Gene expression is the process by which information encoded in a gene is used to synthesize a functional gene product, typically a protein. The process can be summarized in several key steps:
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Transcription:
- Initiation: The enzyme RNA polymerase binds to the promoter region of a gene on the DNA, unwinding the DNA double helix to expose the coding sequence.
- Elongation: RNA polymerase synthesizes a single strand of messenger RNA (mRNA) by adding RNA nucleotides that are complementary to the DNA template strand.
- Termination: RNA polymerase continues until it reaches a termination sequence, at which point it detaches from the DNA and releases the newly formed mRNA transcript.
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RNA Processing (in eukaryotes):
- Capping: A modified guanine nucleotide is added to the 5' end of the mRNA, which helps protect it from degradation and assists in ribosome binding during translation.
- Polyadenylation: A tail of adenine nucleotides (poly-A tail) is added to the 3' end of the mRNA for stability and export from the nucleus.
- Splicing: Introns (non-coding regions) are removed from the mRNA, and exons (coding regions) are joined together to produce a mature mRNA molecule that will be translated.
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Translation:
- Initiation: The mature mRNA is transported to the ribosome, where it binds to the small ribosomal subunit. The first tRNA, carrying the amino acid methionine, binds to the start codon (AUG) on the mRNA.
- Elongation: Ribosomes facilitate the binding of tRNA molecules to the mRNA strand, where the anticodon of tRNA pairs with the corresponding mRNA codon. Amino acids are linked together in a growing polypeptide chain.
- Termination: When a stop codon (UAA, UAG, UGA) on the mRNA is reached, the ribosome releases the finished polypeptide chain, which then folds into a functional protein.
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Post-Translational Modifications (if applicable):
- The newly synthesized polypeptide may undergo various modifications (e.g., phosphorylation, glycosylation, methylation) to become fully functional. These modifications can affect the protein's activity, localization, and stability.
In summary, gene expression involves the transcription of DNA into mRNA, processing of the mRNA, translation of the mRNA into a polypeptide at the ribosome, and subsequent modifications that result in a functional protein. This process is tightly regulated and crucial for cellular function and organismal development.