Sex-linked disorders are genetic disorders caused by genes located on the sex chromosomes (usually the X chromosome), so their inheritance is linked to the sex of the individual. Haemophilia is an X-linked recessive disorder in which the blood does not clot properly (a clotting factor is missing), so there is prolonged bleeding even from minor injuries. Colour blindness (red-green) is also an X-linked recessive disorder in which a person cannot distinguish between red and green colours. Because these genes are on the X chromosome, such disorders are more common in males (who have only one X chromosome).

Semiconservative replication is the method of DNA replication in which each of the two strands of the parent DNA acts as a template, and the newly formed DNA molecule contains one old (parental) strand and one newly synthesised strand. During replication, the enzyme helicase unwinds and separates the two strands; then the enzyme DNA polymerase adds new nucleotides to each template strand following complementary base pairing, forming two identical daughter DNA molecules. This mode was experimentally proved by Meselson and Stahl.

Transcription is the process of copying the genetic information from one strand of DNA into a molecule of messenger RNA (mRNA). Only one strand of the DNA (the template strand) is copied. The enzyme RNA polymerase binds to the promoter region and moves along the template, adding ribonucleotides according to complementary base pairing (A of DNA pairs with U of RNA, and so on), forming the RNA. The three steps are initiation, elongation and termination. In eukaryotes the RNA is then processed (splicing) before it leaves the nucleus.