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 O2 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: 10-26 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