§ The growth and reproduction of all organisms depend on the division and enlargement of cells.
§ The mechanisms of division and multiplication of cells together constitute cell reproduction.
CELL CYCLE
- It is the life period of a cell during which a cell synthesizes a. G1 phase (Gap 1 or Antephase): First growth phase. It is
DNA (replication), grows & divides into 2 daughter cells. the interval between mitosis and DNA replication.
- Cell growth (cytoplasmic increase) is a continuous process Main events:
but DNA synthesis occurs only at a specific stage. • Continuous growth of cell.
- Duration of cell cycle varies in each organism and each cell • Cell becomes metabolically active.
type. E.g. Duration of a typical eukaryotic cell cycle (e.g. • Prepares machinery for the DNA replication.
human cell) is about 24 hrs. In Yeasts, it is 90 minutes. • Synthesizes RNA and proteins.
Phases of Cell Cycle b. S (Synthetic) phase:
• In this, DNA replication takes place.
Cell cycle includes 2 basic phases: Interphase & M Phase.
• Amount of DNA per cell doubles. But chromosome
number is not increased.
• In animal cells, replication begins in the nucleus, and
the centriole duplicates in the cytoplasm.
c. G2 phase (Gap 2):
• Second growth phase. Cell growth continues.
• Synthesis of RNA and proteins continues.
• Cell is prepared for mitosis.
2. M Phase (Mitosis phase)
- It represents the actual cell division (mitosis).
- In human cell cycle, it lasts for only about an hour.
- M Phase includes karyokinesis (nuclear division) and
cytokinesis (division of cytoplasm).
- Some cells do not show division. E.g. heart cells.
- Many other cells divide only occasionally to replace
1. Interphase (resting phase) damaged or dead cells.
- It is the phase between two successive M phases. - The cells that do not divide further exit G1 phase and enter
- It includes cell growth and DNA synthesis. an inactive stage called quiescent stage (G0). Such cells
- It lasts more than 95% of the duration of cell cycle. remain metabolically active but do not proliferate.
Interphase has 3 phases:
MITOSIS
- It is the cell division occurring in somatic cells. o Centrosomes begin to move towards opposite poles of
- It is also called as equational division as the number of the cell. Each centrosome radiates out microtubules
chromosomes in the parent and progeny cells is same. called asters. The two asters together with spindle
- Mitosis is generally seen in diploid cells. It also occurs in fibres forms mitotic apparatus.
haploid cells of some lower plants and some social insects. • Cells at the end of prophase do not show Golgi complexes,
- It involves major reorganization of all cell components. endoplasmic reticulum, nucleolus & nuclear envelope.
The karyokinesis of mitosis has 4 stages: Prophase, 2. Metaphase
Metaphase, Anaphase & Telophase. • The nuclear envelope completely disintegrates. Hence the
1. Prophase chromosomes spread through the cytoplasm of the cell.
• Chromosome condensation is completed. They can be
• It is the longest phase in mitosis.
observed and studied easily under the microscope. They
• It follows the S and G2 phases of interphase.
will have two sister chromatids.
• In the S & G2 phases, DNA molecules are intertwined.
• Chromosomes come to lie at the equator. The plane of
• Characteristic events:
alignment of the chromosomes at metaphase is called the
o Chromosomal materials (chromatin fibres) are untangled
metaphase plate.
and condensed to form mitotic chromosomes. They are
• The spindle fibres from both poles are connected to
seen to be composed of two chromatids attached
together at the centromere. chromatids by their kinetochores in the centromere.
1
§ The mechanisms of division and multiplication of cells together constitute cell reproduction.
CELL CYCLE
- It is the life period of a cell during which a cell synthesizes a. G1 phase (Gap 1 or Antephase): First growth phase. It is
DNA (replication), grows & divides into 2 daughter cells. the interval between mitosis and DNA replication.
- Cell growth (cytoplasmic increase) is a continuous process Main events:
but DNA synthesis occurs only at a specific stage. • Continuous growth of cell.
- Duration of cell cycle varies in each organism and each cell • Cell becomes metabolically active.
type. E.g. Duration of a typical eukaryotic cell cycle (e.g. • Prepares machinery for the DNA replication.
human cell) is about 24 hrs. In Yeasts, it is 90 minutes. • Synthesizes RNA and proteins.
Phases of Cell Cycle b. S (Synthetic) phase:
• In this, DNA replication takes place.
Cell cycle includes 2 basic phases: Interphase & M Phase.
• Amount of DNA per cell doubles. But chromosome
number is not increased.
• In animal cells, replication begins in the nucleus, and
the centriole duplicates in the cytoplasm.
c. G2 phase (Gap 2):
• Second growth phase. Cell growth continues.
• Synthesis of RNA and proteins continues.
• Cell is prepared for mitosis.
2. M Phase (Mitosis phase)
- It represents the actual cell division (mitosis).
- In human cell cycle, it lasts for only about an hour.
- M Phase includes karyokinesis (nuclear division) and
cytokinesis (division of cytoplasm).
- Some cells do not show division. E.g. heart cells.
- Many other cells divide only occasionally to replace
1. Interphase (resting phase) damaged or dead cells.
- It is the phase between two successive M phases. - The cells that do not divide further exit G1 phase and enter
- It includes cell growth and DNA synthesis. an inactive stage called quiescent stage (G0). Such cells
- It lasts more than 95% of the duration of cell cycle. remain metabolically active but do not proliferate.
Interphase has 3 phases:
MITOSIS
- It is the cell division occurring in somatic cells. o Centrosomes begin to move towards opposite poles of
- It is also called as equational division as the number of the cell. Each centrosome radiates out microtubules
chromosomes in the parent and progeny cells is same. called asters. The two asters together with spindle
- Mitosis is generally seen in diploid cells. It also occurs in fibres forms mitotic apparatus.
haploid cells of some lower plants and some social insects. • Cells at the end of prophase do not show Golgi complexes,
- It involves major reorganization of all cell components. endoplasmic reticulum, nucleolus & nuclear envelope.
The karyokinesis of mitosis has 4 stages: Prophase, 2. Metaphase
Metaphase, Anaphase & Telophase. • The nuclear envelope completely disintegrates. Hence the
1. Prophase chromosomes spread through the cytoplasm of the cell.
• Chromosome condensation is completed. They can be
• It is the longest phase in mitosis.
observed and studied easily under the microscope. They
• It follows the S and G2 phases of interphase.
will have two sister chromatids.
• In the S & G2 phases, DNA molecules are intertwined.
• Chromosomes come to lie at the equator. The plane of
• Characteristic events:
alignment of the chromosomes at metaphase is called the
o Chromosomal materials (chromatin fibres) are untangled
metaphase plate.
and condensed to form mitotic chromosomes. They are
• The spindle fibres from both poles are connected to
seen to be composed of two chromatids attached
together at the centromere. chromatids by their kinetochores in the centromere.
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