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3.4.1 DNA, genes and chromosomes

AQA A-level Biology 3.4.1 practice on DNA, genes and chromosomes, with free MCQs, clues and worked explanations drawn from the Genetic information, variation and relationships between organisms section of specification 7402.

Specification route
3.4.1
Question bank
25 questions
Course stage
AS / Year 12

Sample questions

QUESTION 1 · 3.4.1 · LEVEL 1

How is the DNA in the nucleus of eukaryotic cells structurally distinct from the DNA in prokaryotic cells?

  • It is very long, linear and associated with proteins called histones.
  • It is short, circular and not associated with proteins.
  • It consists of a single, continuous loop of naked DNA.
  • It is bound by a capsule and forms multiple small plasmids.
Show clue

Think about how eukaryotic cells pack massive amounts of genetic material into the nucleus.

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Answer: It is very long, linear and associated with proteins called histones.

In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. In contrast, prokaryotic DNA is short, circular and not associated with proteins.

QUESTION 2 · 3.4.1 · LEVEL 1

Which organelles in eukaryotic cells contain DNA that resembles prokaryotic DNA?

  • Mitochondria and chloroplasts
  • Golgi apparatus and lysosomes
  • Rough endoplasmic reticulum and smooth endoplasmic reticulum
  • Nucleus and cell vacuole
Show clue

These energy-converting organelles are thought to have evolved from free-living bacteria.

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Answer: Mitochondria and chloroplasts

The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.

QUESTION 3 · 3.4.1 · LEVEL 2

Biologically, what is the precise definition of a gene?

  • A base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA.
  • A long, linear molecule of DNA associated with histone proteins.
  • A single triplet of DNA bases that codes for a specific amino acid.
  • The complete set of proteins that a cell is able to produce.
Show clue

Genes contain the instructions for building structural and functional molecules.

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Answer: A base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA.

A gene is a base sequence of DNA that codes for: the amino acid sequence of a polypeptide; a functional RNA.

QUESTION 4 · 3.4.1 · LEVEL 1

What term is used to describe the fixed position that a gene occupies on a particular DNA molecule?

  • Locus
  • Intron
  • Codon
  • Centromere
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This word comes from the Latin for 'place'.

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Answer: Locus

A gene occupies a fixed position, called a locus, on a particular DNA molecule.

QUESTION 5 · 3.4.1 · LEVEL 2

The genetic code is described as 'degenerate'. What does this specific property mean?

  • Most amino acids are coded for by more than one triplet.
  • Each triplet codes for only one specific amino acid.
  • The same specific base triplets code for the same amino acids in all organisms.
  • Each base in the sequence is read only once in its triplet.
Show clue

There are $64$ possible triplets but only $20$ common amino acids.

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Answer: Most amino acids are coded for by more than one triplet.

The genetic code is universal, non-overlapping and degenerate. 'Degenerate' means multiple triplet codes can code for the same amino acid, providing some protection against mutations.

QUESTION 6 · 3.4.1 · LEVEL 1

Within eukaryotic genes, what are the non-coding sequences of DNA that separate the coding sequences?

  • Introns
  • Exons
  • Multiple repeats
  • Plasmids
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These sequences are 'interrupting' the gene and are removed during splicing.

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Answer: Introns

Within genes, exons code for amino acid sequences and are separated by non-coding sequences called introns.

QUESTION 7 · 3.4.1 · LEVEL 2

A specific sequence of DNA bases reads $TAC$. Due to the non-overlapping nature of the genetic code, how is this sequence read by the cellular machinery?

  • It is read strictly as one distinct triplet, coding for a single amino acid.
  • It is read as three separate triplets: $T, A,$ and $C$.
  • It is read overlapping with adjacent bases to form multiple triplets (e.g., $TA, AC$).
  • It is read continuously until a stop codon acts as a spacer.
Show clue

Non-overlapping means each base is part of only one triplet.

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Answer: It is read strictly as one distinct triplet, coding for a single amino acid.

The genetic code is universal, non-overlapping and degenerate. Non-overlapping means that each base in the sequence is read only once, as part of one specific distinct triplet.

QUESTION 8 · 3.4.1 · LEVEL 2

What structural feature do both mitochondria and chloroplasts share with prokaryotic cells regarding their genetic material?

  • Their DNA is short, circular, and not associated with proteins.
  • Their DNA is long, linear, and bound to histone proteins.
  • Their DNA is composed of RNA rather than deoxyribonucleic acid.
  • Their genetic material is entirely enclosed within a secondary nucleus.
Show clue

These organelles are theorized to have evolved from ancient engulfed bacteria.

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Answer: Their DNA is short, circular, and not associated with proteins.

The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.

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