Transcription and Translation - Protein Synthesis From DNA - Biology

Overview of Transcription and Translation

• Transcription and translation are two essential processes for gene expression in cells.

Transcription

• Definition: The process of converting DNA into mRNA (messenger RNA).

• Steps: Transcription occurs in three main stages:

  • Initiation:

    • RNA polymerase binds to the promoter region of DNA, specifically the TATA box (located 25 nucleotides upstream).

  • Elongation:

    • RNA polymerase separates DNA strands and synthesizes mRNA by adding nucleotides from the 5' end to the 3' end.

    • The template strand (anti-sense strand) is used for synthesis, while the non-template strand (sense strand) is not used.

    • Differences: Uracil (U) replaces thymine (T) in mRNA.

  • Termination:

    • RNA polymerase and the mRNA strand detach from the DNA after the transcription is complete.

    • Poly A polymerase caps the 3' end with a poly A tail, and a cap is added to the 5' end for protection against degradation.

    • The final product is pre-mRNA which contains both:

      • Introns: Non-coding sequences that are removed during RNA splicing.

      • Exons: Coding sequences that remain and are used for protein synthesis.

Translation

• Definition: The process of using mRNA to build proteins.

• Steps: Similar to transcription, translation also involves three stages:

  • Initiation:

    • Begins with the start codon (AUG), corresponding to the anticodon UAC on the tRNA, which carries the amino acid methionine.

    • The tRNA enters the ribosome at the P site (peptidyl site).

  • Elongation:

    • Another tRNA enters the A site, pairing its anticodon with the corresponding mRNA codon.

    • A covalent bond forms between adjacent amino acids, extending the polypeptide chain.

    • The tRNA in the P site moves to the E site (exit site) and is released.

  • Termination:

    • Occurs when a stop codon (UAA, UAG, UGA) is encountered, triggering the entry of a release factor.

    • This prompts the disassembly of the ribosome complex and releases the newly synthesized protein.

• After translation, proteins are sent to the Golgi body for processing and modification, involving folding into specific shapes to perform targeted functions.