diversity/selection

• diploid cell: full set of chromosomes

  haploid: half set of chromosomes

  gametes: sex cells
• genetic variation due to:

  • crossing over
  • independent segregation
• Crossing over:

  • alleles are exchanged
  • occurs in prophase one

  A) homologous chromosomes

  B) sister chromatids

  C) chromosomes associate

  D) chiasma

  E) new combination of alleles

  
• Independent segregation:

  • number of possibilities = 2^n
  • n= number of chromosomes
  • random assortment
• when describing each division:

  • meiosis 1 = homologous chromosomes separate
  • meiosis 2 = chromatids separate
• meiosis:

  • diploid parent chromosomes are replicated
  • there are two nuclear divisions
  • in meiosis 1, separation of homologous chromosomes via spindle fibres
  • in meiosis 2, separation of sister chromatids via spindle fibres
  • producing four genetically different haploid cells
• Describe the process of crossing over and explain how it increases genetic diversity (4)

  homologous chromosomes associate

  chiasma forms

  alleles are exchanged

  producing a new combination of alleles
• Define ‘non-coding base sequences’ and describe where the non-coding multiple repeats are positioned in the genome. (2)

  DNA that doesn't code for polypeptides

  positioned between genes
• show three differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell (3)

  DNA in plant associate with histones, prokaryote not

  DNA in plant are longer, prokaryotes are shorter

  DNA in plants contain introns whereas prokaryotes dont
• Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. Give two reasons why (2)

  triplets code for same amino acid (degenerate)

  occurs in introns
• Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells (5)

  nucleotide structure is identical

  nucleotides joined by phosphodiester bonds

  prokaryote has circular dna whereas eukaryote has linear

  eukaryote dna is longer

  eukaryote dna contains introns eukaryotic dna dont

  eukaryote dna dont have introns but prokaryotes do
• What is a homologous pair of chromosomes

  two chromosomes carrying same gene
• mutations: change in base sequence
• Types of mutations:

  • substitution
  • addition
  • deletion
  • inversion - AAT= TAA
  • duplication
  • translocation
• substitution: one or more bases swapped

  deletion: one or more bases deleted

  addition: one or more bases added

  duplication: one or more bases repeated

  inversion: sequence of bases reversed

  translocation: moved from one location in genome to another
• explain how a single base substitution causes a change in structure of polypeptide (3)

  substitution causes amino acid sequence to change so the primary structure changes as well as the tertiary structure therefore there would be a change in bonds
• variation: differences between two things

  alleles: different form of same gene

  genotype: alleles of gene

  phenotype: characteristics of organism

  gene: section of DNA which codes for polypeptides
• genetic drift: random event which causes a change in frequencies of alleles

  • founders effect
  • genetic bottleneck
  • decrease genetic diversity
• Founder effect:

  • describes what happens when organisms from population start new colony
  • reduces gene pool
  • rare allele in original may be common in new
  • most likely to have genetic disorder

  
• Genetic bottleneck:

  • causes big reduction to population
  • large number of organisms die before reproducing
  • reduces different alleles