Cels Module 3

• What are the key features of the genetic code?

  • single bases that make up triplets called a codon, and each codon encodes one amino acid.
  • 61 of 64 codons specify one amino acid, most amino acids have more than one codon.
  • UAA, UAG and UGA are stop codons
  • AUG is the start codon
• What is the role of mRNA in protein synthesis?

  mRNA carries the genetic information from DNA to the ribosomes, where it serves as a template for protein synthesis.
• What is tRNA?

  • A single strand of RNA
  • 70-80 nucleotides in length
  • At least one tRNA for each amino acid
  • Has a region which can bind an amino acid and a region that can interact with mRNA
• What are the steps in ‘charging’ a tRNA?

  • An enzyme recognises both an amino acid and the matching tRNA and joins them together.
  • There are 20 different enzymes, 1 for each amino acid.
• What is translation?

  the synthesis of proteins by ribosomes using mRNA as a set of instructions
• What do ribosomes contain?

  rRNA and proteins
• Where are the ribosomes located?

  Bound to the rER - to synthesise proteins that are used within the plasma membrane or are exocytosed from the cell

  Free in the cytosol - synthesise proteins that are released into the cytosol and used within the cell
• What is the process of initiation?

  • a specific tRNA (carrying methionine) will bind to the small ribosomal subunit that will then identify the 5’ G-cap and attach to the mRNA.
  • it will then move along the mRNA (5’ to 3’ direction) until it finds the AUG codon.
  • The complex stops, with the initiator tRNA carrying the first methionine positioned in the P site.
  • the large ribosomal subunit attaches
• Elongation

  1. Charged tRNA with anticodon complementary to A site codon lands in A site

  2. Ribosome breaks bond binding amino acid and tRNA in P site, transfers it to amino acid in P site, forming peptide bond, while tRNAs bound to mRNA (in P and A sites) and ribosome moves 3 nucleotides down mRNA

  3. In E site, anticodon of tRNA detaches from mRNA codon and uncharged tRNA expelled

  4. New charged tRNA with complementary anticodon to next A site codon enters and process repeats
• What is the process of termination?

  • When the ribosome reaches a stop codon (UAA, UAG GUA) a protein called release factor enters the A site.
  • The release factor breaks the bind between the P site tRNA (using water) and the final amino acid, the polypeptide chain will detach from the tRNA and be released.
  • the small and large ribosomal sub-units dissociate from the mRNA and each other.
• What is genetic variation?

  Genetic variation refers to the differences in the genetic makeup of individuals within a population.
• What is phenotypic variation?

  Differences in physical traits among individuals of the same species. Determined by what happens at protein level and environmental influences
• What are sex linked traits?


  Traits controlled by genes located on the sex chromosomes, so the inheritance patterns differ between males and females
• How does recombination occur for linked genes?

  Crossing over
• Crossing over leads to recombination between linked genes because the alleles are switched between genes so new combinations of alleles are formed and this enhances diversity
• What is a population?

  localised group of individuals of the same species
• What is a gene pool?


  total aggregate of genes in the population at one time
• Why do we need to estimate frequencies of genotypes in a population?

  • to predict how ,any individuals will inherit a genetic disease
  • to estimate the proportion of individuals who are carriers of a genetic disease
• How do allele frequencies change?

  • non-random mating (assortative mating and inbreeding)
  • random genetic drift
  • bottleneck effect
  • founder effect
  • natural selection
  • gene flow or migration
  • mutation
• What is random genetic drift?


  a random change in allele frequencies due to sampling error over generations. Happens faster and less controlled in smaller populations
• What is the bottleneckeffect?

  genetic drift due to an uncontrolled circumstance / random event
• What is the founder effect?


  The founder effect is the loss of genetic variation that occurs when a small group of individuals establishes a new population. the new population will be smaller and have limited diversity
• What is stabilising selection?

  Stabilizing selection is a type of natural selection that favors the average phenotype and reduces variation in a population.
• What is directional selection?


  Selection that favors individuals at one extreme of a trait distribution.
• What is disruptive selection?


  Selection that favors extreme phenotypes over intermediate ones.
• What is a cline?

  the gradual geographical change in genetic/phenotypic composition. (Individuals of the same species will favour different traits depending on where they are living and evolve to suit their environment)
• What are 2 ways of changing gene pools over time?

  • mutation - slow and usually disadvantageous
  • migration - an individual from another population successfully mates and contributes gametes to the gene pool