DNA, genes and chromosomesAQA A-Level Biology: Mind map

Exam questions on DNA, genes and chromosomes

1. A student compares the DNA in a bacterium, Escherichia coli, with the DNA in a human liver cell.
   Describe how the DNA in the nucleus of the liver cell differs from the DNA in the bacterium.2 marks
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.
   The rRNA genes are transcribed but their products are never translated into polypeptides. Explain why these sequences are still called genes.2 marks
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.
   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