Biology Translation Overview

Translation Overview

• Translation is the process of converting mRNA into a protein.
• It involves the machinery of the ribosome and transfer RNA (tRNA).

Central Dogma Analogy

• The process is analogous to a cookbook (DNA), instructions (mRNA), and final product (protein/cookie).
• The analogy shows how DNA codes for mRNA, and how mRNA codes for protein.

mRNA and Proteins

• mRNA is translated into proteins.
• Proteins are composed of amino acids.
• There are 20 different amino acids.

Amino Acids

• Each amino acid has unique properties (size, charge, etc.).
• Amino acids are linked together by peptide bonds.
• The product formed is called a polypeptide.

General Structure of Amino Acid

• An amino acid has an amino group (+H₃N), carboxyl group (COO-), a central carbon atom (α-carbon), and a radical group (R-group/side chain).

Amino Acid Classification

• Amino acids can be classified into nonpolar, polar, and charged groups.

Peptide Bond Formation

• Peptide bonds are formed via a dehydration reaction.
• This reaction removes a water molecule.

Protein Structure Levels

• Proteins have four levels of structure: primary, secondary, tertiary, and quaternary.

Primary Structure

• Linear sequence of amino acids.
• Amino acids are linked by peptide bonds.

Secondary Structure

• Hydrogen bonding between amino acid backbones causes alpha helices and beta-pleated sheets.
• An alpha helix is like a spiral staircase.
• A beta-pleated sheet is like folded paper.

Secondary Structure: Triple Helix

• Three polypeptide chains are intertwined by hydrogen bonds to form a triple helix.
• Often found in collagen.

Tertiary Structure

• Interactions among side chains (R groups) fold the protein into a three-dimensional shape:
  • Hydrophobic interactions between nonpolar amino acids.
  • Hydrophilic interactions between polar amino acids and water.
  • Salt bridges between ionized R groups of basic and acidic amino acids.
  • Hydrogen bonds between polar R groups.
  • Disulfide bonds between cysteine amino acids.

Quaternary Structure

• Multiple polypeptide chains interact to form a functional protein complex.
• Hemoglobin, made up of four polypeptide chains, is an example.

Protein Structure and Function

• The specific shape of a protein directly influences its function.
• Even small changes in the protein's structure can have a significant impact on its activity.
• Prions are misfolded proteins that cause other proteins to adopt their abnormal conformation, leading to disease.

Translation Machinery

• Ribosomes are the sites of protein synthesis.
• They have two subunits (large and small).
• Transfer RNAs (tRNAs) carry specific amino acids to the ribosome to match to mRNA's codons.

Universal Genetic Code

• mRNA uses codons (three nucleotides) to specify amino acids.
• The genetic code is universal, which means the same codons specify the same amino acids in all organisms.
• It contains 64 codons, but only 20 amino acids.
• Many codons specify the same amino acid.

Codon Recognition

• The anticodon on a tRNA molecule binds with the codon on the mRNA.
• This ensures the correct amino acid is added to the growing polypeptide chain.

Wobble Hypothesis

• The third base of a codon can sometimes tolerate variations in base pairings.
• This allows a single tRNA to recognize multiple codons, accounting for why the genetic code is redundant.
• The wobble position is a beneficial feature that ensures accuracy in amino acid incorporation.

Translation Steps: Initiation

• Ribosome assembles on mRNA.
• Initiation factors help assemble the ribosome.
• The ribosome's small subunit binds to the mRNA’s Shine-Dalgarno sequence.
• The first tRNA (carrying methionine) binds to the mRNA's Start codon (AUG). -This tRNA fits into the ribosome's P site.

Translation Steps: Elongation

• Charged tRNAs in the complex, with the help of elongation factors, bind to the A site.
• A peptide bond forms between the amino acids in the A and P sites.
• Translocation occurs: the ribosome moves to the next codon.
• The tRNA in the P site moves to the E site (exits). The tRNA in the A site moves to the P site.

Translation Steps: Termination

• The ribosome encounters a stop codon (UAG, UAA, or UGA).
• Release factors (RFs) bind to the A site instead of a tRNA.
• The polypeptide chain is released.
• The ribosome disassembles.