Lecture 13: Translation and Mutations - High Yield Review

Translation

• Translation involves using RNA produced during transcription to produce proteins, which are made up of amino acids.
• Occurs on free ribosomes in the cytosol or on the rough endoplasmic reticulum (RER).
• Components:
  • Messenger RNA (mRNA) produced during transcription.
  • Transfer RNAs (tRNAs) also produced via transcription.
  • Ribosomes (composed of ribosomal RNA, rRNA, also from transcription).
  • Protein factors (instead of transcription factors).

Messenger RNA (mRNA)

• Read codon by codon, where each codon is three bases/nucleotides.
• Each codon codes for an amino acid (AA).
• Start codon (AUG) codes for methionine.
• Stop codons (UGA, UAG, UAA) do not code for an amino acid; they signal the machinery to stop.

Transfer RNA (tRNA)

• Carries amino acids.
• Recognizes codons on the mRNA through codon-anticodon interactions.
• Interacts with ribosomes.
• Adapter molecules that attach amino acids to the tRNA using aminoacyl tRNA synthetase enzymes.
• Aminoacyl tRNA synthetases use ATP to provide energy for adding the amino acid to the tRNA.

Ribosomes

• Workbenches of protein synthesis.
• Hold the mRNA in place.
• Have sites for tRNA association to produce polypeptides.

Phases of Translation

• Initiation
• Elongation
• Termination

Initiation

1. Small subunit of the ribosome identifies and binds to the Kozak sequence (in eukaryotes), which is upstream from the start codon (AUG).
2. Initiation factor proteins (IFs) are involved.
3. The small subunit moves to find the start codon (AUG).
4. tRNA carrying methionine binds to the start codon via codon-anticodon interactions.
   • If the codon is AUG, the anticodon on the tRNA will be UAC.
5. The large subunit of the ribosome joins, forming the E, P, and A sites.

Elongation

1. The next tRNA enters the A site, carrying the appropriate amino acid, facilitated by codon-anticodon interactions.
2. A peptide bond forms between the amino acid on the tRNA in the P site and the amino acid on the tRNA in the A site.
   • The ribosome has peptidyl transferase activity, which catalyzes the formation of the peptide bond.
3. The ribosome moves over one codon; everything shifts over.
4. The tRNA in the A site moves to the P site.
5. The tRNA in the P site moves to the E site.
6. A new tRNA bearing the appropriate amino acid enters the A site.
7. This process continues until a stop codon is reached.

Termination

1. A stop codon enters the A site; it does not code for an amino acid but codes for a protein release factor.
2. Everything disassociates.
   • The ribosome comes apart.
   • The protein disassociates from the tRNA.
   • The mRNA is eventually degraded if another ribosome isn't translating it.
3. The final product is a protein.

Post-Translational Modifications

• Occur after translation.
• Include:
  • Hydroxylation: Addition of hydroxyl groups (OH).
  • Methylation: Attachment of a methyl group (CH3).
  • Acetylation: Attachment of a two-carbon molecule called an acetyl group.
  • Phosphorylation: Attachment of phosphate groups.
  • Glycosylation: Attachment of a sugar (e.g., glucose).
  • Lipidation: Attachment of a lipid (e.g., a fatty acid).

Mutations

• Include chromosomal mutations

Types of Cells Affected

• Somatic mutations: Affect body cells (e.g., skin cells).
  • Only affect daughter cells of the mutated body cell.
• Germline mutations: Occur in gametes (egg or sperm).
  • Affect offspring.

Causes of Mutations

• Spontaneous mutations: Happen spontaneously due to errors in replication or cell division (chromosomal segregation).
• Induced mutations: Result from external agents like radiation or chemical mutagens.

Point Mutations

• Affect a single nucleotide.
• Types:
  • Frameshift mutations: Addition or subtraction of a single nucleotide, affecting the reading frame.
  • Substitution mutations: Inserting the wrong base.
    • Silent: The amino acid produced is the same due to redundancy in the genetic code; no effect on the protein.
    • Missense: A different amino acid is produced; may negatively affect the protein.
    • Nonsense: Produces a stop codon, stopping translation and resulting in a shorter, non-functional protein.

Trinucleotide Repeat Expansions

• Three bases repeat over and over, expanding with each round of cellular replication.
• Due to errors in DNA replication, forming a slippage loop.
• Associated with progressive neuromuscular skeletal diseases, such as:
  • Fragile X syndrome
  • Huntington's disease
  • Myotonic dystrophy

Splice Site Mutations

• Affect the borders of introns, where splicing occurs.
• Can cause introns to remain in the mRNA transcript, which can be problematic during translation.

Chromosomal Mutations

• Occur during cell division.
  • Can be due to issues with crossing over or chromosomal separation.
• Types:
  • Deletions: Missing piece of a chromosome.
  • Duplications: Piece of one chromosome is duplicated onto another.
  • Inversions: Piece of a chromosome breaks off and reconnects upside down.
  • Translocations: Piece of a chromosome moves to another chromosome; can be reciprocal or non-reciprocal.

Nondisjunction

• Due to issues with chromosomal separation during anaphase.
• Results in aneuploidy (too much or not enough genetic information).
  • Monosomy: Missing an entire chromosome
  • Trisomy: An extra copy of a chromosome