AQA A-Level Biology: DNA, Genes and Inheritance

A gene in a eukaryotic cell codes for a polypeptide.

Describe and explain how a polypeptide is synthesised from the base sequence of this gene. In your answer refer to both transcription in the nucleus and translation at the ribosome.

(6 marks)

In a species of pea plant, stem height and flower colour are each controlled by a single gene. The two genes are located on different (unlinked) chromosomes.

• The allele for tall stems (T) is dominant to the allele for dwarf stems (t).
• The allele for purple flowers (P) is dominant to the allele for white flowers (p).

Two plants, each heterozygous at both genes (tall, purple), were crossed.

(a) Using a genetic diagram, determine the expected phenotypic ratio of the offspring. (4 marks)

(b) The cross produced a total of 160 offspring. Calculate the expected number of offspring in each phenotype class. (2 marks)

(6 marks)

Scientists investigated how DNA replicates. Bacteria were grown for many generations in a medium in which the only source of nitrogen was the heavy isotope ¹⁵N, so that all of their DNA was "heavy". The bacteria were then transferred to a medium containing only the lighter isotope ¹⁴N and allowed to divide.

At intervals, DNA was extracted and spun in a density gradient. DNA settles at a depth that depends on its density, forming a band: heavy DNA (both strands ¹⁵N) sits deepest, light DNA (both strands ¹⁴N) sits highest, and DNA of intermediate density sits between them.

The results are shown below.

Generations in ¹⁴N medium	Position of DNA band(s)
0	One band — heavy
1	One band — intermediate
2	Two bands — half intermediate, half light

(a) Interpret what the band seen after 1 generation shows about the strands of each new DNA molecule. (2 marks)

(b) Explain how the full set of results supports semi-conservative replication rather than conservative replication. (3 marks)

(5 marks)

Flower colour in an ornamental plant is controlled by a single gene that has three alleles:

• Cᴿ — produces red pigment
• Cᵂ — produces white pigment
• c — produces no pigment (yellow)

The relationships between the alleles are:

• Cᴿ and Cᵂ are codominant with each other — a plant with both alleles has pink flowers.
• Cᴿ and Cᵂ are each dominant to c.
• A plant that is cc has yellow flowers.

A pink-flowered plant was crossed with a red-flowered plant that is known to carry the c allele.

(a) Using a genetic diagram, determine the genotypes and the phenotypic ratio of the offspring. (4 marks)

(b) Explain what is meant by the term codominant. (1 mark)

(5 marks)

A short section of an mRNA molecule has the base sequence:

AAA – GAA – GUA

Part of the genetic code is shown in the table. (These are mRNA codons.)

Codon	Amino acid
AAA	Lysine
AAG	Lysine
GAA	Glutamate
GAG	Glutamate
GUA	Valine
GUG	Valine
GCA	Alanine

A gene mutation causes a single base substitution: the last base of the third codon changes from A to G, so the third codon becomes GUG.

(a) Deduce the effect of this substitution on the sequence of amino acids in the polypeptide. (2 marks)

(b) Using the idea that the genetic code is degenerate, explain why this substitution has the effect you described, and why a different single base substitution could change the polypeptide. (2 marks)

(4 marks)

Meiosis produces gametes that are genetically different from one another.

Explain how meiosis brings about this genetic variation. In your answer refer to both independent segregation (assortment) of homologous chromosomes and crossing over.

(3 marks)