FROM GENE TO PROTEIN: TRANSLATION & MUTATIONS Chapter 17

Recall: Central dogma of molecular biology DNA  RNA  Protein Steps of gene expression Transcription, RNA processing (eukaryotes), Translation

Translation Components necessary: 1. tRNA 2. Ribosome Made of RNA and protein subunits Larger in eukaryotes Anatomy: P site – holds tRNA carrying growing peptide A site – holds tRNA carrying next a.a. in chain E site – exit site for discharged tRNA

Prokaryotes vs. Eukaryotes Transcription and translation coupled in prokaryotes Happen simultaneously DNA is already in cytoplasm Transcription and translation are separate in eukaryotes DNA is in nucleus, ribosomes in cytoplasm mRNA must be edited first

Translation: Initiation mRNA binds to the ribosome Initiator tRNA binds to start codon on mRNA (AUG) 1st a.a. = methionine Large ribosomal subunit binds to complex

Translation: Elongation tRNAs bring in appropriate amino acid to growing chain based on mRNA codon This process continues until a stop codon is reached (UGA, UAA, UAG)

Translation: Termination Peptide synthesis continues until a stop codon is reached Peptide is released Where peptide goes depends on its role

The Genetic Code Made of 3 letter codes: codons (found on mRNA) Table is used to determine which amino acid each codon codes for It is the same in almost all organisms Redundant: more than one codon for some AA’s

Transcription & Translation Summary

When Protein Synthesis Goes Wrong: Gene Mutations Changes to the DNA sequence resulting in production of malfunctioning or nonfunctioning protein. Differ from chromosomal mutations since only single nucleotides are affected.

Types of Gene Mutations Substitution: wrong nucleotide in place Silent – doesn’t change amino acid, protein Missense – changes amino acid, protein Nonsense – changes amino acid to stop codon Insertion or deletion: nucleotide added or removed Frameshift

Substitution: Sickle Cell Anemia Caused by a single nucleotide substitution in one of the polypeptides that makes up hemoglobin (Hgb) Hgb folds incorrectly, causing RBC’s to become sickle shaped They cannot carry O 2 as effectively

Deletion: Cystic Fibrosis Most common mutation that causes CF is the result of a deletion in CFTR gene Mutation causes faulty CFTR protein This protein transports Cl - ions across cell membrane Causes mucus buildup in lungs, digestive tract

Insertion: Huntington’s Disease Caused by a CAG repeat Normal Huntington gene: repeats Mutant gene: >27 repeats Autosomal dominant Causes nervous system degeneration leading to loss of motor function, dementia

Causes of Mutations Mutagens: physical or chemical agents that cause mutations by Acting like a normal nucleotide Causing DNA to be miscopied Causing the cell to produce chemicals that have mutagenic potential--peroxides

Not all mutations are harmful… Increase variation, drives evolution Example: Mutation in gene CCR5 (important in immune function) Caused by a deletion causes HIV resistance in homozygotes; delayed onset of HIV infection in heterozygotes Currently a research study being conducted to genetically modify T-cells so that they have this mutation