10: CELL CYCLE AND CELL DIVISION

CHAPTER 10: CELL CYCLE AND CELL DIVISION

CELL CYCLE:
The sequence of events by which a cell duplicates its genome, synthesizes the other
constituents of the cell and eventually divides into two daughter cells is termed cell cycle.

➢ The period required to complete one cell cycle (from beginning of one cell division to the
beginning of next) is called generation time.

➢ It is 24hrs in human cells and 90 minutes in yeast.

➢ Cell growth continues throughout cell cycle through increase in cytoplasm.

PHASES OF CELL CYCLE

Consists of two phases- Interphase and M-phase.

➢ Passage- genetically controlled by proteins cyclins and cyclin dependent protein kinase
(CDKs).

➢ The transition from G1 to S and from G2 to M is carried out by CDKs.

➢ Interphase has two regulatory mechanisms called check points- decision about cell
division.

➢ First check point, G1 cyclin lies in between G1 and S.

➢ Second check point Mitotic cyclin lies between G2 and M.

INTERPHASE

➢ It is an interval between two successive M phases in which growth and synthetic activities
prepare a cell and its nucleus to divide again.

➢ It is also called inter-mitosis.

➢ The interphase cell is metabolically active and at the end of interphase, the cell becomes
ready for equitable division into two equal daughter cells.

➢ Interphase occupies 75-95% of the generation time (22:15hours out of 24 hours in human
beings) ➢ It has three stages- G1(Gap 1), S (Synthesis) and G2 (Gap 2).

G1 Phase (Gap 1 or first growth phase):

➢ It is phase between end of M-phase of previous cell cycle and initiation of DNA
replication.

➢ Longest phase of interphase.

➢ Cell is metabolically active and grows continuously; nucleus grows only to a smaller
extent.

➢ RNA and proteins are synthesised.

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➢ Each chromosome has a single DNA chromatin thread.

➢ The fate of cell in G1 phase depends upon the available factors which has three options-

i) to continue cell cycle and enter S-phase ii) stop cell cycle and enter G0 phase for
undergoing differentiation iii) get arrested in G1 phase whence it may enter G0 phase or re-
enter cell cycle.

➢ Available factors are decided at check point called G1 cyclin.

S- Phase (Synthesis):

➢ Chromosomes replicate

➢ DNA content-doubles (1C to 2C for haploid cells and 2C to 4C for diploid cells)

➢ Chromosome changed from single stranded to double stranded.

➢ At the end of S-phase, each chromosome has two chromatids that are attached to each
other at centromere.
G2 phase (Gap 2) or second growth phase:

➢ Synthesis of DNA stops.

➢ Cell size increases; nucleus grows in size.

➢ Formation of RNAs and protein continues which are required for multiplication of cell
organelles, spindle formation and cell growth.

➢ Prepares the cell to undergo division.

➢ Second check point called mitotic cyclin lies between G2 and M phase and causes
transition from G2 to M phase.

M-PHASE OR MITOTIC PHASE

➢ M-phase is the final phase of the cell cycle.

➢ Represents the phase of actual division.

➢ Consists of karyokinesis (division of nucleus) and cytokinesis (division of cell cytoplasm).

➢ After M-phase, a cell may re-enter fresh cycle or pass into G0 phase.

G0 phase (Quiescent Stage):

➢ It is non cycling quiescent stage of cell where cells do not divide, leave the cell cycle and
exit G1 phase.

➢ Cells remain metabolically active but do not grow or differentiate.

➢ The cells function as reserve cells which can join cell cycle any time.

✓ Such cells divide occasionally, as needed to replace cells that have been lost because of
injury or cell death.

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SIGNIFICANCE OF CELL DIVISION
▪ Cell Multiplication- cell division is a means of cell multiplication or formation of new cells
from pre-existing cells.
▪ Continuity- It maintains continuity of living matter generation after generation.
▪ Asexual Reproduction- Cell division is a means of asexual reproduction in lower
organisms.
▪ Sexual Reproduction- requires a special type of cell division called meiosis.
▪ Growth- Growth of an organism involves cell growth and cell division.
▪ Repair- Cell division is a means of repair and healing of injured regions of the body where
old or worn-out cells are replaced by new ones.
CHROMOSOME
A chromosome is a long DNA molecule packaged into threadlike structures located inside
the nucleus of each cell.
MITOSIS

➢ Mitosis is a mode of cell division in which chromosomes replicate and equally distributed
into two daughter nuclei and the daughter cells are genetically similar to their parent cell.

➢ Their nuclei come to have the same number and type of chromosomes as are present in
the mother cell.

➢ Mitosis occurs during the formation of body cells and therefore, it is also called somatic
cell division.

➢ This phase starts with the nuclear division(karyokinesis) and usually ends with division of
cytoplasm(cytokinesis).

➢ Mitosis phase is also called as equational division since the number of chromosomes in
the parent and progeny cells is the same.
KARYOKINESIS

➢ karyon-nucleus, kinesis-movement

➢ Karyokinesis is the name of nuclear division.

➢ In mitosis, karyokinesis is also called indirect nuclear division because the nucleus
passes through a complicated sequence of events before forming two daughter nuclei.

➢ It has been divided into four phases- prophase, metaphase, anaphase and telophase.

PROPHASE

➢ It is the first phase of mitotic karyokinesis in which chromatin fibres condense to form
chromosomes.

➢ In the beginning of prophase, animal cells have two centrosomes close together and the
two begin to shift towards the opposite sides.

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➢ Both the centrosomes radiate out fine microtubular fibrils called astral rays. Each group of
astral rays along with its centriole pair is called aster.

➢ chromatin fibres thicken and shorten to form long stainable chromosomes.

➢ Chromosomal material- untangle during chromatin condensation.

➢ In mid prophase, each chromosome has two strands or chromatids.

➢ Asters further move away from each other due to elongation of microtubular fibrils
between them.

➢ Sister chromatids are attached to each other by centromere.

➢ In late prophase, nucleolus disappears completely.

➢ Nuclear envelope breaks into small vesicles.

KEY FEATURES:
1.Chromosomal material condenses to form compact mitotic chromosomes.
2.Chromosomes are seen to be composed of two chromatids attached together at the
centromere.
3. Centrosome which had undergone duplication during interphase, begins to move towards
opposite poles of the cell.
4. Each centrosome radiates out microtubules called asters. The two asters together with
spindle fibres forms mitotic apparatus.
METAPHASE

➢ Metaphase is characterized by all the chromosomes coming to lie at the equator.

➢ The plane of alignment of the chromosomes at metaphase is referred to as the

metaphase plate.

➢ Kinetochores (small disc-shaped structures present at the surface of the centromeres)

serve as the sites of attachment of spindle fibres to the chromosomes.
KEY FEATURES:
▪ Spindle fibres attach to kinetochores of chromosomes.
▪ Chromosomes are moved to spindle equator and get aligned along metaphase plate
through spindle fibres to both poles.
ANAPHASE
▪ Each chromosome is split simultaneously and the two daughter chromatids begin their
migration towards the two opposite poles.
▪ As each chromosome moves away from the equatorial plate, the centromere of each
chromosome remains directed towards the pole.
▪ Anaphase A is the stage where the chromosomes separate and pass to the opposite poles;
centromere divides into two and each chromosome have its own centromere.

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▪ In Anaphase B, the spindle elongates so that the spindle poles move away from one
another.
KEY FEATURES:

➢ Centromeres split and chromatids separate.

➢ Chromatids move to opposite poles.

TELOPHASE

➢ Chromosomes undergo de-condensation or unfolding.

➢ At the beginning of telophase, the chromosomes that have reached their respective poles
decondense and lose their individuality.

➢ The individual chromosomes can no longer be seen and each set of chromatin material
tends to collect at each of the two poles.
KEY FEATURES:

➢ Chromosomes cluster at opposite spindle poles and their identity is lost as discrete
elements.

➢ Nuclear envelope develops around the chromosome clusters at each pole forming two
daughter nuclei.

➢ Nucleolus, Golgi complex and ER reforms.

CYTOKINESIS

➢ Cytokinesis is the division of protoplast of a cell into two daughter cells after the nuclear
division.

➢ In cytokinesis, the cell itself is divided into two daughter cells by the separation of
cytoplasm at the end of which cell division gets completed.

➢ In an animal cell, the furrow appeared in the plasma membrane gradually deepens and
ultimately joins in the centre dividing the cell cytoplasm into two.

❖ In plant cells, formation of new cell wall begins with the formation of cell-plate that
represents the middle lamella between the walls of two adjacent cells.
SIGNIFICANCE OF MITOSIS

➢ Growth: Somatic cells are formed by mitosis and therefore, mitosis is essential for growth
and development of all multicellular organisms.

➢ Maintenance of genetic constitution: Mitosis involves replication and equitable distribution

of all the chromosomes so that all the cells of a multicellular organism have the same
number and type of chromosomes.

➢ Regeneration: Mitosis maintains genetic similarity in all somatic cells and thus fragment or
part of the body can regenerate the whole body.

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➢ Reproduction: It helps in vegetative propagation and asexual reproduction of organisms.
➢ Repair: cells of upper layers of skin epidermis, lining of gut, old RBC’s and WBC’s
regularly die. The same are replaced by new cells formed through mitosis.

➢ Healing: An injury or wound is healed by repeated mitotic divisions of the surrounding

healthy cells.
Cancer: Uncontrolled mitotic division leads to cancer.
MEIOSIS

➢ Meiosis is a double division which occurs in a diploid cell and gives rise to four haploid
cells, each having half the number of chromosomes as compared to the parent cell.

➢ Since meiosis reduce the number of chromosomes to half, it is also called reduction
division.

➢ Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing
organisms whereas fertilization restores the diploid phase.

➢ The cells undergoing meiosis are called meiocytes and they occur in gonads (testis,
ovary) and sporangia.
KEY FEATURES OF MEIOSIS:

➢ Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and
meiosis II but only a single cycle of DNA replication.

➢ Meiosis I is initiated after the parental chromosomes have replicated to produce identical
sister chromatids at the S phase.

➢ Meiosis involves pairing of homologous chromosomes and recombination between non-

sister chromatids of homologous chromosomes.

➢ Four haploid cells are formed at the end of meiosis II.

MEIOSIS I

➢ It is the first division of meiosis where a diploid cell divides into two daughter cells each
having half the number of replicated chromosomes

➢ It is also called heterotypic division because it changes the pattern or type of cell from
diploid to haploid form

➢ Like mitosis, it is also studied under four stages- prophase I, metaphase I, anaphase I
and telophase I
PROPHASE I

➢ Prophase I is divided into five sub- phases – leptotene, zygotene, pachytene, diplotene
and diakinesis.

➢ Leptotene- It is thin thread substage where the chromatin fibres condense to form
elongated chromosomes.

➢ There are two chromosomes of each type and they are called homologous chromosomes.

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➢ Zygotene- It is the substage of pairing of homologous chromosomes.

➢ During this stage, chromosomes start pairing together and this process of association is
called synapsis.

➢ Pachytene- It is the substage of chromosome thickening and crossing over.

➢ Crossing over is the exchange of genetic material between two homologous

chromosomes.

➢ Diplotene- It is the substage in which homologous chromosomes separate except in the

region of chiasmata.

➢ The points of attachment between homologous chromosomes after their separation in

diplotene appear X-shaped and are called chiasmata.

➢ Diakinesis- It is the substage which is marked by terminalization of chiasmata.

➢ During this phase the chromosomes are fully condensed and the meiotic spindle is
assembled to prepare the homologous chromosomes for separation.

➢ By the end of diakinesis, the nucleolus disappears and the nuclear envelop also breaks
down.
METAPHASE I

➢ The bivalent chromosomes align on the equatorial plate

➢ The microtubules from the opposite poles of the spindle attach to the kinetochore of
homologous chromosomes
ANAPHASE I

➢ It is the stage in which the homologous chromosomes separate and pass towards the
different poles.

➢ Sister chromatids remain associated at their centromeres.

➢ Two groups of dyad chromosomes are formed each having half the number of
chromosomes present in parent cell.
TELOPHASE I

➢ Chromosomes elongate

➢ The nuclear membrane and nucleolus reappear

➢ Spindle fibres and astral ray degenerate

➢ Interphase occurs prior to meiosis

➢ Chromosome replication occurs once but meiosis has two M-phases each with its own
karyokinesis and Cytokinesis.

➢ The transition period between Meiosis I and Meiosis II is short and without DNA
replication. It is called interkinesis.

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MEIOSIS II

➢ It is second division of meiosis in which replicated or doublestranded chromosomes split

and single-stranded chromosomes pass into daughter cells.

➢ After the end of meiosis I, the two sister chromatids of a chromosome become genetically
different due to crossing over.

➢ The daughter cells formed after meiosis II are neither similar to each other nor similar to
the parent.
PROPHASE II

➢ The centriole pairs develop asters and move to the regions of future spindle poles

➢ Nucleolus and nuclear envelope degenerate

➢ Chromosomes shorten; show divergent chromatids

METAPHASE II

➢ At this stage the chromosomes align at the equator.

➢ The microtubules from opposite poles of the spindle get attached to the kinetochores.

ANAPHASE II

➢ The centromere of each chromosome divides into two so that there is one centromere for
each chromatid.

➢ The two chromatids separate completely and are called daughter or new chromosomes.

➢ The daughter chromosomes move towards opposite poles of the cell.

TELOPHASE II

➢ The four groups of chromosomes arrange themselves into haploid nuclei.

➢ Groups of chromosomes once again get enclosed by a nuclear envelope.

➢ Astral rays and spindle fibres disappear.

SIGNIFICANCE OF MEIOSIS

➢ Sexual reproduction: Meiosis is essential in the life cycle of all sexually reproducing
organisms

➢ Maintenance of chromosome number: Fertilization doubles the chromosome number

during the fusion of gametes. Meiosis halves the number of chromosomes.

➢ Crossing over: It introduces new combination of traits or variation.

EXERCISE:
Q. Define cell cycle.
Q. Write a short note on Interphase.

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Q. Give two points of difference between G1phase and G2 phase.
Q. What are chromosomes?
Q. What is mitosis? Give a brief description of mitosis phase.
Q. What are kinetochores?
Q. Why is mitosis also called equational division?
Q. Give 3 points of significance of Mitosis.
Q. Why is meiosis also called reductional division?
Q. Describe Prophase I of Meiosis.
Q. Give two points of significance of meiosis.
Q. What is crossing over?
Q. Give 5 points of difference between Mitosis and Meiosis.

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