Genes, chromosomes and mutations

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Genes and Chromosomes

A gene is a section of DNA that carries the instructions for one trait, such as eye colour or the shape of a protein.
Genes are found on chromosomes — long, thread-like structures made of DNA, held inside the nucleus of a cell.
Each chromosome carries many genes lined up along its length, not just one.

A normal set of chromosomes compared with a set showing a structural change.

Mutations and DNA Replication

DNA replication before cell division is not always 100% accurate; sometimes the wrong code is inserted.
A permanent change in the DNA sequence is called a mutation.
Once a mutation occurs, it is copied each time DNA replicates, so all resulting cells carry the mutation.

Effects of Mutations

Mutations can have harmful, beneficial, or neutral effects on the organism.
A mutation may have no effect, or it can cause a protein to be made incorrectly, affecting its function.
Usually, loss of protein function is detrimental and makes survival harder.

Harmful Mutations

Example: Albinism is a mutation in the gene for melanin, a protein in skin and eyes.
Albinism can result in no melanin production or a significant decline, affecting camouflage.
For an alligator, albinism makes it harder to blend in, negatively impacting survival.

An albino alligator.

Beneficial Mutations

In rare cases, a mutation can help an organism survive in its environment.
Example: A rabbit with albinism might be better camouflaged in a snowy arctic environment.
Humans' ability to see more colors is the result of a beneficial mutation from long ago.
Organisms with beneficial mutations are more likely to survive and pass the mutation to offspring.

Neutral Mutations

Some mutations have no effect on survival; they are neutral.
Example: Heterochromia (two different colored eyes) does not help or harm survival.
Neutral mutations can still be passed on to future generations.

A dog with heterochromia (two different eye colours).

Causes of Mutations

Mutations can happen spontaneously or be caused by mutagens in the environment.
Mutagens are substances that cause genetic mutations, such as chemicals in tobacco smoke.
Some mutagens can also cause cancer; tobacco smoke is linked to lung cancer.

Mutations and Traits

Mutations can cause changes in traits, such as an extra thumb or toe.
A slight change in the order of chemicals in chromosomes can lead to extra digits.
These changes occur as copying errors when cells divide, and they can be harmful, beneficial, or neutral.

A foot with an extra toe, illustrating a mutation that causes changes in traits.

Genes and DNA

DNA contains information that encodes inherited traits, such as eye color and allergies.
The nucleus contains genetic material organized into long DNA molecules.
The difference in DNA between individuals is called variation. Even a slight difference in DNA can lead to noticeable differences within a species, and between different species.

Nucleus and Genetic Material

The nucleus is enclosed by a double membrane (nuclear membrane) with pores.
Inside the nucleus, a nucleolus is often visible as a darker area.
Eukaryotic cells have a nucleus; prokaryotic DNA floats in the cytoplasm without a membrane.

A diagram and micrograph of a cell nucleus, showing the nuclear membrane, DNA, nucleolus, and nuclear pore.

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Practice questions

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1.What is a mutation?
Easy
- AA permanent change in the sequence of DNA
- BA temporary error in protein synthesis
- CA change in the number of chromosomes only
- DA change in the environment that affects an organism
2.Mutations are always harmful to an organism.
Easy
True or false?
3.Which of the following is an example of a neutral mutation?
Easy
- AAlbinism in a dark forest
- BHeterochromia (two different colored eyes)
- CA mutation causing a protein to not work
- DA mutation that helps an organism survive in its environment
4.A mutation that causes a protein to be made incorrectly can affect how well the protein works.
Easy
True or false?
5.What is a mutagen?
Easy
- AA type of protein that causes mutations
- BSomething in the environment that causes genetic mutations
- CA permanent change in DNA
- DAn organelle that contains DNA
6.Match each mutation to its effect on the organism.
Easy
- Albinism in a dark forest alligator
- Color vision in humans
- Heterochromia
- Harmful
- Beneficial
- Neutral
7.Mutations can only be caused by errors during DNA replication.
Easy
True or false?
8.A mutation occurs in a gene that codes for melanin in a rabbit. If the rabbit lives in a snowy arctic environment, what is the likely effect of this mutation?
Easy
- AHarmful, because the rabbit will stand out against the snow
- BBeneficial, because the white fur will help the rabbit blend in
- CNeutral, because fur color does not affect survival
- DHarmful, because the rabbit will be unable to produce any fur

Revision notes

## Genes, Chromosomes, and Mutations ### Key Ideas - **Genes** are segments of DNA located on **chromosomes** that carry instructions for making proteins. - **Mutations** are changes in the DNA sequence. They can be caused by errors during DNA replication, damage from environmental factors, or mobile genetic elements. ### Types of Mutations - **Substitution**: one base is replaced by another. - **Insertion**: extra base(s) are added. - **Deletion**: base(s) are removed. - **Duplication**: a segment of DNA is copied and inserted. - **Chromosomal rearrangements**: large segments break and rejoin differently (e.g., fusion of chromosomes). ### Effects of Mutations - **No effect**: mutation occurs in non-coding DNA or does not change the protein. - **Harmful**: alters protein function, often causing disease or reduced fitness. - **Beneficial**: provides an advantage (e.g., new function, adaptation). - Most mutations that change a protein are harmful (~70% in one study). ### Mutation vs. DNA Damage - **DNA damage** is a physical alteration (e.g., break, chemical adduct) that can be repaired using the undamaged strand as a template. - **Mutation** is a permanent change in the base sequence that cannot be recognized or repaired once both strands are altered. Mutations are replicated when the cell divides. ### Role in Evolution - Mutations are the **ultimate source of genetic variation**. - They provide raw material for natural selection. - Gene duplication allows one copy to keep its original function while the other evolves a new function. - Chromosomal rearrangements can accelerate speciation by reducing interbreeding. - Mobile DNA elements (e.g., transposons) can create new genes or alter existing ones. ### Key Takeaway Mutations can be helpful, harmful, or neutral. They are essential for evolution but can also cause diseases like cancer. Understanding mutations helps explain how traits change over time.

Gene: A segment of DNA on a chromosome that contains instructions for making a protein.
Chromosome: A structure made of DNA and proteins that carries genetic information; humans have 46 chromosomes in each cell.
Mutation: A change in the DNA sequence of an organism's genome.
Substitution: A type of mutation where one nucleotide base is replaced by another.
Insertion: A mutation where one or more nucleotide bases are added into the DNA sequence.
Deletion: A mutation where one or more nucleotide bases are removed from the DNA sequence.
DNA damage: A physical alteration in DNA structure (e.g., break, chemical adduct) that can be repaired using the complementary strand.
Genetic variation: Differences in DNA sequences among individuals in a population; mutations are the ultimate source.
Natural selection: The process by which organisms with traits better suited to the environment tend to survive and reproduce more.
Gene duplication: A mutation that copies a segment of DNA, creating an extra copy of a gene that can evolve new functions.

Flashcards

What is a mutation?

A mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA.

What are the main causes of mutations?

Mutations result from errors during replication, mitosis, meiosis, or damage to DNA, which may trigger error-prone repair or cause errors during replication.

What are the three types of mutations that can change DNA sequence?

Substitution, insertion, or deletion of segments of DNA.

Can mutations be caused by mobile genetic elements?

Yes, mutations may result from substitution, insertion or deletion of DNA due to mobile genetic elements like transposons.

Do mutations always produce detectable changes in an organism's phenotype?

No, mutations may or may not produce detectable changes in the observable characteristics (phenotype) of an organism.

What are the possible effects of a mutation in a gene?

Mutations in genes can have no effect, alter the product of a gene, or prevent the gene from functioning properly or completely.

How do mutations affect evolution?

Mutation is the ultimate source of all genetic variation, providing raw material on which evolutionary forces such as natural selection can act.

What is the difference between DNA damage and a mutation?

DNA damage is a physical alteration in DNA structure that can be repaired, while a mutation is a change in base sequence that cannot be recognized by repair enzymes once present in both strands.

Can DNA damage lead to mutations?

Yes, DNA damages often cause errors during replication or repair, and these errors are a major source of mutation.

What happens to a cell if DNA damage is not repaired?

Transcription may be prevented, translation blocked, DNA replication blocked, or the cell may die.

How do mutations affect protein function?

Mutations can alter protein function and regulation, and may be harmful, neutral, or beneficial.

What percentage of amino acid polymorphisms are estimated to be harmful?

About 70% of amino acid polymorphisms are estimated to be harmful, with the remainder neutral or marginally beneficial.

What is a gene duplication?

A duplication of a large section of DNA, often through genetic recombination, providing raw material for evolving new genes.

How do gene duplications contribute to evolution?

Duplications increase redundancy, allowing one copy to acquire a new function while the other performs the original function.

What is an example of gene duplication in humans?

The human eye uses four genes for light sensing (three for color vision, one for night vision), all arising from a single ancestral gene.

How can mutations create new genes from noncoding DNA?

Occasionally, mutations can create new genes from previously noncoding DNA.

What are chromosomal rearrangements?

Changes in chromosome number or structure where segments of DNA break and rearrange, such as fusions or translocations.

Give an example of a chromosomal fusion in human evolution.

In Homininae, two chromosomes fused to produce human chromosome 2; other apes retain separate chromosomes.

What role do chromosomal rearrangements play in speciation?

They can accelerate divergence by making populations less likely to interbreed, preserving genetic differences.

What are transposons?

Sequences of DNA that can move about the genome, making up a major fraction of genetic material in plants and animals.

How can transposons affect genes?

When they move, they can mutate or delete existing genes, producing genetic diversity.

What is an example of a transposon in humans?

The Alu sequence, with more than a million copies in the human genome, can regulate gene expression.

What happens to nonlethal mutations in a gene pool?

They accumulate and increase genetic variation; some may be reduced by natural selection, while favorable ones may accumulate.

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