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SB027: BIOLOGICAL DIVERSITY & ENVIRONMENT

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CHAPTER 1 : CELL DIVISION

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CHAPTER 1 : CELL DIVISION

1.1 The Concept of Cell Division

1.2 The Cell Cycle

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CONCEPT OF CELL DIVISION

•Modern cell theory: ‘All new cells are derived from other cell’.

•Genetic material inherited from one generation to the next.

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Importance of cell division

• Unicellular organism divides & forms duplicate offspring.

• Produce progeny from some multicellular organisms (plants that grow from cuttings).

• Sexually reproducing organisms develop from a single cell.

• Cell renewal & repair.

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• Cell division involves karyokinesis & cytokinesis

• two types of karyokinesis:

1. Mitosis - daughter cells having the same number of chromosomes as the parent.

2. Meiosis – daughter cells having only half the number of chromosomes found in the parent cell.

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• A cell’s endowment of DNA, its genetic information, is called its genome.

• A dividing cell duplicates its DNA.• The two copies separated to

opposite ends.• Only then split into daughter cells.• So that each daughter cell ends up

with a complete genome.

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• replication and distribution of so much DNA is manageable because the DNA molecules are packaged into chromosomes.

• DNA is associated with histone proteins that maintain the structure.

• Every eukaryotic species has unique number of chromosomes.

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•This DNA – protein complex, called chromatin, is organized into a long, thin fiber.

•Then DNA duplicates.

•Chromatin condenses: densely coiled and folded, making the chromosome much shorter and so thick.

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• Each duplicated chromosome has two sister chromatids, containing identical copies of the chromosome’s DNA molecule.

• In its condensed form, chromosome has a narrow ‘waist’ at a specialized region called the centromere.

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THE CELL CYCLE

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THE CELL CYCLE

1. The cell cycle is the complete sequence of events in the life of an individual diploid cell.

2. Mitotic (M) phase (10%) alternates with much longer interphase (G1, S and G2) 90%.

3. The four stages of the cell cycle; i. G1 – The first growth phase ii. S Phase iii. G2 –second growth phase iv. Mitosis

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THE CELL CYCLE

i. G1 – The first growth phase

• Longest phase.• Volume of cytoplasm increase• Protein synthesis.• Increase number of organelles - In form of growing chromatin (long, thin)

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ii. S phase- DNA synthesis phase.

• - The cell’s DNA replicates & now consist of two identical chromatids (sister chromatids).

iii. G2 – The second growth phase

• - grows more• - Completes preparations for cell division.• - Energy stores are increased.

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iv. Mitosis

• Divides the nucleus & distributes its chromosomes to the daughter nuclei.

• Followed by cytokinesis - divides the cytoplasm.

• Accounts for about 10% of the cycle.