Translation: From RNA to Protein

• Translation: 


  Synthesis of Polypeptides from mRNA
• Synthesis of Polypeptides from mRNA


  Translation is the process by which the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids, forming a polypeptide.

  Key points:

  1. Genetic Code:
  2. Codon-Amino Acid Correspondence
  3. Directionality
• Genetic Code:


  • The mRNA sequence is read in groups of three nucleotides (codons)
  • Each codon specifies a particular amino acid or a stop signal
• Codon-Amino Acid Correspondence:


  • 61 codons code for 20 different amino acids
  • 3 stop codons (UAA, UAG, UGA) signal the end of translation
• Directionality:


  • mRNA is read from 5' to 3' end
  • Polypeptide is synthesized from N-terminus to C-terminus
• Translation involves the coordinated action of mRNA, ribosomes, and tRNA
• Messenger RNA (mRNA):


  • Carries the genetic information from DNA to the ribosome
  • Contains start codon (AUG) and stop codons (UAA, UAG, UGA)
  • Binds to the small subunit of the ribosome at the 5' end
•  Ribosomes:


  • Cellular machines where translation occurs
  • Composed of two subunits: small subunit and large subunit
• Large subunit:

  • Contains the peptidyl transferase center for peptide bond formation
  • Has three sites: A (aminoacyl), P (peptidyl), and E (exit) sites
• Small subunit:


  • Binds to mRNA
  • Helps in decoding the genetic message
• Transfer RNA (tRNA):


  • Brings amino acids to the ribosome
  • Has a specific anticodon that matches the codon on mRNA
  • Two tRNAs can bind simultaneously to the large subunit:
  • One in the A site (incoming amino acid)
  • One in the P site (growing polypeptide chain)
• The Translation Process:


  1. Initiation
  2. Elongation
  3. Termination
• Initiation:

  • mRNA binds to the small ribosomal subunit
  • Initiator tRNA (carrying methionine) binds to the start codon (AUG)
  • Large ribosomal subunit joins
• Elongation


  • tRNAs enter the A site, matching codons on mRNA
  • Peptide bonds form between amino acids
  • Ribosome moves along mRNA, tRNAs shift from A to P to E site
• Termination:


  • Stop codon enters A site
  • Release factors trigger release of the completed polypeptide
• Complementary Base Pairing between tRNA and mRNA


  The interaction between tRNA and mRNA is crucial for the accurate translation of genetic information into proteins. This interaction is based on complementary base pairing between codons and anticodons.
• Codons


  • Three-nucleotide sequences on mRNA
  • Specify particular amino acids or stop signals
  • Read in the 5' to 3' direction on mRNA
• Anticodons


  • Three-nucleotide sequences on tRNA
  • Complementary to mRNA codons
  • Located on the anticodon loop of tRNA
• Base Pairing


  • Follows RNA base pairing rules: A-U, G-C
  • Anticodon on tRNA pairs with codon on mRNA in antiparallel orientation
• Features of the Genetic Code

  The genetic code is the set of rules by which information encoded in genetic material is translated into proteins. Triplet Code, Degeneracy & Universality
• The anticodon is complementary and antiparallel to the codon, allowing for precise matching during translation.
• Triplet Code:


  • Each codon consists of three nucleotides
  • Reasons for a triplet code: a) Provides enough combinations (4^3 = 64) to code for all 20 amino acids b) Allows for start and stop signals c) Provides redundancy, which can help minimize the impact of mutations
• Degeneracy:


  • Multiple codons can code for the same amino acid
  • Also known as the "redundancy" of the genetic code
  • Helps protect against the effects of point mutations
  • Example: Six different codons (UUU, UUC, UUA, UUG, CUU, CUC) all code for leucine
• Additional features of the genetic code:


  1. Non-overlapping: Each nucleotide is part of only one codon
  2. Unambiguous: Each codon specifies only one amino acid (or stop signal)
  3. Continuous: Codons are read sequentially without gaps or punctuation
• Using the Genetic Code Table


  The genetic code table is a crucial tool for translating mRNA sequences into amino acid sequences. Here's how to use it effectively:

  1. Read the mRNA sequence from 5' to 3' direction.
  2. Divide the sequence into codons (groups of three nucleotides).
  3. For each codon, find the corresponding amino acid in the table.
  4. Write down the amino acid sequence from N-terminus to C-terminus.
• Ribosome Structure:


  • A site (Aminoacyl site): Accepts incoming charged tRNA
  • P site (Peptidyl site): Holds tRNA with growing polypeptide chain
  • E site (Exit site): Where uncharged tRNA leaves the ribosome
• Stepwise Movement of the Ribosome and Polypeptide Elongation


  The elongation phase of translation involves the stepwise movement of the ribosome along the mRNA and the formation of peptide bonds between amino acids.
• Elongation Process


  a) Codon Recognition:

  b) Peptide Bond Formation:

  c) Translocation:
• Codon Recognition:


  • Charged tRNA enters the A site
  • Anticodon of tRNA base-pairs with the mRNA codon
• Peptide Bond Formation:


  • Peptidyl transferase (part of the large ribosomal subunit) catalyzes peptide bond formation
  • The growing polypeptide chain transfers from the tRNA in the P site to the amino acid on the tRNA in the A site
• Translocation


  • Ribosome moves three nucleotides along the mRNA (one codon)
  • tRNA in P site moves to E site and exits
  • tRNA with growing polypeptide moves from A site to P site
  • Next codon enters A site
• Energy Requirement


  GTP hydrolysis provides energy for tRNA movement and ribosome translocation
• Example of Elongation cycle:

  1. tRNA with Methionine in P site, next codon (e.g., CCU for Proline) in A site
  2. tRNA carrying Proline enters A site
  3. Peptide bond forms between Methionine and Proline
  4. Ribosome moves, empty tRNA exits from E site, Met-Pro chain now in P site
  5. Next codon enters A site, cycle repeats
• Mutations that Change Protein Structure


  Mutations are changes in the DNA sequence that can alter the resulting protein structure. These changes can have various effects on protein function, ranging from negligible to severe.
• Types of mutations that can affect protein structure:


  1. Point Mutations: Changes in a single nucleotide
  2. Insertions: Addition of one or more nucleotides
  3. Deletions: Removal of one or more nucleotides
  4. Frameshift Mutations: Insertions or deletions that alter the reading frame
• Point Mutations


  Point mutations can be categorized into three types based on their effect on the protein:

  1. Silent Mutations
  2. Missense Mutations
  3. Nonsense Mutations
• Silent Mutations:


  • No change in amino acid (due to genetic code degeneracy)
  • Usually have no effect on protein structure or function
• Missense Mutations:


  • Change one amino acid to another
  • Can have varying effects on protein structure and function
• Nonsense Mutations


  • Create a premature stop codon
  • Result in a truncated protein
• Effects of mutations on protein structure:


  1. Primary Structure: Direct change in amino acid sequence
  2. Secondary Structure: Can disrupt alpha helices or beta sheets
  3. Tertiary Structure: May alter protein folding and 3D shape
  4. Quaternary Structure: Can affect interactions between protein subunits