Gene expression
This topic focuses on the concept of gene expression. Remember that this is the conversion of genes to proteins to express the gene's coded characteristics. In this, three terms are important to differentiate:
- Genome - the collection of an organism's genes.
- Transcriptome - the collection of an organism's transcribed genes.
- Proteome - the collection of an organisms's proteins.
These may all seem equivalent, but this is not the case. Not all genes are transcribed, and not all proteins are produced from their transcribed genes. Thus, the end phenotype exhibited by a person only matches their proteome, not their transcriptome or genome. The changes from genome to proteome result from changes in gene expression. A person's patterns of gene expression is called their epigenome, whilst the study of epigenomes is called epigenetics.
Nucleosomes
This gene expression is first controlled by nucleosomes, where acetylation, methylation or phosphorylation of the histones can coil or uncoil the nucleosomes.
- Coiling prevents transcription, resulting in decreased gene expression.
- Uncoiling enables transcription, resulting in increased gene expression.
Transcription
The next opportunity to control gene expression is during the initiation of transcription, the process of which we have previously covered. Here, it was discussed that transcription is regulated by promoter sequences, enhancer or repressor proteins, and transcription factors. As a result, whether transcription is initiated or not determines whether a protein is eventually produced.
Once mRNA is produced and released into the cytoplasm, ribosomes can indefinitely translate the strand to produce an infinite number of polypeptides. As this is not desirable, it is prevented by two mechanisms:
- mRNA is stored for translation when it is desired.
- Following translation to the desired output, mRNA is degraded by nuclease enzymes.
This thus regulates gene expression at the last possible moment by controlling the degree to which it occurs.