MoBio Overview of Gene Expression Chapter 4

An organism may contain many types of somatic cells, each with distinct shape and function. However, they all have the same genome. The genes in a genome do not have any effect on cellular functions until they are expressed. Different types of cells express different sets of genes, thereby exhibiting various shapes and functions.

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Figure 4-A-1. Essential steps involved in the expression of protein genes.

Gene expression means the production of a protein or a functional RNA from its gene. Several steps are required:

Transcription: A DNA strand is used as a template to synthesize a complementary RNA strand, which is called the primary transcript.

RNA processing: This step involves modifications of the primary transcript to generate a mature mRNA (for protein genes) or a functional tRNA or rRNA.

For RNA genes (tRNA and rRNA), the expression is complete after a functional tRNA or rRNA is generated. However, protein genes require additional steps:

Nuclear transport: mRNA has to be transported from the nucleus to the cytoplasm for protein synthesis.

Protein synthesis: In the cytoplasm, mRNA binds to ribosomes, which can synthesize a polypeptide based on the sequence of mRNA.

The central dogma

According to the above process, the flow of genetic information is in the following direction:

DNA > RNA > Protein.

This rule was dubbed the central dogma, because it was thought that the same principle would apply to all organisms. However, we now know that for RNA viruses, the flow of genetic information starts from RNA.