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

chromosome gene controlling skin colour gene controlling tongue rolling gene controlling eye colour gene controlling blood group 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)

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

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

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 allele for white skin colour allele for dark skin colour

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

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

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

Chromosomes become visible Nuclear membrane disappears First Meiotic Division

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

 Homologous chromosomes line up in the middle of cell randomly

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

 Nuclear membrane reforms  Followed by cytoplasmic cleavage

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

Occurrence of meiosis  Plants: anthers and ovules  Mammals: testes and ovaries

Comparison between Mitosis & Meiosis MitosisMeiosis Number of division 12 No. of daughter cells produced Type of cells produced Chromosome number of daughter cells 2 Somatic (body) cells Same as parent cell (diploid) 4 Gametes (sex cells) Half of parent cell (haploid)

MitosisMeiosis Pairing of homologous chromosomes NoYes Occurrence Role Growing tissues For Growth & Replacement Reproductive tissues (gonads) For Gamete Formation Comparison between mitosis & meiosis

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

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

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