Genes, chromosomes and mutations

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Genes and Chromosomes

**Genes** are segments of **DNA** that carry instructions for making proteins.
**Chromosomes** are long, coiled strands of DNA that contain many genes.
Humans have 23 pairs of chromosomes (46 total) in each body cell.
Genes are located at specific positions on chromosomes called **loci**.
Each gene has a specific sequence of **nucleotides** (A, T, C, G) that codes for a protein.

What is a Mutation?

A **mutation** is a change in the DNA sequence of an organism's genome.
Mutations can occur in genes (coding regions) or in non-coding regions.
They can be caused by errors during DNA replication, or by damage from environmental factors like radiation or chemicals.
Mutations are the ultimate source of all **genetic variation**.

Types of Mutations

**Substitution**: one nucleotide is replaced by another.
**Insertion**: one or more nucleotides are added to the DNA sequence.
**Deletion**: one or more nucleotides are removed from the DNA sequence.
**Duplication**: a segment of DNA is copied and repeated.
**Chromosomal rearrangement**: large segments of DNA break and rejoin in a different order.

Effects of Mutations

Mutations can be **harmful**, **beneficial**, or **neutral** (no effect).
Harmful mutations may cause genetic disorders or increase cancer risk.
Beneficial mutations can provide an advantage, such as resistance to disease.
Most mutations in protein-coding genes are harmful (about 70% of amino acid changes are damaging).
Neutral mutations do not affect the organism's fitness and can accumulate over time.

Mutations and Evolution

Mutations create **genetic diversity**, which is the raw material for **natural selection**.
Beneficial mutations can become more common in a population over generations.
Harmful mutations are usually removed by natural selection.
Gene duplication allows one copy to perform the original function while the other evolves a new function.

DNA Damage vs. Mutation

**DNA damage** is a physical alteration to DNA structure (e.g., a break or chemical change).
DNA damage can often be repaired by enzymes using the undamaged strand as a template.
A **mutation** is a permanent change in the DNA sequence that cannot be repaired once both strands are altered.
DNA damage can lead to mutations if not repaired correctly during replication.

DNA double helix showing base pairs (A-T, C-G) and sugar-phosphate backbone.

A chromosome is made of tightly coiled DNA. Genes are specific segments along the DNA.

Common types of mutations: substitution, insertion, deletion, duplication, and chromosomal rearrangement.

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Practice questions

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1.What is a mutation?
Easy
- AAn alteration in the nucleic acid sequence of the genome
- BA change in the number of chromosomes in a cell
- CA repair of damaged DNA
- DA type of cell division
2.Where are genes located?
Easy
- AOn chromosomes
- BIn the cytoplasm
- CIn ribosomes
- DOn the cell membrane
3.Which of the following is NOT a cause of mutations?
Easy
- AErrors during DNA replication
- BDamage to DNA
- CTranscription of mRNA
- DErrors during meiosis
4.A mutation occurs in a gene that codes for an enzyme. What is the most likely effect on the organism?
Medium
- AThe mutation will be harmful because it changes the protein
- BThe mutation will be beneficial because it creates new traits
- CThe mutation will have no effect because enzymes are not important
- DThe mutation will always cause the organism to die
5.What is the difference between DNA damage and a mutation?
Medium
- ADNA damage can be repaired using the complementary strand; a mutation cannot be repaired once it is present in both strands
- BDNA damage is always harmful; a mutation is always beneficial
- CDNA damage occurs only in non-coding regions; mutations occur only in genes
- DThere is no difference; they are the same thing
6.Which of the following is an example of a mutation that can be passed to offspring?
Medium
- AA mutation in a skin cell caused by UV radiation
- BA mutation in a sperm cell
- CA mutation in a liver cell
- DA mutation in a brain cell
7.A segment of DNA is duplicated, resulting in an extra copy of a gene. Over time, this extra gene may acquire a new function. This process is an example of:
Hard
- AGene duplication as a source of new genes
- BA point mutation that changes one base
- CA deletion mutation that removes the gene
- DChromosomal fusion
8.What role do transposons play in mutations?
Hard
- AThey can move within the genome and mutate or delete existing genes
- BThey repair damaged DNA
- CThey prevent mutations from occurring
- DThey are always harmful and cause disease

More questions

9.Which of the following best describes the relationship between mutations and evolution?
Medium
- AMutations provide the genetic variation on which natural selection acts
- BMutations are always harmful and slow down evolution
- CEvolution causes mutations to occur
- DMutations and evolution are unrelated
10.What is a gene?
Easy
- AA segment of DNA that codes for a protein
- BA type of chromosome
- CA molecule that carries amino acids
- DA structure that produces energy
11.A mutation occurs in a non-coding region of DNA. What is the most likely effect?
Medium
- ANo effect on the organism's phenotype
- BThe organism will develop a disease
- CThe mutation will be passed to offspring
- DThe mutation will change the amino acid sequence of a protein
12.In humans, chromosome 2 is the result of a fusion of two ancestral chromosomes. This type of mutation is called:
Hard
- AA chromosomal rearrangement
- BA point mutation
- CA gene duplication
- DA transposon insertion
13.A student observes that a certain species of bird has two different beak shapes in the same population. Which of the following is the most likely source of this variation?
Medium
- AMutation
- BNatural selection
- CSelective breeding
- DEnvironmental factors
14.In a population of bacteria, a mutation occurs that allows the bacteria to break down a new antibiotic. Which of the following best describes the effect of this mutation on the population over time?
Hard
- AThe mutation will likely spread because it provides a survival advantage in the presence of the antibiotic.
- BThe mutation will disappear because most mutations are harmful.
- CThe mutation will have no effect because it is a DNA damage, not a mutation.
- DThe mutation will cause all bacteria to die because it is a change in the genome.
15.Which of the following is an example of a mutation?
Easy
- AA change in the DNA sequence from ATCG to ATTG
- BA break in one strand of DNA
- CA chemical adduct attached to a DNA base
- DA cell dividing to produce two identical daughter cells
16.The table below shows the number of mutations observed in a gene across three generations of a laboratory strain of fruit flies. Which statement is best supported by the data? | Generation | Number of mutations | |------------|---------------------| | 1 | 2 | | 2 | 5 | | 3 | 8 |
Medium
- AMutations accumulate over time.
- BMutations are always harmful.
- CMutations occur at a constant rate.
- DMutations are caused by natural selection.
17.A mutation occurs in a skin cell of a human. Which of the following is the most likely outcome?
Medium
- AThe mutation will be passed to offspring only if it occurs in a gamete.
- BThe mutation will be passed to all daughter cells and to offspring.
- CThe mutation will be repaired by enzymes because it is DNA damage.
- DThe mutation will cause the skin cell to become cancerous immediately.
18.A scientist compares the DNA sequences of two related species and finds that a particular gene has been duplicated in one species. Which of the following is the most likely evolutionary advantage of this duplication?
Hard
- AOne copy can acquire a new function while the other performs the original function.
- BThe duplicate gene will be expressed at twice the level, improving fitness.
- CThe duplication prevents mutations from occurring in the original gene.
- DThe duplication causes the species to evolve into a new species immediately.
19.Where are genes located in a cell?
Easy
- AOn chromosomes
- BIn the cytoplasm
- COn ribosomes
- DIn the cell membrane
20.A researcher treats a culture of bacteria with a chemical that causes DNA damage. After several generations, she observes that some bacteria now have a new mutation. Which of the following best explains this observation?
Hard
- ADNA damage led to errors during DNA replication, producing mutations.
- BThe chemical directly changed the DNA sequence into a mutation.
- CThe bacteria evolved the mutation in response to the chemical.
- DThe mutation was already present and the chemical selected for it.
21.Which of the following best describes the relationship between DNA damage and mutation?
Medium
- ADNA damage can cause mutations if it is not correctly repaired.
- BDNA damage and mutation are the same thing.
- CMutations cause DNA damage.
- DDNA damage is always repaired, so it never leads to mutations.
22.A mutation results in the substitution of one amino acid for another in a protein. What is the most likely effect on the organism?
Medium
- AThe effect could be harmful, neutral, or beneficial.
- BThe effect will always be harmful.
- CThe effect will always be beneficial.
- DThe protein will not be produced at all.
23.In a certain plant species, a mutation causes the flowers to produce a toxin that deters herbivores. However, the mutation also reduces the number of seeds produced. Which of the following is the most likely outcome in a population where herbivores are common?
Hard
- AThe mutation will increase in frequency because the benefit of toxin outweighs the cost of fewer seeds.
- BThe mutation will decrease in frequency because it reduces seed production.
- CThe mutation will have no effect on the population because it is a non-coding mutation.
- DThe mutation will cause the plant species to go extinct.
24.Which of the following is a source of mutations?
Easy
- AErrors during DNA replication
- BPhotosynthesis
- CProtein synthesis
- DCellular respiration
25.A scientist treats bacterial cells with a chemical that causes DNA damage. After several generations, some bacteria have a new mutation. What is the most likely sequence of events?
Easy
- ADNA damage is repaired incorrectly, leading to a permanent base change that is replicated.
- BThe chemical directly changes the base sequence without any replication.
- CDNA damage is always repaired correctly, so no mutation occurs.
- DThe chemical kills all cells except those with pre-existing mutations.
26.A student grows bacteria on a plate containing an antibiotic. Most bacteria die, but a few survive and form colonies. Which statement best explains the survival of these bacteria?
Medium
- AThe antibiotic caused mutations that made the bacteria resistant.
- BThe bacteria already had a mutation that made them resistant before exposure to the antibiotic.
- CThe bacteria learned to break down the antibiotic during exposure.
- DThe antibiotic activated a resistance gene in all bacteria.
27.A researcher compares the DNA sequences of a gene in two related species. Species A has one copy of the gene, while species B has two very similar copies located next to each other on the same chromosome. What is the most likely explanation for this difference?
Hard
- ASpecies B experienced a duplication of the gene after diverging from the common ancestor.
- BSpecies A lost one copy of the gene through a deletion mutation.
- CBoth species inherited two copies from a common ancestor, but species A lost one.
- DThe extra copy in species B arose from a point mutation.
28.A student examines a diagram of a chromosome and identifies a specific segment that codes for a protein. What is this segment called?
Medium
- AGene
- BAllele
- CGenome
- DChromatid
29.In a population of bacteria, a mutation occurs that allows the bacteria to break down a new antibiotic. Which of the following best describes the effect of this mutation on the population?
Hard
- AThe mutation will immediately become common in the population because all bacteria need it.
- BThe mutation will only become common if the antibiotic is present, because it provides a survival advantage.
- CThe mutation will disappear because it is harmful.
- DThe mutation will have no effect on the population.
30.A scientist wants to estimate the mutation rate in a gene. She sequences the gene in 1000 offspring from a single pair of parents and counts the number of new mutations that appeared in the offspring. Which additional information is needed to calculate the mutation rate per base pair per generation?
Hard
- AThe length of the gene in base pairs
- BThe age of the parents
- CThe number of chromosomes in the organism
- DThe temperature at which the organisms were raised
31.A student sets up an experiment to test the effect of UV radiation on mutation frequency in yeast. She exposes yeast cells to UV light for different times and then counts the number of colonies that grow on a medium that selects for a specific mutation. What is the independent variable in this experiment?
Medium
- AThe number of colonies with the mutation
- BThe exposure time to UV light
- CThe type of yeast used
- DThe temperature of the growth medium
32.A researcher treats a culture of bacteria with a chemical that causes DNA damage. After several generations, she observes that some bacteria now have a new mutation. Which of the following is the most likely explanation?
Hard
- AThe chemical directly changed the base sequence of the DNA.
- BThe chemical caused DNA damage that was repaired incorrectly, leading to a mutation.
- CThe chemical killed all bacteria without the mutation.
- DThe mutation was already present but only became visible after the chemical treatment.
33.What is the name for a change in the base sequence of DNA?
Easy
- AMutation
- BDNA damage
- CReplication
- DTranslation
34.Which of the following is a possible source of mutations?
Easy
- AErrors during DNA replication
- BEating a balanced diet
- CRegular exercise
- DGetting enough sleep
35.A scientist compares the DNA sequences of a gene in two related species. Species A has one copy of the gene, while species B has two very similar copies located next to each other on the same chromosome. What is the most likely explanation for the presence of two copies in species B?
Medium
- AA mutation caused the gene to be duplicated.
- BA mutation deleted the original copy in species A.
- CThe two copies arose from a fusion of two chromosomes.
- DThe gene was acquired from a virus.
36.A population of bacteria is exposed to an antibiotic. Most bacteria die, but a few survive and reproduce. After several generations, the entire population is resistant to the antibiotic. Which statement best explains the survival of the bacteria?
Hard
- AThe antibiotic caused mutations in the bacteria that made them resistant.
- BSome bacteria already had a mutation that made them resistant before the antibiotic was added.
- CThe bacteria learned to break down the antibiotic and passed that knowledge to their offspring.
- DThe antibiotic triggered a stress response that made all bacteria temporarily resistant.
37.A researcher treats a culture of bacteria with a chemical that damages DNA. She then allows the bacteria to grow for many generations. Which of the following is the most likely long-term effect on the population?
Medium
- AAll bacteria will die because the damage cannot be repaired.
- BSome bacteria will have new mutations because errors during repair of the damage changed the DNA sequence.
- CThe bacteria will become resistant to the chemical by learning to avoid it.
- DThe DNA damage will be passed directly to offspring without any change in sequence.
38.The table below shows the number of mutations observed in a specific gene across three generations of fruit flies. The flies were raised in identical conditions. | Generation | Number of flies examined | Number of new mutations in the gene | |------------|-------------------------|-------------------------------------| | 1 | 1000 | 2 | | 2 | 1000 | 3 | | 3 | 1000 | 2 | Which statement is best supported by the data?
Hard
- AMutations occur at a constant rate in this gene.
- BMutations are caused by environmental factors in generation 2.
- CThe mutation rate is decreasing over time.
- DMost mutations are harmful.
39.A scientist sequences the DNA of a gene from 500 offspring of a single pair of parents. She finds 4 new mutations that were not present in either parent. What is the mutation rate per offspring for this gene?
Medium
- A4 per 500 offspring
- B4 per 1000 offspring
- C1 per 125 offspring
- D1 per 500 offspring
40.A student sets up an experiment to test the effect of UV radiation on mutation frequency in yeast. She exposes yeast cells to UV light for different times and then counts the number of colonies that grow on a plate containing a toxic chemical that only resistant mutants can survive. The results are shown below. | UV exposure time (minutes) | Number of resistant colonies | |----------------------------|------------------------------| | 0 | 0 | | 1 | 5 | | 2 | 12 | | 3 | 25 | | 4 | 48 | Which conclusion is best supported by the data?
Hard
- AUV radiation kills most yeast cells.
- BLonger UV exposure increases the number of mutations.
- CThe toxic chemical causes mutations.
- DYeast cells become resistant to UV over time.

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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