Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
More About Genes: Repair, Mutation, Recombination, and Cloning

Chromosomes are by no means inert, static structures that rigidly preserve Genetic information in its original form. They constantly undergo various types of modifications. Some of these changes are random and easily repaired. For instance, during Replication, when the duplex strands unwind and separate at high speed, single-strand breaks may occur in the DNA. These breaks are repaired by the action of DNA polymerase I and DNA ligase. Similarly, an incorrectly incorporated nucleotide that fails to form a proper complementary base pair can be excised and replaced thanks to the proofreading activity of DNA polymerase (Section 28.14). In this chapter, we will see that DNA alterations can also be triggered by various environmental factors. If The Cell fails to repair the resulting damage, it leads to a heritable change known as a mutation.

Chromosomes also undergo Other types of changes and rearrangements that occur as part of their normal biological functioning. The fusion of an egg and a sperm in eukaryotes is accompanied by genetic recombination, resulting in offspring with novel Gene combinations. Furthermore, genes or segments of genes can relocate from one chromosomal site to another. Genes can also be exchanged and recombined during viral infection of Cells.

Yet, despite constant damage, repair, exchange, and transposition of genes, the intra-species identity of living organisms is maintained across generations with exceptional fidelity. This is made possible by the remarkable ability of certain Enzymes to protect or restore specific nucleotide sequences within each of the Organism's chromosomes. The discovery of these enzymes has also enabled the laboratory construction of recombinant—essentially novel—DNA molecules, in which genes from one organism are combined with those of another. These breakthroughs ushered in a new era in biochemical genetics and gave rise to a new field: Introduction/32.html">Genetic Engineering.

Let us examine some types of DNA modifications.