-phase- promoting factor
(MPF) is responsible for signaling the entrance
into M phase.

Mitosis test
metaphase anaphase telophase

Mitosis test
G2 prophase Early metaphase

Results of cell division
• The cell cycle results in two new cells, which
are genetically the same.
• They also are the same as the original cell that
no longer exists.
• A human cell has 46 chromosomes.
• When that cell divides, it produces two new
cells, each with 46 chromosomes.

Results of cell division
• The cell cycle is important for reproduction in
some organisms.
• It is important for growth in multicellular
organisms.
• The cell cycle also helps replace worn-out or
damaged cells and repair damaged tissues.

Replacement
• Cell division continues even after an organism is fully
grown.
• Cell division replaces cells that wear out or are
damaged.
• The outermost layer of your skin is always rubbing or
flaking off.
• A layer of cells below the skin’s surface is constantly
dividing.
• This produces millions of new cells each day to replace
the ones that rub off.
• Everyday billions of red blood cells die and need to be
replaced, replacement .

Repair
• Cell division is also important for repairing damage.
• When a bone breaks, cell division produces new bone
cells.
• These new cells patch the broken pieces of bone back
together.
• Not all damage can be repaired, because not all cells
continue to divide.
• Some nerve cells stop the cell cycle in interphase.
• Injuries to nerve cells often cause permanent damage.

Uncontrolled Mitosis
• If mitosis is not controlled, unlimited cell
division occurs causing cancerous tumors
• Oncogenes are special proteins that increase
the chance that a normal cell develops into a
tumor cell.
• This lead to cancer.

Meiosis
•
• Formation of Gametes
• (Eggs & Sperm)

Meiosis
• Preceded by interphase which includes
chromosome replication
• Two meiotic divisions ---
• Meiosis I
• Meiosis II
• Called Reduction- division
• Original cell is diploid (2n)
• Four daughter cells produced are haploid (1n)

Facts About Meiosis
• Daughter cells contain half the contain the
number of chromosomes as the as the original
cell
• Produces gametes (eggs & sperm)
• Occurs in the testes in males
(Spermatogenesis)
• Occurs in the ovaries in females (Oogenesis)

Meiosis Facts
• Start with 46 double stranded chromosomes
(2n)
• After 1 division - 23 double chromosomes (n)
• After 2nd division - 23 single stranded
chromosomes (n)
• Occurs in our germ cells that produce gametes

Why Do we Need Meiosis?
• It is the fundamental basis of sexual
reproduction sexual reproduction
• Two haploid (1n) gametes are brought
together through fertilization to form a
diploid(2n) zygote

Fertilization – “Putting it all together”
2n=46
zygote

Oogenesis
The polar bodies die… only one ovum (egg) is
produced from each primary oocyte

Meiosis
• Sex cells divide to produce gametes (sperm or egg).
• Gametes have half the number of chromosomes.
• Occurs only in gonads (testes or ovaries).
• Male: spermatogenesis
• Female: oogenesis
• Meiosis is similar to mitosis with some chromosomal
differences.

Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
• Centriole pairs also replicate.

Meiosis I
Prophase I
Metaphase !

Meiosis I
Anaphase I
Telophase I

Prophase 1
• It is the longest phase of meosis 1.
• This phase is further divided into 4 sub stages.
• They are: i) Leptotene
• ii) Zygotene
• iii) Pachytene
• iv) Diplotene
• v) Diakinesis

Homologous pair
• Pair of chromosomes (maternal and paternal)
that are similar in shape and size.
• Homologous pairs (tetrads) carry genes
controlling the same inherited traits.
• Each locus (position of a gene) is in the same
position on homologues.
• Humans have 23 pairs of homologous
chromosomes.

Homologous pair
Paternal Maternal

Leptotene
• condensation of the already
replicated chromosomes, this procedure
continues throughout this stage
• The chromosomes become visible by using
electron microscopy, which can distinguish
between sister chromatids
• The chromosome here looks like “beads in a
string’’, and the beads are called
chromomeres.

Leptotene
This is called bouquet stage because all the
telomeres tend to contact the nuclear envelope
in one spot so that the looped chromosomes
balloon out from that point like flower petals.

Zygotene
• Synapsis between homologous
chromosome begins.
• This synapsis makes numerous piont of
contact called ‘’synaptonemal complex’’
• After the homologous pairs synapse they are
either called tetrads or bivalents.

Zygotene
Early zygotene
Late zygotene
Synaptonemal complex

Pachytene
• Once the synapse is formed it is called a
bivalent (where a chromatid of one pair is
synapsed/attached to the chromatid in a
homologous chromosomes) and crossing over
can occur.
• Subsequently, the synapses snap completing
the crossing over of the genetic information.
• Bivalent or tetrad which was formed in
zygotene is clearly visible.

Pachytene
• Longest phase of prophase 1.
• Appearance of recombination nodule at the
site of crossing over.
• Enzyme recombinase helps in the process
recombination.

Diplotene
• During this phase the two homologous
chromosomes begin to migrate apart as
the 'synaptonemal complex disintegrates
between the two chromosomal arms and they
begin to repel one another.
• This allows the two chromosome to move
apart, held only by the chiasma(ta).
• In oocytes of some vertebrates, this can last
for some year.

Diakinesis
• Final stage of meiotic prophase I
• Termination of the condensing of the chaisma.
• Chromosomes are at their most condensed form
during diakinesis.
• The homologous chromosomes in a bivalent are
still connected by at least 1 chiasma
• The nucleolus disappears, the nuclear envelope
disintegrates and the centrioles move to the
equator, whilst the spindles migrate.

Metaphase I
• Shortest phase
• Bivalent chromosomes align on the equatorial
plate.
• Microtubules from the opposite poles of the
spindle attach to the pair of homologous
chromosomes.

Metaphase I
• INDEPENDENT ASSORTMENT OCCURS:
• Orientation of homologous pair to poles is
random.
• Variation occurs.
• Number of combinations: 2^𝑛
Example:2^𝑛 = 4, if n=2
• thus 2^𝑛= 4 combinations

Question?
• In terms of Independent Assortment -how many
different combinations of sperm could a human
male produce?
• Formula: 2n
• Human chromosomes: 2n = 46
n = 23
• 223 = 8 million combinations

Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromeres.

Telophase I
• Each pole now has haploid set of
chromosomes.
• Cytokinesis occurs and two haploid daughter
cells are formed.

Meiosis II
• No interphase II (or very short - no more DNA
replication)
• Meiosis II is similar to mitosis.

Prophase II
• Same as prophase in mitosis.
• Initiated immediately after cytokinesis.
• Nuclear membrane disappears by the end.

Metaphase II
• Chromosomes align at the equator
• Microtubules from opposite poles of the
spindle get attached to the kinetochores

Anaphase II
• Simultaneous splitting of the centromere of each
chromosome (which was holding the sister
chromatids together), allowing them to move
toward opposite poles.

Telophase II
• Cytokinesis occurs.
• Two groups of chromosomes once again get
enclosed by a nuclear envelope.
• Formation of tetrad of cells.
• Four haploid daughter cells (gametes)
produced.

Question?
• How many meiotic division have to occur for
producing 16 zygote in Homo sepians?

Non-disjunction
• Non-disjunction is the failure of homologous
chromosomes, or sister chromatids, to
separate during meiosis.
• Non-disjunction results with the production
of zygotes with abnormal chromosome
numbers.

Occurance of Non-disjunctions
• Two types
• The first is called Monosomy
• organism having Monosomy have only one
chromosome in the 23rd set.
• The second is called Trisomy
• an organism having Trisomy has three
chromosomes in the 18th set.

Common Non-disjunction Disorders
• Down’s Syndrome – Trisomy 21
• Turner’s Syndrome – Monosomy 23 (X)
• Kleinfelter’s Syndrome – Trisomy 23 (XXY)
• Edward’s Syndrome – Trisomy 18

What are the respective functions of the
separation of homologous chromosomes and
of the separation of identical chromatids during
meiosis?
• The separation of homologous chromosomes in meiosis
I has two main functions: to reduce to a half the total
number of chromosomes, generating haploid daughter
cells at the end of the process, and to make the
recombination of genetic material possible, since the
separation is random, that is, each pair of daughter cells
can be different from the other pair, carrying different
combinations of chromosomes from its progenitors.
(Furthermore, if crossing over is considered, each of the
four resulting cells can be different from the others.)
• The separation of identical chromatids during meiosis II
has the same function as it has in mitosis: to separate
the chromosomes already duplicated into the daughter

Does interphase occur once again
between meiosis I and meiosis II?
• There is no interphase or DNA duplication
between the phases of meiosis. Only a short
stage called interkinesis occurs.

Is the interphase of meiosis different
from the interphase of mitosis?
• The interphase that precedes meiosis is
similar to the interphase that precedes
mitosis. During them, the main event is DNA
replication (chromosome duplication).

What is the difference between sexual
spores and gametes? Do humans have
sexual spores or gametes?
• Sexual spores are structures generated from
meiosis with ploidy (the number of
chromosomes) reduced to a half compared to
the mother cell of the spore.
• Spores germinate and give birth to
gametophytes, organisms that form gametes
via mitosis. The meiosis that generates sexual
spores is called sporic meiosis. It is the type of
meiosis that occurs in plants, for example.

What is the difference between sexual
spores and gametes? Do humans have
sexual spores or gametes?
• Gametes are also cells that contain half the number
of chromosomes of the normal cell of the species,
but they are specially designed for fertilization.
• Gametes can be produced through gametic meiosis
or by mitosis in gametophytes originated from sexual
spores.
• The process used in humans as well in most animals
is gametic meiosis. There are no spores or alternation
of generations.

Question?
How many meiotic
division are required to
produce 4016 pollen
grain in cyperaceae
plant?

Source
• Internet
• Fundamentals of life science by pranav kumar
• Ncert
• Concept of botany by Bendre Pandey
• Trueman’s biology

Cell cycle & cell division

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