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I.1.7 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). Mitochondria are structures within cells that convert the energy from food into a form that cells can use.

The DNA molecule has three basic components: the pentose sugar, deoxyribose; a phosphate group; and four types of nitrogenous bases. Two of the bases, cytosine and thymine, are single carbon–nitrogen rings called pyrimidines. The other two bases, adenine and guanine, are double carbon–nitrogen rings called purines . The four bases are commonly represented by their first letters: C, T, A, and G. DNA bases pair up with each other, A with T and C with G, to form units called base pairs.

DNA basic structure

Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

DNA is a double helix ( Watson and Crick Model) formed by base pairs attached to a sugar-phosphate backbone.


An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases.Only about 1% to 2% of human DNA actually encodes proteins.
Double Helix
DNA condensation
The DNA is wound around a histone protein core to form a nucleosome . About 140 to 150 DNA bases are wound around each histone core, and then 20 to 60 bases form a spacer element before the next nucleosome complex. The nucleosomes in turn form a helical solenoid; each turn of the solenoid includes about six nucleosomes. The solenoids themselves are organized into chromatin loops, which are attached to a protein scaffold. Each of these loops contains approximately 100,000 base pairs (bp), or 100 kilobases (kb), of DNA.

Type- There are several major types of DNA, viz single-copy DNA, satellite DNA, and dis-
persed repetitive DNA. The latter two categories are both classes of repeated DNA sequences.

Single-copy DNA sequences are seen only once (or possibly a few times) in the genome. Single-copy DNA accounts for about half of the genome and includes the protein-coding genes. Protein-coding DNA (exons) represents only a small fraction of all single-copy DNA, most of which is found in introns or in DNA sequences that lie between genes.

Repetitive DNA sequences are repeated over and over again in the genome, often thousands of times. There are two major classes of repetitive DNA: dispersed repetitive DNA and satellite DNA.

Dispersed repeats  tend to be scattered singly throughout the genome; they do not occur in tandem. Dispersed repetitive DNA makes up about 45% of the genome, and these repeats fall into several major categories. The two most common categories are short inter- spersed elements (SINEs) and long interspersed elements (LINEs).

Satellite repeats are clustered together in certain chromosome locations, where they occur as tandem repeats (i.e., the beginning of one repeat occurs immediately adjacent
to the end of another). Satellite DNA accounts for approximately 8% to 10% of the genome and can be further subdivided into several categories. a-Satellite DNA occurs as tandem repeats of a or longer. This type of satellite DNA is found near the cen- tromeres of chromosomes.

Minisatellites are blocks of tandem repeats (each 14 to 500 bp long) whose total length is much smaller, usually a few thousand base pairs. Another category, microsatellites, are smaller still: the repeat units are to 13 bp long, and the total length of the array is usually less than a few hundred base pairs. Minisatellites and micro- satellites are of special interest in human genetics because they vary in length among individuals, making them highly useful for gene mapping. A minisatellite or microsatellite is found at an average frequency of one per kb in the human genome; altogether they account for about 3% of the genome.

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