C.8.1

=Describe the structure of nucleotides and their condensation polymers (nucleic acids).=

An excellent animation describing the structure of DNA at http://207.207.4.198/pub/flash/24/menu.swf View the structure of DNA animation which shows the structure of three units (deoxyribose sugar, phosphate and nitrogenous base) that make up a nucleotide.

There are two types of nucleic acids; **DNA** and **RNA**.
It consists of four types of nucleotides; adenine, cytosine, guanine, and thymine It consists of four types of nucleotides; adenine, cytosine, guanine, and uracil.
 * DNA (Deoxyribonucleic Acid)**
 * RNA (Ribonucleic Acid)**

**Nucleotides**
Nucleotides are the monomers that comprise nucleic acids. They are made up of a **nitrogen-containing organic base**, **five-carbon sugar** (a pentose sugar), and one or more **phosphate groups**. In DNA, then five-carbon sugar molecule is deoxyribose. In RNA, the five-carbon sugar molecule is ribose.

Figure1. Nucleotide In Figure 1, ribose is the pentose sugar, the phosphate group attached on the left hand side by the ester bond, and on the right hand side the nitrogen base it attached by the glycoside bond.

However, nucleotides can carry one of five different types of nitrogen-containing bases, which are **adenine** (A), **cytosine** (C), **guanine** (G), **thymine** (T) and **uracil** (U). Thymine (T), is present in DNA molecules, while in RNA, thymine is replaced by uracil (U).



**Nucleic Acids**
Nucleic acids are polymers of nucleotides formed by condensation reactions. Nucleotides joined together to form a polynucleotide. To do this two nucleotides (condense) to form a **phoshodiester bond**, which is covalent bond formed between the C5' hosphate group of one nucleotide and -OH group on C3' on the sugar of another nucleotide. This forms what is called the sugar-phosphate backbone of DNA (or RNA).

. Figure2. Condensation of Nucleotides

The figure below shows a section of a nucleic acid molecule showing two different nucleotides and their three parts (base, phosphate group and sugar).

Hydrogen bonding between Bases
The bases of two nucleic acid strands hydrogen bond with one another to form a DNA double helix. Hydrogen bonds are the strongest type of intramolecular bonds, formed when hydrogen is bonded to a small highly electronegative element (H-N, H-F, H-O). As the electron pair in the covalent bond is drawn from the H atom to the more highly electronegative atom, all that remains is the proton in the nucleus as there are no inner electrons. The proton attracts an a non bonding pair of electrons from a F, N or O atom on another molecule resulting in a much stronger dipole-dipole interaction called a hydrogen bond. Figure: hydrogen bonds formed between the bases thymine and adenine on two different nucleic acid strands.

Figure: hydrogen bonds (.....) formed between cytosine and guanine.