seminar cell anddddd molecularr

1. CELL AND MOLECULAR BIOLOGY
Topic : Cell division
Aswathy Jayan
1st
PG
Reg.no : 1066

2.  All cells come from pre-existing cells.
 In prokaryotes, which do not have a well defined nucleus,
daughter cells are formed by fission in which cell
components of the parent cell are equally divided
 In eukaryotic cells is different where cells divide by a
complex process of mitosis.
 Eukaryotic cell division takes place through a series of
orderly events known as the cell cycle.

3. PHASES OF CELL CYCLE
Cell cycle is divided into 4 phases
 G1 phase (gap 1) : this is the period of active RNA and protein
synthesis when the cell is preparing itself for DNA synthesis
and chromosome replication and is perhaps the longest phase
lasting about 10 hours.
 S phase: this is marked by synthesis of DNA and centrosomes,
lasting about 9 hours, hence called synthetic phase.
 G2 phase (gap 2) : during this phase the cell is preparing for cell
division and requires high energy input, lasting four hours of
duration. During this period necessary proteins like tubulin,
cyclins etc, are synthesized which are needed for mitosis.
 Mitosis : this is the phase when actual division of the cell is
accomplished and it lasts for about an hour.

4. MITOSIS – CELL DIVISION
 Chromosomes in the cell nucleus are present in all animals, plants, and protists.
 Additional continuous elements in the cytoplasm include centrioles,
mitochondria, Golgi apparatus, and plastids, all of which undergo division
across generations.
 Chromosomes: Chromosomes do not arise de novo; they remain intact
throughout the life of the nucleus.
-Their basic structures duplicate during nuclear divisions and form the genetic
material of the cell.
-The behaviour of chromosomes during mitosis and meiosis is consistent across
protists, plants, and animals, though there may be differences in the accessory
apparatus responsible for chromosome distribution.
 Interphase (End of G Phase): Interphase is the stage between two successive
₂
division cycles, involving growth and active metabolism.
- At the end of G phase, the cell prepares to divide into two equal halves with
₂
precise distribution of DNA.

5. - It is a dynamic process that includes chromosome assortment and
cytoplasmic division.
- The interphase was once considered a "resting stage," but it is now
recognized as the most active stage due to intense metabolic activity.
- - The nucleus is granular during this stage, often referred to as the
"metabolic nucleus.“
 Mitosis: All undifferentiated eukaryotic cells undergo mitotic divisions
continuously.
- Mitosis is divided into four stages:
 1. Prophase: Chromosomes condense.
2. Metaphase: Spindle formation occurs.
3. Anaphase: Chromosomes move towards the poles.
4. Telophase: Nucleus reforms, and cytokinesis takes place.
- The chromosomes undergo changes in form during mitosis, but the
chromosomes remains unaltered, which is closely related to chemical
alterations.

6. PROPHASE
 Earliest Stage of Mitosis: Prophase is the first and longest stage of
mitosis, lasting a few hours.
 Chromosomes appear as thick, short filaments. Each chromosome
consists of two arms bound by a centromere.
 During early prophase, each arm of the chromosome is duplicated
into sister chromatids, although the centromere itself does not
duplicate.
 The two DNA strands, which are coiled into minor coils, start
condensing and coiling around each other, forming more tightly
bound structures.
 Chromosomes split longitudinally, and the two chromatids coil
around each other. Coiling becomes progressively tighter.
 The nucleolus disappears during prophase, although in some
protists, it remains intact.

7.  Centriole pairs move to opposite sides of the nucleus, acting as
centers from which spindle fibers extend.
 A spindle structure composed of microtubules begins to form,
originating from the centrioles. These microtubules extend
throughout the cell.
 The nuclear envelope starts to break down into small fragments,
persisting until the end of anaphase and reassembling at telophase.
 The spindle is formed by microtubules, while asters (structures
produced by centrioles) appear. The centrioles do not directly
participate in cell division but help organize the spindle.
 Prophase is an energy-intensive stage, requiring ATP for the process
to proceed. If cellular respiration stops in early prophase, mitosis is
halted.
 The amount of proteins, RNA, and phospholipids in the cell
increases significantly during prophase.
 Nucleolar RNA may be transferred to the chromosomes during
prophase.

8. METAPHASE
 Duplicated chromosomes move towards the equatorial plane, where they
align.
 Chromosomes are easily countable at this stage.
 Spindle fibers are clearly visible, converging towards the poles.
 Spindle fibers are attached to the centromeres in the central plane.
 Use of Colchicine: Cytologists use colchicine to inhibit mitosis at
metaphase for chromosome counting and morphology studies.
 Size Variation: Chromosome size varies in different species; larger
chromosomes are at the periphery, and smaller ones are at the center.
 Chromosomes become thick, short, and maximally contracted
(chromonema).
 The centromere divides into two, each connected to a chromatid.
 The spindle is stretched longitudinally.

9.  Chromosomes assume a cylindrical shape due to tight coiling of
chromonema.
 Changes in the form of chromosomes (coiling and uncoiling) are
attributed to the changes in chromonemata.
 The chromosomes shorten by 3 to 6 times from prophase to metaphase.
ANAPHASE
 Anaphase is the shortest phase of the cell division cycle.
 The phase is marked by the separation of sister chromatids, which move
toward opposite poles of the spindle.
 The separation of sister chromatids occurs due to the breakdown of
cohesin proteins, which hold the chromatids together.
 The spindle fibers do not act like elastic rubber bands (no
contraction). They consist of proteins that change to a soluble form
when movement begins.
 Some researchers suggest spindle fibers may function similarly to
the ATPase activity of myosin filaments, contributing to chromatid
movement.

10.  At the end of anaphase, chromosomes gather around the mitotic
centers with centromeres positioned almost equidistant from
centrioles.
 The nuclear envelope starts forming around the chromosome cluster.
 Chromosomes begin to disperse after nuclear envelope formation.
 During late anaphase, the cleavage furrow begins to form, and the
membrane starts constricting at the equator.
 Proper execution of anaphase is critical; errors may result in cell
death.
 In some cases, chromatids may form without centromeres, preventing
attachment to spindle fibers, which can lead to the loss of
chromosomes and cell death.
 Cytokinesis: Cytokinesis begins before telophase starts.

11. TELOPHASE
 Arrival of daughter chromosomes at the poles of the cell.
 Demolition of the mitotic apparatus, leading to the gradual
disappearance of spindle fibers.
 Chromatids uncoil and return to thread-like structures similar to
interphase.
 Nuclear envelope begins to form around the chromosomes.
 Nucleolus reappears, beginning the production of rRNA.
 Centrioles reform and occupy positions at right angles to each other.
 Endoplasmic reticulum concentrates at the poles, and small vesicles from
it contribute to the formation of the new cell membrane at the equatorial
plane.
 Cytoplasm moves away from the equator, pushing the cell membrane
inwards.
 Division of the cytoplasm completes, forming two daughter cells through
cytokinesis.
 Telophase concludes when cytokinesis is complete, and DNA replication
begins again.

13. CHROMOSOME MOVEMENT DURING MITOSIS
 Prophase Events: - Chromosomes condense, and centromeres appear on
them.
- Kinetochores form on centromeres for spindle fiber attachment.
- Centrioles move to opposite poles of the cell.
- The spindle begins to form, and the nuclear envelope starts breaking
down.
 Centrioles and Spindle Formation:- Centrioles, composed of microtubule
triplets, play a crucial role in spindle formation.
- Centrioles replicate during the S phase and move to opposite poles.
- Spindle fibers originate from the centriolar pair.
 Chromosome Movement and Microtubules: - Chromosome movement
during mitosis is driven by microtubules made of tubulin.
- Movement towards poles occurs through three types of forces:

14. 1. Polymerization/Depolymerization: Microtubules lengthen and shorten,
facilitating chromosome movement.
2. Sliding Microtubules: Microtubules slide past each other, assisted by
dynein arms.
3. Contraction: Actomyosin contractions help chromosomes move apart.
 Polymerization and Depolymerization: - In anaphase, polar microtubules
lengthen, and kinetochore microtubules shorten, aiding chromosome
movement.
- Colchicine experiments show that microtubule depolymerization results
in chromosome movement toward the poles.
 Sliding Microtubular Model:- Proposed by J.R. McIntosh in 1969, this
model suggests force generation through the sliding of adjacent
microtubules.
- Initially, microtubules were thought to be oriented antiparallel, but later
research shows they are arranged in parallel.
- The sliding model is less supported due to this finding.

15.  Actomyosin as a force-generating system: Proposed by A. Forer in
1974 to explain chromosome movement.
 Actin and myosin in the mitotic spindle: Forer suggested that actin
and myosin may be present in the mitotic spindle.
 Sliding filament model: Actin and myosin slide past each other,
similar to muscle contraction.
 Chromosome movement: The model focuses on chromosome
movement, not spindle elongation.
 Formation of actomyosin: Actin and myosin filaments slide past each
other to form actomyosin, and ATP is consumed in the process.
 ATP dependency: Chromosome movement is ATP-dependent.
 Microtrabecular network: Recent suggestions indicate that mitotic
forces may also be generated by the contractile activities of a
microtrabecular network, which coexists with microtubules.

16. CYTOKINESIS
 Cytokinesis is the division of the cytoplasm during cell division.
- There are three types of cytoplasmic division:
Constriction: Common in animal cells.
Formation of separate zones: Also observed in animal cells.
Formation of a division plate: Found in plant cells.
 Cytokinesis in animal cells often involves both constriction and separate
zones.
 In sea urchin eggs, the cleavage furrow forms in the equatorial plane
during cytokinesis.
 In plant cells, cytokinesis occurs through the formation of a cell
plateduring anaphase.
 The spindle transforms into a phragmoplast, which helps form the new
cell wall.
 The cell plate becomes the middle lamella, linking the daughter cells.
- The duration of mitosis is not fixed and varies depending on the cell,
tissue, environmental conditions, and physiological state of the cytoplasm.

17. - In root and shoot tips, division occurs once in a few hours, but it can vary
with environmental factors.
REPRODUCTION IN CELLS
 Cell Division and Asexual Reproduction: - Cell division is a process of
growth and cell multiplication.
- It is a form of asexual reproduction.
 Sexual Reproduction:- Involves the production of specialized sex cells
(gametes).
- Gametes unite to form a zygote through fertilization.
- This process results in genetic variation in offspring.
 Reproduction in Unicellular Organisms:- Unicellular organisms often
reproduce by fission (asexual reproduction).
- Some organisms, like Chlamydomonas, use syngamy, where two
individuals unite to produce a new individual.
- In certain cases, such as conjugation, there is a temporary fusion for
exchanging genetic material.

18.  Reproduction in Prokaryotes: - Prokaryotes (e.g., bacteria) reproduce by
binary fission.
- The bacterial cell grows, replicates its chromosome, and divides into two
daughter cells.
- The process of chromosome replication and cell division is synchronized.
 Reproduction in Eukaryotes:- Sexual reproduction in eukaryotes
involves genetic material from two individuals (parents).
-Chlamydomonas exhibits a primitive form of sexual reproduction, where
gametes are formed and fuse to create a new individual.
- Isogametes are produced (identical gametes in shape and size but
different in behaviour).
 Gamete Production:- Gametes are produced in specialized cells located
in the gonads.
- Somatic cells are diploid (2n), while gametes are haploid (n) to ensure the
formation of a diploid zygote upon fertilization.
 Meiosis:- Meiosis is a reduction division that reduces the chromosome
number by half.

19. - It ensures that each gamete (sperm and ovum) has half the number of
chromosomes (haploid).
- Meiosis divides homologous chromosome pairs, distributing one set to
each gamete.
MEIOSIS
 Meiosis involves reduction division, producing gametes with half the
number of chromosomes, ensuring fertilization restores the diploid
number.
- Meiosis occurs before fertilization in animals, whereas in plants, it
may be separated by one or more cell generations.
- In unicellular organisms, meiosis may follow fertilization.
Meiocyte: A cell that undergoes meiosis, with changes influenced by
the gonadal environment.
- Meiosis has two stages: Meiosis I and Meiosis II, each containing
several phases.

20. Meiosis I
Prophase 1: The prophase of meiotic division I is a complex
process, much slower than mitosis. The beginning of prophase is
marked by an increase in the nuclear volume in which the
chromosomes appear as long, coiled and thin threads. This is the
Leptotene stage of prophase I. The chromosomes appear as
undivided structures, but in reality most of the DNA of cells has
been doubled in the preceding prophase I. According to Brown,
DNA synthesis continues until the leptotene stage and it
constiutes G, period of the cycle.
Zygotene Stage: This stage is characterised by pairing of
homologous chromosomes in a specific manner that is unique for
meiosis. The attractive force that brings about pairing of
homologous chromosomes is still not clear. This pairing is known
as synapsis and the homologous pairs are called bivalents.

21.  Pachytene Stage: In this stage the nucleolus grows in size and
the bivalents coil around each other so that the synapsed
chromosomes appear as short and thick structures, which split
longitudinally in such a manner that every bivalent pair now
looks a pack of four strands. This is known as the tetrad stage.
Each chromatid of a tetrad undergoes a process of coiling around
its fellow and becomes shorter and thicker due to strain. Each
homologous chromosome has an independent centromere, thus
each chromatid is provided with a centromere. The most
important event in pachytene stage is the formation of a
chiasma, when the two sister chromatids of each homologous
pair exchange segments. The chromatids may break at various
points and may join with the broken ends of the chromatids. This
process is known as crossing over. Pachytene stage lasts for a
longer time and the end is marked by a force of repulsion
between chromatids.

22.  Diplotene Stage: At diplotene the paired chromosomes begin
to pull apart, but they do not separate completely since they are
held together at the points of interchange. This stage has
tremendous significance in genetics, since an exchange occurs
between non sister chromatids. The crossing over permits
exchange of genes to form recombinant chromatids. In molecular
biology crossing over has been used as an experimental tool for
mapping the chromosomes.
 Diakinesis: In diakinesis the chromosomes become much
shorter and thicker and the chiasma disappears. The
homologous chromosomes are pulled apart toward the periphery
of the nucleus, but complete separation of chromatids does not
take place. The homologues are held together only at their ends,
to form wide loop-like structures. Besides, the nucleolus and the
nuclear envelope disappear and the spindles are fully formed.
The chromosome bivalents arrange themselves in the
metaphase plate.

23.  Metaphase 1: This stage follows diakinesis of prophase and
resembles mitotic metaphase. The homologues are arranged in
the equatorial plane and remain attached to the spindle fibres
through centromeres, which face toward the poles.
 Anaphase 1: The sister chromatids of each homologue attached
by their centromeres thove toward their respective poles. The
chiasmata is completely broken and the non-sister chromatidds
separate, which differ from the paternal and maternal
chromosomes. In contrast to mitotic anaphase here we have each
chromosome consisting of two chromatids, one of which is a
recombinant.
 Telophase 1: Telophase is for a brief duration, during which the
chromatids uncoil, elongate and persist for some time in a
condensed state. The nuclear envelope starts reappearing around
each group of chromatids, resulting in two separate nuclei. In
some organisms, after the formation of nuclear envelope, each
daughter nucleus undergoes a period of rest or interphase before
the second meiotic division starts. There is no DNA synthesis
between two meiotic divisions.

24. CROSSING OVER
 Is the exchange of genetic material between non-
sister chromatids of homologous chromosomes,
resulting in new combinations of alleles and
contributing to genetic diversity.

25. Meiosis II
 Meiosis II is similar to mitosis, but differs because the
chromosomes exist in a doublet condition (as chromatids).
 Prophase II: - Chromatids separate except at the centromere.
- The nucleolus and nuclear envelope disappear.
- Chromatids become free in the cytoplasm, coil, and the spindle
begins to form.
 Metaphase II: - Chromatids are arranged in the equatorial plane.
- Spindle fibers attach to the chromatids at their centromeres.
 Anaphase II: - Centromeres divide.
- Chromatids are pulled apart towards opposite poles.
 Telophase II: - Nuclear envelope forms around each set of
chromatids at the poles.
- Chromatids uncoil, and the nucleolus reappears.

26.  Cytokinesis occurs, resulting in four daughter cells.
- Each of the four daughter cells is haploid, containing one chromatid from
the original tetrad.
 Genetic variation: - Two daughter cells contain chromatids with
recombinant DNA.
- The other two daughter cells have the original parental character.

28. SIGNIFICANCE OF MEIOSIS
 Formation of gametes − Meiosis form gametes that are essential for sexual
reproduction.
 Genetic information − Meiosis switches on the genetic information for the
development of gametes.
 Maintenance of chromosome number − Meiosis maintains the fixed
number of chromosomes in sexually reproducing organisms.
 Assortment of chromosomes − In meiosis paternal and maternal
chromosomes assort independently. It causes reshuffling of chromosomes
and the traits controlled by them.
 Crossing over − It introduces new combination of traits or variations.
 Mutation − Mutations take place due to irregularities of meiotic division.

29. REFERENCE
 S C Rastogi (2005) cell biology (3rd
ed.)
 Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., &
Walter, P. (n.d.). Molecular Biology of the Cell (4th ed.).
 https://www.shaalaa.com/question-bank-solutions/what-is-
the-significance-of-meiosis_8076