Genetics

DNA replication is semi-conservative because each of the two double-stranded DNA molecules contains one original template strand and one newly synthesized strand. This differs from the conservative model, where the original double helix would remain entirely intact while generating a completely new double-stranded copy.

Helicase is the enzyme responsible for breaking hydrogen bonds between nitrogenous bases to unwind the DNA double helix during replication. DNA polymerase, on the other hand, does not unwind the DNA but instead synthesizes the new complementary DNA strands by adding nucleotides.

Jacob and Monod's model explained how gene expression is regulated in prokaryotes, specifically the lac operon, a set of genes involved in lactose metabolism. A diagram for this topic isn't available yet — the worked example below walks the same reasoning step by step.

• !Many students confuse the lac operon with the discovery of DNA structure, which was actually achieved by Watson and Crick.
• !Zygotene is incorrect because it is the earlier stage where homologous chromosomes pair up but have not yet initiated crossing over.
• !Option A is incorrect because 46 chromosomes are retained after mitosis, not after the first meiotic division.

Answer each from memory — you'll see instantly whether you're right and why.

Run a focused 10-question mini-mock on Genetics and see it stick.