Practice Paper 2 (HL IB Biology)

Leber Congenital Amaurosis (LCA)  is a term given to a group of diseases that are caused by mutations in at least 25 genes. These diseases are all related to vision loss and blindness as the mutations all affect the functioning of the retina in the eye. It is often expressed in young children.

One of these genes is called CEP290. When functioning, this gene codes for a protein in the retina and helps to transport proteins in photoreceptor cells to aid with vision. If someone inherits two mutated copies of this gene then they are not able to produce this protein, and this causes them to have LCA10 and vision loss which could ultimately result in blindness if left untreated. 

The most common form of mutation in CEP290 is called a splice mutation. 

The way that this mutation works is that an area of non-coding intron DNA is mutated to turn some of that genetic code into a "cryptic" exon. This means that the non-coding DNA is incorrectly translated and the resulting protein has an additional section of amino acids that results in a non-functioning protein. 

A type of therapy for this condition is in clinical trials. This is called EDIT-101 and uses gene editing to cut this section of DNA from the affected cells. 

The gene editing method in EDIT-101 uses CRISPR technology.

Outline the function of the following molecules in this process:

Cas9

[1]

sgRNA (guide RNA)

[1]

Other than the mechanism described above, suggest two other possible outcomes of a mutation at a site on the DNA that controls splicing.

In the EDIT-101 procedure, a virus is loaded with the sgRNA and the Cas9 and then a small amount of this virus is inserted into the back of each of the patient's eyes using a syringe.

Describe the sequence of events that occurs after the patient has been injected with the sample. 

In a 2024 study, 12 adults and 2 children were injected with EDIT-101 in order to test for adverse side-effects and to identify if the treatment worked to improve patient vision. 

Suggest a reason why so few people participated in this trial. 

[2]

Nine participants experienced a "meaningful improvement" in their vision following the trial and zero participants experienced "serious" adverse side-effects from the treatment.

Explain why it may not be the case that the EDIT-101 medicine can immediately be rolled out to a much larger number of patients. 

[3]

One effect of an ageing population is the increase in deaths and health complications from dehydration in the elderly, many of whom lack a thirst response when partially dehydrated. 

A study was performed to examine the effect of ethanol ('alcohol') consumption on the urine output of a group of elderly men. 

Ethanol is a known diuretic.

Define the term diuretic and outline its mode of action. 

In the study referred to in part a), a group of 20 elderly men (age range 65-75) were given the same diet and were optimally hydrated before the study. All the men consumed no alcohol for 48 hours prior to the experiment. At the beginning of the study,

• one subgroup was given alcoholic beer, another non-alcoholic beer
• one subgroup was given alcoholic wine, another non-alcoholic wine
• one subgroup was given strong spirits, another plain water
• for the men given alcoholic drinks, an equivalent volume of ethanol was consumed in each case
• for the men given non-alcoholic drinks, the same volume was consumed as for their alcohol-containing counterparts

In each case, the men's average cumulative urine output was measured over the 4 hours following consumption of the drinks. The results are shown below.

Calculate the percentage difference between the urine produced in the first hour by men drinking alcoholic beer versus those drinking alcoholic wine. 

The researchers conducting the study concluded that drinking beer of any variety contributed more to dehydration than any other choice of drink. 

Evaluate this conclusion.

Study the following cladogram showing the evolutionary relationship between different mammalians.

Identify the mammalians that are the most closely related according to this cladogram.

State the purpose of the nodes in the cladogram.

Identify the mammalian group that were the first to branch off and form an independent group from the others.

Scientists investigated the effect of exercise on muscle fibres in mice. The mice were split into two groups. Group A undertook regular exercise for 8 weeks, Group B was not exercised. After 8 weeks the scientists compared how long each group of mice could carry out prolonged exercise.

[1]

[5]

The scientists then investigated the effect of cyanide on the uptake of oxygen in mitochondria isolated from mice cells. Cyanide acts as an inhibitor of several enzymes involved in respiration. The scientists compared the mice's mitochondria with mitochondria isolated from plant cells. They prepared a flask of suspension of each type of mitochondria and recorded the oxygen concentration in each flask over time.

• After 10 minutes they added ADP and a respiratory substrate to each suspension.
• After 20 minutes they added cyanide to each suspension.

Their results are shown in the graph below. During time A to C the results of the two flasks overlap.

[2]

[2]

One mark is available for clarity of communication throughout this question.

Deamination (the removal of an amino group from a molecule) and gluconeogenesis (the production of glucose from non-carbohydrate sources) are two reactions that occur in the liver.

From the information given, suggest with reasons, which metabolic reactions these are classified as.

Explain the properties of water that are key for the survival of an oak tree.

[7]

One mark is available for clarity of communication throughout this question.

Outline the equivalent features of selective breeding and evolution by natural selection.

Explain how natural selection can account for the development of antibiotic resistant bacterial strains.

[5]