4.2 DNA and protein synthesis

• 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 cells 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
• compare and contrast the structure of tRNA and mRNA

  both single polynucleotide strand

  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
• how mRNA is formed by transcription in eukaryotic cells

  hydrogen bonds between DNA bases break

  only one DNA strand acts as a template

  free RNA nucleotides align next to their complementary bases on the template strand

  in RNA uracil is used in place of thymine (pairing with adenine in DNA)

  RNA polymerase joins adjacent RNA nucleotides

  this forms phosphodiester bonds via condensation reactions

  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

  mRNA attaches to a ribosome and the ribosome moves to a start codon (AUG)

  tRNA brings a specific amino acid

  tRNA anticodon binds to complementary mRNA codon

  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

  using energy from hydrolysis of ATP

  tRNA released after amino acid joined polypeptide

  ribosome moves along mRNA to form the polypeptide until stop codon is reached
• 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
• 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
• 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