Lecture notes
Nucleic Acids
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- AQA
Unit 1 - Subtopic - Nucleic acids (AQA A-Level Biology)
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Nucleic AcidsStructure of DNA and RNA
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are
both polynucleotides.
Formation of DNA: A condensation reaction between the phosphate group of one nucleotide and the pentose of another
form a dinucleotide with a phosphodiester bond. Many nucleotides join making a single polynucleotide chain.
Definition of DNA: A double helix with 2 anti-parallel DNA polynucleotide chains held together by hydrogen bonds between
complementary base pairs
DNA RNA
Number of Strands 2 anti-parallel strands One strand
Length Long Short
Pentose Sugar Deoxyribose Ribose
Nitrogenous Bases Adenine, Thymine, Adenine, Uracil,
Cytosine, Guanine Cytosine, Guanine
Function Holds genetic information Transfers genetic information from DNA to
ribosomes
The simplicity of DNA led scientists to doubt that it carried the genetic code.
DNA Replication
Semi-conservative replication:
1. DNA helicase breaks down hydrogen bonds between complementary base pairs, causing the double helix to unwind.
2. Each of the separated original DNA strands acts as a template. Free DNA nucleotides are attracted to their
complementary base pairs on the template strands.
3. The adjacent nucleotides are joined forming a phosphodiester bond via a condensation reaction. DNA polymerase
catalyses the joining together of adjacent nucleotides.
4. The two sets of new DNA molecules contain one original strand and one newly synthesised strand.
Watson-Crick Theory:
Watson and Crick discovered the structure of DNA in 1953. They hypothesised that DNA replicates either conservatively or
semi-conservatively.
1. Conservative replication: The original DNA strand remains intact after replication, and the two newly synthesised strands
of DNA join.
2. Semi-Conservative replication: Each replicated DNA molecule contains one original DNA strand and one newly
synthesised DNA strand.
Meselson and Stahl conducted an experiment which proved DNA replicated semi-conservatively.
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