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- PROPERTIES OF DNA
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PROPERTIES OF DNA
DNA strands that form the helix have opposite orientations: one is in the direction 5′-3’y its complement in the 3′-5 ‘. The breaking of hydrogen bonds by heat, alkali or various chemical, produces the physical separation of the two strands of DNA in a process called denaturation. The heat denaturation is complete at 90 ° C and excess alkali pHs 11.3. In both cases the process is reversible, and the disappearance of the denaturing agent occurs renaturation of the molecule, ie the re-acquisition of lost-helical structure. ‘
The denaturation process is followed by increased absorption of ultraviolet light (260 nm) called the hyperchromic effect.
Another interesting concept to define is the melting temperature (Tm), which is the temperature at which half the molecules of nucleic acid solution, have become denatured state. This temperature depends on the number of GC pairs that exist in the nucleic acid molecule. When higher the higher the Tm.
FUNCTIONS OF THE NUCLEIC ACIDS
Its main function is the preservation of information of the cell or organism that contains it and the transmission of this information to replicate.
GENETIC ENDOWMENT OF VIRUSES Viruses can have DNA duplex, single-stranded DNA, single stranded RNA or RNA duplex. Occasionally, the viral genome available biological information needed by the cell machinery to work for him which parasites, and other information has to make itself different functions for replication.
GENETIC ENDOWMENT OF BACTERIA
Bacteria, like eukaryotic organisms possess two types of nucleic acids above: DNA, a component of its single chromosome, and the various ribonucleic acids (rRNA, tRNA, mRNA). In addition, bacteria also have a certain amount of extra DNA that is commonly found circularize and repeated several times, called extrachromosomal DNA or plasmid DNA and, although it may not contain specific information (cryptic plasmid) normally encodes factors antimicrobial resistance, as b-lactamases, and so on.
CHROMATIN AND CHROMOSOMES
DNA is never naked. In eukaryotes interacts with a variety of proteins, and spiral in to form the chromatin condenses and chromosomes.
Chromosomes are the higher order packing of DNA and can be visualized by light microscopy. The structural unit below the chromosome is the chromatin fiber which can be viewed under the electron microscope. This fiber is composed of 6-7 nucleosomes per turn. Each nucleosome is a disc composed of an octamer of basic proteins called histones, which DNA is wound externally.
As noted earlier, the DNA consists of two complementary helices linked by weak hydrogen bonds can break and re-form simply by heating and cooling. These processes are called respectively denaturation and renaturation of DNA.
Based on this concept and very schematic, replication of DNA in nature is to:
1. Separation of the two strands that form the double helix, which are responsible for enzymes and proteins found in eukaryotic cells.
2. Union of primers (primers or primers) RNA in a separate strand. (3 ‘® 5′)
3. Polymerase binding to the places where the ‘first’ to start copying progressively the DNA strand as the polymerase requires a primer that tells you where to start, being unable to copy single-stranded DNA.
4. Constitution of the new copies always consist of a mother and one daughter strand, so that the process is called semiconservative replication of DNA.
Thus, a parental DNA after replication, daughter molecules produce two exactly alike.
DNA replication fork with all the enzymes involved in the synthesis. There must be a ‘first’ for DNA polymerase of E. coli initiate chains de novo. This ‘first’ is provided by an RNA polymerase called primase, which in association with a protein complex called the primosome synthesizes a small strand of RNA. DNA polymerase III can use this ‘first’ to continue DNA synthesis. A protein called helicase (originally called rep) is required to unlink and open the DNA helix to allow replication. -Binding proteins are responsible for stabilizing ssDNA regions that form a simple chain transiently during the replication process. DNA polymerase can synthesize DNA in the 5 ‘® 3′, so that one strand must be synthesized discontinuously, this leads to the formation of short DNA strands with gaps between them which must be completed by the action of DNA polymerase I and united by the action of a DNA ligase.