What are chromosomes?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 nu

mber 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 What would happen if gametes are formed by gametes are formed by

mitosis?mitosis?Male (2n) Female (2n)

Sperm (2n) Egg (2n)

Zygote (4n)

Sperm/Egg (4n)

Zygote (8n)

Mitosis

Fertilization

Mitosis

Fertilization

MeiosisMeiosis

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 DivisionFirst 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 DivisionSecond 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 meiosisOccurrence of meiosis

Plants: anthers and ovulesMammals: testes and

ovaries

Comparison Comparison between Mitosis & between Mitosis &

MeiosisMeiosisMitosis Meiosis

Number of division

1 2

No. of daughter cells produced

Type of cells producedChromosome number of daughter cells

2

Somatic (body) cells

Same as parent cell (diploid)

4

Gametes (sex cells)

Half of parent cell (haploid)

Mitosis Meiosis

Pairing of homologous chromosomes

No Yes

Occurrence

Role

Growing tissues

For Growth & Replacement

Reproductive tissues

(gonads)For Gamete Formation

Comparison Comparison between mitosis & between mitosis &

meiosismeiosis

Significance of meiosisSignificance of meiosis

Produce haploid gametes which, after fertilization, can restore normal diploid conditions

Produce genetic variations

Sources of genetic Sources of genetic variationvariation

• Different combination of the

genes of each gamete

• Random fusion of gametes

during fertilization

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