▶️ Answer/Explanation
(a)
- Chromosomes condense and become visible – they shorten and thicken by supercoiling.
- The nuclear membrane breaks down, so the chromosomes are free in the cytoplasm.
- Spindle fibers (made of microtubules) begin to form.
- Spindle fibers attach to the centromeres of the chromosomes.
- The chromosomes line up at the middle of the cell (equator).
- The centromeres split, and the sister chromatids separate.
- The chromatids are pulled to opposite sides (poles) of the cell.
A new nuclear membrane forms around each group of chromosomes.
The cell is now ready to split into two identical daughter cells.
(b)
- The DNA is copied (replicated) so each chromosome will have two identical strands.
- An enzyme called helicase unwinds and separates the DNA strands.
- The bases are exposed, and new strands are made using the original strands as templates.
- The enzyme DNA polymerase adds new nucleotides using complementary base pairing (A with T, C with G).
- The result is two identical DNA molecules – one for each future cell.
- RNA is also made (transcription), where mRNA is copied from DNA.
- The mRNA leaves the nucleus and goes to the ribosomes to make proteins.
Introns (non-coding sections) are removed from the mRNA before it leaves.
Interphase is all about preparing the cell for division — copying DNA and making RNA.
(c)
- The Y chromosome has a gene that tells the embryo to grow testes.
- These testes start to produce testosterone, the male sex hormone.
Testosterone causes male body parts like the penis and other male features to develop.
Markscheme
a
a. supercoiling/condensation of chromosomes
b. breakup of nuclear membrane
c. growth of spindle/microtubules
d. attachment of spindle/microtubules to chromosomes/centromeres
e. chromosomes line up at the equator/middle
f. division of centromeres
g. separation of sister chromatids/chromosomes
OR
sister chromatids/chromosomes move to opposite poles
h. reformation of nuclear membranes around chromosomes at each pole
b
a. DNA is replicated
b. unwinding of the double helix/DNA (by helicase)
OR
supercoiled DNA is relaxed (by gyrase)
c. hydrogen bonds broken
OR
strands separate
d. synthesis of new strands of DNA (by DNA polymerase)
e. DNA replication occurs semi-conservatively
OR
existing strand used as a template (for a new strand of DNA)
f. copying base sequences occurs through complementary base pairing
g. DNA is transcribed
h. synthesis of RNA/mRNA (by RNA polymerase)
i. copy of a gene produced/only one gene transcribed
j. introns are removed
k. RNA/mRNA exported/moves to the cytoplasm
c
a. gene/genetic information on Y chromosome causes embryo to develop testes
b. (developing) testes in embryo secrete testosterone
c. testosterone causes male genitalia/penis to develop
▶️ Answer/Explanation
(a)
Interphase is the phase where the cell is preparing for division. Key processes include:
- DNA replication – the DNA is copied so each new cell will have a full set.
- Cell growth – the cell gets bigger and prepares materials for division.
- Organelle duplication – mitochondria and other structures are copied.
Protein synthesis – the cell makes proteins needed for the next phase.
(b)
(i)
(ii)
(c)
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of cells made | 2 | 4 |
| Chromosome number | Same as original (diploid, 2n) | Half of original (haploid, n) |
| Genetic identity | Genetically identical | Genetically different |
| Purpose | Growth, repair, and body cells | Production of gametes (sperm/egg) |
(d)
The mitotic index tells us how many cells are dividing in a sample. It’s calculated using:
Mitotic Index = (Number of cells undergoing mitosis) ÷ (Total number of cells observed)
This can also be expressed as a percentage.
It helps doctors estimate how fast cancer cells are growing and how well treatment might work.
Markscheme
a
a. DNA replication
b. cell growth
c. duplication of organelles/mitochondria / production of microtubules/protein synthesis
Accept first two answers only
b
i prophase
The stage should be clearly labelled. If more than one stage is shown the candidate does not receive a mark.
ii anaphase
The stage should be clearly labelled. If more than one stage is shown the candidate does not receive a mark.
c
a. mitosis produces two daughter cells while meiosis four
b. mitosis produces cells with same number of chromosomes (2n) while in meiosis they are halved (n)
c. mitosis produces body cells but meiosis produces gametes OR mitosis produces genetically identical cells but meiosis does not
Allow answers in a table. Reference to both has to be present for the mark
d
number of cells (seen under the microscope) undergoing mitosis divided by the total number of cells (observed in sample area)
Can be given as a %
▶️ Answer/Explanation
(a)
Answer: It is a eukaryotic cell.
Reason: The cell has a nucleus and membrane-bound organelles like mitochondria or endoplasmic reticulum, which are only found in eukaryotic cells. Prokaryotes (like bacteria) don’t have a nucleus or other membrane-bound organelles.
(b)(i)
Answer: Mitosis
Mitosis ensures that each new nucleus gets a complete and identical set of chromosomes.
(b)(ii)
- The cell cycle is made up of different phases: G₁, S, G₂, and mitosis (M phase).
- It is controlled by special proteins called cyclins, which rise and fall at specific times.
- Each cyclin binds to an enzyme called a cyclin-dependent kinase (CDK).
- The CDK-cyclin complex activates or deactivates other proteins by phosphorylation.
- These proteins help move the cell from one phase to the next.
- Checkpoints make sure the cell only divides if it’s ready (e.g., DNA is correctly copied).
Both internal signals (like DNA damage) and external signals (like growth factors) affect the control of the cycle.
Markscheme
a. Eukaryotic because nucleus/membrane-bound organelles/named organelle other than ribosome (e.g., mitochondria, vesicles) internal membranes (ER)/multicellular.
b(i). Mitosis
b(ii).
- Cell cycle is a sequence of stages (G1, S, G2, and mitosis)
- Control by cyclins, which fluctuate in concentration during the cycle
- Different cyclins regulate entry into different stages
- Cyclin-dependent kinases (CDKs) are activated by cyclins
- CDKs phosphorylate proteins to trigger cell cycle events
- Internal and external conditions affect regulation
▶️ Answer/Explanation
(a)
The actual size of the bacterium is 1 µm = 1000 nm.
Let’s assume the measured image length of the bacterium = 4.5 cm = 45 mm = 45,000 nm.
\[
\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} = \frac{45{,}000\ \text{nm}}{1{,}000\ \text{nm}} = 45{,}000\times
\]
(b) Binary fission
Bacteria reproduce asexually by binary fission, where the cell duplicates its DNA and splits into two genetically identical cells.
(c)
- Antibiotics are effective against bacteria because they target bacterial processes such as cell wall synthesis, ribosomes, or DNA replication.
- Antibiotics are not effective against viruses because viruses do not have a cell wall, use host cell machinery, and are not alive in the same way.
(d)(i)
Saprotrophic organisms are organisms that:
- Secrete enzymes onto dead organic matter, and
- Absorb the nutrients or products of digestion.
This is a form of external digestion.
(d)(ii)
Example roles:
- Decompose dead organisms
- Recycle nutrients
Break down organic material into inorganic substances (e.g., returning nitrogen or carbon to the soil or atmosphere)
Markscheme
a. 45 000× (accept answers in the range of 44 000 to 46 000)
b. Binary fission
c. Effective against bacteria, but not viruses
d(i). An organism that secretes enzymes into dead organic matter and absorbs its nutrients/products of digestion
d(ii). Decomposer / recycle nutrients / break down organic material into inorganic material
Do not just accept “recycle” alone.
▶️ Answer/Explanation
a. Stages of the Cell Cycle:
- Interphase is the longest phase.
- Interphase includes G₁, S, and G₂ phases.
- In G₁ and G₂, the cell grows, performs normal functions (like protein synthesis), and replicates organelles.
- In the S-phase, DNA replication takes place.
- Mitosis is the division of the nucleus/genetic material.
Cytokinesis is the division of the cytoplasm, forming two daughter cells.
b. Translation in Cells:
- mRNA binds to a ribosome.
- tRNA molecules bring specific amino acids and match their anticodons with codons on the mRNA.
- Peptide bonds form between the amino acids, creating a growing polypeptide chain.
- The ribosome moves along the mRNA until it reaches a stop codon.
The completed polypeptide is released.
c. Dipeptide Formation (Condensation Reaction):
- Two amino acids join through a condensation reaction, releasing a molecule of water.
- The carboxyl group (–COOH) of one amino acid reacts with the amino group (–NH₂) of the other.
- This forms a peptide bond (–CO–NH–) between them.
The result is a dipeptide.
Markscheme
a. Stages of the Cell Cycle:
1. Interphase is the longest phase.
2. Interphase includes \( G_1 \), \( S \), and \( G_2 \).
3. In \( G_1 \) and \( G_2 \), the cell performs normal functions (e.g., protein synthesis, growth, organelle replication).
4. \( S \)-phase (synthesis) is when DNA replicates.
5. Mitosis is the division of the nucleus/genetic material.
6. Cytokinesis is the division of the cytoplasm, forming two daughter cells.
Award [3 max] if all three stages (interphase, mitosis, cytokinesis) are not mentioned.
b. Process of Translation:
1. mRNA attaches to the ribosome.
2. tRNA carries specific amino acids and binds to mRNA codons via anticodons.
3. Peptide bonds form between amino acids, elongating the polypeptide chain.
4. The ribosome moves along mRNA until a stop codon is reached.
5. The polypeptide is released upon encountering the stop codon.
c. Dipeptide Formation:
1. A condensation reaction joins two amino acids, eliminating \( H_2O \).
2. The carboxyl group (\( \text{COOH} \)) of one amino acid reacts with the amino group (\( \text{NH}_2 \)) of another.
3. A peptide bond (\( \text{CO-NH} \)) forms, producing a dipeptide.
4. This occurs at ribosomes during protein synthesis.