Genes and healthEdexcel A-Level Biology A: Topic test

20 questions, 54 marks

Edexcel A-Level Biology A

Genes and health topic test

Total 54 marks

Name

Class

Date

1
Each alveolus of a healthy human lung has a wall made of a single layer of flattened epithelial cells and is surrounded by a network of capillaries.
(a)
Which feature most directly helps to maintain a steep concentration gradient for oxygen between the air in the alveolus and the blood?
[1 mark]
AThe alveolar wall is only one cell thick
BThere are millions of alveoli in each lung
CBlood flowing through the capillaries continually carries oxygen away
DThe epithelial cells are flattened
(b)
In a lung disease the alveolar wall becomes three times as thick and the difference in oxygen concentration between the alveolar air and the blood falls to half of its normal value. The surface area is unchanged. According to Fick's law, the rate of diffusion of oxygen is now
[1 mark]
Aone-sixth of its normal value
Bone-third of its normal value
Cone-half of its normal value
Dsix times its normal value
(c)
In emphysema, the walls between neighbouring alveoli break down so that several alveoli merge into one larger air space. Explain why this reduces the rate of gaseous exchange.
[2 marks]
Total for question 1: 4 marks
2
Red blood cells take up glucose from the blood plasma through carrier proteins in their cell surface membranes. The concentration of glucose is always higher in the plasma than inside the cell.
(a)
Which statement about the uptake of glucose by red blood cells is correct?
[1 mark]
AGlucose moves against its concentration gradient using energy from ATP
BGlucose diffuses directly through the phospholipid bilayer
CGlucose enters the cell in vesicles by endocytosis
DGlucose moves down its concentration gradient through carrier proteins without the use of ATP
(b)
When the glucose concentration in the plasma is raised from 5 to 20 mmol dm⁻³, the rate of uptake rises only slightly. What is the most likely explanation?
[1 mark]
AThe red blood cell runs out of ATP
BAll the carrier proteins are working at their maximum rate
CThe glucose molecules become too large to pass through the carriers
DThe concentration gradient is reversed
(c)
Sodium ions are moved out of red blood cells against their concentration gradient by carrier proteins. Explain why this process needs ATP.
[2 marks]
Total for question 2: 4 marks
3
Lactase is an enzyme that hydrolyses the disaccharide lactose in the small intestine. It is a globular protein made of a single polypeptide chain and is secreted by the cells of the intestine.
(a)
Explain how the primary structure of lactase determines its specific three-dimensional shape.
[3 marks]
(b)
Explain how lactase speeds up the hydrolysis of lactose and why it does not catalyse the hydrolysis of maltose.
[4 marks]
Total for question 3: 7 marks
4
A scientist studies a human gene that codes for a polypeptide of 120 amino acids. Before the cell divides, all of its DNA is copied, and at other times the gene is expressed to make the polypeptide.
(a)
Describe how DNA is replicated and explain why each of the two new DNA molecules is identical to the original.
[6 marks]
(b)
Describe how the information in this gene is used to make the polypeptide, including the minimum number of bases in the coding region of the mRNA.
[6 marks]
Total for question 4: 12 marks
5
During DNA replication, DNA polymerase occasionally inserts an incorrect nucleotide opposite a base in the template strand. Uncorrected errors become mutations in the daughter DNA.
(a)
A substitution changes an mRNA codon from GAA to GAG, but the amino acid sequence of the polypeptide is unchanged. Which feature of the genetic code explains this?
[1 mark]
AThe code is non-overlapping
BThe code is degenerate
CThe code is a triplet code
DThe first codon of a gene is always a start codon
(b)
A single nucleotide is deleted near the start of the coding region of a gene. Why is this usually more damaging than a substitution of one nucleotide?
[1 mark]
AIt always creates a stop codon at the start of the gene
BIt prevents the ribosome from binding to the mRNA
CIt stops DNA polymerase from copying the rest of the gene
DIt shifts the reading frame, so every codon after the deletion is changed
(c)
Explain how a substitution mutation in a gene can lead to a non-functional protein.
[2 marks]
Total for question 5: 4 marks
6
A man and a woman are each heterozygous carriers of the recessive cystic fibrosis allele, f. They are expecting a child.
(a)
What is the probability that the child will be a heterozygous carrier, like both of the parents?
[1 mark]
A50%
B25%
C75%
D100%
(b)
Which statement describes the phenotype of a heterozygous carrier of the cystic fibrosis allele?
[1 mark]
AThey have cystic fibrosis because they have one recessive allele
BThey have two recessive alleles and mild symptoms
CThey do not have cystic fibrosis but can pass the recessive allele to their children
DThey have two dominant alleles
(c)
The couple are offered a prenatal test. Give one advantage and one disadvantage of using chorionic villus sampling (CVS) rather than amniocentesis.
[2 marks]
Total for question 6: 4 marks
7
A student cuts identical cylinders from a sweet potato, weighs them and places one in each of a series of sucrose solutions for 30 minutes. The cylinder in 0.0 mol dm⁻³ sucrose solution increases in mass by 6%, the cylinder in 0.4 mol dm⁻³ shows no change in mass and the cylinder in 0.6 mol dm⁻³ decreases in mass by 5%.
(a)
Explain the changes in mass of the cylinders in the 0.0, 0.4 and 0.6 mol dm⁻³ solutions.
[3 marks]
(b)
Describe how the structure of the cell surface membrane allows water, but not sucrose, to enter the cells, and explain what would happen to the membrane if the cylinders were heated to 80 °C for five minutes before the investigation.
[4 marks]
Total for question 7: 7 marks
8
Phenylketonuria (PKU) is caused by a recessive allele of the gene for the enzyme phenylalanine hydroxylase, which converts the amino acid phenylalanine to tyrosine. In one family both parents are unaffected, but their first child has PKU. The couple are expecting a second child.
(a)
Explain how a mutation in the gene for phenylalanine hydroxylase can result in a non-functional enzyme and a build-up of phenylalanine.
[6 marks]
(b)
Using a genetic diagram, calculate the probability that the second child will have PKU, and discuss the issues that the couple may consider if a prenatal test is offered.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).