3.4 Exam qs

• 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
• Explain how change in the DNA base sequence for a protein may result in a change in the structure of the protein (3)

  • change in primary structure
  • change in hydrogen, disulfide or ionic bonds
  • alters tertiary structure
• 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
• 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
• 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
• 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
• 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
• 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
• A mutation can lead to the production of a non-functional enzyme. Explain how. (6)

  • change in base sequence of gene
  • change in primary structure
  • change in hydrogen, ionic or disulfide bonds
  • change in tertiary structure
  • change in active site
  • substrate not complementary
• Define gene mutation and explain how a gene mutation can have:

  • no effect on individual
  • a positive effect on an individual (4)

  • change in the base sequence of chromosomes
  • results in the formation of new allele

  (Has no effect because)

  • genetic code is degenerate so amino acid sequence may not change
  • does change amino acid but no effect on tertiary structure
  • new allele is recessive so does not affect phenotype

  (Has positive effect because)

  • results in change in polypeptide that positively changes the properties of protein
  • may result in increased survival chances
• The genetic code uses four different DNA bases. What is the maximum number of different DNA triplets that can be made using these four bases ? (1)

  64
• 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
• Contrast the structures of DNA and mRNA molecules

  • DNA double stranded, mRNA single-stranded
  • DNA very long and RNA short
  • Thymine in DNA and uracil in RNA
  • deoxyribose in DNA and ribose in RNA
  • DNA has base pairing and mRNA does not
  • DNA has introns and mRNA does not
• Explain how the structure of DNA is related to it’s functions (6).

  • sugar-phosphate backbone so provides strength
  • long molecule so can store lots of information
  • helix so compact
  • base sequence allows information to be stored
  • double stranded so replication can occur semi-conservatively
  • weak hydrogen bonds for replication
• A phylogenetic group is a group of organisms that have similar evolutionary history/share common ancestors.
• Courtship behaviour enables...

  • members of the same species to recognise each other
  • identify mate of opposite gender
  • indicates readiness to mate
  • stimulates mating and release of gametes
  • pair bond
• Biodiversity is the variety of different species in an ecosystem or on Earth.
• Species diversity is the number of different species and the number of individuals in each species in a particular area at any one time. It is also known as the index of diversity.
• Describe how a gene is a code for the production of a polypeptide (3)

  • base sequence
  • in triplets
  • determine order/sequence of amino acid sequence
• 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
• Explain how a change in the DNA base sequence for a protein may result in a change in the structure of the protein (3)

  • change in primary structure
  • change in hydrogen/ionic/disulfide bonds
  • alters tertiary structure
• Describe how the process of meiosis results in haploid cells (4)

  • DNA replication (during late interphase)
  • Two divisions
  • Separation of homologous chromosomes (in first division)
  • Separation of (sister) chromatids (in second division)
  • Produces 4 (haploid) cells
• Compare and contrast DNA in eukaryotic cells with the DNA in prokaryotic cells (5)

  Comparisons

  • nucleotide structure is identical
  • nucleotides joined by phosphodiester bond
  • DNA in mitochondria/chloroplasts similar structure to DNA in prokaryotes

  Contrasts

  • eukaryotic DNA is longer
  • eukaryotic DNA contain introns, prokaryotic DNA does not
  • eukaryotic DNA is linear, prokaryotic DNA is circular
  • eukaryotic DNA is associated with histones, prokaryotic DNA is not
• Haemoglobins are chemically similar molecules found in many different species

  Differences in the primary structure of haemoglobin molecules can provide evidence of phylogenetic (evolutionary) relationships between species.

  Explain how. (5)

  • mutations change base/nucleotide sequence
  • causing change in amino acid sequence
  • mutations build up over time
  • fewer differences in base/nucleotide sequence between closely related species
  • closely related species have recent common ancestor