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