DNA : deoxyribose nucleic acid
- 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
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:
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:
|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.