Exam questions · Biology
DNA and Protein Synthesis
- 36 exam questions
- 116 marks
- 54 quick checks
The Structure of DNA
Just this lesson-
1 State [2 marks]
State the two base pairs found in DNA.
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Model answer
A with T (adenine with thymine), and C with G (cytosine with guanine).
Mark scheme
- A with T — 1 mark
- C with G — 1 mark
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2 Name [3 marks]
Name the three parts of a nucleotide.
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Model answer
A sugar, a phosphate group and a base.
Mark scheme
- Sugar — 1 mark
- Phosphate group — 1 mark
- Base — 1 mark
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3 Complete [2 marks]
One strand of DNA has the base sequence G G C A T T. Write the base sequence of the complementary strand.
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Model answer
C C G T A A.
Mark scheme
- Applies A-T and C-G pairing — 1 mark
- C C G T A A — 1 mark
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4 Calculate [3 marks]
A sample of DNA contains 28% adenine. Calculate the percentage of guanine in the sample.
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Model answer
Thymine is also 28%, so adenine and thymine make up 56%. The remaining 44% is cytosine and guanine, which are equal, so guanine is 22%.
Mark scheme
- T is the same as A (28%) — 1 mark
- 100 - 56 = 44% for C and G — 1 mark
- G = 22% — 1 mark
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5 Explain [3 marks]
Explain why the hydrogen bonds between the two strands of DNA are important.
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Model answer
The hydrogen bonds hold the two strands together by joining the complementary bases, so the double helix is stable. They are weak enough to be broken easily, so the strands can be separated when DNA is copied or when a gene is read.
Mark scheme
- Hold the two strands together — 1 mark
- They are weak, so the strands can separate — 1 mark
- Needed for copying or reading the genes — 1 mark
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6 Describe [6 marks]
Describe the structure of DNA, and explain how the structure allows a DNA molecule to be copied accurately.
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Model answer
DNA is a polymer made of nucleotides. Each nucleotide has a sugar, a phosphate group and one of four bases. The sugar and phosphate form a backbone, and two strands are coiled into a double helix. The bases of the two strands are joined by hydrogen bonds, with A always pairing with T and C always pairing with G. When DNA is copied the weak hydrogen bonds break, so the strands separate, and each strand acts as a template. Because of complementary base pairing, each base on the template can only join with one type of base, so the new strand is an exact copy.
Mark scheme
- Polymer of nucleotides — 1 mark
- Sugar, phosphate group and base — 1 mark
- Two strands in a double helix — 1 mark
- Complementary base pairing (A-T, C-G) joined by hydrogen bonds — 1 mark
- Weak bonds let the strands separate — 1 mark
- Each strand acts as a template, so the copy is exact — 1 mark
Quick check
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1
Which base pairs with thymine?
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A: Adenine
Adenine always pairs with thymine.
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2
Which base pairs with cytosine?
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C: Guanine
Cytosine always pairs with guanine.
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3
What holds the two strands of DNA together?
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B: Hydrogen bonds between bases
Weak hydrogen bonds join the paired bases of the two strands.
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4
What is the repeating unit of DNA called?
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D: Nucleotide
DNA is a polymer made of repeating units called nucleotides.
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5
Which of these is a correct nucleotide?
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A: A sugar, a phosphate group and a base
A nucleotide is made of one sugar, one phosphate group and one base.
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6
What is the complementary sequence to T T G A?
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C: A A C T
T pairs with A, G pairs with C and A pairs with T, so the sequence is A A C T.
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7
A DNA sample has 35% adenine. What percentage is guanine?
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B: 15%
T is 35% as well, so 70% is A and T. The remaining 30% is C and G in equal amounts, so G is 15%.
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8
Which scientists built the model of the double helix in 1953?
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D: Watson and Crick
James Watson and Francis Crick built the model, using X-ray images taken by Rosalind Franklin.
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9
Why is it an advantage that the hydrogen bonds between bases are weak?
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C: The strands can be separated when needed
Weak bonds mean the strands can be separated easily, so DNA can be copied and its genes read.
The Genome and the Human Genome Project
Just this lesson-
1 State [1 mark]
State what is meant by the genome of an organism.
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Model answer
The entire DNA of an organism.
Mark scheme
- All of an organism's DNA — 1 mark
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2 State [2 marks]
State two things that the Human Genome Project found.
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Model answer
Any two from: the order of all of the bases in human DNA; that humans have about 20 000 genes; the positions of many genes; that people are more than 99% identical in their DNA.
Mark scheme
- First finding — 1 mark
- Second finding — 1 mark
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3 Describe [3 marks]
Describe how the Human Genome Project was used to find the order of bases in human DNA.
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Model answer
DNA from volunteers was broken into small pieces. The order of bases in each piece was read by sequencing machines. Computers then put the overlapping pieces together to give the full sequence.
Mark scheme
- DNA broken into small pieces — 1 mark
- Sequence of each piece read by machines — 1 mark
- Computers assembled the full sequence — 1 mark
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4 Explain [3 marks]
Explain how a genetic test could help a person who has several relatives with breast cancer.
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Model answer
The test can show whether she has a variant of a gene, such as BRCA1, that increases the risk of cancer. If she has the variant, she can have regular screening, so that any cancer is found early and treated more successfully. She could also decide to take other preventive steps.
Mark scheme
- Test for a gene variant such as BRCA1 — 1 mark
- Shows a higher risk — 1 mark
- Early screening or prevention — 1 mark
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5 Explain [4 marks]
Explain two ways in which knowing the genome helps doctors to treat patients.
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Model answer
Doctors can use genetic tests to find the variants that make a person more likely to develop a disease, so they can give early screening or lifestyle advice. They can also choose the best drug and dose using the person's genes, so that the treatment works well and has fewer side effects.
Mark scheme
- Predict risk of disease — 1 mark
- Developed: screening, lifestyle advice — 1 mark
- Personalised medicine — 1 mark
- Developed: best drug and dose — 1 mark
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6 Discuss [6 marks]
Discuss the potential benefits and problems of using information from the Human Genome Project in medicine.
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Model answer
Knowing the genome lets doctors find the genes that cause inherited disorders, test people for variants that raise their risk of disease and choose treatments that suit a person's genes. This could lead to earlier diagnosis and better treatment, and it helps researchers to develop new drugs and gene therapies. There are also problems. Genetic information is private, and insurers or employers might use it to discriminate against people. People could become very anxious if they are told they are likely to develop a disease for which there is no cure. Testing is also expensive, so not everyone may benefit. Overall the benefits are large if the information is kept private and properly regulated.
Mark scheme
- Finds genes for disorders / predicts risk — 1 mark
- Personalised treatment or new drugs — 1 mark
- Developed example — 1 mark
- Privacy or discrimination — 1 mark
- Anxiety or cost — 1 mark
- A reasoned conclusion — 1 mark
Quick check
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1
Roughly how many genes are in the human genome?
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B: 20 000
The Human Genome Project found about 20 000 genes.
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2
What was the aim of the Human Genome Project?
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A: To find the order of all of the bases in human DNA
The project aimed to find the order of all of the bases in human DNA.
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3
What is a genome?
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C: All of an organism's DNA
The genome is all of the DNA of an organism.
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4
Which statement about the DNA of two people is correct?
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D: It is more than 99% identical
Any two people have DNA that is more than 99% identical.
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5
Which is a benefit of the Human Genome Project?
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A: It helps to identify genes that cause inherited disorders
Knowing the genes helps scientists find which genes cause disorders, and so predict and treat them.
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6
What is personalised medicine?
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B: Treatment chosen using the patient's own genes
Personalised medicine uses information from a patient's genes to choose the most suitable treatment.
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7
Which is a possible problem of genetic testing?
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C: Discrimination by insurers or employers
Insurers or employers could use genetic information to discriminate against people.
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8
Roughly how many base pairs are in the human genome?
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D: 3 billion
There are about 3 billion base pairs.
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9
Only a small part of the human genome is made up of genes. What is the rest called?
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B: Non-coding DNA
DNA that does not code for a protein is non-coding DNA, and some of it controls when genes are used.
Extracting DNA from Fruit
Just this lesson-
1 State [2 marks]
State two pieces of safety equipment or precautions needed when using ethanol.
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Model answer
Any two from: wear eye protection, keep ethanol away from naked flames, do not taste it, keep the lid on the bottle.
Mark scheme
- First precaution — 1 mark
- Second precaution — 1 mark
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2 State [1 mark]
State what the extracted DNA looks like.
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Model answer
White, stringy strands.
Mark scheme
- White stringy / cloudy strands — 1 mark
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3 Explain [2 marks]
Explain why the fruit is mashed before the extraction solution is added.
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Model answer
Mashing breaks open the cells, including the cell walls. This lets the detergent and salt reach the cell membranes and the DNA.
Mark scheme
- Breaks open the cells / cell walls — 1 mark
- So the solution reaches the DNA — 1 mark
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4 Explain [3 marks]
Explain why detergent is used in the extraction of DNA.
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Model answer
The detergent dissolves the lipids in the cell membrane and the nuclear membrane. This breaks the membranes down, so the contents of the cell and the nucleus are released. The DNA is then free in the solution.
Mark scheme
- Breaks down the cell membrane — 1 mark
- And the nuclear membrane — 1 mark
- Releases the DNA — 1 mark
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5 Explain [3 marks]
Explain why ice-cold ethanol is added to the filtered mixture.
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Model answer
DNA does not dissolve in ethanol, so it comes out of the solution and forms a white precipitate. The ethanol is cold so the DNA separates more easily and the enzymes that could break it down are slowed.
Mark scheme
- DNA is insoluble in ethanol — 1 mark
- So it forms a precipitate that can be seen — 1 mark
- Cold: easier separation / slows enzymes — 1 mark
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6 Evaluate [6 marks]
A student compares the amount of DNA extracted from a strawberry, a banana and an onion. Describe how the student could make the investigation a fair test, and evaluate the method.
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Model answer
The student should use the same mass of each fruit, the same volumes of detergent solution and ethanol, the same mashing and mixing times and the same temperature of ethanol. The type of fruit is the independent variable, and the amount of DNA is the dependent variable, which can be measured by the length of the strands or by scoring. The method is quick, cheap and safe, and the DNA is visible. However, the DNA is not pure, and it is difficult to measure the amount accurately, so results are mostly qualitative and could be biased. The student could repeat the experiment to calculate means, and weigh the dried DNA.
Mark scheme
- Same mass of fruit and volumes of solutions — 1 mark
- Same times and temperature — 1 mark
- Independent and dependent variables identified — 1 mark
- A strength, such as quick or cheap — 1 mark
- A weakness, such as not pure or hard to measure — 1 mark
- An improvement, such as repeating and a mean — 1 mark
Quick check
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1
What is the purpose of the detergent in DNA extraction?
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B: To break down the cell and nuclear membranes
Detergent breaks down the cell membranes and the nuclear membrane, so the DNA is released.
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2
Why is the mixture filtered?
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C: To remove the solid debris
Filtering removes the solid pieces of fruit and cell debris from the liquid that contains the DNA.
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3
Why is ethanol added at the end?
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A: DNA is insoluble in it, so it precipitates
DNA is insoluble in ethanol, so it forms a visible precipitate.
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4
What does extracted DNA look like?
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D: White stringy strands
It appears as white, stringy strands at the boundary between the filtrate and the ethanol.
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5
What is the main hazard of ethanol?
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B: It is flammable
Ethanol is flammable and so must be kept away from flames.
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6
Which step breaks open the cell walls?
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A: Mashing the fruit
Mashing crushes the fruit and breaks open the cells.
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7
What is the role of salt in the extraction solution?
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C: It helps the DNA to clump together
Salt helps the DNA to clump together, which makes the strands easier to see.
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8
Why is the ethanol added slowly down the side of the tube?
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D: To form a separate layer on top
Adding it slowly makes a separate layer on top of the filtrate, and the DNA appears at the boundary.
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9
A student says: "The DNA dissolved in the ethanol, which is why strands appeared." What is wrong with this?
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B: DNA does not dissolve in ethanol, so it formed a precipitate
DNA is insoluble in ethanol. It appeared because it came out of the solution as a precipitate.
From Genes to Proteins (Higher tier)
Just this lesson-
1 State [2 marks]
State what a triplet is, and what it codes for.
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Model answer
A triplet is a group of three bases in DNA. It codes for one amino acid.
Mark scheme
- Three bases — 1 mark
- One amino acid — 1 mark
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2 Calculate [2 marks]
The coding section of a gene has 600 bases. Calculate the number of amino acids in the protein it codes for.
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Model answer
Each amino acid is coded for by 3 bases, so 600 ÷ 3 = 200 amino acids.
Mark scheme
- Divides by 3 — 1 mark
- 200 amino acids — 1 mark
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3 Explain [3 marks]
Explain why four different bases can code for 20 different amino acids.
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Model answer
The bases are read in groups of three, called triplets. Four bases in groups of three give 64 different combinations, which is more than enough for 20 amino acids.
Mark scheme
- Bases read in groups of three — 1 mark
- 64 combinations possible — 1 mark
- More than the 20 amino acids needed — 1 mark
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4 Describe [3 marks]
Describe how the order of bases in a gene decides the structure of a protein.
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Model answer
The order of the bases is read in triplets, and each triplet codes for an amino acid, so the order of the bases decides the order of amino acids in the chain. The chain then folds into a specific shape, which is the structure of the protein.
Mark scheme
- Triplets code for amino acids — 1 mark
- Order of bases gives the order of amino acids — 1 mark
- Chain folds into a specific shape — 1 mark
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5 Explain [4 marks]
An enzyme is a protein. Explain why a change in the order of its amino acids might stop it working.
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Model answer
The order of amino acids decides how the chain folds, so a change might alter the shape of the enzyme. This could change the shape of the active site, so the substrate would no longer fit into it. The enzyme could no longer catalyse the reaction.
Mark scheme
- The order decides how the chain folds — 1 mark
- Shape of the enzyme changes — 1 mark
- The active site is a different shape, so the substrate does not fit — 1 mark
- The reaction is not catalysed — 1 mark
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6 Explain [6 marks]
Explain how a gene codes for a protein, and why the protein can only do its job if it has the right shape.
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Model answer
A gene is a section of DNA, and the order of its bases is read in triplets. Each triplet codes for one amino acid, so the order of bases decides the order of amino acids in a polypeptide chain. The chain then folds up to give a specific three-dimensional shape. The shape is important because it allows the protein to carry out its function. For example, an enzyme has an active site that matches the shape of its substrate, and an antibody has a shape that fits one antigen. If the order of amino acids changes, the shape may change and the protein may no longer work.
Mark scheme
- Order of bases read in triplets — 1 mark
- Triplet codes for an amino acid — 1 mark
- Order of amino acids decides the polypeptide — 1 mark
- The chain folds into a specific shape — 1 mark
- The shape suits the function, with an example — 1 mark
- A change in order can change the shape / function — 1 mark
Quick check
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1
What are proteins made from?
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C: Amino acids
Proteins are made from chains of amino acids.
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2
How many bases code for one amino acid?
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B: 3
A triplet of three bases codes for one amino acid.
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3
What decides the order of amino acids in a protein?
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A: The order of bases in the gene
The order of bases in the gene is read in triplets, and this decides the order of amino acids.
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4
A polypeptide has 50 amino acids. How many bases code for it, at least?
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D: 150
Each amino acid needs three bases, so \(50 \times 3 = 150\).
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5
What happens to a polypeptide chain after it is made?
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B: It folds into a specific shape
It folds to give a specific three-dimensional shape.
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6
Why can a protein with the wrong shape fail to work?
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C: It cannot fit the molecule it works with
The shape allows it to fit its substrate or other molecule, so if the shape changes it may not fit.
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7
How many different triplets can be made from the four bases?
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A: 64
With four choices for each of three places, there are \(4 \times 4 \times 4 = 64\) triplets.
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8
Why can bacteria be made to produce human insulin?
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D: All organisms use the same genetic code
All living things use the same genetic code, so bacteria read the human gene and make the same protein.
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9
A gene has 3 more bases inserted in the middle of its coding section. Roughly what happens to the polypeptide?
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B: It will have one extra amino acid
Three extra bases add one triplet, so the polypeptide will have one extra amino acid, which may change how it folds.
Protein Synthesis (Higher tier)
Just this lesson-
1 State [2 marks]
State the names of the two stages of protein synthesis.
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Model answer
Transcription and translation.
Mark scheme
- Transcription — 1 mark
- Translation — 1 mark
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2 Complete [2 marks]
The DNA coding sequence for part of a gene is A A A C C T T T T. Write the sequence of the mRNA that is made from it, using U for uracil.
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Model answer
U U U G G A A A A.
Mark scheme
- Applies the base pairing rule — 1 mark
- U U U G G A A A A — 1 mark
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3 Describe [3 marks]
Describe the role of RNA polymerase in protein synthesis.
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Model answer
RNA polymerase binds to non-coding DNA in front of a gene. It moves along the gene and makes a complementary strand of mRNA from the coding DNA.
Mark scheme
- Binds to non-coding DNA in front of the gene — 1 mark
- Moves along the coding DNA — 1 mark
- Makes a complementary mRNA — 1 mark
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4 Explain [3 marks]
Explain why mRNA is needed in protein synthesis.
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Model answer
DNA stays in the nucleus, but the ribosomes where proteins are made are in the cytoplasm. mRNA is a copy of the gene that can leave the nucleus. It carries the code to the ribosome.
Mark scheme
- DNA cannot leave the nucleus — 1 mark
- Ribosomes are in the cytoplasm — 1 mark
- mRNA carries the code — 1 mark
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5 Describe [4 marks]
Describe what happens at the ribosome during translation.
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Model answer
The mRNA attaches to the ribosome, and the ribosome reads it in groups of three bases called codons. tRNA molecules bring the amino acids that match each codon, using their anticodons. The ribosome joins the amino acids together in the right order to make a polypeptide.
Mark scheme
- mRNA attaches to the ribosome — 1 mark
- Read as codons — 1 mark
- tRNA brings amino acids matching the codons — 1 mark
- Amino acids are linked into a polypeptide — 1 mark
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6 Explain [6 marks]
Explain how the information in a gene is used to make a protein.
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Model answer
In transcription, RNA polymerase binds to non-coding DNA in front of the gene. The DNA strands separate, and RNA polymerase makes a complementary mRNA strand from the coding DNA of the gene. The mRNA leaves the nucleus and attaches to a ribosome. In translation, the ribosome reads the mRNA in codons of three bases. tRNA molecules, each carrying a specific amino acid, pair their anticodons with the codons. The amino acids are linked together in order to form a polypeptide, which folds into the protein.
Mark scheme
- RNA polymerase binds in front of the gene — 1 mark
- Complementary mRNA made from the coding DNA — 1 mark
- mRNA leaves the nucleus and attaches to a ribosome — 1 mark
- Read in codons — 1 mark
- tRNA brings amino acids matching the codons — 1 mark
- Amino acids linked to form a polypeptide — 1 mark
Quick check
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1
Where does transcription happen?
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A: In the nucleus
Transcription happens in the nucleus, where the DNA is.
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2
Which molecule carries the code from the nucleus to the ribosome?
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C: mRNA
mRNA is a copy of the gene that carries the code to the ribosome.
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3
Which base is found in RNA but not in DNA?
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D: Uracil
Uracil replaces thymine in RNA.
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4
What is the mRNA for the DNA sequence C G T?
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B: G C A
C pairs with G, G with C and T with A, so the mRNA is G C A.
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5
What does tRNA do?
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A: Carries an amino acid to the ribosome
tRNA carries a specific amino acid to the ribosome and matches its anticodon to the codon.
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6
What is a codon?
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C: Three bases on mRNA that code for one amino acid
A codon is a group of three bases on mRNA that codes for one amino acid.
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7
Which enzyme makes mRNA?
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B: RNA polymerase
RNA polymerase makes mRNA from the coding DNA of a gene.
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8
Which statement about translation is correct?
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D: It makes a polypeptide from the code in mRNA
In translation the ribosome reads the mRNA, and amino acids are joined in the order of the codons.
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9
A cell needs to make a lot of one protein quickly. Which statement best explains how it can?
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A: Many ribosomes can read the same mRNA at the same time
Many ribosomes can read the same mRNA at once, so lots of copies of the polypeptide are made at the same time.
Genetic Variants and Their Effects (Higher tier)
Just this lesson-
1 State [2 marks]
State what is meant by a genetic variant.
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Model answer
A genetic variant is a difference in the base sequence of DNA between individuals. It is caused by a mutation.
Mark scheme
- Difference in the base sequence — 1 mark
- Between individuals / caused by a mutation — 1 mark
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2 Describe [3 marks]
Describe how a variant in the non-coding DNA in front of a gene can affect the phenotype.
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Model answer
The variant can change how well RNA polymerase binds to the DNA in front of the gene. This changes the amount of mRNA made, and so the quantity of protein produced. The change in the amount of protein can change the phenotype.
Mark scheme
- Affects the binding of RNA polymerase — 1 mark
- Changes the quantity of mRNA / protein — 1 mark
- This changes the phenotype — 1 mark
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3 Describe [3 marks]
Describe how a variant in the coding DNA of a gene can affect the activity of a protein.
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Model answer
The variant can change the sequence of amino acids in the protein. This can change how the polypeptide folds, so its shape is different. The changed shape can alter its activity, for example an enzyme might not fit its substrate.
Mark scheme
- Changes the amino acid sequence — 1 mark
- Changes the shape of the protein — 1 mark
- Changes the activity — 1 mark
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4 Explain [3 marks]
A substitution in a gene changes one base, but the protein made is exactly the same. Explain how this can happen.
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Model answer
Several different triplets code for the same amino acid. The new triplet therefore codes for the same amino acid as the original one. The amino acid sequence is unchanged, so the protein is the same.
Mark scheme
- Different triplets can code for the same amino acid — 1 mark
- The amino acid is unchanged — 1 mark
- So the protein is the same — 1 mark
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5 Explain [4 marks]
Sickle cell disease is caused by a change of one base in the haemoglobin gene. Explain how one base change can cause the disease.
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Model answer
The change of one base alters a triplet, so a different amino acid, valine, replaces glutamic acid in the polypeptide. This changes the shape of the haemoglobin molecule. At low oxygen concentrations the haemoglobin molecules stick together, which distorts the red blood cells into a sickle shape and stops them from carrying oxygen properly.
Mark scheme
- Triplet changed, so a different amino acid — 1 mark
- Shape of haemoglobin changes — 1 mark
- Molecules stick together at low oxygen — 1 mark
- Cells distorted so they carry less oxygen — 1 mark
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6 Compare [6 marks]
Compare the effects on the phenotype of variants in the non-coding DNA and in the coding DNA of a gene.
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Model answer
A variant in the non-coding DNA in front of a gene can alter how well RNA polymerase binds, which changes how much mRNA and so how much protein is made. The protein itself is normal, so only the quantity changes. An example is lactase persistence, where a variant keeps the lactase gene switched on in adults. A variant in the coding DNA can change the amino acid sequence, which changes the shape and activity of the protein. An example is sickle cell disease, where one amino acid is changed in haemoglobin. In both cases the phenotype can change, but in a different way.
Mark scheme
- Non-coding: changes RNA polymerase binding — 1 mark
- Non-coding: quantity of protein changes, protein is normal — 1 mark
- Example of non-coding variant — 1 mark
- Coding: changes the amino acid sequence — 1 mark
- Coding: changes protein shape and activity — 1 mark
- Example of coding variant — 1 mark
Quick check
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1
What does RNA polymerase bind to?
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B: Non-coding DNA in front of a gene
RNA polymerase binds to the non-coding DNA in front of a gene.
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2
A variant in non-coding DNA reduces RNA polymerase binding. What is the most likely effect?
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A: Less of the protein is made
Less binding means less mRNA is made, so less of the protein is produced.
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3
A variant in coding DNA changes one amino acid. What may happen?
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C: The shape and activity of the protein may change
A different amino acid can change how the polypeptide folds, so the protein's shape and activity can change.
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4
Which type of mutation swaps one base for another?
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D: Substitution
A substitution replaces one base with a different base.
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5
Why might a substitution have no effect on the protein?
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B: Different triplets can code for the same amino acid
Several triplets code for the same amino acid, so the amino acid may stay the same.
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6
What is a frameshift?
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A: An insertion or deletion that changes how the following triplets are read
An insertion or deletion that is not a multiple of three changes the way every following triplet is read.
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7
What is the cause of sickle cell disease?
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D: A single base change in the haemoglobin gene
A change in one base in the haemoglobin gene means valine replaces glutamic acid, changing the shape of haemoglobin.
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8
Lactase persistence is caused by a variant near the lactase gene that keeps it switched on. What type of variant is this?
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C: A variant in non-coding DNA that changes how much protein is made
The variant is in the non-coding DNA, and it changes the quantity of protein, not its structure.
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9
Three bases are deleted from the middle of a gene's coding DNA. What is the most likely result?
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B: One amino acid is lost
Deleting exactly three bases removes one codon, so one amino acid is lost but the rest are read correctly. It is not a frameshift.