Construct an explanation of how mutations in DNA can change proteins and traits, and may be harmful, beneficial, or neutral (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).

What this topic is asking

The Tennessee LS3 standards ask you to explain how mutations in DNA can change proteins and traits. For the Biology I EOC that means knowing what a mutation is, the basic types (substitution, insertion, deletion), how a change in the DNA sequence flows through to the protein, why a mutation can be harmful, beneficial, or neutral, and how mutations are the ultimate source of new genetic variation for evolution. Items often connect mutations forward to natural selection.

What a mutation is

Because the order of bases is the genetic code, changing the order can change the message, and therefore the protein the gene builds. This is the link back to DNA structure and protein synthesis: a mutation alters the code that translation reads.

The types of point mutation

The three common small (point) mutations are:

• Substitution. One base is swapped for another. This changes at most one codon, so it often changes only one amino acid (and sometimes none, because several codons code for the same amino acid).
• Insertion. An extra base is added. This shifts the reading frame, so every codon after the insertion is changed.
• Deletion. A base is removed. Like an insertion, this shifts the reading frame and changes every codon after it.

From mutation to protein to trait

The chain the EOC tests is: a mutation changes the DNA sequence, which can change the mRNA codons, which can change the amino-acid sequence, which can change the protein's shape, which can change its function, which can change the organism's trait. Because a protein's job depends on its shape (the macromolecules standard), even a single amino-acid change can disable a protein, as in sickle-cell disease, where one substitution changes hemoglobin.

Harmful, beneficial, or neutral

A mutation is not automatically "bad." Its effect depends on what it does to the protein and on the environment:

• Harmful. The protein works less well or not at all, reducing survival or causing a disorder.
• Beneficial. The new trait improves survival or reproduction in that environment (for example, antibiotic resistance in bacteria, or a coloration that camouflages an animal).
• Neutral. The mutation has no noticeable effect on the protein or on survival (for example, a substitution that codes for the same amino acid).

Crucially, whether a mutation is beneficial or harmful can depend on the environment: the same trait can help in one setting and hurt in another. This is the bridge to natural selection.

Mutations as the source of variation

Mutations are the only source of genuinely new alleles. Meiosis and sexual reproduction shuffle existing alleles into new combinations, but they do not create new ones; mutation does. This new variation is the raw material on which natural selection acts (the LS4 standards), so without mutation there would be no new variation for evolution to work with. Only mutations in gamete-forming cells are passed to offspring; a mutation in a body (somatic) cell affects only that individual.

Try this

Q1. State the three types of point mutation and which two cause a frameshift. [2]

• Cue. Substitution, insertion, and deletion; insertion and deletion cause a frameshift.

Q2. Explain why mutations are important for evolution. [2]

• Cue. Mutations are the only source of new alleles (new variation); natural selection then acts on that variation, so mutations supply the raw material for evolution.