Replication, of DNA molecules

The region in a replication unit where DNA synthesis is initiated will be referred to as the initiation site. It is stressed that initiation sites in both prokaryote and eukaryote chromosomes are deoxynucleotide sequences no other structural linkers provide this site. This is well illustrated in prokaryote cells where DNA synthesis is initiated at a genetically defined site in the DNA (Caro and Berg, 1969 Makover, 1968 Mosig and Werner, 1969). Similarly in eukaryote cells, the establishment of a heritable pattern of replication of DNA molecules through each S phase combined with the demonstration that contiguous DNA molecules contain multiple replication units, indicates that a component(s) of the initiation mechanism must be able to select a deoxynucleotide sequence as the site of initiation of DNA synthesis. 

Describe the stractural differences between DNA and RNA. The monomers in nucleic acids are called nucleotides. What are the three parts of a nucleotide The compounds adenine, guanine, cytosine, and thymine are called the nucleic acid bases. What structural features in these compounds make them bases DNA exhibits a double-helical structure. Explain. Describe how the complementary base pairing between the two individual strands of DNA forms the overall double-helical structure. How is complementary base pairing involved in the replication of DNA molecule during cell division Describe how protein synthesis occurs. What is a codon, and what is a gene The deletion of a single base from a DNA molecule can constitute a fatal mutation, whereas substitution of one base for another is often not as serious a mutation. Explain. 

FIGURE 11.21 Replication of DNA gives identical progeny molecules because base pairing is the mechanism determining the nucleotide sequence synthesized within each of the new strands during replication. 

All multicellular life starts as a single cell. Copies of the DNA in that cell must eventually occupy almost every one of the trillions of cells in a human body. For that to happen, the DNA in the original cell must replicate itself many times. The key to this replication is the famous double helix. When two strands of DNA— let s call them X and Y—separate, each strand can assemble the other. X builds a new Y, forming a fresh double helix. Y does the same thing. This doubles the number of DNA molecules. This mechanism depends on the two strands of DNA being able to hold together under normal conditions, yet unwind easily. That is where hydrogen bonds come in. 

Pieces of DNA, for example genes or cDNA probes, are multiplied using a method based on in vivo replication in bacterial cells. The sequence of DNA is incorporated into a vehicle or vector which transports it into a cell host, usually E. coli. As the bacteria culture grows, the vector also replicates so producing more of the required sequence of DNA. Two of the naturally occurring types of DNA molecule which can be used as vectors are plasmids and viral chromosomes. 

DNA is the carrier of the genetic information of all cells and many viruses. Two important points follow from this simple statement. First, DNA must be replicated in order for cell division to take place, hi cell division, one cell becomes two and each must have its own store of genetic information. The genetic information is stored in chromosomes and each chromosome contains a single molecule of DNA. Secondly, the replication of DNA must be a very precisely controlled process. The goal is for each daughter cell to have exactly the same genetic information as the parent cell, which requires that each chromosomal DNA molecule replicated in the course of... 

Now suppose that you are a cell in possession of one copy of your genome, a small collection of DNA molecules. When you sense a round of cell division about to happen, you have two related tasks. The first is to replicate your DNA molecules exactly, so as to have two copies, one for each daughter cell. Unlike the publisher of Rowling s books, you need only one new copy, not millions, but you will need to continue to replicate the DNA molecules at each round of cell division. The second task is to translate the DNA molecule language into that of RNA and then into that of proteins. A new complement of RNA and protein molecules is required for the new cell and for the process of cell division. So the task of a cell facing cell division is not unlike that of a book publisher in some basic ways. 

Replication of DNA is an enzymatic process that starts with the partial unwinding of the double helix. Just before the cell division, the double strand begins to unwind. As the strands separate and bases are exposed, new nucleotides line up on each strand in a complementary fashion, A to T, and C to G. Two new strands now begin to grow, which are complementary to their old template strands. Two new identical DNA double helices are produced in this way, and these two new molecules can then be passed on, one to each daughter cell. As each of the new DNA molecules contains one strand of old DNA, and one new, the process is called semiconservative replication. 

The relatively weak links between the complementary bases in the DNA strands allow the helix to split at cell division and be replicated to yield an identical pair of DNA molecules as each half acts as a template. Thus, DNA is able to undergo replication. 

It serves to control the passage of small molecules into and out of the cell. Its outer surface carries receptors for recognition of various materials. The inside surface of bacterial membranes contains enzymes that catalyze most of the oxidative metabolism of the cells. Bacterial cell membranes are sometimes folded inward to form internal structures involved in photosynthesis or other specialized reactions of metabolism such as oxidation of ammonia to nitrate.2 In E. coli replication of DNA seems to occur on certain parts of the membrane surface, probably under the control of membrane-bound enzymes. The formation of the new membrane which... 

Many bacterial cells contain self-replicating, extrachromosomal DNA molecules called plasmids. This form of DNA is closed circular, double-stranded, and much smaller than chromosomal DNA its molecular weight ranges from 2 X 106 to 20 X 106, which corresponds to between 3000 and 30,000 base pairs. Bacterial plasmids normally contain genetic information for the translation of proteins that confer a specialized and sometimes protective characteristic (phenotype) on the organism. Examples of these characteristics are enzyme systems necessary for the production of antibiotics, enzymes that degrade antibiotics, and enzymes for the production of toxins. Plasmids are replicated in the cell by one of two possible modes. Stringent replicated plasmids are present in only a few copies and relaxed replicated plasmids are present in many copies, sometimes up to 200. In addition, some relaxed plasmids continue to be produced even after the antibiotic chloramphenicol is used to inhibit chromosomal DNA synthesis in the host cell. Under these conditions, many copies of the plasmid DNA may be produced (up to 2000 or 3000) and may accumulate to 30 to 40°/o of the total cellular DNA. 

One of the simplest molecules found to inhibit the replication of DNA viruses in animals is phosphonofomiic acid (PFA), CH3O5P (1). Both PFA (as the trisodium salt CNa OsP, foscarnet) and its homologue pliosphonoacctic acid (PAA), C2H5O5P (2). were developed by Astra Pharmaceuticals and show selective inhibition of DNA polymerase in various herpes viruses. 

DNA fragments can be introduced into bacterial cells, most or all will lack the ability for self-replication and will quickly be lost. However, two types of DNA molecule are known which can replicate autonomously in bacterial cells bacteriophages (see Topic 14) and plasmids. Plasmids are small circular double-stranded DNA molecules that exist free inside bacterial cells, often carry... 

PLASMID A self-replicating, extrachromosomal DNA molecule in the cytoplasm of some cells characterized most extensively in bacteria. 

Each original DNA molecule has now been replicated to form two molecules. The cycle is repeated from step 2, each time doubling the number of DNA molecules. This is why it is called a chain reaction, since the number of molecules increases exponentially. Typically PCR is run for 20-30 cycles. 

Replication of DNA la the process by whkh identical DNA copies arc made and genetic infonnatioit U preserved- This occura when the [>NA douUe helix unwinds, complementaiy deoxyribonurleoiides line up in order, and two new DNA molecules are produced. 

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