Genetic information

• AQA A Level Biology Topic 4 Genetic information, variation and relationships between organisms
• Topics


  • 4.1 DNA, genes and chromosomes
  • 4.2 DNA and protein synthesis
  • 4.3 Genetic diversity can arise as a result of mutation or during meiosis
  • 4.4 Genetic diversity and adaptation
  • Required practical 6
  • 4.5 Species and taxonomy
  • 4.6 Biodiversity within a community
  • 4.7 Investigating diversity
• DNA in eukaryotic cells

  Longer, linear, associated with histone proteins, contains introns

  
• DNA in prokaryotic cells


  Shorter, circular, not associated with proteins, no introns
• Chromosome


  Long, linear DNA + its associated histone proteins, in the nucleus of eukaryotic cells

  
• Gene


  A sequence of DNA (nucleotide) bases that codes for the amino acid sequence of a polypeptide or a functional RNA

  
• Locus


  Fixed position a gene occupies on a particular DNA molecule

  
• Genetic code


  Triplet code, universal, non-overlapping, degenerate
• Non-coding base sequences


  DNA that does not code for amino acid sequences / polypeptides, found between genes and within genes (introns)
• Introns


  Base sequence of a gene that doesn't code for amino acids, in eukaryotic cells

  
• Exons


  Base sequence of a gene coding for amino acid sequences (in a polypeptide)

  
• Genome


  The complete set of genes in a cell (including those in mitochondria and /or chloroplasts)
• Proteome


  The full range of proteins that a cell can produce (coded for by the cell's DNA / genome)
• Transcription


  Production of messenger RNA (mRNA) from DNA, in the nucleus
• Translation


  Production of polypeptides from the sequence of codons carried by mRNA, at ribosomes
• Transcription in eukaryotic cells

  Hydrogen bonds between DNA bases break

  2. Only one DNA strand acts as a template

  3. Free RNA nucleotides align next to their complementary bases on the template strand

  4. RNA polymerase joins adjacent RNA nucleotides

  5. This forms phosphodiester bonds via condensation reactions

  6. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

  
• Translation


  mRNA attaches to ribosome and ribosome moves to start codon

  2. tRNA brings specific amino acid

  3. tRNA anticodon binds to complementary mRNA codon

  4. Ribosome moves, another tRNA binds, peptide bond forms

  5. tRNA released, ribosome moves along mRNA

  6. Until stop codon reached
• Role of ATP, tRNA and ribosomes in translation

  ATP: hydrolysis provides energy for amino acid joining and peptide bond formation

  tRNA: transports specific amino acid, anticodon binds to mRNA codon

  Ribosomes: bind mRNA, catalyse peptide bond formation, move along mRNA
• Gene mutation is a change in the base sequence of DNA
• Mutagenic agent


  A factor that increases rate of gene mutation, eg. ultraviolet (UV) light or alpha particles
• How a mutation can lead to a non-functional protein or enzyme

  Changes sequence of base triplets in DNA, so changes sequence of codons on mRNA

  2. Changes sequence of amino acids in the polypeptide

  3. Changes position of bonds, so changes protein tertiary structure

  4. For enzymes, active site changes shape so substrate can't bind
• Substitution mutation


  Base / nucleotide in DNA replaced by a different base / nucleotide, changes one amino acid or no change (due to degenerate genetic code)
• Deletion mutation


  One nucleotide / base removed from DNA sequence, changes sequence of amino acids, changes tertiary structure
• Homologous chromosomes


  Same length, same genes at same loci, but may have different alleles
• Diploid cell


  Has 2 complete sets of chromosomes, represented as 2n
• Haploid cell


  Has a single set of unpaired chromosomes, represented as n

  
• Meiosis


  Meiosis I: Homologous chromosomes separate, crossing over occurs

  2. Meiosis II: Chromatids separate

  Outcome: 4 genetically varied daughter cells, normally haploid
• n


  Unpaired chromosomes
• Cell division by meiosis

  1. Interphase: DNA replicates → 2 copies of each chromosome (sister chromatids), joined by a centromere

  2. Meiosis I: Separates homologous chromosomes, chromosomes arrange into homologous pairs, crossing over between homologous chromosomes, independent segregation of homologous chromosomes

  3. Meiosis II: Separates chromatids, outcome = 4 genetically varied daughter cells, daughter cells are normally haploid (if diploid parent cell)
• Meiosis halves the number of chromosomes
• Crossing over


  1. Homologous pairs of chromosomes associate / form a bivalent

  2. Chiasmata form (point of contact between (non-sister) chromatids)

  3. Alleles / (equal) lengths of (non-sister) chromatids exchanged between chromosomes

  4. Creating new combinations of (maternal & paternal) alleles on chromosomes

  
• Independent segregation


  1. Homologous pairs randomly align at equator → so random which chromosome from each pair goes into each daughter cell

  2. Creating different combinations of maternal & paternal chromosomes / alleles in daughter cells
• Other ways genetic variation is increased

  • Random fertilisation / fusion of gametes
  • Creating new allele combinations / new maternal and paternal chromosome combinations
• Importance of meiosis


  • Two divisions creates haploid gametes (halves number of chromosomes)
  • Diploid number is restored at fertilisation → chromosome number maintained between generations
  • Independent segregation and crossing over creates genetic variation

  
• Recognising meiosis and mitosis in life cycle

  1. Mitosis occurs between stages where chromosome number is maintained

  2. Meiosis occurs between stages where chromosome number halves

  
• Chromosome number mutations


  1. Spontaneously by chromosome non-disjunction during meiosis

  2. Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis

  3. Some gametes have an extra copy (n+1) of a chromosome, others have none (n-1)

  
• Possible chromosome combinations in daughter cells after meiosis

  2^n where n = number of pairs of homologous chromosomes (half the diploid number)

  
• Possible chromosome combinations after random fertilisation

  (2^n)^2 where n = number of pairs of homologous chromosomes (half the diploid number)

  
• Not all mutations change amino acid sequence
• Mutations may change protein tertiary structure