Chapter Summary

Focus of the chapter:

  • The structure of the cell and the DNA molecule
  • DNA as the blueprint for life
  • The difference between mitosis and meiosis
  • Protein production and genes
  • Genetic variation, gene expression, population genetics

DNA is found in all life on Earth, in both prokaryotic (single-celled) and eukaryotic (multicellular) organisms. DNA makes up each chromosome and is the body’s genetic code. The number of chromosomes varies by species (humans have twenty-three pairs).

Nuclear DNA is found within the nucleus of a cell; mitochondrial DNA is found within the cell’s cytoplasm.

The double-sided helix of DNA is made up of four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Each string of DNA is made of pairs of these bases: A pairs with T, and C with G. These pairs form the genetic code that is replicated.

Some chromosomes carry the blueprints for nonsex traits (somatic chromosomes), whereas others carry the traits for male and female (sex chromosomes). Meiosis is the process by which DNA, carried on the chromosomes, replicates itself into haploid gametes that contain half the genetic information of the organism.

Errors in chromosome sorting can be a source of variation; translocations, or the exchange of chromosomal segments, are one example. Genes perform many functions that can be divided into structural and regulatory categories. Structural genes provide the code for body tissues and organs, whereas regulatory genes turn other genes on and off and are responsible for the body’s growth and maintenance.

As the field of modern genetics has developed, it has become clear that the idea of "one trait, one gene" is not completely applicable. Many traits are influenced by more than one gene (polygenic). Some single genes can have multiple effects (pleiotropy).