This document summarizes key concepts about cell division from Chapter 1. It discusses that all new cells are derived from existing cells through cell division, which involves duplicating genetic material. There are two types of cell division: mitosis produces two identical daughter cells, while meiosis reduces the chromosome number by half to produce gametes. The cell cycle is described as having four main phases - G1, S, G2, and Mitosis - where the cell grows and duplicates its DNA before dividing.

1. SB027: BIOLOGICAL DIVERSITY & ENVIRONMENT

2. CHAPTER 1 : CELL DIVISION

3. CHAPTER 1 : CELL DIVISION 1.1 The Concept of Cell Division 1.2 The Cell Cycle

4. CONCEPT OF CELL DIVISION Modern cell theory : ‘ All new cells are derived from other cell’. Genetic material inherited from one generation to the next.

5. 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.

6. Cell division involves karyokinesis & cytokinesis two types of karyokinesis: Mitosis - daughter cells having the same number of chromosomes as the parent. Meiosis – daughter cells having only half the number of chromosomes found in the parent cell.

7. 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.

8. 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.

9. 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.

10. 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 .

12. THE CELL CYCLE

13. THE CELL CYCLE The cell cycle is the complete sequence of events in the life of an individual diploid cell. Mitotic (M) phase (10%) alternates with much longer interphase (G 1 , S and G 2 ) 90%. The four stages of the cell cycle; i. G 1 – The first growth phase ii. S Phase iii. G 2 –second growth phase iv. Mitosis

14. 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)

15. 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.

16. 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.