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DNA : deoxyribose nucleic acid

Key Concepts

Nucleic acids are the carriers of genetic codes in living things.
The two principal types of nucleic acids are deoxyribose nucleic acid (or deoxyribonucleic acid, DNA) and ribose nucleic acid (or ribonucleic acid, RNA).
DNA is a polymer.
Individual monomer units are called nucleotides.
The DNA polymer is composed of:
- a backbone of molecules of the sugar deoxyribose (a 5 carbon sugar)
- phosphate groups linking the deoxyribose sugar molecules together in a chain
- four principal bases (2 are pyrimidines and 2 are purines):
  - cytosine, a pyrimidine, abbreviated to C
  - thymine, a pyrimidine, abbreviated to T
  - adenine, a purine, abbreviated to A
  - guanine, a purine, abbreviated to G
The Watson-Crick model of DNA is that of a double helix of two long DNA molecules held together by hydrogen bonds.
In this "double strand" of DNA, the principal bases occur in pairs with hydrogen bonding between them:
- thymine-adenine pair formed by 2 hydrogen bonds
- cytosine-guanine pair formed by 3 hydrogen bonds

Structure of DNA

Deoxyribose is a 5 carbon sugar molecule and forms the backbone of the DNA polymer chain.

One of the four principal bases can attach to this deoxyribose sugar molecule.

adenine

cytosine

guanine

thymine

The principal base attaches through its nitrogen to the carbon in the ring that is bonded to both the oxygen atom of the ring and an OH (hydroxy) group.

A phosphate group can link 2 deoxyribose sugar molecules by attaching through the other 2 available OH (hydroxy) groups.

These three components

a deoxyribose sugar
a phosphate group
a base

make up a DNA monomer or nucleotide.

These nucleotides link up to form the DNA polymer.

Representing a DNA Polymer Strand

The diagram on the left shows a piece of a strand of DNA.

The sequence of bases shown from top to bottom is:

adenine
guanine
cytosine
thmine

The sequence of phosphate groups and deoxyribose sugars is always the same, only the sequence of bases changes.

It is common to therefore represent a strand of DNA using lines to represent the sugar-phosphate backbone, and the letters A (adenine), C (cytosine), G (guanine), and T (thymine) to represent the bases.

Double Stranded DNA

In a double strand of DNA hydrogen bonds occur between pairs of bases:

guanine is hydrogen bonded to cytosine 3 hydrogen bonds are formed
adenine is hydrogen bonded to thymine 2 hydrogen bonds are formed
Since the sequences of bases on one strand determine the sequence of bases on the other strand, the two strands are referred to as complementary. The diagram to the right shows how a piece of uncoiled double stranded DNA could be represented.