Gene Expression

Gene expression is the use of information encoded in DNA to produce a functional RNA or protein. Cells regulate expression so different genes are active at different times and in different cell types.

Transcription

RNA polymerase binds near the start of a gene, locally separates the DNA strands, and uses one strand as a template to synthesize RNA in the 5-prime to 3-prime direction. RNA bases pair complementarily with the template, with uracil used instead of thymine.

RNA processing in eukaryotes

The initial RNA transcript can be processed before translation. Introns are removed and exons are joined by splicing. Alternative splicing can join exons in different combinations, allowing one gene to contribute to more than one mature RNA and protein product.

Translation

A ribosome reads mRNA codons. Transfer RNAs carry specific amino acids and use anticodons to pair with codons. The ribosome forms peptide bonds as it moves along the mRNA, producing a polypeptide that then folds and may be modified.

Regulating transcription

Regulatory proteins can bind promoters, enhancers, silencers, or other DNA control elements. Chromatin structure also matters: tightly packed chromatin is generally less accessible to transcription machinery than open chromatin.

DNA methylation, especially at regulatory regions, is often associated with reduced transcription. Histone acetylation commonly loosens chromatin and is often associated with increased transcription. The effect depends on genomic context and the proteins recruited.

The lac operon

The lac operon in bacteria coordinates genes used for lactose metabolism. When lactose is absent, a repressor can bind the operator and block efficient transcription. When lactose is present, its derivative allolactose binds the repressor and reduces its ability to bind the operator, allowing transcription when other regulatory conditions are favorable.

Steroid hormones and gene expression

Steroid hormones such as testosterone are lipid-soluble and can cross cell membranes. After binding an intracellular receptor, the hormone-receptor complex can interact with DNA regulatory sequences and alter transcription of target genes.