▶️ Answer/Explanation
Answer: B
Step-by-step reasoning using the dichotomous key:
- Animal is symmetrical?
→ Yes → Go to 2 - Animal has radial symmetry?
→ No (the octopus has bilateral symmetry) → Go to 3 - Animal has an anus?
→ Yes → Go to 4 - Animal has a visibly segmented body?
→ No (the octopus has a soft, unsegmented body) → B
Why B is Mollusca:
The animal shown in B is an octopus, which belongs to the phylum Mollusca.
It is bilaterally symmetrical, has an anus, and does not have visible body segmentation or an exoskeleton—matching all the conditions that lead to B.
▶️ Answer/Explanation
a.
b.i.
A mutation is a change in the DNA/base/nucleotide sequence of an organism’s genetic material.
b.ii.
- Mutations introduce genetic variation within a population.
- Some mutations may give individuals a survival or reproductive advantage in a particular environment.
- These individuals are more likely to survive and reproduce, passing on the beneficial mutations to their offspring.
- Over time, the advantageous traits become more common in the population.
This process is called natural selection, and it drives evolution.
Markscheme
a.
1. Correct base sequence: T, G, A;
2. Strand drawn anti-parallel;
3. Correct shapes used;
Award [2 max] if bonds are not from the correct carbon or if the nucleotides are not joined.
b.i.
Change in genetic makeup/DNA/nucleotide/base sequence.
b.ii.
1. Mutations cause variation among organisms of same species/population;
2. Some variations/mutations make individual more suited to its environment/way of life;
3. Individuals that are better adapted survive and produce offspring;
4. Individuals pass on genetic characteristics/mutation/variation to offspring;
5. Natural selection increases frequency of characteristics/alleles that make individuals better adapted;
▶️ Answer/Explanation
a.
- M. tuberculosis has a cell wall, which human cells lack.
- It contains pili or flagella for movement/attachment.
- It has 70S ribosomes instead of the 80S ribosomes in human cells.
- Its DNA is found in a nucleoid region and is circular and not enclosed in a nucleus.
It also has plasmids, which are small, circular extra-chromosomal DNA.
b.
- TB bacteria carry specific antigens on their surface.
- These antigens are recognized by lymphocytes, a type of white blood cell.
- The immune system activates a specific clone of B lymphocytes (plasma cells) to fight the infection.
- These cells begin to produce antibodies, which are proteins that bind specifically to the TB antigens.
- Each B cell produces only one type of antibody.
- This is called specific immunity.
- Some plasma cells become memory cells, which remain in the body.
These memory cells allow the immune system to respond faster and stronger if the infection occurs again.
c.
- Antibiotics kill bacteria by interfering with specific bacterial processes (e.g., cell wall synthesis).
- Some bacteria may have natural resistance due to mutations.
- When antibiotics are overused, non-resistant bacteria die, while resistant ones survive and multiply.
- These resistant traits can be passed to future generations.
- Resistant bacteria may also transfer resistance genes to other bacteria via plasmids.
- This leads to a population with more antibiotic-resistant bacteria.
Eventually, antibiotics become ineffective, and infections become harder or impossible to treat, posing a major public health risk.
Markscheme
a.
1. Cell wall ✔
2. Pili/flagella ✔
3. 70S ribosomes ✔
4. Nucleoid/circular DNA OR naked DNA ✔
5. Plasmids ✔
As candidates do not need to know the structure of Mycobacterium tuberculosis, all prokaryotic structures are accepted.
Ignore references to membrane bound organelles.
b.
1. Phagocytes/lymphocytes are white blood cells ✔
2. TB bacterium has a specific antigen ✔
3. This antigen is recognised by white blood cells ✔
4. A clone of lymphocytes/plasma cells/B cells are produced ✔
5. Antibodies are produced by lymphocytes ✔
6. Each lymphocyte produces just one type of antibody ✔
7. (This is) specific immunity ✔
8. (Part of the) antibody/immunoglobulin binds to the antigen/specific antibody binds to the specific antigen ✔
9. Antibodies are proteins/immunoglobulins ✔
10. (Some) plasma cells become memory cells ✔
11. Memory cells reproduce quickly ✔
12. Memory cells prevent infection in the future ✔
Allow annotated diagrams to explain the process.
c.
1. Antibiotics block bacterial processes ✔
2. Example of bacterial process ✔ (e.g. cell wall formation)
3. Variations exist naturally in a population/some are naturally resistant to the antibiotic ✔
4. Bacteria that are not resistant to this antibiotic will die/only resistant will survive (when antibiotic given) ✔
5. This characteristic could be passed to next generation ✔
6. (Natural selection) leads to changes in the proportions/frequency in the population ✔
7. Human population will be exposed to antibiotic resistant bacteria and will not have antibiotic to kill them ✔
8. (Antibiotic resistant bacteria) may pass resistance to other bacteria species/types by means of plasmids (so other bacteria species turn resistant too) ✔