Gene expression is the process through which genetic information encoded in DNA is translated into functional proteins. This process primarily involves two key stages: transcription and translation. Below are the detailed steps involved in gene expression:
1. DNA Transcription
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Initiation:
- The process begins with the enzyme RNA polymerase binding to a specific region of the DNA called the promoter, located upstream of the gene that is to be expressed.
- Additional transcription factors may assist this binding to ensure that RNA polymerase correctly initiates transcription.
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Elongation:
- Once RNA polymerase is bound, it unwinds the double helix of DNA and synthesizes a complementary RNA strand, using one of the DNA strands as a template.
- This synthesis occurs in the 5' to 3' direction, meaning that RNA polymerase adds RNA nucleotides, which are complementary to the DNA template.
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Termination:
- Transcription continues until RNA polymerase reaches a termination sequence in the DNA. This sequence signals the end of the gene.
- RNA polymerase detaches from the DNA, releasing the newly formed pre-mRNA (in eukaryotes) or mRNA (in prokaryotes).
2. RNA Processing (Eukaryotes only)
Before mRNA can be translated into protein, it undergoes several modifications:
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5' Capping:
- A modified guanine nucleotide is added to the 5' end of the mRNA. This cap protects the mRNA from degradation and assists in ribosome binding during translation.
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Polyadenylation:
- A sequence of adenine nucleotides (poly-A tail) is added to the 3' end of the mRNA. This tail enhances the stability of the mRNA and regulates its translation.
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Splicing:
- Non-coding regions called introns are removed from the pre-mRNA, and the coding regions called exons are joined together. This mature mRNA, now consisting only of exons, is then ready to be translated.
3. Translation
Once the mRNA is processed and transported out of the nucleus (in eukaryotic cells), it enters the cytoplasm where translation takes place.
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Initiation:
- The small subunit of the ribosome binds to the mRNA at the start codon (AUG).
- A tRNA molecule carrying the amino acid methionine recognizes this start codon and binds to it.
- The large subunit of the ribosome then binds, forming a complete ribosome.
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Elongation:
- The ribosome moves along the mRNA, reading codons (three-nucleotide sequences) one at a time.
- For each codon, a corresponding tRNA with a specific amino acid binds to the ribosome.
- The ribosome facilitates the formation of peptide bonds between adjacent amino acids, creating a growing polypeptide chain.
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Termination:
- The process continues until the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA.
- No corresponding tRNA is available for stop codons; instead, a release factor binds to the stop codon, prompting the ribosome to disassemble and release the completed polypeptide chain.
4. Post-Translational Modifications
After translation, proteins often undergo further modifications to become fully functional:
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Folding:
- The polypeptide chain folds into its unique three-dimensional structure, which is crucial for its function. Chaperone proteins may assist in this process.
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Chemical Modifications:
- Proteins can undergo various modifications, such as phosphorylation, glycosylation, and methylation, which can affect their activity, localization, and stability.
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Activation:
- Some proteins are synthesized in an inactive form and require cleavage or other modifications to become active.
Conclusion
In summary, gene expression involves the transcription of DNA into mRNA, processing of the mRNA in eukaryotic cells, translation of mRNA into a polypeptide chain, and subsequent post-translational modifications to ensure proper protein functionality. Each step is meticulously regulated to ensure appropriate gene expression in response to the cell’s needs.
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