DNA, genes and chromosomesAQA A-Level Biology: Subtopic test

1. 1
   A student compares the DNA in a bacterium, Escherichia coli, with the DNA in a human liver cell.

   (a)

   Which description applies to the DNA molecule in the bacterium?

   [1 mark]

   • ALong, linear and associated with histones
   • BShort, circular and not associated with histones
   • CLong, circular and associated with histones
   • DShort, linear and not associated with proteins

   (b)

   Which structure in the liver cell also contains DNA of the same type as that in the bacterium?

   [1 mark]

   • ARibosomes
   • BThe Golgi apparatus
   • CLysosomes
   • DMitochondria

   (c)

   Describe how the DNA in the nucleus of the liver cell differs from the DNA in the bacterium.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Human cells contain a gene for the polypeptide β-globin, found on chromosome 11. They also contain genes that are transcribed to make ribosomal RNA (rRNA) and transfer RNA (tRNA). Each of these genes is always found at the same position on its chromosome.

   (a)

   What is the name for the fixed position of a gene on a DNA molecule?

   [1 mark]

   • ALocus
   • BCodon
   • CExon
   • DAllele

   (b)

   Which statement about a gene is correct?

   [1 mark]

   • AA gene is a whole DNA molecule
   • BA gene is a sequence of three bases
   • CA gene may code for a functional RNA as well as for a polypeptide
   • DA gene always codes for a polypeptide

   (c)

   The rRNA genes are transcribed but their products are never translated into polypeptides. Explain why these sequences are still called genes.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A human nuclear gene has three exons and two introns arranged alternately (exon, intron, exon, intron, exon). The exons contain 150, 210 and 90 bases, and the introns contain 400 and 1000 bases. Human cells also contain mitochondria, which have their own DNA.

   (a)

   Calculate the percentage of the bases in this gene that are in exons, and state why only the exons affect the amino acid sequence of the polypeptide.

   [3 marks]

   (b)

   Describe the differences between the DNA in the nucleus and the DNA in a mitochondrion of the same cell.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A geneticist sequenced a eukaryotic gene on a plant chromosome and found that the 3000-base gene contained several stretches of non-coding DNA. Elsewhere on the same chromosome, between genes, she found a short base sequence repeated many times. She also extracted DNA from the nucleus and from the chloroplasts of the plant, and from a bacterium.

   (a)

   Using these findings, explain the meaning of the terms gene, locus, exon and intron, and why only part of the plant's nuclear DNA codes for polypeptides.

   [6 marks]

   (b)

   Compare the organisation of the DNA in the bacterium, in the plant nucleus and in the plant chloroplasts.

   [6 marks]

   Total for question 4: 12 marks

End of questions