DNA Structure and FunctionDNAstructure and function (Genetics & Inherited Conditions)
The importance of chromosomes in heredity has been known since early in the twentieth century. Chromosomes consist of both DNA and protein, and in the early twentieth century there was considerable controversy concerning which component was the hereditary molecule. Early evidence favored the proteins. In 1944, however, a series of classic experiments by Oswald Avery, Maclyn McCarty, and Colin MacLeod lent strong support to the proponents favoring DNA as the genetic material. They showed that a genetic transforming agent of bacteria was DNA and not protein. In experiments reported in 1952, Alfred Hershey and Martha Chase provided evidence that DNA was the genetic material of bacteriophages (viruses that infect bacteria). Combined with additional circumstantial evidence from many sources, DNA became favored as the hereditary molecule, and a heated race began to determine its molecular structure.
In 1953, James Watson and Francis Crick published a model for the atomic structure of DNA. Their model was based on known chemical properties of DNA and X-ray diffraction data obtained from Rosalind Franklin and Maurice Wilkins. The structure itself made it clear that DNA was indeed the molecule of heredity and provided evidence for how it might be copied. The molecule resembles a ladder. The “rails” are composed of repeating units of sugar and phosphate, forming a backbone for the molecule. Each...
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The Cell CycleCell cycleDNA replication (Genetics & Inherited Conditions)
In eukaryotic organisms (most organisms other than bacteria), cells progress through a series of four stages between cell divisions. The stages begin with a period of growth (G1 phase), followed by replication of the DNA (S phase). A second period of growth (G2 phase) is followed by division of the cell (M phase). Each of the two cells resulting from the cell division goes through its own cell cycle or may enter a dormant stage (G0 phase). The passage from one stage to the next is tightly regulated and directed by internal and external signals to the cell.
The transition from G1 into S phase marks the beginning of DNA replication. In order to enter S phase, the cell must pass through a checkpoint or restriction point in which the cell determines the quality of its DNA: If there is any damage to the DNA, entry into S phase will be delayed. This prevents the potentially lethal process of beginning replication of a DNA molecule that has damage that would prevent completion of replication. If conditions are determined to be acceptable, a “molecular switch” is thrown, triggering the initiation of DNA replication. What is the nature of this molecular switch? There are many different proteins that participate in the process of DNA replication, and they can have their activity turned off and on by other proteins. Addition or removal of a chemical group called a phosphate is a common mechanism of chemical...
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Origins and Initiation (Genetics & Inherited Conditions)
If the human genome were replicated from one end to the other, it would take several years to complete the process. The DNA molecule is simply too large to be copied end to end. Instead, replication is initiated at many different sites called origins of replication, and DNA synthesis proceeds from each site in both directions until regions of copied DNA merge. The region of DNA copied from a particular origin is called a replicon. Using hundreds to tens of thousands of initiation sites and replicons, the genome can be copied in a matter of hours. The structure of replication origins has been difficult to identify in all but a few organisms, most notably yeast. Origins consist of several hundred base pairs of DNA comprising sequences that attract and bind a set of proteins called the origin recognition complex (ORC). The exact mechanism by which the origin is activated is still under investigation, but a favored model is supported by all of the available evidence.
The ORC proteins are believed to be bound to the origin DNA throughout the cell cycle but become activated at the G1/S boundary through the action of kinases. Kinases add phosphate groups to one or more of the six ORC proteins, activating them to initiate DNA replication. Different replicons are initiated at different times throughout S phase. It is unclear how the proposed regulatory system distinguishes between replicons that have been replicated in a...
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DNA SynthesisDNAsynthesis (Genetics & Inherited Conditions)
The DNA synthesis machinery is not able to synthesize a strand of DNA from scratch; rather, a short stretch of RNA is used to begin the new strands. The synthesis of the RNA is catalyzed by an enzyme called primase. This short piece of RNA, or primer, is extended using DNA nucleotides by the enzymes of DNA synthesis, called DNA polymerases. The RNA primer is later removed and replaced by DNA. Nucleotide monomers align with the exposed template DNA strand one at a time and are joined by the DNA polymerase. The joining of nucleotides into a growing DNA chain requires energy. This energy is supplied by the nucleotide monomers themselves. A high-energy phosphate bond in the nucleotide is split, and the breakage of this high-energy bond provides the energy to drive the polymerase reaction.
The two strands of DNA are not synthesized in the same way. The two strands are oriented opposite one another, but DNA synthesis only occurs in one direction: 5′ to 3′. Therefore, one strand, called the leading strand, is synthesized continuously in the same direction that the replication fork is moving, while the lagging strand is synthesized away from the direction of fork movement. Since the lagging-strand DNA synthesis and fork movement are in opposite directions, this strand of DNA must be made in short pieces that are later joined. Lagging-strand synthesis is therefore said to be discontinuous. These short intermediates are...
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Impact and Applications (Genetics & Inherited Conditions)
DNA replication is a fundamental cellular process: Proper cell growth cannot occur without it. It must be carefully regulated and tightly controlled. Despite its basic importance, the details of the mechanisms that regulate DNA replication are poorly understood. Even with all of the checks and balances that have evolved to ensure a properly replicated genome, occasional mistakes do occur. Attempting to replicate a genome damaged by chemical or other means may simply lead to death of a single cell. Far more ominous are genetic errors that lead to loss of regulating mechanisms. Without regulation, cell growth and division can proceed without normal limits, resulting in cancer. Much of the focus for the study of cell growth and regulation is to set a foundation for the understanding of how cancer cells develop. This knowledge may lead to new techniques for selective inhibition or destruction of cancer cells.
Manipulation of DNA replication and cell cycle control are the newest tools for progress in genetic engineering. In early 1997, the first successful cloning of an adult mammal, Dolly the sheep, raised important new issues about the biology and ethics of manipulating mammalian genomes. The technology now exists to clone human beings, although such experiments are not likely to be carried out. More relevant is the potential impact on agriculture. It is now possible to select for animals that have the most desirable...
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Further Reading (Genetics & Inherited Conditions)
Abstracts of Papers Presented at the 2007 Meeting on Eukaryotic DNA Replication and Genome Maintenance: September 5-September 9, 2007. Arranged by Stephen Bell and Joachim Li. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory, 2007. The laboratory regularly publishes abstracts of papers presented at its annual meeting on eukaryotic DNA replication.
Cann, Alan J. DNA Virus Replication. New York: Oxford University Press, 2000. Gives an analysis of protein-protein interactions in DNA virus replication, covering all major DNA virus groups: hepatitis B virus, papillomavirus, herpes simplex virus, Epstein-Barr virus, Kaposi’s sarcoma herpesvirus (KSHV), human cytomegalovirus, and adenoviruses. Illustrated.
Cotterill, Sue, ed. Eukaryotic DNA Replication: A Practical Approach. New York: Oxford University Press, 1999. Serves as a comprehensive lab manual that describes key aspects of current techniques for investigating DNA replication in eukaryotes. Contains more than one hundred reliable protocols, including methods for studying the origin of replication, replication proteins, and the synthesis of telomeres.
DePamphilis, Melvin L., ed. Concepts in Eukaryotic DNA Replication. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 1999. A broad account of the basic principles of DNA replication and related functions such as DNA repair and protein phosphorylation. One...
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Web Sites of Interest (Genetics & Inherited Conditions)
National Center for Biotechnology Information: “What Is a Cell?”. http://www.ncbi.nlm.nih.gov/About/primer/Genetics_cell.html. The site’s science primer includes a page providing basic information about cells, including an explanation of DNA replication.
Scitable. http://www.nature.com/scitable. Scitable, a library of science-related articles compiled by the Nature Publishing Group, features several articles about DNA replication. Users can retrieve these articles by typing the words “DNA replication” into the site’s search engine.
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