The Genome of a cell or organism is the same as that of the entire organism. However, the proteome of an organism is much greater than that of each cell (unless the organism is unicellular). How do you account for the presence of more variation among of proteins than there are in genes?
The simple answer would be "there are more proteins than genes", but I'm more interested in why. Is it sufficient to say that during gene expression a single gene codes for multiple proteins (Alternative Splicing) and ribosomes translating mRNA to polypeptide chains (Posttranslational modification)? Could someone expand more on this?
Answer
There are several concepts to understand:
- Alternative splicing
- From one gene, several proteins can be constructed depending on which introns are spliced.
- Tissue-specific genetic expression
- Genes are differentially expressed (and differentially spliced) depending on the tissue.
- Post-transcriptional modifications
- Those are modifications that are made directly on the proteins (in opposition with alternative splicing which are modifications made on the RNA)
- Structure of the proteins
- Proteins may fold (to what we call primary, secondary, tertiary and quaternary structures) differently in different environment.
Therefore there is variance in proteome between cells although there is no variance in genome between cells (except mutations and red blood cells). Moreover, one gene sequence can give rise to many different proteins due to the processes of alternative splicing and PMT.
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