DNA and Mutations

Once students understand how DNA holds the instructions for making proteins and how DNA gets passed down from parents to offspring, students apply what they know to analyze how sometimes changes happen to the sequence of nucleotides, which results in the phenomena of genetic mutations. These mutations can be neutral, harmful, or beneficial.

• DNA gets passed down from parents to offspring, focusing on how sometimes there are permanent changes to an individual’s DNA. These are called mutations.
• There are several ways mutations can happen.
• Mutations can be neutral, harmful, or beneficial.

• What can cause a mutation?
• Why are some mutations called neutral or silent mutations?
• Why are some mutations beneficial?
• Why are some mutations harmful?

Misconception: All mutations are harmful.

Fact: Some mutations are harmful, but many are “silent,” which means they do not have a positive or a negative effect on the organism, while some mutations are beneficial.

Asexual reproduction : reproduction that requires only one parent (e.g., binary fission, budding, and fragmentation)

Chromosome : a threadlike structure of DNA and protein; found in the nucleus of eukaryotic cells; a discrete package of genetic material

Daughter cell : a cell formed by the division of a parent cell

Heredity : the passing on of traits from parents to offspring

Reproduction : the ability of a mature organism to have offspring

Mutation : any permanent change to an organism’s DNA

Drinking Milk

In 2007, an international team of scientists went to Africa with a question: what caused the majority of Africans to be able to drink milk?

This question may seem silly, but it is something that researchers have been investigating around the world. This is because not everyone can digest the primary sugar in milk, which is called lactose. A protein called lactase is responsible for breaking down lactose, cutting the lactose molecule in two. Babies are born with this protein functioning so they can digest milk.

However, research has shown that in our early ancestors, the genes that produce lactase somehow got switched off after babies stopped drinking their mothers’ milk. In some people today, their genes that code for lactase are still switched off as they become adults. These people are lactose intolerant, which means they cannot digest lactose.

However, in some populations around the world today, there are many people who are able to drink milk as adults because their bodies continue producing lactase.

So what makes this possible? The answer is mutations, which are permanent changes to an individual’s DNA. The scientists investigating their question gathered samples of DNA from people living in the region and looked for structural changes to the genes that coded for the protein lactase.

Structural Changes to DNA

Babies are born able to digest milk. However, some people lose that ability as they grow up. Mutations are relatively common. A mistake occurs every 100,000 or so nucleotides, resulting in about 120,000 mistakes each time a cell divides. The cell is able to repair most changes, but sometimes a change gets through. If a change occurs in an egg or sperm cell, it will be passed down to offspring.

In this lesson, students use a model to investigate how mutations to genes can result in changes to proteins, which can affect the structures and functions of an organism and thereby change traits. Students analyze their models to explain the phenomenon of how a mutation to the gene for hemoglobin affects how the cell makes this protein.