Variant Effects

Variant Effects 🧬

Every time DNA is copied, small mistakes can creep in. A change in the DNA base sequence is called a **variant** (or mutation). So does every variant cause a disease? Almost never, as it turns out. This module (Higher tier) works out when a variant matters and when it changes nothing at all.

Most change nothing ⚖️

A **variant** is a change in the order of DNA bases. Most variants have **no effect at all** on the organism, for two reasons: • Much of our DNA is **non-coding**, so a change there does not alter any protein's amino acids. • The genetic code is **degenerate**: several different codons code for the **same** amino acid, so a changed base often still gives the same amino acid. Only some variants go on to change the phenotype.

Why so little effect?

Why do most DNA variants have no effect on the phenotype?

• Much DNA is non-coding, and several codons code for the same amino acid
• DNA repairs every variant immediately, so none survive
• Variants only ever occur in unused chromosomes
• All variants change the protein, but the body ignores them

A variant in a coding region 🔧

**(3.10B)** A variant in a **coding** region can change the **amino acid sequence** of the protein. A different amino acid can change how the chain **folds**, altering the protein's **shape**. If the shape changes, the function can be lost: an enzyme's **active site** may no longer fit its substrate, for example.

A variant in a non-coding region 🎚️

**(3.9B)** A variant in a **non-coding** region does not change the amino acids directly. But non-coding DNA includes the stretches where **RNA polymerase binds** to begin transcription. A variant there can change **how well RNA polymerase binds**, and so change **how much** of the protein is made. More or less protein can still change the phenotype.

Match each term to its meaning

Trace the variant

An interactive activity.

True about variants

Select ALL THREE statements about DNA variants that are TRUE.

• A variant is a change in the DNA base sequence
• Most variants have no effect on the phenotype
• A coding-region variant can change the protein's shape and function
• Every variant causes a genetic disease
• Non-coding variants can never affect the phenotype
• A variant always changes the amino acid it codes for

The non-coding route

How can a variant in a non-coding region of DNA still affect the phenotype?

• By altering how RNA polymerase binds, changing how much protein is made
• By changing the amino acid sequence of the protein
• By deleting a whole chromosome
• It cannot. Non-coding variants never matter

Match each term to its definition

Variant summary

A variant is a change in the DNA base sequence. Most variants have _____ effect on the phenotype, partly because the genetic code is _____. A variant in a _____ region can change an amino acid and so the protein's shape. A variant in a non-coding region can change how _____ polymerase binds, altering how much protein is made.