aqa biology exam questions and answers 2023 with
complete solution
1)What name is used for the non-coding sections of a gene?
Introns
Some gene mutations affect the amino acid sequence. Some mutations do not. Explain
whether mutation 1 affects the amino acid sequence.
No effect as it still codes for the same amino acid (Glycine)
How mutation 2 could lead to the formation of a non-functional enzyme.
Change in amino acid sequence
Change in hydrogen and ionic bonds which alters tertiary structure
Substrate is no longer complementary to enzyme active site so no enzyme-substrate
complexes formed
Gene mutations occur spontaneously.
During which part of the cell cycle are gene mutations most likely to occur? Suggest an
explanation for your answer.
S phase is where DNA replication occurs
2) One hypothesis for the cause of cancer of the colon (large intestine) is that
Clostridium bacteria present in the gut can convert bile steroids into cancer-causing
substances. Explain the presence of bile in the colon.
Secreted by the liver, stored in the gall bladder and released into the small intestine;
Bile passes unchanged from small intestine to colon;
A statistical test showed there was a significant difference between the cancer patients
and the controls in each of the four categories.
Explain how the results could be used to support the hypothesis that Clostridium
bacteria convert bile steroids into substances which cause colon cancer.
Difference between the two patients types is not due to chance alone
Patients who have high concentration of bile steroids and a high number of bacteria has
a significantly higher percentage of cancer
Patients with low steroids and low bacteria has a significantly higher percentage of
control patients;
Explain how the results indicate that other factors may be involved in causing colon
cancer.
Some patients with low levels of both factors have cancer;
Human cells contain genes that control their growth and division. One of these genes
codes for a protein that prevents cell division. The substances formed from bile steroids
by Clostridium bacteria may cause gene mutation. Describe and explain how these
substances could cause colon cancer.
Mutation e.g base deletion causes a change in base sequence of gene therefore a
different mRNA strand is transcribed (change in mRNA) so amino acid sequence is
changed which produces a different protein which is non-functional. This leads to
uncontrolled cell division.
3)Describe how altered DNA may lead to cancer.
1 DNA altered by a mutation
2 Mutation causes a change in base sequence of gene controlling cell division which
, leads to a change in amino acid sequence
3 This changes protein structure so a non-functional protein is produced.
4 This leads to uncontrolled mitosis thus a malignant tumour is produced
4) Explain why fragments of DNA from cancer cells may be present in blood plasma
Cancer cells die so they break open which releases DNA into the blood
Explain why the nucleic acid on the test strip will only bind to altered DNA
Normal DNA and altered DNA have different base sequences;
DNA only binds to a complementary base sequence;
This test strip will allow cancers to be detected at a very early stage. Explain why
cancer is more likely to be treated successfully if the disease is detected at a very early
stage.
Fewer cancerous cells so less cell damage thus fewer locations to treat
Explain how examining mRNA enables scientists to discover whether cancer is present.
mRNA base sequence has changed
Gene (proto-oncogene) has mutated so cancer gene (oncogene) is active and tumour
suppressor gene is inactive
4)Explain how this mutation leads to the formation of non-functioning PAH.
1 Mutation causes a change in bases sequence which leads to a change in amino acid
sequence
2. Change in hydrogen and ionic bonds alters tertiary structure of enzyme
3. Substrate no longer complementary to active site so doesn't bind thus no enzyme-
substrate complexes formed
Use the information in the diagram to give two symptoms you might expect to be visible
in a person who produces non-functioning PAH.
1. Lack of skin pigment so pale skin
2. Lack of muscle coordination
One mutation causing phenylketonuria was originally only found in one population in
central Asia. It is now found in many different populations across Asia. Suggest how the
spread of this mutation may have occurred.
Migration and interbreeding
5) Treatment with isoniazid leads to the osmotic lysis of this bacterium. Use information
in the diagram to suggest how.
1. Cell wall not formed
2. Lower water potential in bacterium;
3. Water enters via osmosis and causes osmotic lysis
Human cells also produce fatty acids. Isoniazid does not affect the production of these
fatty acids. Use information in the diagram to suggest one reason why isoniazid does
not affect the production of fatty acids in human cells.
Human cells lack enzyme B
A mutation in the gene coding for enzyme B could lead to the production of a non-
functional enzyme. Explain how.
Mutation causes a change in base sequence of gene which leads to a change in amino
acid sequence
This changes the hydrogen and ionic bonds which changes the tertiary structure
Substrate is no longer complementary to enzyme active site so no enzyme-substrate
complexes are formed
complete solution
1)What name is used for the non-coding sections of a gene?
Introns
Some gene mutations affect the amino acid sequence. Some mutations do not. Explain
whether mutation 1 affects the amino acid sequence.
No effect as it still codes for the same amino acid (Glycine)
How mutation 2 could lead to the formation of a non-functional enzyme.
Change in amino acid sequence
Change in hydrogen and ionic bonds which alters tertiary structure
Substrate is no longer complementary to enzyme active site so no enzyme-substrate
complexes formed
Gene mutations occur spontaneously.
During which part of the cell cycle are gene mutations most likely to occur? Suggest an
explanation for your answer.
S phase is where DNA replication occurs
2) One hypothesis for the cause of cancer of the colon (large intestine) is that
Clostridium bacteria present in the gut can convert bile steroids into cancer-causing
substances. Explain the presence of bile in the colon.
Secreted by the liver, stored in the gall bladder and released into the small intestine;
Bile passes unchanged from small intestine to colon;
A statistical test showed there was a significant difference between the cancer patients
and the controls in each of the four categories.
Explain how the results could be used to support the hypothesis that Clostridium
bacteria convert bile steroids into substances which cause colon cancer.
Difference between the two patients types is not due to chance alone
Patients who have high concentration of bile steroids and a high number of bacteria has
a significantly higher percentage of cancer
Patients with low steroids and low bacteria has a significantly higher percentage of
control patients;
Explain how the results indicate that other factors may be involved in causing colon
cancer.
Some patients with low levels of both factors have cancer;
Human cells contain genes that control their growth and division. One of these genes
codes for a protein that prevents cell division. The substances formed from bile steroids
by Clostridium bacteria may cause gene mutation. Describe and explain how these
substances could cause colon cancer.
Mutation e.g base deletion causes a change in base sequence of gene therefore a
different mRNA strand is transcribed (change in mRNA) so amino acid sequence is
changed which produces a different protein which is non-functional. This leads to
uncontrolled cell division.
3)Describe how altered DNA may lead to cancer.
1 DNA altered by a mutation
2 Mutation causes a change in base sequence of gene controlling cell division which
, leads to a change in amino acid sequence
3 This changes protein structure so a non-functional protein is produced.
4 This leads to uncontrolled mitosis thus a malignant tumour is produced
4) Explain why fragments of DNA from cancer cells may be present in blood plasma
Cancer cells die so they break open which releases DNA into the blood
Explain why the nucleic acid on the test strip will only bind to altered DNA
Normal DNA and altered DNA have different base sequences;
DNA only binds to a complementary base sequence;
This test strip will allow cancers to be detected at a very early stage. Explain why
cancer is more likely to be treated successfully if the disease is detected at a very early
stage.
Fewer cancerous cells so less cell damage thus fewer locations to treat
Explain how examining mRNA enables scientists to discover whether cancer is present.
mRNA base sequence has changed
Gene (proto-oncogene) has mutated so cancer gene (oncogene) is active and tumour
suppressor gene is inactive
4)Explain how this mutation leads to the formation of non-functioning PAH.
1 Mutation causes a change in bases sequence which leads to a change in amino acid
sequence
2. Change in hydrogen and ionic bonds alters tertiary structure of enzyme
3. Substrate no longer complementary to active site so doesn't bind thus no enzyme-
substrate complexes formed
Use the information in the diagram to give two symptoms you might expect to be visible
in a person who produces non-functioning PAH.
1. Lack of skin pigment so pale skin
2. Lack of muscle coordination
One mutation causing phenylketonuria was originally only found in one population in
central Asia. It is now found in many different populations across Asia. Suggest how the
spread of this mutation may have occurred.
Migration and interbreeding
5) Treatment with isoniazid leads to the osmotic lysis of this bacterium. Use information
in the diagram to suggest how.
1. Cell wall not formed
2. Lower water potential in bacterium;
3. Water enters via osmosis and causes osmotic lysis
Human cells also produce fatty acids. Isoniazid does not affect the production of these
fatty acids. Use information in the diagram to suggest one reason why isoniazid does
not affect the production of fatty acids in human cells.
Human cells lack enzyme B
A mutation in the gene coding for enzyme B could lead to the production of a non-
functional enzyme. Explain how.
Mutation causes a change in base sequence of gene which leads to a change in amino
acid sequence
This changes the hydrogen and ionic bonds which changes the tertiary structure
Substrate is no longer complementary to enzyme active site so no enzyme-substrate
complexes are formed
