IB DP Biology- D2.1 Cell and nuclear division- IB Style Questions For HL Paper 2 -FA 2025
▶️ Answer/Explanation
(a)
• Causes: A substitution mutation occurs when a single nucleotide is replaced with a different one. This may arise from errors during DNA replication or due to exposure to mutagenic agents such as UV radiation or chemical pollutants.
• Consequences: A substitution alters one codon in the gene and may result in:
– Silent mutation: No change in the amino acid produced.
– Missense mutation: Production of a different amino acid, which can change protein function.
– Nonsense mutation: Creation of a stop codon, leading to a shortened and likely non-functional protein.
If the mutation occurs in a body cell (somatic), it may contribute to diseases such as cancer. If it occurs in a gamete, it can be inherited by offspring.
(b)
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of Divisions | One | Two |
| Chromosome Number | Maintained (diploid → diploid) | Halved (diploid → haploid) |
| Daughter Cells | Two, identical | Four, genetically varied |
| Genetic Variation | No crossing over | Crossing over occurs |
| Purpose | Growth, repair, asexual reproduction | Formation of gametes for sexual reproduction |
(c)
Gene expression during transcription is controlled by transcription factors that bind to DNA and regulate whether RNA polymerase can initiate transcription.
• Promoters: Specific DNA sequences where RNA polymerase binds; transcription factors may enhance or inhibit this process.
• Enhancers and silencers: Regulatory sequences where activator or repressor proteins bind to increase or decrease transcription rates.
Epigenetic mechanisms also influence transcription:
• DNA methylation: Adding methyl groups to DNA usually reduces transcription.
• Histone modification: Acetylation or methylation of histones affects DNA packing, thereby controlling how accessible genes are.
These regulatory processes determine which proteins are produced, shaping the organism’s traits (phenotype). Environmental factors such as diet, hormones, or toxins can also influence transcriptional regulation.