Genetics

Nucleic Acid Structure

The nucleic acid, DNA and RNA are build from nucleotides monomers. The nucleotides link together by phosphodiester bonds to make the complex structure of DNA or RNA molecules.

DNA and RNA differ in terms of the nitrogenous bases they contain, the sugar component of their nucleotides, and whether they are double or single stranded. 

DNA (deoxyribonucleic acid) contain the bases adenine, guanine, cytosine, and thymine. The sugar found in nucleotide is deoxyribose, and DNA molecules are usually double stranded.

On the other hand, RNA (ribonucleic acid) contain different bases (then DNA) these include adenine, guanine, cytosine, and uracil (instead of thymine, although tRNA contains a modified form of thymine). sugar in RNA is ribose, and most RNA molecules are single stranded.

DNA Structure

The discovery that DNA is the genetic material set into motion a fierce competition to determine the precise structure of DNA. DNA molecule is very large and generally composed of two strands (polypeptide chains) coiled together to form a double helix 2.0 nm in diameter.

Each chain contain purine and pyrimidine deoxyribonucleoside joined by phosohodiester linkages. That is a phosphoric acid molecule forms a bridge between a 3’-hydroxyl of one sugar and 5’-hydroxyl of an adjacent sugar. Purine and pyrimidine bases are attached to the 1’-carbon of the deoxyribose sugars and extend toward the middle of the cylinder formed by two chains. They are stacked on top of each other in the center, one base pair every 0.34 nm. The purine adenine (A) of one strand is always paired with pyrimidine thymine (T) of the opposite strand by two hydrogen bonds. The purine guanine (G) pairs with cytosine (C) by three hydrogen bonds. This AT and GC base pairing means that two strands in a DNA double helix are complementary. In other words, the bases in one strand match up with those of the other according to specific base pairing rules.

Because the sequences of bases in these strands encode genetic information, considerable effort has been devoted to determining the base sequences of DNA and RNA from many organisms, including a variety of microbes.

When the DNA strands twist about one another, a wide major groove and narrow minor groove are formed by the backbone. Each base pair rotates 36 degree around the cylinder with respect to adjacent pairs so that there are 10 base pairs per turn of the helical spiral. Each turn of the helix has a vertical length of 3.4 nm. The helix is right-handed, that is, the chains turn counterclockwise as they approach a viewer looking down the longitudinal axis. The two backbones are anti parallel, which means they run in opposite directions with respect to the orientation of their sugars.

One end of each strand has an exposed 5’ – hydroxyl group, often with phosphates attached. Whereas the other end has a free 3’-hydroxyl group. If one end of double helix is examined, the 5’ end of one strand and the 3’ end of the other are visible. In a given direction one strand is oriented 5’ to 3’ and the other, 3’ to 5’.

RNA Structure

RNA differs chemically from DNA, and is usually single stranded rather than double stranded. However, an RNA strand can coil back on itself to form a hairpin-shaped structure with complementary base pairing and helical organization. The three different types of RNA are – messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA) differ from one another in function, site of synthesis in eukaryotic cells, and structure.

Gaurav Singh

Editor in Chief Medical Microbiology & RDT Labs - RDT Labs Magazine | BSc Medical Microbiology | MSc Microbiology

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