What is the difference between a missense and a nonsense mutation?

A missense mutation changes a codon so that it specifies a different amino acid, whereas a nonsense mutation changes a codon into a stop signal, truncating the protein prematurely.

Are mutations always harmful?

No. Many mutations are neutral and have no detectable effect, some are harmful, and a few are beneficial; it is this variation, including the rare advantageous change, that provides the raw material for evolution.

Mutation and Recombination

Mutation introduces new variants into a sequence and recombination shuffles existing ones, and together they generate the genetic variation on which inheritance and evolution depend.

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Definition

A mutation is a heritable change in the nucleotide sequence of DNA, and recombination is the exchange or reassortment of DNA segments between molecules, both of which alter the genetic information passed to descendants.

Scope

This topic covers the classes of mutation, including point substitutions, insertions and deletions, frameshifts, and chromosomal rearrangements, the distinction between spontaneous and induced mutation, mutagens and the Ames test, the random nature of mutation as shown by fluctuation analysis, and homologous recombination as a mechanism for exchanging and repairing DNA. It treats the origin and exchange of sequence variants; the patterns of their inheritance and frequency in populations are covered elsewhere.

Core questions

What are the main classes of mutation and how do they affect a gene's product?
How do spontaneous and induced mutations differ in origin and rate?
How did fluctuation analysis show that mutations arise at random rather than in response to selection?
How does homologous recombination exchange genetic material and repair breaks?

Key concepts

Point mutations: missense, nonsense, silent
Insertions, deletions, and frameshifts
Spontaneous versus induced mutation and mutagens
The Luria-Delbrück fluctuation test
Homologous recombination and crossing over

Mechanisms

Mutations arise from replication errors, spontaneous chemical changes such as deamination, and damage by radiation or chemical mutagens; recombination proceeds through the alignment of homologous sequences, strand exchange, and resolution of the resulting joint molecules, a process that also serves to repair double-strand breaks.

Clinical relevance

Mutation underlies inherited disease and the somatic changes that drive cancer, mutagen testing informs assessment of chemical and radiation hazards, and recombination is exploited in gene targeting and CRISPR-based genome editing through homology-directed repair.

History

Muller demonstrated in 1927 that X-rays induce mutations, Luria and Delbrück's 1943 fluctuation test showed that bacterial mutations arise spontaneously and independently of selection, and McClintock's discovery of transposable elements revealed that genomes can rearrange themselves.

Key figures

Hermann Muller
Salvador Luria
Max Delbrück
Barbara McClintock

Seminal works

lutherbach1943

Frequently asked questions

What is the difference between a missense and a nonsense mutation?: A missense mutation changes a codon so that it specifies a different amino acid, whereas a nonsense mutation changes a codon into a stop signal, truncating the protein prematurely.
Are mutations always harmful?: No. Many mutations are neutral and have no detectable effect, some are harmful, and a few are beneficial; it is this variation, including the rare advantageous change, that provides the raw material for evolution.