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CH 10 – CELL CYCLE & CELL DIVISION

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CELL CYCLE AND CELL DIVISION
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Growth and reproduction are characteristics of living cells and organisms
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Cell cycle includes three processes cell division, DNA replication and cell growth in coordinated
way
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(e
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, in Yeast cell cycle is of 90 minutes, in human 24 hrs
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Lasts for 95% of total duration of cell
cycle
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M phase (mitosis phase)

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The actual cell division or mitosis occurs
Lasts for 95% of total duration of cell
cycle
Starts with nuclear division and ends
with cytoplasmic division
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G1 phase (Gap 1 Phase)
Corresponds to the interval between mitosis and initiation of DNA replication
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S phase (synthesis phase)
period during which DNA synthesis or replication takes place
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(only amount of DNA is doubled, no of
chromosomes remain same
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CH 10 – CELL CYCLE & CELL DIVISION

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G2 phase (Gap 2 Phase)
Proteins are synthesised in preparation for mitosis while cell growth continues
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these cells enter in an inactive phase
called G0 or quiescent phase from G1 phase
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Mitosis or M phase
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In animals, mitotic cell division is only seen in the diploid somatic cells while in the plants
mitotic divisions can be seen in both haploid and diploid cells
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Mitosis is divided into the following four stages:
 Prophase
 Metaphase
 Anaphase
 Telophase

Prophase
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It follows the S and G2 phases of interphase
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In prophase Chromosomal material condenses to form compact mitotic chromosomes
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Cell organelles like Golgi complexes, endoplasmic reticulum, nucleolus and the nuclear
envelope disappear
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The chromosomes are spread through the cytoplasm of the cell
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CH 10 – CELL CYCLE & CELL DIVISION

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This is the stage at which morphology of chromosomes is most easily studied
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centromere serve as the sites of attachment of spindle fibres to the chromosomes
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the metaphase is characterised by all the chromosomes coming to lie at the equator with one
chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole
and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole
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Anaphase
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At the onset of anaphase, each chromosome arranged at the metaphase plate is split
simultaneously and the two daughter chromatids begin to move towards the two opposite poles
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Nuclear envelope assembles around the chromatin network
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Cytokinesis
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After karyokinesis the cell itself is divided into two daughter cells by a separate process called
cytokinesis
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The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into
two
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In cell plate method wall formation starts in
the centre of the cell and grows outward to meet the existing lateral walls
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At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed
between the two daughter cells
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g
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(should be coenocytic)

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CH 10 – CELL CYCLE & CELL DIVISION

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Significance of mitosis
(1) Mitosis results in the production of diploid daughter cells with identical genetic complement
usually
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(3) Cell growth results in disturbing the ratio between the nucleus and the cytoplasm
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(4) mitosis is important in cell repair
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(5) Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a
continuous growth of plants throughout their life
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Anurag mittal
Mob
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wordpress
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It is responsible for formation of haploid gametes, which during sexual reproduction form diploid
zygote by fusion
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Interphase of meiosis is similar to interphase of mitosis
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It has been further subdivided into the following five phases based on chromosomal behavior
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(Bouquet phase)

Zygotene

Homologous chromosomes start pairing together and form
synaptonemal complex or tetrad by synapsis
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Crossing over occurs between non-sister chromatids of the
homologous chromosomes
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Recombined chromosomes of the tetrad separate from
each other except at the sites of crossovers
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Diakinesis

terminalisation of chiasmata
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Meiotic spindle is assembled to prepare the homologous
chromosomes for separation
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6

CH 10 – CELL CYCLE & CELL DIVISION

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Metaphase I:
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The bivalent chromosomes align on the equatorial plate
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Anaphase I:
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The homologous chromosomes separate, while sister chromatids remain associated at their
centromeres
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cytokinesis follows telophase I
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The stage between the two meiotic divisions
is called interkinesis and is generally short lived
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Meiosis II
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Meiosis II resembles a normal mitosis
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The nuclear membrane disappears by the end of prophase II
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Metaphase II:
At this stage the chromosomes align at the equator and the microtubules from opposite poles of
the spindle get attached to the kinetochores of sister chromatids
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Chromosomes move toward opposite poles of the cell
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cytokinesis follows resulting in the formation of four haploid daughter cells)
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(2) It also increases the genetic variability in the population of organisms from one generation to the
next
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