Section 4 - Unit 8: DNA, Genes & Protein Synthesis

Question 1

In which of the following are hydrogen bonds present in: DNA, mRNA, tRNA (3 marks)

Answer 1

• DNA

- tRNA

Question 2

Give two ways in which the structure of a molecule of tRNA differs from the structure of a molecule of mRNA (2 marks)

Answer 2

• tRNA has an amino acid binding site

- tRNA has hydrogen bonds between base pairs

Question 3

Describe the part played by mRNA in protein synthesis (3 marks)

Answer 3

Any three from:

• Carries coded information about the sequence of amino acids
• Copied from DNA/gene
• Code is in sequence of bases for one amino acid
• Moves out of nucleus
• Into the ribosomes

Question 4

Explain how a gene codes for a protein (2 marks)

Answer 4

• Base sequence on DNA

- Determines sequence of amino acids

Question 5

Explain what homologous chromosomes are (2 marks)

Answer 5

• Pairs of chromosomes
• With genes for same features
• At same loci

Question 6

Define gene (2 marks)

Answer 6

• A base sequence of DNA that codes for the amino acid sequence of a polypeptide
• And a functional RNA (including ribosomal RNA and tRNAs).

Question 7

State two differences between prokaryotic and eukaryotic DNA (2 marks)

Answer 7

• Short vs long

- Circular vs linear

Question 8

What is the name of a position of a gene on a chromosome? (1 mark)

Answer 8

• Locus

Question 9

Describe how mRNA is produced in the nucleus of a cell (6 marks)

Answer 9

• Helicase
• Breaks hydrogen bonds
• One DNA strand acts as template
• RNA nucleotides attracted to exposed bases
• According to base pairing rule
• RNA polymerase joins (RNA) nucleotides together
• Pre-mRNA spliced to remove introns

Question 10

Explain why the genetic code is described as being universal (1 mark)

Answer 10

• In all organisms, the same triplet codes for the same amino acid

Question 11

What is the maximum number of different DNA triplets that can be made (1 mark)

Answer 11

64

Question 12

Name the process that removes base sequences from pre-mRNA to form mRNA (1 mark)

Answer 12

Splicing

Question 13

Explain why the mutation of an exon can lead to the production of a non-functional protein (3 marks)

Answer 13

Any 3 from:

• Mutation changes triplets
• Which change the amino acid sequence after this
• And affects the hydrogen bonding
• Which changes the tertiary structure of protein (so non-functional)

Question 14

Explain what effect a mutation of the intron has on the protein (2 marks)

Answer 14

• Introns non-coding

- So no effect on protein

Question 15

Explain why the genetic code is described as being degenerate (1 mark)

Answer 15

• An amino acid can be coded for by more than one triplet

Question 16

Explain what is meant by a codon (2 marks)

Answer 16

• Three bases on mRNA

- That code for an amino acid

Question 17

Explain the role of RNA polymerase during transcription (1 mark)

Answer 17

• To join nucleotides together to form mRNA

Question 18

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

Answer 18

• Change in amino acid sequence
• Leads to a change in hydrogen/ionic/disulphide bonds
• Which alters tertiary structure

Question 19

Explain the difference between pre-mRNA and mRNA (1 mark)

Answer 19

• Pre-mRNA contains introns vs mRNA only contains exons

Question 20

Starting with mRNA, describe how the process of translation leads to the production of a polypeptide (4 marks)

Answer 20

Any four from:

• mRNA attaches to ribosome
• The codon on mRNA
• Binds to an anti-codon on tRNA
• Each tRNA brings a specific amino acid
• Bases on mRNA determines order of amino acids
• Formation of peptide bonds by condensation reactions

Question 21

State the role of RNA polymerase in transcription (1 mark)

Answer 21

• Attaches nucleotides to form a strand

Question 22

Describe the role of tRNA in the process of translation (3 marks)

Answer 22

Any 3 from:

• Anticodon complementary to codon
• Specific amino acid
• Carried to ribosome
• Correct sequence of amino acids along polypeptide

Question 23

Explain how a change in a sequence of DNA bases could result in a non-functional enzyme

Answer 23

• Change in sequence of amino acids/primary structure
• Change in hydrogen/ionic/disulfide bonds leads to change in tertiary structure/active site of enzyme
• Substrate cannot bind/no enzyme-substrate complexes form

Question 24

Use your knowledge of DNA structure and function to explain how it’s possible for humans and animals to be different even though they have very similar percentages of each base (2 marks)

Answer 24

• Different genes
• So triplets are in a different order
• So different amino acids are coded for

Question 25

Explain how a cancer drug could prevent DNA replication (2 marks)

Answer 25

- Drug has similar shape | - So complementary base pairing cannot take place

Question 26

Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of one base for another (3 marks)

Answer 26

- Deletion causes frame shift - So changes many amino acids - Substitution alters one codon - But degenerate nature means that the same amino acid can be coded for

Question 27

Describe how two amino acids differ from one another (1 mark)

Answer 27

- Different R groups

Question 28

Suggest how a mutation can lead to the production of a protein that has one amino acid missing (1 mark)

Answer 28

- Loss of 3 bases

Question 29

Name the enzyme used to convert mRNA into DNA (1 mark)

Answer 29

- Reverse transcriptase

Question 30

Describe how a polypeptide is formed by translation of mRNA (6 marks)

Answer 30

- 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) - The ribosome moves along the mRNA to form the polypeptide

Question 31

Describe the role of a ribosome in the production of a polypeptide (3 marks)

Answer 31

- mRNA binds to ribosome - Idea of two codons/binding sites - (Allows) tRNA with anticodons to bind/associate - (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules) - Moves along (mRNA to the next codon)/translocation described