Covering Lessons 06-08: DNA structure, complementary base pairing, semiconservative DNA replication, mitosis, meiosis, chromosome-number stability and why accurate replication matters for continuity of species.
Replication accuracy preserves hereditary information, mitosis preserves chromosome number within an organism, and meiosis halves chromosome number before fertilisation restores it.
Do not confuse mitosis with meiosis. Mitosis maintains chromosome number in somatic cells; meiosis reduces chromosome number in gamete formation.
Crossing over creates new combinations of existing alleles. It does not create new alleles, and mutation is not the focus of this checkpoint.
Which description of a nucleotide is correct?
Which statement best matches the Watson-Crick model of DNA?
What is the best explanation of semiconservative DNA replication?
Why does complementary base pairing matter in DNA replication?
Which outcome is most directly associated with mitosis?
Which statement correctly compares mitosis and meiosis?
What is the main result of meiosis I?
Which process contributes to variation during meiosis?
If an organism has a diploid chromosome number of 16, how many chromosomes would each gamete usually contain after meiosis?
Which statement best explains how IQ2 links to continuity of species?
Explain how complementary base pairing and semiconservative replication help preserve hereditary information. (4 marks)
Complementary base pairing means adenine pairs with thymine and cytosine pairs with guanine, so each original DNA strand can act as a template during replication (1 mark). This guides accurate placement of new nucleotides in the new strand (1 mark). In semiconservative replication, each daughter DNA molecule contains one original strand and one newly synthesised strand (1 mark). Together, these features help preserve the original base sequence and therefore hereditary information when cells replicate (1 mark).
Compare mitosis and meiosis in terms of chromosome number, purpose and effect on continuity of species. (5 marks)
Mitosis produces daughter cells that maintain the same chromosome number as the parent cell, while meiosis reduces chromosome number to produce haploid gametes (1 mark). Mitosis is used for growth, tissue repair and asexual reproduction in some organisms (1 mark). Meiosis is used in sexual reproduction so gametes carry half the chromosome number (1 mark). Mitosis supports continuity within an individual organism by preserving genetic stability in somatic cells (1 mark). Meiosis supports continuity across generations because fertilisation can restore the diploid number instead of causing chromosome number to double each generation (1 mark).
A student says, "Mitosis would be a better way to make gametes because it is more accurate than meiosis." Evaluate this claim using chromosome-number reasoning and the role of variation in sexual reproduction. (5 marks)
The claim is incorrect because gametes must be haploid, not diploid (1 mark). If gametes were produced by mitosis, they would keep the full chromosome number, so fertilisation would double chromosome number in the zygote and this would continue across generations (1 mark). Meiosis solves this by halving chromosome number before fertilisation restores the diploid number (1 mark). Meiosis also contributes to variation through crossing over and independent assortment, which mitosis does not provide in the same way (1 mark). Therefore meiosis, not mitosis, is essential for stable sexual reproduction and continuity of species across generations (1 mark).
Checkpoint 2 complete - IQ2 Cell Replication