dna and its structure and rna
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DNA and its structureand RNA
Prepared By NISAR ALI MSC MicrobiologyUniversity of Swat [email protected] 03479682053
What is DNA?DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).
The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.
DNA StructureDeoxyribonucleic acids are very large molecules, usually composed of two polynucleotide chains coiled together to form a double helix 2.0 nm in diameter.
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Each chain contains purine and pyrimidine deoxyribonucleosides joined by phosphodiester bridges.That is, two adjacent deoxyribose sugars are connected by a phosphoric acid molecule esterified to a 3 - hydroxyl of one sugar and a 5 -hydroxyl of the other. ′ ′Purine and pyrimidine bases are attached to the 1 -carbon of the deoxyribose ′sugars and extend toward the middle of the cylinder formed by thet wo chains. They are stacked on top of each other in the center, one base pair every 0.34 nm. The purine adenine (A) is always paired with the pyrimidine thymine (T) by two hydrogen bonds. The purine guanine (G) pairs with cytosine (C) by three hydrogenBonds.
A-DNA is thought to be one of three biologically active double helical structures along with B-DNA and Z-DNA.
It is a right-handed double helix fairly similar to the more common and well-known B-DNA form, but with a shorter more compact helical structure whose base-pairs are not perpendicular to the helix-axis as in B-DNA.
It appears likely that it occurs only in dehydrated samples of DNA, such as those used in crystallographic experiments, and possibly it's also assumed by DNA-RNA hybrid helices.
The same helical conformation is the most commonly seen one in double-stranded RNA's.
In B-DNA, the most common double helical structure, the double helix is right-handed with about 10–10.5 nucleotides per turn.
The double helix structure of DNA contains a major groove and minor groove, the major groove being wider than the minor groove.
Given the difference in widths of the major groove and minor groove, many proteins which bind to DNA do so through the wider major groove.
Z-DNA is one of the many possible double helical structures of DNA. It is a left-handed double helical structure in which the double helix winds
to the left in a zigzag pattern (instead of to the right, like the more common B-DNA form).
Z-DNA is thought to be one of three biologically active double helical structures along with A- and B-DNA.
Z-DNA is quite different from the right-handed forms.
The Z-DNA helix is left-handed and. The major and minor grooves, unlike A- and B-DNA, show little difference in width.
Formation of this structure is generally un favorable, although certain conditions can promote it; such as alternating purine-pyrimidine sequence (especially poly (dGC)2), or high salt and some cations (all at physiological temperature, 37 °C, and pH 7.3-7.4).
The Z-DNA conformation has been difficult to study because it does not exist as a stable feature of the double helix.
Instead, it is a transient structure that is occasionally induced by biological activity and then quickly disappears.[9]
RNA StructureBesides differing chemically from DNA, ribonucleic acid is usually single stranded rather than double stranded like most DNA.
An RNA strand can coil back on itself to form a hairpin-shaped structure with complementary base pairing and helical organization.
Cells contain three different types of RNA—messenger RNA, ribosomal RNA, and transfer RNA—that differ from one another in function, site of synthesis in eucaryotic cells, and structure.