8 - DNA, Genes & Protein Synthesis

• What is a gene?

  A section of DNA that contains a code for making a polypeptide and functional RNA.
• What is an allele?

  A different form of the same gene
• What are a Homologous pair of chromosomes?

  A pair of chromosomes with the same genes, but might have different alleles.
• What is the locus?

  The location of a particular gene on a chromosome
• What is the start codon?

  At the start of every gene, there are three bases which help to initiate translation.
• What is the stop codon?

  A stop codon causes ribosomes to detach and therefore stop translation
• What is the genome?

  An organism's complete set of DNA in one cell
• What is the proteome?

  The full range of proteins a cell can produce.
• What are some comparisons between DNA in a eukaryote and a prokaryote?

  1. The structure of the nucleotides are identical
  2. Nucleotides joined by phosphodiester bonds
  3. DNA in mitochondria/chloroplasts is similar to DNA in prokaryotes
• What are some differences between DNA in a eukaryote and a prokaryote?

  1. Eukaryotic DNA is longer
  2. Eukaryotic DNA contains introns
  3. Eukaryotic DNA is linear; prokaryotic DNA is circular
  4. Eukaryotic is associated with histones
• What are some comparisons between DNA in the nucleus and chloroplast of a eukaryote?

  IN CHLOROPLASTS

  1. DNA shorter
  2. Fewer genes
  3. DNA is circular, not linear
  4. Not associated with histones, unlike nuclear DNA
  5. Introns absent but present in nuclear DNA
• What does degenerate mean in terms of the genetic code?

  More than 1 codon/triplet of bases code for the same amino acid (i.e. ACG & ACC could both code for the same amino acid)
• What does universal mean in terms of the genetic code?

  The same triplet codes for the same amino acid in all organisms (i.e. animals, plants, fungi)
• What does non-overlapping mean in terms of the genetic code?

  Each base is part of only one triplet
• What's the advantage of the genetic code being degenerate?


  Even if there is a gene mutation, the codon may still code for the same amino acid
• What's the advantage of the genetic code being non-overlapping?

  If there is a mutation, it will only effect one codon and therefore one amino acid. This reduces the potential change in tertiary structure of the protein
• What is a codon?

  A triplet of bases on mRNA that codes for an amino acid.
• What is the difference between an exon and an intron?

  • Introns are the non-coding sections of DNA (do not code for a protein).
  • Exons are the coding sections of DNA (do code for proteins).
• Name the two types of RNA involved in protein synthesis?

  mRNA & tRNA
• • mRNA is a copy of a gene from DNA. mRNA is created in the nucleus and then enters the cytoplasm where it attaches to a ribosome.
  • DNA is too large to leave the nucleus and would be at risk of being damaged by enzymes. mRNA is much shorter because it is only the length of one gene, and can therefore leave the nucleus as it is small enough to fit through the nuclear pores.
  • mRNA is single-stranded and every three bases in the sequence code for a specific amino acid, these three bases are called codons.

  
• What are the key components of a tRNA molecule?

  • Hydrogen bonds
  • Anticodons
  • Amino acid attachment site

  
• • tRNA is found in the cytoplasm. It is single-stranded but folded to create a cloverleaf shape, held in place by hydrogen bonds. The function of tRNA is to attach to one of the 20 amino acids and transfer this amino acid to the mRNA to create the polypeptide chain.
  • Specific amino acids attach to specific tRNA molecules and this is determined by 3 bases found on the tRNA which are complementary to the 3 bases on mRNA. These are called the anticodon because they are complementary to the codon on mRNA.
• What are the 2 stages of protein synthesis, where do they occur & the products?

  Stage 1 : Transcription

  Location: Nucleus

  Product: mRNA (or pre-mRNA)

  
  

  Stage 2: Translation

  Location: The cytoplasm attached to a ribosome

  Product: polypeptide chain
• What are the 2 enzymes involved in transcription & their function?

  DNA helicase - breaks the hydrogen bonds between DNA strands.

  
  

  RNA polymerase - joins adjacent RNA nucleotides.
• In eukaryotic organisms, pre-mRNA is modified before leaving the nucleus. What modification is made?

  The introns are removed (spliced) by a splicesome.
• Why does splicing not occur in prokaryotic organisms?

  Prokaryotes do not contain introns in their DNA.
• What are the steps of transcription?

  1. The DNA helix unwinds to expose the bases to act as a template.
  2. Only one chain of the DNA acts as a template.
  3. The unwinding is catalysed by DNA helicase. DNA helicase breaks the hydrogen bonds between bases.
  4. Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases.
  5. The enzyme RNA polymerase bonds together the RNA nucleotides to create a new RNA polymer chain. One entire gene is copied.

  
• What are these steps of translation?

  1. mRNA attaches to a ribosome in the cytoplasm. The ribosome attaches at the start codon.
  2. The tRNA molecule with the complementary anticodon to the start codon aligns opposite the mRNA, which is held in place by the ribosome.
  3. The ribosome will move along the mRNA molecule to enable another complementary tRNA to attach to the next codon on the mRNA.

  
• What are the steps of translation?

  4. The two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP.

  5. This continues until the ribosome reaches the stop codon at the end of the mRNA molecule. The stop codon does not code for an amino acid and therefore the ribosome detaches and translation ends.