Transcription and Translation in Protein Synthesis

Steps of Transcription

  • Definition: Transcription is the process by which the information in a gene's DNA is copied to messenger RNA (mRNA).

  • Enzymes Involved:

    • RNA Polymerase: The primary enzyme that synthesizes RNA by unwinding the DNA helix and synthesizing the corresponding mRNA strand using the DNA template.

  • Steps of Transcription:

    1. Initiation: RNA polymerase binds to the promoter region of the gene.

    2. Elongation: RNA polymerase unwinds the DNA and synthesizes the mRNA strand.

    3. Termination: RNA polymerase reaches a termination signal in the DNA, leading to the release of the newly synthesized mRNA.

Ribosomes and Protein Synthesis

  • Module 11 Lesson 3: Presented by Pallavi Rallapalli.

The Genetic Code

  • Polypeptides: Chains of amino acids that form proteins.

  • Genetic Code: Refers to all of the mRNA codons that carry instructions for synthesizing proteins.

  • Codon: A sequence of three nucleotide bases on the mRNA that codes for a specific amino acid.

How to Read Codons

  • There are 64 possible combinations of mRNA codons due to the variation in nucleotide sequences.

  • Reading Method:

    1. Start from the first base of the codon (the inside).

    2. Move outward to each subsequent base until arriving at the corresponding amino acid.

Start and Stop Codons

  • Start Codon: Every protein synthesis begins with the codon AUG, which codes for the amino acid Methionine.

  • Stop Codons: The synthesis ends when one of the following stop codons is reached:

    • UGA

    • UAA

    • UAG

Wobble Effect

  • Defines the phenomenon that occurs when there is more than one codon for a given amino acid.

  • It allows for mutations in the DNA sequence without necessarily affecting the amino acid sequence of the resulting protein, thus providing redundancy in the genetic code.

Translation

  • Translation: The process by which ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains.

  • Decoding mRNA Codons: Translation interprets the nucleotide language of mRNA into the amino acid language of proteins.

Steps in Translation

  1. Ribosome Attachment: The ribosome attaches to the mRNA strand.

  2. Code Reading: The ribosome reads the mRNA code and signals transfer RNA (tRNA) to bring the corresponding amino acids.

  3. tRNA and Anticodon: Each tRNA carries an anticodon that complements the mRNA codon.

    • Example: If the mRNA codon is AUG, the corresponding tRNA anticodon will be UAC, which brings Methionine to the ribosome.

Post-Translational Modifications

  • Phosphorylation: The addition of phosphate groups to amino acids, which can alter protein function.

  • Cleavage: The removal of specific amino acids from the polypeptide chain to activate the protein.

  • Glycosylation: The addition of carbohydrate chains to amino acids, important for protein stability and function.

  • Lipidation: The attachment of lipid molecules to amino acids, particularly for proteins that will be positioned within cell membranes.