All multicellular organisms start life as a single cell, fertilized egg and grow by addition of new cells. one phenomenon of production of daughter cells from parent cells is known as cell division or cell replication. Cell reproduction has three major parts: first, involves replication of parental DNA; second, separation of duplicated DNA into two equally sized groups of chromosomes and third, division.of pore cells.
 |
| Cell Cycle |
The sequence of events by which "duplicates its genome, synthesizes other cell contents and eventually divides into two daughter cells is termed as cell cycle. All the events of the cell cycle occur in a c0-ordinated manner and are under genetic control. A typical eukaryotic somatic cell has two main stages: a long undividing stage called interphase (1-phase) and a shorter nucleus dividing stage called mitotic (M-phase) phase. These two main phases are followed by a still shorter phase of cytoplasmic division called cytokinesis (C-phase).
Interphase is the metabolically active stage and is a period of intense synthesis and growth. This phase is divided into three periods- first gap phase (G1-phase), synthesis phase S-phase) and second gap/growth phase (G2-Phase). GI phase is a stage of initial growth of a newly formed
cell in which the cell carries on active metabolic activities in preparation for DNA replication. In G1-phase, a cell has three options: to continue cell cycle and enter S-phase, stop cell cycle and enter Go, phase for undergoing differentiation and get arrested in G1-phase when it may enter Go Phase or re-enter cell cycle. Some cells such as nerve cells and erythrocytes that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (Go) of the cell cycle. They remain metabolically active but no longer proliferate unless called on to do so.
S or synthesis phase marks the period during which DNA synthesis or replication takes place. 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. However, there is no increase in the chromosome number. In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
G2 phase is a stage of further growth of the cell and preparation for its division. During this stage, synthesis of RNAs and proteins continue The M-phase is itself composed of two tightly coupled processes, karyokinesis in which the cell's chromosomes are divided between the two daughter cells and cytokinesis, in which the cell's cytoplasm divides in half forming distinct cells Activation of each phase is dependent on the proper progression and completion of the previous one, Thus the correct sequence of these phases in cell cycle is G1--S--G2--M.
MITOSIS
Mitosis involves the exact replication of a parent cell followed by its division into two daughter cells which are identical and contain the same number of chromosomes as in parent celis. It was first observed by Strassburger (1870) in plant cells and Fleming term mitosis (Gk. mitos = thread) In plants mitosis occurs in the meristematic cells of root or shoot tip.
Prophase is the first phase of mitosis followed by G2 phase. It involves following events-
(a) Initiation of condensation of chromosomal materials.
(b) Movement of centrioles towards opposite poles of the cell.
(c) At the end of prophase, endoplasmic reticulum, nuclear membrane, Golgi complex disappears. Metaphase starts with complete disappearance of the nuclear membrane. The most suitable stage for study of morphology of chromosomes. In involves
(a) Condensation of chromosomal materials into compact and distinct chromosomes made up of two sister chromatids attached with spindle fibres with kinetochores.
(b) Chromosomes arrange at the centre of a cell called metaphase plate
Anaphase involves the
(a) Splitting of each chromosome at centromere into two sister chromatids.
(b) Two chromatids start moving towards opposite poles.
Telophase is the last stage of mitosis which involves
(a) Chromosomes reach at opposite poles and lose their identity as discrete units.
(b) Nuclear membrane reassembles around the chromosome clusters.
(c) Nucleolus, Golgi complex and ER reappear.
Cytokinesis is the division of cytoplasm of a cell after karyokinesis (division of chromosome) into two daughter cells. In animal cells, the appearance of furrows in plasma membranes deepens gradually and joins to divide cytoplasm into two parts. In plant cells, wall formation starts at the centre and grows outwards to meets lateral walls. The formation of a cell wall begins with the formation of a cell plate.
Significance of mitosis
• Production of diploid daughter cells with identical genetic complement.
• Cell division helps to restore the nucleo-cytoplasmic
• Ratio Old or worn out cells are replaced by new cells formed through mitosis Helps to increase the number of cells within an organism
MEIOSIS
Meiosis is a specialised kind of cell division that reduces the chromosome number by half which results in the production of haploid daughter cells. It ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase. Meiotic events can be grouped into different phases as given in the flow chart. The key features of meiosis are as follows:
Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but have 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 them Four haploid cells are formed at the end of meiosis.
During Leptotene, the chromosome becomes distinct and visible under microscope. Compaction of chromosomes continues throughout the leptotene phase.
During Zygotene stage, chromosomes start pairing together (synapsis). The paired chromosomes are called homologous chromosome. Synaptonemal complex formed by a pair of homologous chromosomes is called bivalent or a tetrad.
During the Pachytene stage, crossing over between non-sister chromatids of homologous chromosomes occurs for exchange of genetic materials. The crossing over is enzyme – mediated process which involves enzyme recombinase.
Diplotene is recognized by dissolution of synaptonemal complex and tendency to separation of bivalent except at the site of crossing over. This forms an X-like structure called chiasmata.
Diakinesis is marked by terminalisation of chiasmata. The nuclear membrane breaks and the nucleolus disappears.
It is initiated immediately after cytokinesis before the chromosome gets elongated.
In prophase II, the nuclear membrane disappears and the chromosome becomes compact.
At metaphase II stage, the chromosomes align at equator and microtubules attach with kinetochores of sister chromatids.
Anaphase II starts with splitting of centromeres of each chromosome to move towards opposite poles.
Meiosis ends with Telophase II in which two groups of chromosomes get enclosed by nuclear membrane followed by cytokinesis to form tetrad of cells
Significance of meiosis
Leads to the formation of sex cells or gametes capable of engaging in fertilisation. Provides opportunities for new combination of genes to occur in the gametes by two ways-independent
assortment of chromosomes and crossing over Switches on the genetic information for the
development of gametes and switches off the sporophytic information Chromosomal and gene mutations can take place by irregularities of meiotic divisions.
Different between mitosis and meiosis
• Mitosis takes place in the somatic cells of the body DNA replication takes place during interphase I The duration of prophase is short, usually of a few hours.
• There is no synapsis.
• The two chromatids of a chromosome do not exchange Segments during prophase.
• A -Synaptonemal complex is absent
• Crossing over is absent
• Chiasmata are absent
• Division of the centromeres takes place during anaphase
• Anaphasic chromosomes are single stranded.
• The chromosome number remains constant at the end of mitosis.
• The genetic constitution of the daughter cells is identical to that of parent cells.
• Mitosis helps in multiplication of cells
• Mitosis takes part in healing and repair
Meiosis
• There are two cell divisions, the first and the second meiotic divisions
• Meiosis takes place in the germ cell.
• DNA replication takes place during interphase I but not during nepas
• Prophase is comparatively longer and may take days.
• Synapsis of homologous chromosomes takes place during prophase
• Chromatids of two homologous chromosomes exchange
• Segments during crossing over.
• Synapsed homologous chromosomes develop a synaptonemal complex.
• Crossing over or exchange of similar segments between non sister chromatids of homologous chromosomes usually takes place during pachytene stage.
• Chiasmata or visible connections between homologous chromosomes of bivalent are observed during diplotene, diakinesis (prophase 1) and metaphase.
• There is no centromeric division during anaphase . Centromeres Divide only during anaphase lI
• Chromosomes are double stranded in anaphaseI buy single stranded in anaphase l.horn
• The chromosome number is reduced from the diploid to the haploid.
• The genetic constitution of the daughter cells differs from that of the parent cell. The chromosomes of daughter cells usually contain a mixture of maternal and paternal genes.
• Multiplication of cells is not involved
What is colchicine ?
It is an alkaloid widely used in plant breeding for doubling the chromosome number. Colchicine
extracted from the corms of autumn crocus (Colchicum autumnale). The alkaloid does not allow the formation of spindle because it prevents assembly of microtubules It is, therefore, called 'mitotic poison
Colchicine holds the cells in metaphase. The enzyme ribonuclease is prophase poison Colchicine does not inhibit chromosome replication. As a result the colchicine treated meristematic cells show doubling of chromosomes. This property of increasing the number of chromosome sets or get es is called polyploidy.