Gene expression is the process through which the information encoded in a gene is used to synthesize a functional gene product, typically a protein. This process involves several key steps, which can be summarized as follows:
-
DNA Structure: Genes are segments of DNA (deoxyribonucleic acid) located on chromosomes within the cell nucleus. DNA is composed of two strands forming a double helix, with sequences of nucleotides (adenine, thymine, cytosine, and guanine) that encode genetic information.
-
Transcription: The first major step in gene expression is transcription, where a specific segment of DNA is copied into messenger RNA (mRNA). This process involves several steps:
- Initiation: RNA polymerase, the enzyme responsible for transcription, binds to a specific region of the gene called the promoter. This region signals the start of the gene.
- Elongation: RNA polymerase then unwinds the DNA strands and synthesizes a complementary RNA strand by adding RNA nucleotides that are complementary to the DNA template strand (A pairs with U, and C pairs with G in RNA).
- Termination: Transcription continues until RNA polymerase reaches a termination signal, at which point it stops synthesizing RNA, and the newly formed mRNA strand is released.
-
RNA Processing (in eukaryotes): Before the mRNA can be translated into a protein, it undergoes several modifications:
- Capping: A methylated cap is added to the 5' end of the mRNA. This helps protect the mRNA from degradation and assists in ribosome binding during translation.
- Polyadenylation: A poly-A tail, consisting of a long stretch of adenine nucleotides, is added to the 3' end of the mRNA, which also protects mRNA and aids in its export from the nucleus.
- Splicing: Introns (non-coding regions) are removed from the pre-mRNA, and the remaining exons (coding regions) are joined together to form the mature mRNA.
-
Translation: The mature mRNA is transported from the nucleus to the cytoplasm, where translation occurs. Translation involves several components:
- Ribosomes: These are the cellular machinery that facilitate the translation of mRNA into proteins. They consist of two subunits (large and small) and can be free-floating in the cytoplasm or attached to the endoplasmic reticulum.
- tRNA (transfer RNA): Each tRNA molecule carries a specific amino acid and has an anticodon that pairs with the corresponding codon on the mRNA. The ribosome facilitates the binding of tRNA to mRNA as it translates the mRNA sequence into a polypeptide chain.
- Initiation: The small ribosomal subunit binds to the mRNA at the start codon (AUG), and the first tRNA, carrying methionine, binds to the start codon.
- Elongation: The ribosome moves along the mRNA, reading its codons and facilitating the binding of complementary tRNA molecules. This process adds the corresponding amino acids to the growing polypeptide chain.
- Termination: When the ribosome encounters a stop codon (UAA, UAG, or UGA), translation stops. The completed polypeptide chain is released, and the ribosomal subunits dissociate.
-
Protein Folding and Post-Translational Modifications: After translation, the newly synthesized polypeptide may undergo further folding into its functional three-dimensional structure, and it may receive post-translational modifications, such as phosphorylation or glycosylation, to become a fully functional protein.
Overall, gene expression represents a vital biological process that converts the genetic code into functional products essential for the growth, development, and functioning of an organism.
1 answer