BIO BOTANY [3 Mark each]

The replication of chromosomes in the absence of nuclear division and cytoplasmic division resulting in numerous copies within each cell is called endomitosis. Chromonema do not separate to form chromosomes, but remain closely associated with each other. Nuclear membrane does not rupture. So no spindle formation. It occurs notably in the salivary glands of Drosophila and other flies. Cells in these tissues contain giant chromosomes (polyteny), each consisting of over thousands of intimately associated, or synapsed, chromatids. Example: Polytene chromosome.

Plants:

1. Centrioles are absent
2. Asters are not formed
3. Cell division involves formation of a cell plate
4. Occurs mainly at meristem.

Animals:

1. Centrioles are present
2. Asters are formed
3. Cell division involves furrowing and cleavage of cytoplasm
4. Occurs in tissues throughout the body.

The significance of Meiosis:

• Meiosis maintains a definite constant number of chromosomes in organisms.
• Crossing over takes place and exchange of genetic material leads to variations among species. These variations are the raw materials to evolution. Meiosis leads to genetic variability by partitioning different combinations of genes into gametes through independent assortment.
• Adaptation of organisms to various environmental stress.

In plants, phragmoplast are formed between the daughter cells. Cell plate is formed between the two daughter cells, reconstruction of cell wall takes place. Finally the cells are separated by the distribution of organelles, macromolecules into two newly formed daughter cells.

Between closed and open mitosis:

1. In closed mitosis, the nuclear envelope remains intact and chromosomes migrate to opposite poles of a spindle within the nucleus. Example: Yeast and slime molds.
2. In open mitosis, the nuclear envelope breaks down and then reforms around the 2 sets of separated chromosome. Example: Most plants and animals cells.

Karyokinesis:

1. Involves division of nucleus.
2. Nucleus develops a constriction at the center and becomes dumbellshaped.
3. Constriction deepens and divides the nucleus into two.

Cytokinesis:

1. Involves division of cytoplasm.
2. Plasma membrane develops a constriction along nuclear constriction.
3. It deepens centripetally and finally divides the cell into two cells.

S Phase – Synthesis phase – cells with intermediate amounts of DNA Growth of the cell continues as replication of DNA occur, protein molecules called histones are synthesised and attach to the DNA. The centrioles duplicate in the cytoplasm. DNA content increases from 2C to 4C.

The checkpoint called the restriction point at the end of G1 it determines a cells fate whether it will continue in the cell cycle and divide or enter a stage called G0 as a quiescent stage and probably as specified cell or die.

The role of nucleus in the cell:

• Control activities of the cell.
• Genetic information copied from cell to cell while the cell divides.
• Hereditary characters are passed onto new individuals when gametic cells fuse together in sexual reproduction.