Nucleic acids
- Polymers
- DNA and RNA are types of nucleic acid
- deoxyribonucleic acid
- ribonucleic acid
- Individual unit called a nucleotide
Nucleotide structure
- Three components:
- Pentose sugar
- Deoxyribose in DNA
- Ribose in RNA
- Phosphate group
- Organic/nitrogenous base that always contains nitrogen
- Pentose sugar
Nitrogenous bases
- Purines
- Adenine and Guanine
- Double ringed structure*
- Pyrimidines
- Thymine and Cytosine
- Single ringed structure*
- Purines always pair with pyrimidines.
- Paired by weak, base-specific hydrogen bonds between DNA strands.
- A joins to T (2 hydrogen bonds) C joins to G (3 hydrogen bonds)
*you don’t have to know the ring structure or even the purines vs the pyrimidines, but it is helpful to understand why complementary base pairing occurs!
Carbon atoms in the deoxyribose sugar are numbered 1′, 2′, 3′, 4′, and 5′.
Phosphate groups are attached to the 5′- and 3′-carbon atoms of each sugar to form the backbone chain of DNA.
One end of the chain carries a free phosphate group attached to the 5′-carbon atom; this is called the 5′ end of the molecule.
The other end has a free hydroxyl (-OH) group at the 3′-carbon and is called the 3′ end of the molecule.
When two DNA strands assemble in a double helix, the two strands always face in opposite directions; the 5′ end of one strand is paired with the 3′ end of the other strand.