topic 4 EXAM QUESTIONS

Question 1

Describe how a gene is a code for the production of a polypeptide. Do not
include information about transcription or translation in your answer. (3 marks)

Answer 1

1. (Because) base/nucleotide sequence;
2. (In) triplet(s);
3. (Determines) order/sequence of amino acid sequence/primary
   structure (in polypeptide);

Question 2

Describe how a phosphodiester bond is formed between two nucleotides
within a DNA molecule. (2)

Answer 2

1. Condensation (reaction)/loss of water;
2. (Between) phosphate and deoxyribose;
3. (Catalysed by) DNA polymerase;

Question 3

Define ‘non-coding base sequences’ and describe where the non-coding
multiple repeats are positioned in the genome.
(2)

Answer 3

DNA that does not code for protein/polypeptides
OR
DNA that does not code for (sequences of) amino acids
OR
DNA that does not code for tRNA/rRNA;
Accept the idea of not transcribed for ‘does not
code for’.
Do not credit ‘DNA that does not code for an amino
acid’.
Ignore reference to introns.
2. (Positioned) between genes;

Question 4

The nucleus and a chloroplast of a plant cell both contain DNA.
Give three ways in which the DNA in a chloroplast is different from DNA in
the nucleus. (3)

Answer 4

In chloroplasts
1. DNA shorter;
Accept smaller
2. Fewer genes;
3. DNA circular not linear;
Accept DNA in a loop not linear
Accept no chromosomes (in chloroplast) unlike nucleus
4. Not associated with protein/histones, unlike nuclear DNA;
5. Introns absent but present in nuclear DNA;

Question 5

Some DNA nucleotides have the organic base thymine, but RNA
nucleotides do not have thymine. RNA nucleotides have uracil instead of
thymine.
Give one other difference between the structure of a DNA nucleotide and
the structure of an RNA nucleotide. (1)

Answer 5

Deoxyribose in DNA and ribose in RNA;

Question 6

Compare and contrast the DNA in eukaryotic cells with the DNA in
prokaryotic cells. (5)

Answer 6

Comparisons
1. Nucleotide structure is identical;
Accept labelled diagram or description of nucleotide
as phosphate, deoxyribose and base
2. Nucleotides joined by phosphodiester bond;
OR
Deoxyribose joined to phosphate (in sugar, phosphate backbone);
3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA
in prokaryotes;
Accept shorter than nuclear DNA/is circular not
linear/is not associated with protein/histones unlike
nuclear DNA;
Contrasts
4. Eukaryotic DNA is longer;
5. Eukaryotic DNA contain introns, prokaryotic DNA does not;
6. Eukaryotic DNA is linear, prokaryotic DNA is circular;
7. Eukaryotic DNA is associated with / bound to protein / histones,
prokaryotic DNA is not;

Question 7

What is a homologous pair of chromosomes? (1)

Answer 7

(Two chromosomes that) carry the same
genes;

Question 8

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

Answer 8

1. (mRNA attaches) to ribosomes
   OR
   (mRNA attaches) to rough endoplasmic
   reticulum;
2. (tRNA) anticodons (bind to) complementary
   (mRNA) codons;
3. tRNA brings a specific amino acid;
4. Amino acids join by peptide bonds;
5. (Amino acids join together) with the use of ATP;
6. tRNA released (after amino acid joined to
   polypeptide);
7. The ribosome moves along the mRNA to form
   the polypeptide;

Question 9

Describe ‘gene mutation’ and explain how a gene mutation can have:
-no effect on an individual
-a positive effect on an individual (4)

Answer 9

(Definition of gene mutation)
1. Change in the base/nucleotide (sequence of
chromosomes/DNA);
2. Results in the formation of new allele;
(Has no effect because)
3. Genetic code is degenerate (so amino acid
sequence may not change);
OR
Mutation is in an intron (so amino acid sequence
may not change);
4. Does change amino acid but no effect on tertiary
structure;
5. (New allele) is recessive so does not influence
phenotype;
(Has positive effect because)
6. Results in change in polypeptide that positively
changes the properties (of the protein)
OR
Results in change in polypeptide that positively
changes a named protein;
7. May result in increased reproductive success
OR
May result in increased survival (chances);

Question 10

Mutation can result in an increase in genetic variation within a species.
Describe and explain the other processes that result in increases in genetic variation within a species. (4)

Answer 10

1. Independent segregation of homologous
   Chromosomes/pairs;
2. Crossing over between homologous
   chromosomes/pairs;
3. Random fertilisation of gametes;
4. (Produces) new combinations of alleles;