Stem cells, differentiation and variation

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   All the cells in an organism contain the same genes, yet a red blood cell precursor and a muscle cell have very different structures.

   Explain how differential gene expression leads to cell specialisation.

   Mark scheme

   • Some genes are switched on (activated) and others switched off in each cell type (1)
   • Transcription factors bind to DNA and control which genes are transcribed (1)
   • Only the active genes are transcribed into mRNA and translated (1)
   • So only certain proteins are made, and these determine the structure and function of the cell (1)
2. Question 23 marks

   Explain how DNA methylation can prevent a gene from being expressed.

   Mark scheme

   • Methyl groups are added to cytosine bases in the DNA (often in the promoter region) (1)
   • This prevents transcription factors / RNA polymerase from binding (1)
   • So the gene is not transcribed and no mRNA (or protein) is produced (1)