Quick questions on Cellular control and gene expression: mutations, the lac operon, transcription factors and Hox genes - OCR A-Level Biology A

A gene mutation is a change in the base sequence of DNA:

In prokaryotes, genes are controlled in operons: a cluster of structural genes with shared control regions. The lac operon controls lactose metabolism in E. coli:

In eukaryotes, transcription factors are proteins that bind to DNA (at promoters and enhancers) to switch transcription on or off by helping or preventing RNA polymerase binding. Hormones can act through them: for example, a steroid hormone enters a cell and binds a receptor that acts as a transcription factor, switching specific genes on. Gene expression can also be controlled by epigenetic changes such as DNA methylation (silencing genes) and histone modification.

Homeobox genes contain a conserved homeobox sequence coding for a homeodomain that binds DNA, so the proteins act as transcription factors controlling other genes. Hox genes are a group of homeobox genes that control the body plan during development, switching on the genes that determine which structures form where along the body axis (for example where limbs develop). They are highly conserved across very different animals, evidence of common ancestry.

Apoptosis is programmed cell death: the cell breaks down in a controlled way (the cytoskeleton breaks, the membrane blebs, the DNA fragments, and the cell breaks into vesicles that are engulfed by phagocytes), without releasing harmful contents. It is essential in development (for example removing the webbing between fingers) and in removing damaged or infected cells. Too little apoptosis can lead to cancer; too much can cause degenerative disease.

Explain why a substitution mutation may have no effect on the protein produced. [2 marks]

Describe the role of the repressor protein in the lac operon when lactose is absent. [2 marks]

State what is meant by apoptosis. [1 mark]