Mutations + High Yield Organisms

🧬 Mutations — Core Types

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What is a point mutation?

A change of a single nucleotide pair in DNA.

What is a transition mutation?

Substitution of a purine for another purine (A↔G) or pyrimidine for another pyrimidine (C↔T).

What is a transversion mutation?

Substitution of a purine for a pyrimidine or vice versa.

What is a silent mutation?

A base change that does not alter the amino acid sequence.

What is a missense mutation?

A base change that substitutes one amino acid for another.

What is a nonsense mutation?

A base change that converts a codon to a stop codon, truncating the protein.

What is a frameshift mutation?

Insertion or deletion of bases not in multiples of three, shifting the reading frame.

What is the effect of a 3-bp insertion or deletion?

Adds or removes one amino acid but maintains the reading frame.

What is a neutral mutation?

A missense mutation that changes the amino acid without altering protein function.

🧬 Sources of Spontaneous Mutation

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Sources

What is a tautomeric shift?

Spontaneous isomerization of a base to an alternative form that pairs incorrectly (A–C or G–T).

What is deamination?

Removal of an amino group (e.g., C→U), causing C:G → T:A transition if unrepaired.

What enzyme removes uracil formed by cytosine deamination?

Uracil-DNA glycosylase.

What is depurination?

Spontaneous loss of a purine base, leaving an abasic site.

What happens if depurination is unrepaired?

A random base is inserted opposite the missing site during replication.

What is oxidative DNA damage?

Reactive oxygen species convert guanine to 8-oxo-G, which mispairs with adenine.

What is strand slippage?

DNA polymerase slips on repetitive sequences, leading to insertions or deletions.

What human diseases result from strand slippage?

Trinucleotide repeat expansion disorders such as Huntington’s disease (CAG repeats).

🧪 Mutagenesis & Repair

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What is a mutagen?

Any chemical or physical agent that increases mutation rate.

How do base analogs cause mutations?

They resemble normal bases but pair incorrectly (e.g., 5-bromouracil pairs with A or G).

How do alkylating agents cause mutations?

Add alkyl groups to bases, leading to mispairing or strand breaks.

What is the purpose of DNA repair pathways?

Detect and correct DNA damage before it becomes permanent.

What enzyme proofreads newly synthesized DNA?

DNA polymerase III via 3′→5′ exonuclease activity.

What is mismatch repair?

System that identifies and corrects mismatched bases after replication.

How are newly synthesized strands recognized in bacteria?

The parental strand is methylated; the new strand is not (MutH–MutL–MutS system).

What is base-excision repair?

Glycosylase removes the damaged base, AP endonuclease cuts the backbone, DNA polymerase fills the gap, and ligase seals it.

What is nucleotide-excision repair?

Removes bulky lesions such as thymine dimers by excising a short single-stranded DNA segment.

🧠 High-Yield Concepts & Mechanisms

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Why is DNA replication semi-discontinuous?

Because the lagging strand is synthesized in Okazaki fragments.

What enzyme introduces negative supercoils?

DNA gyrase (topoisomerase II).

Which enzyme seals nicks after primer removal?

DNA ligase.

What is the function of the σ-factor?

Recognizes promoter regions (–10 and –35) and initiates transcription.

What sequence aligns the start codon in bacteria?

The Shine–Dalgarno sequence.

What molecule provides the energy for tRNA charging?

ATP.

Which elongation factors use GTP?

EF-Tu and EF-G.

How many high-energy bonds are used per amino acid added?

Approximately four (2 ATP + 2 GTP).

What ensures the correct amino acid is attached to a tRNA?

Aminoacyl-tRNA synthetase proofreading.

What is wobble base pairing?

Flexible pairing at the 3rd codon position allowing one tRNA to read multiple codons.

Why is telomerase needed in eukaryotes?

It extends chromosome ends to solve the lagging-strand end-replication problem.

What kind of enzyme is telomerase?

A reverse transcriptase that uses an RNA template.

What is coupled transcription-translation?

Process in prokaryotes where translation begins on mRNA still being transcribed.

What are polysomes?

Multiple ribosomes translating a single mRNA molecule simultaneously.

How do tautomeric shifts cause mutations?

They lead to transient mispairing during replication.

What causes Huntington’s disease at the DNA level?

Expansion of CAG trinucleotide repeats through strand slippage.

What is the overall flow of genetic information?

DNA → RNA → Protein (Central Dogma).