What Is Gene Expression?

Source - Nature

Gene Expression

Gene expression refers to the process by which genetic information encoded in DNA is used to synthesize proteins or other functional RNA molecules. The expression of a gene is regulated at multiple levels, including transcription, RNA processing, translation, and post-translational modifications.

Transcription

Transcription is the process by which the genetic information encoded in DNA is transcribed into mRNA by RNA polymerase. This process occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. The mRNA molecule contains a copy of the coding sequence of a gene, which specifies the amino acid sequence of the protein.

(Source - Study mind)

The process of transcription begins when RNA polymerase binds to the DNA molecule at the promoter region, which is a specific sequence of nucleotides that signals the start of a gene. RNA polymerase then unwinds the DNA double helix, exposing the template strand. The RNA polymerase reads the template strand from the 3' to 5' direction and synthesizes the complementary RNA molecule in the 5' to 3' direction.

Transcription can be regulated by various factors, including transcription factors and epigenetic modifications, which can influence the accessibility of the DNA to the RNA polymerase.

Post-transcriptional Modification

The post-transcriptional modification also includes alternative splicing, where different combinations of exons can be joined together to form different mRNA isoforms from a single gene, allowing for greater diversity in the proteome.

After transcription, the mRNA molecule undergoes several modifications to produce a functional mRNA that can be translated by the ribosome. The first modification is the addition of a 5' cap, which is a modified guanine nucleotide added to the 5' end of the mRNA molecule. The 5' cap helps in the recognition of the mRNA molecule by the ribosome and protects the mRNA from degradation by exonucleases.

The second modification is the addition of a poly(A) tail at the 3' end of the mRNA molecule. The poly(A) tail is a stretch of adenine nucleotides that helps in the stability of the mRNA molecule and its export to the cytoplasm.

The third modification is splicing, where the non-coding introns are removed, and the exons are joined together to form a mature mRNA. Splicing is mediated by a complex of proteins and small nuclear RNA (snRNA) molecules called the spliceosome.

Translation

Translation is the process by which the mRNA molecule is translated into a polypeptide chain by the ribosome. The ribosome reads the mRNA codons in groups of three, called codons, and matches them with the corresponding amino acids brought by the tRNA molecules. The polypeptide chain grows as each amino acid is added to the previous one in a sequence specified by the mRNA codons.

During translation, the ribosome also interacts with other molecules, including initiation factors, elongation factors, and release factors, which help in the initiation, elongation, and termination of translation.

The process of translation begins when the ribosome binds to the mRNA molecule at the start codon, which is AUG. The tRNA molecule carrying the amino acid methionine binds to the start codon, and the ribosome moves along the mRNA molecule, reading the codons and adding the corresponding amino acids to the growing polypeptide chain.

Protein synthesis can be regulated at various stages, including mRNA stability, translation initiation, and post-translational modifications. These regulatory mechanisms allow cells to respond to changes in their environment and regulate gene expression.

(Source - BioNinja)

Post-translational Modification

After translation, the polypeptide chain undergoes several modifications to form a functional protein. These modifications include folding, glycosylation, phosphorylation, acetylation, and cleavage, among others.

The final structure of a protein can also be influenced by other factors, such as protein-protein interactions, post-translational modifications, and environmental factors, such as pH and temperature.

Protein folding is the process by which the polypeptide chain attains its native conformation, which is essential for its biological function. Protein folding is facilitated by chaperones, which are proteins that help in the correct folding of the polypeptide chain.

Glycosylation involves the addition of sugar molecules to the protein, which can affect its stability and function. Phosphorylation, acetylation, and cleavage are post-translational modifications that can regulate the protein's activity, localization, and stability.

In summary, protein synthesis is a complex process that involves several steps, including transcription, post-transcriptional modification, translation, and post-translational modification. Each step is essential to ensure that the final protein product is functional and performs its intended biological function

Written by - Narayanamanikandan B