Meiosis genetic variation Learning Objective Why is meiosis
Meiosis & genetic variation Learning Objective: • Why is meiosis necessary? • What happens during meiosis? • How does meiosis create genetic variation? Starter: what is the difference between mitosis and meiosis?
Cell Division (Mitosis & Meiosis) • The division on cells involves firstly, the division of the nucleus and secondly the division of the cells as a whole. The division of the nucleus of a cell occurs in one of two ways: • Mitosis: produces two new daughter cells that are identical to each other and has the same number of chromosomes as the parent cell. • Meiosis: produces four daughter nuclei, each with half the number of chromosomes as the parent cell.
Outcomes: • Know that cell division by meiosis results in the formation of gametes. • Can describe the importance of meiosis in creating variation by independent assortment of chromosomes and crossing over.
Why is meiosis necessary? • Every diploid cell of an organism has two sets of chromosomes: one set from each parent. • During meiosis these pairs separate so that only one set enters the each (haploid) gamete. • When these gametes fuse at fertilisation the diploid number of chromosomes is restored.
The process of meiosis • Cell division 1: • Cell division 2: • This brings about variation because there is independent serration of homologous chromosomes & recombination of homologous chromosomes by crossing over.
Replication of chromosomes occurs prior to division Homologous chromosomes centromere chromatids chromosome
Meiosis consists of two divisions Meiosis 1 Meiosis 2
Gametes are formed by meiosis: Homologous chromosomes associate
Outcomes: • Know that cell division by meiosis results in the formation of gametes. • Describe the importance of meiosis in creating variation by independent assortment of chromosomes and crossing over. • Know that cell division by mitosis results in an increase in number of identical cells for growth and repair. • Identify and name the stages of mitosis in diagrams and photomicrographs. • Describe the cell cycle and relate it to an understanding of cancer and its treatment.
Independent segregation increases variation maternal paternal Random assortment in meiosis II
or maternal paternal Random assortment in meiosis II
Crossing over increases variation recombinant chromosome chiasma In the first division of meiosis the homologous chromosomes associate
Crossing over increases variation The exchange (of alleles) between chromatids/chromosomes B B b b G g B, G B, g b, G b, g All gametes have a different combination of alleles on the chromosomes
Outcomes: • Know that cell division by meiosis results in the formation of gametes. • Can describe the importance of meiosis in creating variation by independent assortment of chromosomes and crossing over.
The Triplet Code • Features of the triplet code: Ø A few amino acids have only triplet code. Ø Code is known as degenerated code because most amino acids have more than one triplet code. Ø The code is Non-overlapping (each base in the sequence is read only once. Ø The code is universal.
DNA and Chromosomes Allele Each gene exists in two or more different forms each of which is called and allele. Each individual inherits one allele from each parent. Any differences in the base sequence of an allele of a single gene may result on different sequence of amino acid being coded, which will lead to production of different polypeptides (proteins). Sometime the protein produced may not function properly or not function at all because it doesn't fit into the enzyme substrate.
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