DNA and Meiosis

Biology unit 2 as level aqa new, dna and meiosis.

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Structure of DNA.

DNA - chemical that determines inherited characteristics in form of genetic code. 3 basic components form a nucleotide by condensation reaction:

  • Pentase sugar = deoxyribose.
  • Phosphate group.
  • Organic nitrogenous base, belonging to 1 of 2 groups:
    • Single-ring bases (pyrimidines) - cytosine and thymine.
    • Double-ring bases (purines) - adenine and guanine. These are longer.

2 mononucleotides join by condensation between sugar of 1 and phosphate of another. DNA is made of 2 strands of nucleotides joined by hydrogen bonds between bases.

  • If rungs are to be same length, 1 short and 1 long base must be used in each pair.
  • A with T by 2 H bonds. C with G by 3 H bonds.
  • For each complete turn of helix there are 10 base pairs.
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Function of DNA.

  • Pass genetic info from cell to cell and down generations.
  • Very stable so can pass without change.
  • 2 separate strands have H bonds so can split for replication and protein synthesis.
  • Carries lots of genetic info as large mol.
  • Base pairs are in cylinder to protect from corruption by chemicals.

Proving That DNA is Hereditary:

Use pneumonia and mice. Bacterium exists in harmful and safe strains. Harmful has info to make toxin but not the means. Safe can make it but doesn't know how. Info transferred to safe form and given to mouse. Pneumonia develops.

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The Triplet Code.

Gene - section of DNA that contains coded info for making a single polypeptide and hence protein. Each gene has a specific position on a chromosome - locus. A polypeptide is a sequence of amino acids.

There must be a minimum of 3 bases coding for DNA because there are only 20 amino acids naturally, each must have its own code of bases but there are only 4 bases. In eukaryotes much of the DNA does not code for an amino acid - these sections are called introns and occur within genes and as multiple repeats between them.

Triplet Code:

  • Few amino acids have 1 triplet code - usually 2 - 6.
  • Code is degenerate as most amino acids have more than 1 triplet code.
  • Always read in 1 direction.
  • Start of sequence always same triplet (methionine).
  • 3 triplets don't code for amino acids but are stop codes.
  • Non-overlapping.
  • Universal.
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Prokaryotes have smaller DNA mols, form a circle and are notassociated with protein mols so have no chromosomes. Eukaryotes are the opposite.

Chromosome Structure:

  • Only visible when cell dividing.
  • Dispersed through nucleus.
  • Joined at centromere.
  • Each thread is a chromatid.
  • DNA held in position by proteins.
  • Double helix fixes protein's position.
  • DNA-protein complex coiled and looped to fit in chromosome.
  • Same number for individuals, different between species.
  • Occur in pairs called homologous so there's an even number.
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Homologous Pairs and Alleles.

  • Homologous chromosomes:
  • Egg and sperm fuse, each contributes set of chromosomes.
  • 1 of each pair comes from chromosomes from mother and other from father = homologous pair - diploid number is 46.
  • Always 2 chromosomes that determine same genetic characteristics.
  • Each carries a code (allele).
  • During meiosis, halving of chromosomes done to ensure daughter cell gets 1 set of info for each characteristic.
  • Alleles:
  • Each gene exists in 2+ different forms = allele.
  • Each person inherits 1 allele from each parent - same or different.
  • Each codes for different polypeptide.
  • Differences in base sequence of allele may lead to different sequence of amino acids being coded for which leads to production of different polypeptides and different protein.
  • Different protein may not function.
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Meiosis - type of cell division that produces haploid cells from diploids. leads to 4 daughter nuclei each with half number of chromosomes.

Meiosis is necessary for leading to offspring. If gamete had diploid number then cell would have too many chromosomes . Chromosome pairs separate so 1 from each pair goes.

It occurs in 2 nuclear divisions, 1 after the other:

  • MEIOSIS 1- homo pairs pair up and chromatids wrap round each other. Equivalent portions exchanges in crossing over.1 chrom from each pair goes to1 of 2 cells.
  • MEIOSIS 2- chromatids move apart so 4 cells formed.

Meiosis produces genetic variation by:

  • Independent segregationof homologous chromosomes.
  • Recombination by crossing over.
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Genetic Variation in Meiosis.

  • Independent Segregation:
  • During meiosis 1each chrom lines up next to homo partner randomly.
  • 1 of each pair goes to daughter cell, which 1 goes depends on how they are lined up.
  • Combo of chroms that go is random - independent segregation.
  • Variety from New Genetic Combos:
  • Each member of homo pair has same genes so determines same characteristics.
  • Alleles may differ.
  • Gametes produced - genetically different because of different combos of maternal and paternal chroms.
  • Haploid gametes fuse randomly.
  • Genetic Recombo by Crossing Over:
  • During meiosis 1 each chrom lines up beside homo partner, then:
  • 1. chromatids of each pair twist round each other.
  • 2. tensionscreated break protions off.
  • 3. broken portions rejoinwith partner's chromatids (recombo) - usually equivalent.
  • Happens many times.
  • As 4 cells are produced variety is increased.
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