Cell Division : By
Kalyani Chivukula Cell Division
Cell Cycle : Cell Cycle The sequence of events by which a cell duplicates its genome, synthesises other cell constituents and divides into two daughter cells.
The events of the cell cycle are under genetic control.
The Sequence of Events in a Cell Cycle : The Sequence of Events in a Cell Cycle Cell duplicates its genome
divides into synthesizes other
two daughter cells constituents
Slide 4 : Cell Cycle
Interphase M Phase
G1 Phase S Phase G2 Phase Karyokinesis Cytokinesis
(Gap 1) ( Synthesis) (Gap 2) (Nuclear Division ) (Cytoplasmic
Division)
Prophase Metaphase Anaphase Telophase
Interphase : Interphase During this phase the cell prepares itself for division, grows and the DNA is replicated in it.
This phase occupies more than 95% of the duration of the cell cycle.
M Phase : M Phase M Phase is the phase when the actual cell division or mitosis occurs.
Interphase (the resting phase) is the phase between two successive M phases.
During interphase the cell prepares for division by undergoing cell growth and DNA (Deoxyribonucleic Acid) replication in an orderly manner.
Slide 7 : G1 Phase (Gap 1)
Interphase S Phase (Synthesis)
G2 Phase (Gap 2)
G1 Phase : G1 Phase Corresponds to the interval between mitosis and initiation of DNA replication
The growth of the cell takes place in this phase
During this phase the cell is metabolically active and grows continuously
Its DNA does not replicate
S or Synthesis Phase : S or Synthesis Phase Synthesis or replication of DNA takes place.
The amount of DNA per cell doubles.
There is no increase in the chromosome number.
In animal cells, during this phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
G2 Phase : G2 Phase Proteins are synthesised in preparation for mitosis while cell continues to grow.
Some cells in the adult animals do not appear to exhibit division (e.g., heart cells).
Many other cells divide only occasionally to replace cells that have been lost due to injury or cell death.
These cells do not divide further, these exit G1 phase and enter an inactive stage called quiescent stage of the cell cycle.
Quiescent Stage of the Cell Cycle : Quiescent Stage of the Cell Cycle Cells in this stage remain metabolically active.
The cells no longer proliferate unless called on to do so depending on the requirement of the organism.
Mitosis or M Phase : Mitosis or M Phase Starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis)
Usually ends with the division of cytoplasm (cytokinesis)
Involves a major reorganisation of all components of the cell.
Since the number of chromosomes in the parent and progeny cells is the same, it is also called equational division.
Slide 13 : Prophase
Metaphase
Mitosis
Anaphase
Telophase
Prophase : Prophase Marked by the initiation of condensation of the chromosomal material.
The chromosomal material becomes untangled during the process of chromatin condensation .
The centriole, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell.
Slide 15 : Metaphase Prophase
Slide 16 : Anaphase Telophase
Slide 17 : Completion of prophase is marked by:
Condensation of chromosomal material to form compact mitotic chromosomes.
Chromosomes are seen to be composed of two chromatids attached together at the centromere.
Initiation of the assembly of mitotic spindle, the microtubules, the proteinaceous components of the cell cytoplasm help in the process.
Cells at the end of prophase, do not consist golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
Metaphase : Metaphase Complete disintegration of the nuclear envelope.
Chromosomes spread through the cytoplasm of the cell.
Condensation of chromosomes is completed.
Chromosome is made up of two sister chromatids, which are held together by the centromere
Small disc-shaped structures at the surface of the centromeres (kinetochores) serve as the sites of attachment of spindle fibres to the chromosomes that are moved into position at the centre of the cell.
Slide 19 : Metaphase is characterised by:
Chromosomes come 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 .
The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
KeyFeatures of Metaphase : KeyFeatures of Metaphase 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 is Characterised by : Anaphase is Characterised by Splitting of centromeres and
Separation of chromatids.
Moving of chromatids to opposite poles.
Anaphase : Anaphase Each chromosome arranged at the metaphase plate split simultaneously and the two daughter chromatids (chromosomes) of the future daughter nuclei, begin to move towards the two opposite poles.
As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.
Telophase : Telophase The chromosomes that have reached their respective poles decondense and lose their individuality.
Individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles .
This stage shows the following key events:
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.
Significance of Mitosis : Significance of Mitosis Restricted to the diploid cells.
In some lower plants and in some social insects haploid cells also divide by mitosis.
Results in the production of diploid daughter cells with identical
genetic complement.
Growth of multicellular organisms is due to mitosis.
Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.
To restore the nucleo-cytoplasmic ratio, it becomes essential for the cell to divide
Cell repair - the cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.