Mutations & Gene Regulation

You’ll be able to

Distinguish silent, missense, nonsense, and frameshift mutations by their effects
Explain how the lac operon switches bacterial genes on and off
Describe eukaryotic gene regulation and epigenetic modification of chromatin

When the sequence changes

A mutation is a change in the DNA sequence, and its impact depends on what it does to the codons. A point mutation swaps a single base (a substitution) and comes in three flavors: silent (the new codon still codes for the same amino acid — thanks to the redundant code), missense (the codon now codes for a different amino acid), and nonsense (the codon becomes a premature stop, truncating the protein). An insertion or deletion of one or two bases causes a frameshift: because codons are read in threes, every codon downstream is shifted and rewritten, usually wrecking the entire protein.

Mutation effects at a glance

silent = same aa · missense = different aa · nonsense = premature stop · frameshift = whole downstream frame altered

Substitutions cause silent/missense/nonsense; insertions and deletions cause frameshifts.

Prokaryotic regulation: the lac operon

Bacteria save energy by making enzymes only when needed, and they cluster related genes into an operon — a promoter, an operator (a switch), and the genes, all controlled together. In the lac operon, genes for digesting lactose are normally off: a repressor protein binds the operator and blocks RNA polymerase. When lactose is present, it binds the repressor and changes its shape so it falls off the operator — an inducible operon that switches on only when its substrate is around. (The trp operon works the opposite way — it is normally on and switches off when tryptophan is abundant.)

Eukaryotic regulation and epigenetics

Eukaryotes regulate genes at many points, especially at transcription. Transcription factors bind promoters and distant enhancer sequences to help or hinder RNA polymerase. Access to the DNA itself is also controlled by chromatin packing: DNA methylation (adding methyl groups) generally silences genes by tightening the chromatin, while histone acetylation loosens it and activates transcription. These chemical tags are epigenetic — they change gene expression without altering the DNA sequence, and some can even be inherited by daughter cells.

Worked example

An mRNA codon that normally reads 5′–U G G–3′ (tryptophan) is changed by a single base substitution to 5′–U G A–3′. Classify this mutation and predict its effect on the protein.

1.Only one base changed (G → A in the third position), so this is a point mutation — a substitution.
2.The original codon UGG codes for tryptophan, but the new codon UGA is a stop codon, which codes for no amino acid.
3.Introducing a premature stop codon is a nonsense mutation: the ribosome halts early, producing a shortened, usually non-functional protein.

Answer: It is a nonsense mutation — the substitution turns the tryptophan codon (UGG) into a premature stop codon (UGA), truncating the protein.

Checkpoint

A substitution changes an mRNA codon from 5′–G A A–3′ to 5′–G A G–3′. Both codons specify glutamate. What type of mutation is this?

Watch out

A single-base insertion or deletion is usually far more damaging than a substitution: it causes a frameshift that scrambles every codon downstream. A substitution changes at most one codon; a frameshift rewrites the whole rest of the protein.

Checkpoint

In the lac operon, what happens when lactose becomes available to the bacterium?

On the exam

Nail the epigenetics distinction: methylation and histone modification change whether a gene is expressed, not the DNA sequence. If a question describes a heritable change in expression with an unchanged sequence, the answer is epigenetic regulation.

Answer the 2 checkpoints as you read.