What are Nucleotides?

Nucleotides are composed of 3 units;

1) a Five Carbon Sugar (Pentose)

Deoxyribose in DNA – In DNA the 2′ OH of ribose is replaced by a hydrogen atom

Ribose in RNA – In RNA the there is a hydroxyl group on both the 2′ and 3′ position.

This contributes to RNA’s inherent poor half- life.

2) a Nitrogenous Base

a) Purine:

Adenine (found in DNA and RNA)

Guanine (found in DNA and RNA)

b) Pyrimidines:

Cytosine (found in DNA and RNA)

Thymine (found in DNA only)

Uracil (found in RNA only)

3) a Phosphate Group

In nucleic acids, nucleotides are joined together by phosphodiester bonds.

A phosphodiester bond is a group of strong covalent bonds between the phosphorus atom in a phosphate group and two other molecules over two ester bonds.

Nucleotides are written 5’ → 3’

RNA and DNA are nucleotide polymers, both of which are created in the nucleus.

In DNA, two strands are joined by the hydrogen bonds to make the structure called the double helix.

This model was proposed by Watson and Crick.

The members of each base pair can fit together within the double helix only if the two strands of the helix are antiparallel.

By convention the top DNA strand goes 5’ → 3’ and the bottom 3’ → 5’.

Going in the 5’ → 3’ Direction is referred to as going downstream.

Going in the 3’ → 5’ Direction is referred to as going upstream.

Adenine (A) and Thymine (T) form two (2) hydrogen bonds, while cytosine (C) and guanine (G) form three (3) hydrogen bonds.

The more G-C%, the higher the melting temperature because of this additional hydrogen bond.

Differences Between DNA and RNA

Structurally, DNA and RNA are nearly identical.

However, there are three fundamental differences that account for the very different functions of the two molecules.

1. RNA is a single stranded nucleic acid and no double helix is formed.

2. RNA has a ribose sugar instead of a deoxyribose sugar like DNA.

3. RNA nucleotides has a uracil base instead of thymine.

Other than these differences, DNA and RNA are the same.

Their phosphates, sugars, and bases show the same bonding patterns to form nucleotides and their nucleotides bind to form nucleic acids in the same way.