DNA

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DNA(deoxyribonucleic acid)

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).

DNA Structure

DNA is a polymeric nucleic acid macromolecule of double helix structure composed of three types of units: a five-carbon sugar, deoxyribose; a nitrogen-containing base; and a phosphate group .The bases are of two types, purines and pyrimidines.

dblhelx1In DNA, there are two purine bases, adenine (A) and guanine (G), and two pyrimidine bases, thymine (T) and cytosine (C). Nucleotides, each composed of a base, a phosphate, and a sugar moiety, polymerize into long polynucleotide chains by 5–3 phosphodiester bonds formed between adjacent deoxyribose units In the human genome, these polynucleotide chains (in their double-helix form) are hundreds ofmillions of nucleotides long, ranging in size from approximately 50 million base pairs (for the smallest chromosome, chromosome 21) to 250 million base pairs (for the largest chromosome, chromosome 1). The anatomical structure of DNA carries the chemical information that allows the exact transmission of genetic information from one cell to its daughter cells and from one generation to the next. At the same time, the primary structure of DNA
specifies the amino acid sequences of the polypeptide chains of proteins.

A DNA molecule is composed of two DNA strands held together by hydrogen bonds between the paired bases. The arrowheads at the ends of the DNA strands indicate the polarities of the two strands, which run anti parallel to each other in the DNA molecule.   The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. If we think of each sugar as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′ hydroxyl) on the other , each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. Moreover, the two ends of the chain will be easily distinguishable, as one has a hole (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA chain is indicated by referring to one end as the 3end and the other as the 5end.

The three-dimensional structure of DNA the double helix arises from the chemical and structural features of its two polynucleotide chains. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar-phosphate backbones are on the outside . In each case,  paired with a single-ring base (a pyrimidine); A always pairs with T, and G with C. This complementary base-pairing enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix. In this arrangement, each base pair is of similar width, thus holding the sugar-phosphate backbones an equal distance apart along the DNA molecule. To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs.

DNA is contained in chromosomes

dna-chromosme

In eukaryotes, DNA is packed into linear chromosomes ‡ where as in prokaryotes, DNA is usually contained in a single, circular chromosome.  In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing.  However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division. Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.