D2.2 Gene expressionIB Biology HL: Revision notes

Gene expression is how information in genes affects the phenotype. The usual stages are transcription, translation and the function of the protein product, such as an enzyme.

Transcription is regulated by proteins that bind to specific base sequences in DNA. RNA polymerase binds the promoter, next to the gene. Transcription factors bind the promoter or distant enhancers to increase (or reduce) transcription.

Translation is also regulated by controlling how long mRNA lasts. In human cells mRNA may persist from minutes up to days before being broken down by nucleases. Short-lived mRNA allows protein production to be switched off quickly.

Epigenesis is the development of patterns of differentiation in the cells of a multicellular organism. Epigenetic changes do not alter DNA base sequences, so they change phenotype but not genotype.

• Genome: all the genes of the cell (the same in almost all body cells).
• Transcriptome: all the mRNA being transcribed in a cell at a given time.
• Proteome: all the proteins produced by a cell at a given time.

No cell expresses all its genes; its pattern of expression determines how it differentiates.

Methylation of cytosine in the DNA of a promoter represses transcription, so the gene downstream is not expressed.

Methylation of amino acids in histones (in nucleosomes) can either repress or activate transcription, depending on which amino acid is methylated.

Phenotypic changes can be passed to daughter cells or offspring without changes to the base sequence if tags such as DNA methylation or histone modification remain in place during mitosis or meiosis.

The environment alters tags: air pollution has been linked to changed methyl tags on DNA, for example in white blood cells. Monozygotic twin studies show twins' tags diverge with age and with different lifestyles.

Most but not all epigenetic tags are removed from the ovum and sperm. Retained tags make some genes' expression depend on the parent of origin. Ligers (lion father) grow huge because they inherit strongly expressed paternal growth genes without the matching maternal restraint; tigons (lion mother) have strong restraint and are not oversized.

Hormone example: a steroid such as oestradiol or testosterone enters the cell, binds a receptor, and the complex acts as a transcription factor, switching on target genes.

Biochemical example: in E. coli, lactose binds the lac repressor, which releases the operator, so genes for lactose metabolism are transcribed only when lactose is present. (Alternatively, tryptophan switches off the genes for its own synthesis.)