4.2 DNA and protein synthesis

• Define ‘genome’


  The complete set of genes in a cell (including those in mitochondria and /or chloroplasts)
• Define ‘proteome’


  The full range of proteins that a cell can produce in a given time (coded for by the cell’s DNA / genome)
• Describe the two stages of protein synthesis

  • 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

  
• Compare and contrast the structure of tRNA and mRNA

  Comparison (similarities)

  • ● Both single polynucleotide strand

  Contrast (differences)

  ● tRNA is folded into a ‘clover leaf shape’, whereas mRNA is linear / straight

  ● tRNA has hydrogen bonds between paired bases, mRNA doesn’t

  ● tRNA is a shorter, fixed length, whereas mRNA is a longer, variable length (more nucleotides)

  ● tRNA has an anticodon, mRNA has codons

  ● tRNA has an amino acid binding site, mRNA doesn’t

  
• Describe how mRNA is formed by transcription in eukaryotic cells

  1. 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. ● In RNA, uracil is used in place of thymine (pairing with adenine in DNA)
  5. RNA polymerase joins adjacent RNA nucleotides
  6. This forms phosphodiester bonds via condensation reactions
  7. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

  
• Describe how production of messenger RNA (mRNA) in a eukaryotic cell is different from the production of mRNA in a prokaryotic cell

  ● Pre-mRNA produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells

  ● Because genes in prokaryotic cells don’t contain introns so no splicing in prokaryotic cells
• Describe how translation leads to the production of a polypeptide

  1. mRNA attaches to a ribosome and the ribosome moves to a start codon (AUG)
  2. tRNA brings a specific amino acid
  3. tRNA anticodon binds to complementary mRNA codon
  4. Ribosome moves along to next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond
  5. ● Using energy from hydrolysis of ATP
  6. tRNA released after amino acid joined polypeptide
  7. Ribosome moves along mRNA to form the polypeptide, until a stop codon is reached

  
• Describe the role of ATP in translation

  ● Hydrolysis of ATP to ADP + Pi releases energy

  ● So amino acids join to tRNAs and peptide bonds form between amino acids
• Describe the role of tRNA in translation


  ● Attaches to / transports a specific amino acid, in relation to its anticodon

  ● tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds

  ● 2 tRNAs bring amino acids together so peptide bond can form
• Describe the role of ribosomes in translation

  ● mRNA binds to ribosome, with space for 2 codons

  ● Allows tRNA with anticodons to bind

  ● Catalyses formation of peptide bond between amino acids (held by tRNA molecules)

  ● Moves along (mRNA to the next codon) / translocation
• Describe how the base sequence of nucleic acids can be related to the amino acid sequence of polypeptides when provided with suitable data

  • ● You may be provided with a genetic code to identify which triplets / codons produce which amino acids (example shown)
  • ● tRNA anticodons are complementary to mRNA codons
  • ○ Eg. mRNA codon = ACG → tRNA anticodon = UGC
  • ● Sequence of codons on mRNA are complementary to sequence of triplets on DNA template strand
  • ○ Eg. mRNA base sequence = ACG UAG AAC → DNA base sequence = TGC ATC TTG
  • ● In RNA, uracil replaces thymine