Mutations & Gene Editing

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Gene Mutations: Definition and Causes

A gene mutation is a change in the sequence of base pairs in a DNA molecule; this may result in a new allele.
Mutations occur all the time and at random; they are more likely during DNA replication (S phase of interphase) due to copying errors.
Mutagenic agents are environmental factors that increase the mutation rate of cells.
Radiation can cause chemical changes in DNA, including high-energy radiation such as UV light and ionising radiation such as X-rays, gamma rays and α particles.
Chemical substances can also cause changes to DNA, e.g. benzo[a]pyrene and nitrosamines found in tobacco smoke, and mustard gas.
Mutagens can also come from inside the cell, such as particular enzymes that break down DNA or produce mutagenic substrates.
Some mutations occur at random during DNA replication and repair when errors are not detected by the proofreading process carried out by DNA polymerase.
If DNA polymerase detects a wrong nucleotide, it removes and replaces it before continuing DNA synthesis.

The sickle cell mutation

Types of Gene Mutation

A substitution mutation occurs when a nucleotide base in the DNA sequence is randomly swapped for a different base.
A substitution mutation only changes the amino acid for the triplet where the mutation occurs; it does not have a knock-on effect further along the gene.
An insertion mutation occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence.
A deletion mutation occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence.
Insertions and deletions are frameshift mutations: they change the triplets further on in the DNA sequence, so the entire sequence after the mutation is read incorrectly.
Frameshift mutations may dramatically change the amino acid sequence produced and therefore the ability of the polypeptide to function.

Consequences of Base Substitutions

A base substitution can only change the amino acid for the triplet in which the mutation occurs; it will not have a knock-on effect on the rest of the sequence.
A base substitution can result in single nucleotide polymorphisms (SNPs), which represent a difference in a single DNA nucleotide.
SNPs occur normally throughout a person's DNA, roughly once in every 300 nucleotides, meaning there are about 10 million SNPs in the human genome.
SNPs are commonly found in non-coding regions of DNA between genes and can act as biological markers to help locate genes associated with disease.
Substitution mutations can take three forms: silent mutations (no change to the amino acid sequence due to the degenerate genetic code), missense mutations (alter a single amino acid), and nonsense mutations (create a premature stop codon).
Sickle cell anaemia is an example of a disease caused by a missense substitution mutation changing a single amino acid in the sequence.
Cystic fibrosis is an example of a disease caused by a nonsense mutation, although this is not always the only cause.

Consequences of Insertions and Deletions

Insertions and deletions are two types of point mutations, which involve a change in the DNA base sequence at a single location.
An insertion occurs when an extra nucleotide is incorporated into the DNA sequence during replication.
A deletion mutation occurs when a nucleotide is missed or absent from the replicated strand.
These mutations are often considered more harmful than substitutions because they impact on the way the rest of the sequence is read by mRNA or the ribosome.
Insertions and deletions can cause a frameshift mutation, leading to a complete change to the entire amino acid sequence of a protein after the mutation site.
The mRNA is read in codons (groups of 3 nucleotides), so if an additional 1 or 2 nucleotides are added or removed, the sequence is 'shifted' and the entire mRNA and resulting protein are completely different.
A frameshift mutation can result in the addition of the wrong amino acids and/or the creation of a codon that stops the protein from growing longer.
Although a frameshift mutation during translation is rare (10^-5 to 10^-7 per codon), the effects are generally catastrophic for the resulting protein.

Mutations in Germ and Somatic Cells

The effect of a mutation can vary depending on whether it occurs in a germ cell or a somatic cell.
Germ cells use meiosis to produce gametes; somatic cells use mitosis to produce cells all over the body.
If a mutation occurs in a germ cell, it can be passed on to the offspring and next generation.
Cells involved in inheritance of genetic information — eggs, sperm and zygote — are known as the germ line.
A mutation that occurs in sperm cells could potentially affect the zygote of that offspring, and all cells developed from that zygote will contain the mutation.
Somatic cell mutations are not inherited by offspring; instead, these mutations are associated with cancers.
Cancers arise due to uncontrolled mitosis; cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells).
Cancers start when a mutation occurs in the genes that control cell division; if the mutated gene causes cancer it is referred to as an oncogene.

Randomness and Hotspots of Mutations

Mutations can occur anywhere in the base sequence of a genome on all chromosomes in all organisms.
This is how new strains of viruses or bacteria can come into existence.
Some locations of the genome are more likely to mutate than others.
Uncoiled DNA has a higher probability of encountering mutations than DNA tightly coiled around a histone because it is more exposed.
Many mutations occur in non-coding regions of DNA such as satellite DNA.
Mutation hotspots are regions where mutations are more frequent; one hotspot is where cytosine (C) is followed by guanine (G), called a CpG site.
When methylation occurs at a CpG site, C can mutate into thymine (T) in a substitution mutation; repeated occurrences form a CpG island, associated with cancers such as colorectal cancer.

Mutations and Genetic Variation

Differences between organisms of the same species are known as variation (e.g. coat colour in mammals, body length in fish, flower colour in flowering plants).
Variation results from small differences in DNA base sequences between individual organisms within a population.
Sources of these differences include mutation, meiosis, and random fertilisation during sexual reproduction.
The original source of genetic variation is mutation, which results in the generation of new alleles that can influence evolution of a species.
Mutations in the dividing cells of the sex organs lead to changes in the alleles of the gametes that are passed on to the next generation.
A new allele may be advantageous, disadvantageous or have no apparent effect.
An advantageous allele is more likely to be passed on because it increases the chance that an organism will survive and reproduce; a disadvantageous mutation is more likely to die out.
Mutations in a species are, in the long term, essential for evolution by natural selection; mutation is the only source of variation in asexually reproducing species.

Intentional Changes to Base Sequences

No known mechanisms exist where cells are able to intentionally mutate or change their DNA base sequence.
Proofreading processes exist to change a mutation back into its original sequence, but no mechanism exists for making a deliberate change to a base or sequence of bases with the purpose of changing a trait of the organism.

Inserting a gene into a plasmid

Commercial Genetic Testing

There are two types of genetic testing available: clinical or medical genetic testing and commercial genetic testing.
Clinical genetic testing is carried out through healthcare providers such as doctors, nurse practitioners, or genetic counselors, who determine which test is needed, order the test, collect the DNA sample, and share results with the patient.
Commercial genetic testing provides genetic tests marketed directly to customers; test kits can be bought online or in stores, and customers send a DNA sample and receive results directly from the company.
Commercial genetic testing provides people access to their genetic information without necessarily involving a healthcare provider, which can pose problems.
Commercially available genetic tests are not scientifically validated and can give inaccurate results.
Unexpected information about health, family relationships, or ancestry may be stressful or upsetting.
People may make important decisions about disease treatment based on inaccurate or misunderstood information, and individuals often are not provided with genetic counseling.

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1.Which of the following are mutagenic agents?
Easy
- AX-rays, benzo(a)pyrene, gamma rays
- BX-rays, benzo(a)pyrene, radio waves
- CX-rays, microwaves, radio waves
- DX-rays, benzo(a)pyrene, microwaves
2.Which of the following statements correctly describes the impact of the substitution mutation shown below? Original sequence: AUGGAAAUACCGCCAGGA Mutated sequence: AUGGAAAUACUGCCAGGA
Medium
- AThe codon encoding an amino acid was changed to a stop codon, truncating the polypeptide
- BThe sequence of amino acids may code for a different protein structure
- CThe resulting polypeptide chain will be elongated
- DThere will be no change in the polypeptide chain
3.The risk of developing a cancerous tumour is increased by exposure to which of the following? (select all that apply)
Medium
- AUltraviolet light
- BBenzo[a]pyrene
- CCarbon monoxide
- DX-rays
4.Sickle cell anaemia is caused by a mutation in the gene that codes for haemoglobin. Which of the following correctly explains how this mutation causes sickle cell anaemia?
Medium
- AIt causes valine to be replaced by glutamic acid, altering the structure of the haemoglobin protein.
- BIt produces an allele known as HbA.
- CIt causes glutamic acid to be replaced by valine, altering the structure of the final haemoglobin protein.
- DIt causes an amino acid substitution at the 8th position in the polypeptide.
5.Which type of mutation involves a nucleotide base being randomly swapped for a different base?
Easy
- ASubstitution mutation
- BInsertion mutation
- CDeletion mutation
- DFrameshift mutation
6.Insertion and deletion mutations are often more harmful than substitution mutations because they cause a frameshift.
Easy
True or false?
7.Match the type of mutation with its correct description.
Medium
- Silent mutation
- Missense mutation
- Nonsense mutation
- Creates a premature stop codon
- Does not alter the amino acid sequence
- Alters a single amino acid in the polypeptide chain
8.Place the following steps in the correct order to show how a frameshift mutation affects a polypeptide.
Medium
- A nucleotide is inserted into the DNA sequence
- The reading frame of the mRNA is shifted
- All subsequent codons are read incorrectly
- The amino acid sequence of the polypeptide is drastically altered

Soal lainnya

9.A substitution mutation in a gene changes a codon that normally codes for an amino acid into a premature stop codon. Which type of mutation is this?
Medium
- ANonsense mutation
- BSilent mutation
- CMissense mutation
- DFrameshift mutation
10.Which of the following are examples of mutagenic agents? (select all that apply)
Medium
- AX-rays
- BBenzo[a]pyrene
- CGamma rays
- DNitrosamines
- ECarbon monoxide
11.Match each type of substitution mutation with its effect on the polypeptide.
Medium
- Silent mutation
- Missense mutation
- Nonsense mutation
- Creates a premature stop codon so the polypeptide is incomplete
- Does not alter the amino acid sequence
- Alters a single amino acid in the polypeptide chain
12.Mutations in somatic cells can be inherited by offspring.
Easy
True or false?
13.Order the steps to show how a mutation in a somatic cell can lead to a tumour.
Medium
- A mutation occurs in a gene that controls cell division
- The mutated gene is an oncogene
- The cell divides repeatedly and uncontrollably
- An irregular mass of cells (tumour) forms
14.Which statement best describes the effect of a silent mutation?
Medium
- AIt does not alter the amino acid sequence of the polypeptide.
- BIt creates a premature stop codon.
- CIt changes one amino acid in the polypeptide chain.
- DIt causes a frameshift in the reading frame.
15.Which of the following are true about mutations? (select all that apply)
Medium
- AMutations can occur at random.
- BMost mutations are harmful or neutral.
- CMutations in gametes can be inherited by offspring.
- DAll mutations result in a change to the polypeptide.
- EMutations are always caused by mutagenic agents.
16.The diagram shows a DNA sequence and the amino acids it codes for: AGG ACA CCT GGA Serine Tyrosine Glycine Proline A single mutation occurred that resulted in only one amino acid being produced from this sequence. Which of the following DNA sequences would produce this result?
Hard
- AAGG ACA UGA GGA
- BAGG ACC CCT GGA
- CAGT ACA CCT GGA
- DAGG ACT CCT GGA
17.A substitution mutation changes the codon UAC (tyrosine) to UAU (tyrosine). What type of mutation is this?
Medium
- ASilent mutation
- BMissense mutation
- CNonsense mutation
- DFrameshift mutation
18.A deletion mutation removes one nucleotide from the third codon of a gene. Which of the following best describes the consequence?
Medium
- AA frameshift occurs, altering every amino acid after the deletion site
- BOnly the third amino acid is changed; the rest are unaffected
- CThe mRNA will be read in pairs of bases instead of triplets
- DThe ribosome will skip the mutated codon and continue in the original reading frame
19.Which of the following statements about SNPs are correct? (select all that apply)
Medium
- ASNPs occur once in every 300 nucleotides on average in the human genome
- BSNPs are commonly found in coding regions of DNA between genes
- CSNPs can act as biological markers to locate genes associated with disease
- DThere are roughly 10 million SNPs in the human genome
- ESNPs always cause a change in the amino acid sequence of a protein
20.Mutations in somatic cells can be passed on to offspring.
Easy
True or false?
21.Match each term with its correct description.
Medium
- Silent mutation
- Missense mutation
- Nonsense mutation
- Frameshift mutation
- Creates a premature stop codon
- Does not alter the amino acid sequence
- Shifts the reading frame after the mutation site
- Changes a single amino acid in the polypeptide
22.Place the following steps in the correct order to show how a deletion mutation leads to a non-functional protein.
Medium
- A nucleotide is deleted from the DNA sequence
- The reading frame of the mRNA is shifted
- All codons after the mutation site are read incorrectly
- A different sequence of amino acids is produced
- The protein folds incorrectly and cannot carry out its function
23.During which phase of the cell cycle are copying errors most likely to introduce mutations into DNA?
Medium
- AS phase of interphase
- BG1 phase of interphase
- CProphase of mitosis
- DCytokinesis
24.DNA polymerase can detect and remove a wrongly added nucleotide before continuing DNA synthesis.
Easy
True or false?
25.Which statement best explains why a mutation in a non-coding region of DNA may have no effect on an organism?
Medium
- ANon-coding DNA does not code for a polypeptide, so the amino acid sequence is unchanged
- BNon-coding DNA is always removed by proofreading enzymes
- CNon-coding DNA is transcribed but never translated, so mutations are corrected
- DNon-coding DNA is tightly coiled around histones and cannot mutate
26.A scientist sequences a section of DNA from two individuals. Individual 1 has the sequence CGTACG, while individual 2 has CGTTCG. Which term best describes this difference?
Medium
- ASingle nucleotide polymorphism (SNP)
- BFrameshift mutation
- CInsertion mutation
- DChromosomal mutation
27.In a CpG site, methylation of cytosine can lead to which substitution mutation?
Medium
- ACytosine to thymine
- BThymine to cytosine
- CGuanine to adenine
- DAdenine to guanine
28.Uncoiled DNA has a higher probability of encountering mutations than DNA tightly coiled around histones.
Easy
True or false?
29.Which of the following is a reason why a substitution mutation may not change the amino acid sequence of a polypeptide?
Medium
- AThe genetic code is degenerate, so more than one triplet may code for the same amino acid
- BSubstitution mutations always occur in non-coding DNA
- CSubstitution mutations change the reading frame of the mRNA
- DSubstitution mutations are always repaired by the ribosome
30.Which of the following statements about germ line mutations are correct? (select all that apply)
Hard
- AThey can be passed on to offspring
- BThey occur in cells that produce gametes
- CThey are associated with cancers but not with inheritance
- DThey can affect every cell developed from the zygote
- EThey only affect somatic cells
31.What is a gene mutation?
Easy
- AA change in the sequence of base pairs in a DNA molecule
- BA change in the number of chromosomes in a cell
- CA change in the structure of a protein
- DA change in the sequence of amino acids in a polypeptide
32.Which of the following correctly describes an insertion mutation?
Medium
- AA nucleotide is randomly inserted into the DNA sequence, creating a new triplet and causing a frameshift
- BA nucleotide is randomly deleted from the DNA sequence, causing a frameshift
- CA nucleotide base is swapped for a different base, affecting only one triplet
- DA whole chromosome is duplicated, causing a frameshift
33.Which of the following correctly describes a deletion mutation?
Medium
- AA nucleotide is missing from the replicated strand, causing a frameshift
- BA nucleotide is inserted into the DNA sequence, causing a frameshift
- CA nucleotide base is substituted, affecting only one triplet
- DA codon is inverted, causing a frameshift
34.Which of the following are examples of mutagenic agents?
Medium
- AX-rays, benzo[a]pyrene, gamma rays
- BX-rays, microwaves, radio waves
- CX-rays, benzo[a]pyrene, radio waves
- DX-rays, benzo[a]pyrene, microwaves
35.Which of the following statements about mutations is correct?
Medium
- AMutations occur at random and can be harmful, neutral, or beneficial
- BMutations always cause a change in the polypeptide produced
- CMutations are always harmful to the organism
- DMutations only occur in gametes
36.Which of the following correctly explains how a substitution mutation can lead to a nonsense mutation?
Medium
- AIt creates a premature stop codon, causing the polypeptide chain to be incomplete
- BIt changes one amino acid to another, altering protein function
- CIt does not change the amino acid sequence due to the degenerate genetic code
- DIt causes a frameshift, changing all subsequent amino acids
37.Which of the following correctly explains how a substitution mutation can lead to a missense mutation?
Medium
- AIt alters a single amino acid in the polypeptide chain
- BIt creates a premature stop codon
- CIt does not alter the amino acid sequence
- DIt causes a frameshift, altering all subsequent amino acids
38.Which of the following correctly explains how a substitution mutation can lead to a silent mutation?
Medium
- AIt does not alter the amino acid sequence because the genetic code is degenerate
- BIt creates a premature stop codon
- CIt alters a single amino acid in the polypeptide chain
- DIt causes a frameshift, altering all subsequent amino acids
39.Which of the following are consequences of a frameshift mutation? (select all that apply)
Medium
- AThe entire amino acid sequence after the mutation site is changed
- BThe polypeptide may cease to function
- COnly one amino acid is affected
- DA premature stop codon may be created
- EThe mutation has no effect on the polypeptide
40.Which of the following statements about somatic cell mutations are correct? (select all that apply)
Medium
- AThey are not inherited by offspring
- BThey can lead to cancer
- CThey occur in gametes
- DThey are passed on to the next generation
- EThey occur in body cells
41.Complete the sentence about mutagenic agents.
Medium
Mutagenic agents such as UV light and X-rays are examples of ____ that can increase the rate of mutation.
42.If a substitution mutation changes a single codon, how many amino acids in the polypeptide chain are directly affected?
Medium
43.Name the process that removes and replaces a wrongly added nucleotide during DNA replication.
Easy
44.On average, how many nucleotides of human DNA occur between one single nucleotide polymorphism (SNP) and the next?
Medium
45.Complete the sentence about the effect of a frameshift mutation.
Easy
A frameshift mutation causes a complete change to the entire amino acid sequence of a protein after the ____ site.
46.Complete the sentence about frameshift mutations.
Medium
A frameshift mutation occurs when a nucleotide is inserted or deleted, which shifts the ____ and changes all amino acids after the mutation site.
47.Complete the sentence about gene mutations.
Easy
A gene mutation is a change in the sequence of base pairs in a DNA molecule; this may result in a new ____.
48.Complete the sentence about the genetic code.
Easy
Because the genetic code is ____, more than one triplet code codes for the same amino acid, so some mutations do not change the amino acid sequence.
49.Complete the sentence about mutation hotspots.
Medium
A mutation hotspot occurs where the nucleotide cytosine (C) is followed by guanine (G), known as a ____ site.
50.Complete the sentence about SNPs.
Medium
SNPs occur once in every ____ nucleotides on average, which means there are roughly 10 million SNPs in the human genome.
51.A frameshift mutation occurs during translation at a rate of about 10^-5 per codon. How many codons would you expect to contain a frameshift mutation in a gene of 100,000 codons?
Medium
52.A substitution mutation changes the codon UAC (tyrosine) to UAU. How many amino acids in the polypeptide chain are directly affected?
Hard
53.Sickle cell anaemia is a genetic disease where a DNA triplet base in the gene coding for α-globin in haemoglobin changes from CTC to CAC. This mutation results in sickle shaped red blood cells that can cause a range of different symptoms. Identify the type of mutation that is illustrated in the diagram.
Medium
54.Sickle cell anaemia is a genetic disease where a DNA triplet base in the gene coding for α-globin in haemoglobin changes from CTC to CAC. This mutation results in sickle shaped red blood cells that can cause a range of different symptoms. State a reason for your answer in part i).
Medium
55.List two consequences of the change in shape of the red blood cells in a person suffering from sickle cell anaemia.
Medium
56.Some mutations, such as those causing sickle cell anaemia, may be inherited by offspring. Describe how mutations can be inherited by offspring.
Medium
57.Gene mutations lead to the formation of new alleles in a population. Define the term 'allele'.
Medium
58.If a substitution mutation changes a single codon, how many amino acids in the polypeptide chain are directly affected?
Medium
- A0
- B2
- C3
- D1
59.On average, how many nucleotides of human DNA occur between one single nucleotide polymorphism (SNP) and the next?
Medium
- A270 nucleotides
- B330 nucleotides
- C300 nucleotides
- D360 nucleotides
60.A frameshift mutation occurs during translation at a rate of about 10^-5 per codon. How many codons would you expect to contain a frameshift mutation in a gene of 100,000 codons?
Medium
- A2 codons
- B0 codons
- C3 codons
- D1 codons
61.A substitution mutation changes the codon UAC (tyrosine) to UAU. How many amino acids in the polypeptide chain are directly affected?
Hard
- A2 amino acids
- B0 amino acids
- C3 amino acids
- D1 amino acids

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