DNA and Chromosomes



DNA provides the heritable information for all cells within an organism. It is composed of two complementary strands that run antiparallel to each other. When a cell divides, DNA replicates. One strand, the leading strand, is continuously replicated and the other strand, the lagging strand, is replicated discontinuously in short segments called Okazaki fragments. Both lagging and leading strands are read in the 3′ to 5′ direction, and both are made in the 5′ to 3′ direction. DNA replication is semiconservative, meaning the two new DNA molecules each have one strand of the original molecule and one new strand. DNA is packaged into chromosomes by winding the strands around histones, which then fold back and forth into loops. Prokaryotes lack this packaging, instead condensing the chromosome into a nucleoid. Gene expression can be regulated via condensing or relaxing chromatin, the collection of DNA and proteins in a chromosome. This regulation of gene expression without changes to the genes themselves can be heritable, which is known as epigenetics.

At A Glance

  • DNA is classified as a type of nucleic acid whose structure consists of a double helix that is composed of two antiparallel strands.
  • DNA replicates in a semiconservative manner to generate two daughter DNA molecules, each of which is composed of one strand of the original molecule and one new strand.
  • DNA is wound around proteins called histones, allowing both the DNA and histones to be packaged and tightly condensed to form chromosomes.
  • Gene expression can be regulated by condensing chromatin into heterochromatin or by relaxing it into euchromatin; this regulation can be inherited.