MUTATIONS CAN CHANGE THE MEANING

OF GENES(last subtitle of chapter 11 - THE LANGUAGE OF

LIFE)

2

DNA and the Language of Life (chapter 11 of the book)

•MUTATIONS CAN CHANGE THE MEANING OF GENES

HOW MUTATIONS AFFECT GENES

Two general categories:

•Base substitutions

•Base insertions or deletions

HOW MUTATIONS AFFECT GENES

 

Occasionally, a base substitution causes no change to a protein, but sometimes it results in a change that affects the function of a protein, sometimes drastically (as in the base substitution leading to sickle cell disease).

Base substitutions

Is the replacement of one base or nucleotide with another

HOW MUTATIONS AFFECT GENES

 

Base substitutionsSome base substitutions show no effect on the protein that is made:

In the genetic code, several amino acids have more than one codon.

Both GAA and GAG code for the same amino acid (Glu).

If a mutation to DNA causes the mRNA codon to change from GAA to GAG, no change in the protein product would result.

This is a "silent mutation."

HOW MUTATIONS AFFECT GENES

• MUTATIONS CAN CHANGE THE MEANING OF GENES

HOW MUTATIONS AFFECT GENES

 

Base insertions or deletions

Because mRNA is read as a series of triplets, adding or subtracting nucleotides may alter the triplet groupings of the genetic message.

Therefore, all the nucleotides that are "downstream" of the mutation will be regrouped into different codons.

These new codons code for new amino acids. The result will be a different, and probably nonworking, protein.

Insertion or deletion of one or more nucleotides in a gene is usually more disastrous than the effects of a base substitution.

HOW MUTATIONS AFFECT GENES

• MUTATIONS CAN CHANGE THE MEANING OF GENES

WHAT CAUSES MUTATIONS?

• Mutations may occur when errors are made during DNA replication, or when errors are made during chromosome crossovers in meiosis.

Physical or chemical agents that cause mutations • Physical mutagen: high-energy radiation (such as X-

rays and ultraviolet light) • Chemical mutagen:chemicals that are similar to

normal DNA bases but cause incorrect base-pairing when incorporated into DNA

MUTAGENS:

WHAT CAUSES MUTATIONS?

• Although mutations are often harmful, they can alter a protein in a way that may be beneficial in certain environments

Example: a genetic mutation is responsible for the dark color seen in some females of the tiger swallowtail butterfly species, Papilio glaucus. The mutation may be advantageous when the environment includes a related species, the poisonous black swallowtail Battus philenor. Predators that avoid eating the black swallowtail may also avoid eating its mimic, the dark form of P. glaucus.

WHAT CAUSES MUTATIONS?

• If a mutation is present in an organism's gametes, it can be passed on to its offspring. Mutations are the ultimate source of genetic diversity in the living world

CONCEPT CHECK

Why base subtitution is often less harmful tan a base deletion or insertion?

•A base subtitution potentially affects only one amino acid of the polypetide, but a base deletion or insertion affects all the amino acids “downstream” from where it occurs.

CONCEPT CHECK

How mutation could be helpful rather tan harmful?

•If it alters a protein in a way that benefits an organism in its environment

CONCEPT CHECK

Examples of mutagens

•X-rays, UV light, some chemicals