3.4.2 DNA and protein synthesis

• Describe how mRNA is produced in a plant cell (5)

  • DNA strands seperated by breaking hydrogen bonds
  • template strand is used to make mRNA
  • complementary base pairing (A and U, T and A, G and C, C and G)
  • RNA nucleotides joined by RNA polymerase
  • pre-mRNA formed
  • introns removed to form mRNA
• In eukaryotic cells, transcription results in a molecule of pre-mRNA that is modified to produce mRNA. In a prokaryotic cells, transcription produces mRNA directly.

  Explain this difference (2)

  • DNA of eukaryotic cell has non-coding regions within gene
  • after transcription, these regions are removed from pre-mRNA
• Give the differences between the structure of mRNA and the structure of tRNA (2)

  • mRNA longer/has more nucleotides
  • mRNA is a straight chain molecule but tRNA is a clover-leaf shaped molecule
  • mRNA contains no paired bases but tRNA has some paired bases
• Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer. (3).

  • mRNA binds to ribosome
  • two codons on mRNA
  • allows tRNA with complementary anticodon binds
  • catalyses the formation of peptide bonds between amino acids
  • moves along mRNA to next codon
• In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA.

  Explain why (2)

  • introns in pre-mRNA
  • removal of introns to form mRNA
• The proteome of a cell is the number of different proteins that a cell is able to produce.
• Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide (5)

  • mRNA associates with a ribosome
  • ribosome moves to find the start codon
  • tRNA brings specific amino acid
  • anticodon on tRNA complementary to codon on mRNA
  • ribosome moves along to next codon
  • process repeated and amino acids join by peptide bonds
• The start codon is AUG which codes for the amino acid methionine (MET).
• DNA is a polymer of nucleotides. Each nucleotide contains an organic base.

  Explain how the organic bases help to stabilise the structure of DNA (2)

  • hydrogen bonds between the base pairs holds two strands together
  • many hydrogen bonds provides strength
• A codon is a group of three bases on mRNA that codes for an amino acid.
• Suggest one advantage of showing the genetic code as base sequences on mRNA, rather than triplets on DNA (1)

  • ribosomes assemble polypeptides using mRNA code
• Stop codons signal the end of translation and result in the detachment of polypeptide chain from ribosome.
• Explain how change in a sequence of DNA bases could result in a non-functional enzyme (3)

  • change in primary structure
  • change in hydrogen, ionic or disulphide bonds
  • alters tertiary structure
  • substrate cannot bind
• tRNA is more stable than mRNA because it has hydrogen bonding between complementary base pairs.
• The function of tRNA is to carry specific amino acids to the ribosome
• Transcription is the production of mRNA from DNA.
• Translation is the production of polypeptides from the sequence of codons carried by mRNA.
• Describe how mRNA is formed by transcription in eukaryotes (5)

  • hydrogen bonds between DNA bases break
  • one DNA strand acts as a template
  • free RNA nucleotides align by complementary base pairing
  • in RNA, uracil base pairs with adenine
  • RNA polymerase joins adjacent RNA nucleotides
  • by phosphodiester bonds between adjacent nucleotides
  • introns removed to form mRNA
• Describe how a polypeptide is formed by translation of mRNA (6)

  • mRNA attaches to ribosomes
  • tRNA anticodons bind to complementary mRNA codons
  • tRNA brings a specific amino acid
  • amino acids join by peptide bonds
  • amino acids join together with the use of ATP
  • tRNA released after amino acid joined to polypeptide
  • ribosome moves along mRNA to form polypeptide
• Describe how mRNA is produced from an exposed template strand of DNA

  Do not include DNA helicase or splicing in your answer (3)

  • free RNA nucleotides form complementary base pairs
  • phosphodiester bonds form
  • by RNA polymerase
• Splicing removes introns and joins exons together to form mRNA.
• What is the role of RNA polymerase during transcription ?

  • joins adjacent RNA nucleotides to form mRNA
• mRNA can be converted to cDNA

  Name the enzyme used in this process. (1)

  • reverse transcriptase
• Describe how the production of messenger RNA (mRNA) in a eukaryote cell is different from the production of mRNA in a prokaryote cell (2)

  • pre-mRNA produced only in eukaryote cell
  • splicing only occurs in eukaryote cell
• Describe role of ATP in process of translation in protein synthesis (2)

  • releases/provides energy
  • so peptide bonds form between amino acids