What are chromosomes? Genetic materials found inside the nucleus of a cell Made up of DNA & protein...
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Transcript of What are chromosomes? Genetic materials found inside the nucleus of a cell Made up of DNA & protein...
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
The 2 members of each homologous chromosome separate from each other and move towards the opposite poles of the cell
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
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