Polypeptide Synthesis

Polypeptide synthesis is vital for life as it leads to the production of proteins which are essential for a range of cellular functions, including structural, catalytic, signalling, and transport roles. This note will explore the full scope of polypeptide synthesis, addressing transcription, translation, and the roles of mRNA and tRNA.

Central Dogma of Molecular Biology

• Flow of Information: DNA → RNA → Protein.
• Importance: This process converts genetic codes into functional proteins, leading to the expression of traits.

Key Terminologies

• Transcription:
  • Definition: Replicating DNA to form complementary RNA (mRNA).
• Translation:
  • Definition: Interpreting mRNA into a polypeptide chain.
• Polypeptides and Proteins: These are chains of amino acids that fold into functional proteins crucial for cellular processes.

Introduction to Transcription

Transcription is a enzymatic process that transforms DNA into mRNA, enabling gene expression and protein formation.

• RNA Polymerase: This enzyme initiates transcription by attaching to promoter sites on DNA.
• Transcription Factors: These proteins aid in the correct binding of RNA polymerase to DNA, ensuring accurate transcription initiation.

Key Steps

• Initiation: RNA polymerase binds and unwinds the DNA strand.
• Elongation: RNA nucleotides pair with the DNA template, synthesised in a 5' to 3' direction.

• Termination: RNA polymerase meets a terminator sequence, completing RNA synthesis.

Initial RNA Processing:

• Splicing: This involves the removal of non-coding introns.
• Poly-A Tail Addition: This stabilises mRNA for cellular export.

Introduction to Translation Process

Translation occurs at the ribosomal location, converting mRNA sequences into amino acids through the sequential stages of Initiation, Elongation, and Termination.

Initiation

• The assembly of small and large ribosomal subunits on mRNA occurs at the Start Codon (AUG).
• Initiation Complex: This complex is formed with the aid of initiation factors.

Diagram:

Elongation

• Aminoacyl-tRNA carries amino acids to the ribosomal A site for Codon-Anticodon Pairing.
• Peptide Bonds are established between successive amino acids.

Impact: Errors can lead to defective proteins.

Termination

• Stop Codons (UAA, UAG, UGA) signal the conclusion of translation.
• Release Factors aid in the disassembly of the translational complex.

Roles of mRNA and tRNA

mRNA guides the amino acid sequence, while tRNA decodes the sequence at the ribosome:

• Anticodon Loop: Responsible for ensuring the proper match with mRNA.

Diagrams:

Protein Structure and Function

Overview

• Primary Structure: A linear array of amino acids. Example: Insulin.
• Secondary Structure: Examples include Alpha-helices and Beta-pleated sheets.
• Tertiary Structure: Three-dimensional arrangement of polypeptides. Example: Myoglobin.
• Quaternary Structure: Complex structure with multiple polypeptides. Example: Haemoglobin.

Diagram:

Protein Folding

Chaperone proteins contribute to proper folding with guidance to prevent conditions like Alzheimer's.

Examining Gene Expression Regulation

Gene Expression: The conversion of gene information into protein.

Role of Transcription Factors

• Activators and Repressors: These proteins regulate gene expression.

Mutations

• Types:
  • Nonsense: Introduces premature stops.
  • Missense: Results in amino acid substitution.

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Polypeptide synthesis is a cornerstone of cellular biology. A thorough comprehension of these processes is essential for understanding how genetic information is translated into functional proteins.