What are chromosomes?

2. What are chromosomes? • Genetic materials found inside the nucleus of a cell • Made up of DNA & protein • Appear as very thin & thread-like structures called Chromatins • Each cell in each type of organism has a definite number of chromosomes • Each cell has Diploid Number of chromosomes: 1 set of Paternal Chromosomes + 1 set of Maternal Chromosomes

3. gene controlling skin colour • A gene is a short length of DNA on a chromosome which is a unit determining an inherited character • It consists of a chemical substance called deoxyribonucleic acid (DNA) gene controlling tongue rolling chromosome gene controlling eye colour gene controlling blood group

4. gene controlling skin colour gene controlling tongue rolling • DNA is responsible for telling the cell to make the right type of protein & control the metabolic activities of the cells chromosome gene controlling eye colour gene controlling blood group

5. Chromosomes always exist in pairs in the body (somatic) cells Homologous Chromosomes • Each human somatic cell has 46 chromosomes • 23 pairs of homologous chromosomes

6. allele for white skin colour allele for dark skin colour • Members of homologous chromosomes carry the same genes (controlling the same character) in same loci • But the genes on the members of homologous chromosomes may be of different forms (effects) •  Alleles

7. Alleles • Alternative forms of genes on the same position of the homologous chromosomes which control the same character but have different expressions

8. What would happen if gametes are formed by mitosis? Male (2n) Female (2n) Mitosis Sperm (2n) Egg (2n) Fertilization Zygote (4n) Mitosis Sperm/Egg (4n) Fertilization Zygote (8n)

9. Meiosis • Producing cells with chromosome number half of the parent cell • Two nuclear divisions • Meiosis I and meiosis II • Four haploid cells are produced

10. First Meiotic Division • Chromosomes become visible • Nuclear membrane disappears

11. Homologous chromosomes pair up(not in mitosis) • Crossing-over(not in mitosis) may occur between homologous chromosomes genetic materials exchanged

12. Homologous chromosomes line up in the middle of cell randomly

13. The 2 members of each homologous chromosome separate from each other and move towards the opposite poles of the cell

14. Nuclear membrane reforms • Followed by cytoplasmic cleavage

15. Second Meiotic Division • Separation of chromatids of each chromosome • 4 daughter cells with half of the chromosome number of the parent cell are formed

16. Occurrence of meiosis • Plants: anthers and ovules • Mammals: testes and ovaries

17. Comparison between Mitosis & Meiosis Mitosis Meiosis 1 2 Number of division No. of daughter cells produced 2 4 Gametes (sex cells) Somatic (body) cells Type of cells produced Chromosome number of daughter cells Same as parent cell (diploid) Half of parent cell (haploid)

18. Comparison between mitosis & meiosis Mitosis Meiosis Pairing of homologous chromosomes No Yes Occurrence Reproductive tissues (gonads) Growing tissues Role For Growth & Replacement For Gamete Formation

19. Significance of meiosis • Produce haploid gametes which, after fertilization, can restore normal diploid conditions • Produce genetic variations

20. Sources of genetic variation • Different combination of the genes of each gamete • Random fusion of gametes during fertilization

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