By Dr Ananya Mandal, MD
How does DNA maintain its original form?
DNA carries a vast quantity of genetic information and continuation of species depends on accurate duplication of the DNA. This duplication process, called DNA replication, must occur before a cell can produce two genetically identical daughter cells.
Replication is accompanied by a continual surveillance and repair of genetic information, as DNA is subject to damage by chemicals and radiation from the environment, and by accidents and reactive molecules that occur inside the cell.
There are specialized enzymes that aid in DNA damage repair. These enzymes catalyze some processes that take place within cells and protect the DNA from damage and change.
Mutations and DNA change
Despite these efforts there are still some copying errors and accidental damage, permanent changes, or mutations. Mutations in the DNA often affect the information it encodes. Sometimes these mutations may lead bacteria to become resistant to antibiotics that are used to kill them.
In humans, mutations are often detrimental. These may be responsible for thousands of inherited diseases, and mutations that appear in cells throughout the lifetime of an individual. These may lead to many types of cancer.
DNA repair thus becomes important to prevent mutations and inherited diseases. DNA evolves over millions of years continually dividing. This is what makes each species unique.
DNA sequences in cells thus are maintained from generation to generation with very little change. While this is true, there is evidence that the DNA sequence in chromosomes does change with time and the DNA gets rearranged over time.
The combination of the genes on the genome may change due to such DNA rearrangements. In a population, this sort of genetic variation is important to allow organisms to evolve in response to a changing environment. These DNA rearrangements are caused by a class of mechanisms called genetic recombination.
Homologous DNA recombination
The most important form of genetic recombination is homologous recombination. The process involves the basic facts such as two double double-stranded DNA molecules that have regions of very similar (homologous) DNA sequence come together so that their homologous sequences are in tandem.
Then they can “cross-over”: in a complex reaction, both strands of each double helix are broken and the broken ends are rejoined to the ends of the opposite DNA molecule to re-form two intact double helices, each made up of parts of the two different DNA molecules.
In addition, the site where the DNA exchange has occurred can occur anywhere in the homologous nucleotide sequences of the two participating DNA. Further there may be no change in the nucleotide sequences at the site of exchange. The process of breaking and rejoining occurs so perfectly that not a single nucleotide is lost or gained.
The proteins that carry out this process in different organisms are often very similar to one another in amino acid sequence. Homologous recombination provides many advantages to cells and organisms in allowing an organism to repair DNA that is damaged on both strands of the double helix and correct genetic accidents that occur during nearly every round of DNA replication.
Nonhomologous DNA recombination
In homologous recombination, DNA rearrangements occur between DNA segments that are very similar in sequence. A second, more specialized type of recombination, called site-specific recombination, allows DNA exchanges to occur between DNA double helices that are dissimilar in nucleotide sequence.
The dissimilar sequence may be a mobile unit, commonly a virus. A virus can package its nucleic acid into viral particles that can move from one cell to another through the extracellular environment.
Mobile genetic elements often comprise a good fraction of an organism’s DNA. 45% of the human genome is made up of mobile genetic elements. Because they have a tendency to multiply, mobile DNA elements are sometimes called parasitic DNA. These are important in DNA evolution studies.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)