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How Mutations Create New Variation · Inheritance and Change · MS-LS3-1 · DNA mutations

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Prerequisite: Complete or review How Green Beetles Took Over: Natural Selection

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How Mutations Create New Variation

with Lumi

Lumi stands at a glowing laboratory bench covered with colorful DNA model strips and protein shape cards, carefully swapping out one colored bead on a long DNA strand and holding the result up to the light with a curious, excited expression.

Explain what a mutation is and how it changes the sequence of DNA bases.
Describe how a change in DNA can alter the shape or function of a protein.
Compare mutations that are harmful, neutral, or beneficial to an organism.
Identify that mutations in reproductive cells can be passed to offspring, creating heritable variation.
Predict whether a given mutation is likely to disrupt protein function based on the type and location of the change.

Key terms

Mutation: A permanent change in the DNA base sequence that can alter a protein.
Codon: A group of three DNA bases that specifies one amino acid in a protein.
Substitution: A mutation that swaps one DNA base for another at a single position.
Frameshift: A shift in the reading frame caused by an insertion or deletion that scrambles later codons.
Silent mutation: A base change that still codes for the same amino acid, leaving the protein unchanged.

Why Frameshifts Are So Damaging

DNA is read three bases at a time, so the reading frame is everything. A substitution changes only the single codon where it lands, often swapping just one amino acid. An insertion or deletion is far worse because it pushes every base after it into a new codon, an effect called a frameshift. The sentence THE CAT ATE THE RAT becomes nonsense once one letter is added, and a protein behaves the same way: nearly every amino acid after the change is now wrong, so the protein usually loses its shape and stops working entirely.

Harmful, Neutral, or Beneficial

A mutation's effect depends on how it changes the resulting protein. Harmful mutations, like the single amino-acid change in sickle-cell hemoglobin, distort a protein's shape and reduce survival. Neutral mutations, including silent changes that still code for the same amino acid, leave the protein and the organism unaffected. Beneficial mutations are rarer but powerful, such as a bacterial change that builds an enzyme able to destroy an antibiotic. The same kind of typo can be a disaster, a non-event, or an advantage, depending entirely on its effect on protein function.

Worked examples

Decide which mutation is more likely to disrupt a 900-base gene.

Locate each change: a frameshift deletion sits in the first 10 bases, a substitution sits in the last 5 bases.
Predict the frameshift: deleting a base near the start shifts the reading frame for almost every codon that follows.
Predict the substitution: a change near the very end alters at most one or a few amino acids.
Compare the damage: scrambling nearly the whole protein is far worse than altering its final amino acids.

Answer: The early frameshift deletion is more disruptive because it scrambles almost the entire protein, while the late substitution changes very little.

Every cell in your body carries a long instruction manual written in DNA. The alphabet of DNA uses only four letters — A, T, C, and G — called bases. Groups of three bases form codons, and each codon tells the cell to add a specific amino acid to a growing protein chain. Proteins are the workers of the cell: they act as enzymes, build structures, carry oxygen, and much more. A mutation is a permanent change in the DNA base sequence. Think of it like a typo in the instruction manual. There are several kinds: • A substitution swaps one base for another (A→G, for example). • An insertion adds an extra base. • A deletion removes a base. Insertions and deletions are especially powerful because they shift every codon after them — this is called a frameshift. Imagine the sentence "THE CAT ATE THE RAT" suddenly becoming "THE XCA TAT ETH ERA T" because one letter was inserted. The whole meaning collapses. When the codon sequence changes, the protein built from it may have a different amino acid in one spot — or many different amino acids. Because a protein's job depends on its 3-D shape, even one wrong amino acid can fold the protein incorrectly. A misfolded protein may work poorly, not at all, or sometimes in a completely new way. Mutations can be harmful (sickle-cell disease changes one amino acid in hemoglobin, distorting red blood cells into a rigid sickle shape), neutral (a codon change that still codes for the same amino acid — called a silent mutation — leaves the protein identical), or beneficial (a mutation in a bacterium might create an enzyme that breaks down an antibiotic, helping it survive). Here is the key heritable variation link: mutations that happen in body cells (skin, muscle) die with the organism. But mutations in reproductive cells — sperm or eggs — can be passed to offspring. Every inherited difference between individuals — fur color, enzyme speed, disease resistance — can ultimately trace back to a mutation somewhere in evolutionary history. That is how mutations fuel the variation that natural selection acts upon.

Activity

Sort each mutation scenario card into the correct outcome bin: Harmful, Neutral, or Beneficial.

Practice

Decide whether a silent substitution in a coding region is harmful, neutral, or beneficial.

Explain why a mutation in a skin cell cannot be passed to an organism's offspring.

Common mistakes to avoid

Every mutation is harmful to the organismMutations can be harmful, neutral, or beneficial depending on how they change the protein's function.
Any mutation can be inherited by offspringOnly mutations in reproductive cells like eggs or sperm can be passed on, not those in body cells.

Check your understanding

A mutation changes one DNA base so that a codon now codes for a different amino acid in the middle of an enzyme. What is the MOST LIKELY immediate effect?

A mutation occurs in a skin cell of a rabbit. Which statement about this mutation is correct?

A frameshift deletion occurs in the first 10 bases of a 900-base gene. A substitution occurs in the last 5 bases of the same gene. Which change is MORE likely to disrupt protein function, and why?

Recap

A mutation permanently changes the DNA base sequence and may alter a protein's shape, with effects ranging from harmful to neutral to beneficial, and only mutations in reproductive cells create the heritable variation natural selection acts upon.

Reflect

Why might a single DNA typo sometimes turn out to help a species survive instead of harming it?