DNA Structure, Function, Watson and Crick model & Chargaff’s Rule

DNA functions as the genetic material for cells both prokaryotes and eukaryotes. In eukaryotes, DNA is located in the nucleus separated from the cytoplasm by the nuclear membrane. Because prokaryotes lack internal membrane systems, their DNA is not separated from the rest of the cellular contents. Eukaryotic DNA is bound to proteins, forming a complex known as chromatin.

DNA is also present in mitochondria (less than 0.1% of the overall DNA) and in the chloroplast of plants. Lots of viruses also contain DNA as their genetic material.


Deoxyribonucleic acid is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying hereditary instructions for the development, functioning, growth, and reproduction of all types of lives and lots of viruses.

DNA Structure

DNA is a long, thread-like macromolecule comprised of a large number of deoxyribonucleotides. Deoxyribonucleotide is made up of a nitrogenous base, a sugar, and a phosphate group. The bases of DNA particle carry hereditary info, whereas their sugar and phosphate groups carry out a structural role. The sugar in a deoxyribonucleotide is deoxyribose.


The purine bases in DNA are adenine (A), and guanine (G). Pyrimidine bases are thymine (T) and cytosine (C). DNA is a polymer of many deoxyribonucleotides connected covalently by 3′, 5′ phosphodiester bonds. The 3′- hydroxyl group of the sugar moiety of one deoxyribonucleotide is joined to the 5′- hydroxyl group of the surrounding sugar moiety of deoxyribonucleotide by a phosphodiester linkage.

The Watson-Crick DNA Double Helical Structure

In 1953, James Watson and Francis Crick deduced the three-dimensional structure of DNA. The essential functions of their model of DNA are as follows:

Two helical polynucleotide chains are coiled around a common axis. The chains run in opposite directions (anti-parallel). The purine and pyrimidine bases are on the inside of the helix, whereas the phosphate and deoxyribose units are on the outside. The diameter of the helix is 20 Å. Surrounding bases are separated by 3.4 Å along the helix axis and the helical structure repeats after ten residues on each chain, i.e. at intervals of 34 Å.

The two chains are held together by hydrogen bonds in between complementary pairs of bases:

 Adenine is constantly paired with thymine by the formation of two hydrogen bonds. Guanine is always paired with cytosine by the development of three hydrogen bonds.


The two strands are always complementary to each other. In a double-stranded DNA molecule, the material of adenine equals to that of thymine, and the contents of guanine equal to that of cytosine. The complementary base pairing proves Chargaff’s rule.


The model proposed by Watson and Crick is a B type of DNA (B-DNA) which is a best-handed helix of 10 base pairs per turn, containing grooves of alternate size, known as significant and small grooves.

Other kinds of DNA might likewise occur, such as A-DNA( It is a right-handed double helix relatively similar to the more common B-DNA kind, however with a shorter, more compact helical structure whose base sets are not perpendicular to the helix-axis as in B-DNA.) and Z-DNA( It is a left-handed double helical structure in which the helix winds to the left in a zigzag pattern, instead of to the right, like the more common B-DNA form.).

Under physiologic conditions, DNA is nearly completely in Watson-Crick B kind.


Chargaff’s Rule

Ervin Chargaff discovered that in DNA of all types, the number of purines is the same as that of pyrimidines (A+G= T+C). He observed that in DNA, the content of adenine equals that of thymine (A= T) and the material of guanine equals that of cytosine (G= C). Watson and Crick deduced that adenine should couple with thymine and guanine with cytosine, because of stearic and hydrogen bonding elements.

Adenine can not couple with cytosine; guanine can not pair with thymine. Thus, one member of a base pair in a DNA must constantly be a purine and the other a pyrimidine. This base-pairing constraint discusses that in a double-stranded DNA molecule, the content of A equals that of T and the material of G equals that of C.


[To remember the base pairs here is an interesting key: AT= English preposition at; GC= Government College]

The ratio of purine to pyrimidine bases in the DNA is constantly one, i.e. G+A: T+C = 1. This is that of Chargaff’s guideline.

Organization of DNA
Prokaryotic DNA
  • A prokaryotic cell generally consists of a single chromosome composed of double-stranded circular DNA, which consists of over 4 x 106 base pairs. Since DNA molecules are so large, they need unique organization/packaging to allow them to live within cells. In E. coli, the circular DNA is supercoiled and connected to an RNA-protein core.
Eukaryotic DNA
  • Eukaryotes consist of over 1,000 times the amount of DNA discovered in prokaryotes. Consequently, their technique of arranging or packing DNA is far more complex. A normal human cell has 46 chromosomes, whose total DNA is roughly two meter in length. The packing of DNA in a chromosome represents a 10,000-fold shortening of its length from primary B-form DNA. In resting nondividing eukaryotic cells, the chromosomal product is called chromatin. Chromatin is comprised of nucleosomes.
Functions of DNA

DNA is the storehouse of genetic information. The hereditary information saved in the DNA serves two functions.

  1. It is the source of details for the synthesis of all protein molecules of the cell and
  2. It provides the information inherited by daughter cells or offspring.