Question

are all genes identical on different cells of multicellular organisms? Think about T and B cell receptors.

Answer 1

• The genetic information in the fragment of DNA or a gene, may be expressed by its ability to encode a specific RNA or protein.
• The product thus generated typically “expresses” the genetic information of the gene. As described by the hypothesis of George Wells Beadle and Edward Lawrie Tatum, “One gene one enzyme”; the product of each genetic expression influences the activity of the cell.
• Thus the process of transcription to produce mRNA and translation to produce protein is defined as the central dogma.

DNA - > mRNA - > Protein

• But it is observed that, the cells which contain similar genome, may have completely different functions. For example, in humans, a nerve cell has a similar genome as that of a lymphocyte. But, a neuron helps in signal transduction, while, lymphocyte is responsible for immune response in the body.
• Hence, this functional variation, in cells having same genome (thus similar genetic code), is attributed to activations and expressions of different genes under different conditions.
• Thus, with same genome, the gene of different cell type, have the ability, to control and alter their genetic expression. This in turn changes the functional ability of each cell.
• Each set of differentiated cell read and produce different RNA subsets.

• The gene comprises of the exons, which are the translated regions and the introns, which are the untranslated regions. However, the introns determine the different products of the gene. The introns generally are removed during splicing. But by variation in the splicing process, different exon sequences might be obtained.
• The process of alternative splicing, the gene products may be modified. In alternate splicing alternative introns may be removed to get a different sequence producing an alternative protein.
• Thus, the introns and exons that are spliced, or the retained sequence after the splicing will produce either of the proteins accordingly.

Example:

• Both B-cells and T-cells originate in bone marrow as lymphoid cells. But by genetic variation, they generate receptor proteins that have different specificity and thus have different functions.