I created these notes throughout y12&13 and used them for my A level 2022 exams. I got a grade 9 in GCSE and am awaiting for my a level biology result. These notes were made using the CGP revision guide, AQA Biology textbook and class notes in order to cover everything in the course. I have also ad...
2.1 STRUCTURE OF RNA AND DNA
07 September 2020
18:25
Both DNA and RNA are polynucleotides, a large molecule of many nucleotide units.
Nucleotides are made up of 3 components:
1. A pentose sugar:
-Ribose in RNA
-Deoxyribose in DNA - Has one less oxygen atom.
1. Phosphate group
2. Nitrogen-containing organic base:
- Cytosine, thymine, adenine, guanine, uracil.
These are bonded together due to condensation reactions, forming a nucleotide.
● This releases 1 water molecule per bond.
Many nucleotides can join to form a polynucleotide.
● Between the phosphate group from one nucleotide and the pentose from
another, a phosphodiester bond will form as a result of a condensation
reaction.
● This forms a strong sugar-phosphate backbone.
Ribonucleic acid (RNA) is a single, relatively short polynucleotide chain.
The pentose sugar is always ribose and the organic bases are adenine, guanine,
cytosine and uracil.
● Uracil is used instead of thymine.
Deoxyribonucleic acid (DNA) is made up of two extremely long strands of
polynucleotides.
● Anti-parallel.
The pentose sugar is deoxyribose and the organic bases are adenine, guanine,
cytosine and thymine.
, The two strands of polynucleotides are joined together by weak hydrogen bonds
formed between the complimentary bases.
The base pairings are always:
● A – T The quantities of A and T are the same. |The bases are said to be
● C – G The quantities of C and G are the same.| complimentary to each other.
-Adenine and Guanine are purines.
-Cytosine and Thymine are pyrimidines.
Purines always bind with pyrimidines.
-Purines are double ringed molecules.
-Pyrimidines are single ringed molecules.
The two chains are complimentary to each other, so the sequence of one chain
determines the sequence of the other.
The two chains are anti-parallel.
DNA is a stable molecule.
● Phosphodiester backbone protects the more chemically reactive organic bases
in the double helix.
● Hydrogen bonds link the base pairs, forming rungs between phosphodiester
uprights.
-C-G pairing has 3 hydrogen bonds, so the higher proportion of C-G pairing,
the more stable the DNA molecule.
DNA is the hereditary material responsible for passing information from cell to cell
and generation to generation.
DNA molecule is adapted:
● Very stable structure - Passes from generation to generation without change.
- Rarely mutates.
- Strong phosphodiester bonds in backbone with many weak hydrogen bonds.
● Helix shape allows for the molecule to be compact.
● Base sequence allows for genetic information to be stored.
● Genetic information, the order of bases, is protected by the deoxyribose-
phosphate backbone from outside chemicals and physical forces.
- Protects genetic information from being corrupted.
● The 2 strands are connected by hydrogen bonds - Can separate during DNA
replication and protein synthesis.
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