DNA STRUCTURE

DNA Structure and Composition

DNA is composed of four nucleotides, each formed by a phosphate group, a sugar known as deoxyribose, and one of four nitrogenous bases: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C). The sugar in DNA is a 5-carbon sugar called deoxyribose, with the carbons labeled from 1' to 5'. The phosphate group is attached to the 5' carbon of deoxyribose, while the nitrogenous base attaches to the 1' carbon. Among the bases, there are two classes: pyrimidines (Cytosine and Thymine) and purines (Adenine and Guanine). Purines consist of two chemical rings of carbon and nitrogen, making them larger and heavier than pyrimidines. Pyrimidines, on the other hand, comprise a single carbon-nitrogen ring, and include Thymine, Cytosine, and Uracil (which is found only in RNA).

The base pairing mechanism forms the double-stranded structure of DNA, where Adenine pairs with Thymine and Guanine pairs with Cytosine. In contrast, when RNA binds to DNA, Adenine in DNA pairs with Uracil in RNA. The history of DNA composition began in the 1940s when Erwin Chargaff analyzed DNA bases across various organisms, discovering a consistent proportion between Adenine and Thymine, as well as Guanine and Cytosine, known as 'Chargaff's Rule'. This was followed by significant contributions from Maurice Wilkins and Rosalind Franklin through X-ray diffraction, revealing that DNA is helical, long, thin, and uniform in diameter (~2 nanometers), with repeating units.

In the 1950s, Watson and Crick proposed the double helix structure of DNA, consisting of two linked nucleotide polymers, which create a sugar-phosphate backbone through covalent bonds, with bases projecting outward. The orientation of nucleotides features one end with a free sugar and the other end having a free phosphate. Hydrogen bonds hold the DNA strands together between the bases, leading to a stable ladder-like structure with sugars and phosphates on the outside and bases on the inside. The double helix twists in antiparallel directions, ensuring that the bases in DNA strands are complementary, maintaining the consistency of Chargaff's Rule.

DNA is essential for inheritance, transmitting characteristics from generation to generation and serving as the chemical basis of inheritance found in chromosomes within the cell nucleus. The number of chromosomes varies by species; for instance, bacteria typically possess a single chromosome while humans have 46 (23 inherited from each parent). The genetic information is organized by winding DNA around proteins called histones to form nucleosomes.

DNA duplication is critical for copying genetic information exactly when cells divide. This process, occurring in the nucleus, aims to produce two identical strands from one original DNA strand. The duplication process is semiconservative, where each strand serves as a template for a new strand, resulting in two new DNA molecules—one old and one new. It is bidirectional, beginning at each origin of replication and progressing in both directions, forming replication forks. Prokaryotes are characterized by monofocal initiation of replication, while eukaryotes exhibit multifocal initiation, which enables timely completion of chromosome replication. The synthesis occurs semi-discontinuously; one strand is synthesized continuously while the other is created in segments known as Okazaki fragments, with fragment lengths varying depending on the cell type.