Nucleic Acids - A-Level Biology notes

Chapter 2: Nucleic acids
2.1 Structure of RNA and DNA
Nucleotides are joined together by a phosphodiester bond in a
condensation reaction.



 Mononucleotide
 Dinucleotide
 Polynucleotide



RNA DNA
Ribonucleic acid Deoxyribonucleic acid
Single strand Double strand
Short Long
Ribose pentose sugar Deoxyribose pentose sugar
Adenine – Uracil Adenine – Thymine



Phosphodiester bonds act as a backbone to protect the
more reactive bases. Hydrogen bonds between bases make DNA
more stable.

Function of DNA:
- Holds and passes genetic material from cell to cell
- Large variety of base combinations provides genetic diversity
- Rarely mutates when passed on to offspring
- DNA replication




2.2 DNA replication
The semi-conservative replication has 4 conditions:
Cell division: 1. All 4 bases must be present
1. Nuclear division: where the nucleus splits 2. Both strands of DNA act as a template
(mitosis and meiosis) 3. DNA polymerase enzyme must be resent
2. Cytokinesis: where the whole cell divides 4. A source of chemical energy
DNA replication:
 DNA unwinds
 DNA helicase enzyme splits base pairs by breaking the hydrogen bonds
 Free nucleotides in the pool bind specifically to their complementary bases
 Bases are joined together by DNA polymerase which forms phosphodiester bonds (the backbone) to make a
complete polynucleotide chains of 2 identical strands of DNA