Structure of Nucleic Acids

An unwounded DNA molecule is about 6 feet long. If you stretch out all the strands of DNA in your body and link them together, it would stretch to 110 billion miles! That is 600 round trips to the Sun! So how can these DNA molecules fit in our bodies? Let us take a look at the fascinating nucleic acid structure.

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Nucleic Acid Structure

As we have already studied Nucleic Acids are one of the most important biomolecules present in humans. They store all our genetic information that we pass down to future generations. And they are able to perform their functions, due to the shape and structure they form. Let us study about this peculiar structure of Nucleic Acids,

Just like in all biomolecules, nucleic acids are also made up of repeating monomers. Here these repeating units or monomers have a special name, nucleotides. These are the building blocks of nucleic acids.

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While discussing nucleic acid structure we must look at nucleotides. Nucleotides are monomers that make up nucleic acids. A nucleotide consists of three basic components. These three components are:

1] Sugar

This is typically a 5 carbon sugar. i.e a pentose. These sugars form a bond with the phosphate groups also present in nucleotides. The sugar in the DNA molecule is deoxyribose and the one in RNA is ribose sugar.

2] Phosphate Group

This is the phosphates based on the inorganic compound phosphoric acid i.e. H3PO4. They react with the sugars in the nucleotides to form an ester bond. Nucleic acids may contain one or more of these phosphate groups.

3] Nitrogenous Base

There are in total five nitrogenous bases. However, in the molecule of a nucleic acid, only four are ever present.

  • Adenine
  • Cytosine
  • Guanine
  • Thymine (only present in DNA)
  • Uracil (only present in RNA)

Structure of DNA

nucleic acid structure

DNA is one of the two types of nucleic acids present in our bodies. You all are well versed in DNA and the function it performs. In fact, you have probably even seen a diagram or picture of a DNA molecule. It has a very distinct spiral shape.

The five carbon sugar present in DNA is deoxyribose. And DNA consists of four of the nitrogenous bases, namely Adenine, Cytosine, Guanine and Thymine. They are expressed by the first letters of their name A C G T. Now the nucleotides in DNA forms a link by a dehydration synthesis polymerization method. The link forms between the sugar atom of one nucleotide and the phosphate of the next one. This is how the sugar-phosphate backbone of the molecule forms.

The nitrogenous bases project to the opposite side of the backbone. DNA is composed of two such strands arranged next to each other. The strands are anti-parallel (i.e. in opposite orientation). The nitrogenous base of the two strands links together by hydrogen bonds. Guanine links with Cytosine and Adenine links with Thymine. This linking plays an important role in the replication of the DNA molecule. And this is how DNA gets its double helix (spring-like) structure.

Structure of RNA

The lesser known of the two nucleic acids in our body is Ribonucleic acid. It is also essential in coding and decoding of genes. Instead of deoxyribose RNA contains the five-carbon sugar, Ribose. Also instead of Thymine, it contains the fifth nitrogenous base of Uracil.

Similar to the DNA in RNA to the link is between the sugar and the phosphate group. But they are single-stranded molecules and do not typically form a helix. Unlike DNA which is permanently placed in the nucleus, RNA molecules are more temporary. The molecules are less stable than those of DNA.

Solved Question for You

Q: Adenine pairs with thymine through:

  1. one hydrogen bond
  2. two hydrogen bonds
  3. one phosphate bond
  4. four hydrogen bonds

Sol: The correct answer is “B”, Adenine always pairs with Thymine through two hydrogen bonds in DNA. Similarly Adenine pairs with Uracil through two hydrogen bonds in RNA. Guanine and Cytosine always pair with three hydrogen bonds.

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