Nucleic Acids, DNA & RNA
Nucleotides and Nucleic Acids:
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Polynucleotides:
- A chain of nucleotides linked together by condensation reactions forms a polynucleotide.
- The phosphate group has a phosphodiester (covalent) bond with carbon 5 of the saccharide. The phosphate group also forms a phosphodiester (covalent) bond with carbon 3 of the saccharide above it.
- This forms a sugar-phosphate backbone which runs 5' to 3'.
- The individual nucleotides are released by breaking the bonds of the sugar phosphate backbone in hydrolysis reactions.
Deoxyribonucleic Acid (DNA):
- DNA is a nucleic acid in which the saccharide is deoxyribose.
- There are 4 different nitrogenous bases in DNA:
The Double Helix:
- A DNA molecule is made of two polynucleotide strands wound around each other in a double helix, and can be anywhere from a couple of nucleotides long to millions.
- The strands are held together by hydrogen bonds between the bases.
- The 5' to 3' arrangement of both strands runs in opposite directions - they are antiparallel.
- Complementary base pairing allows DNA to be copied and transcribed/translated.
- Adenine and Thymine bond because they can both form two hydrogen bonds.
- Cytosine and Guanine bond because they can both form three hydrogen bonds.
- This is complementary base pairing.
- It means a small pyrimidine always bonds to a large purine, maintaining a stable distance between the backbones.
- It means the amounts of Adenine and Thymine are always equal.
- It means the amounts of Cytosine and Guanine are always equal.
- Each section of DNA codes for a different protein, based on the base pair sequence.
Ribonucleic Acid (RNA):
- RNA is a short section of DNA, which is able to leave the nucleus through nuclear pores for translation (mRNA).
- Instead of a deoxyribose sugar, RNA used ribose which has an extra oxygen.
- In mRNA, Thymine is replaced with Uracil.
- After protein synthesis, RNA polymers are hydrolysed in the cytoplasm and the nucleotides are released and reused.
DNA Extraction:
- Grind sample in a mortar and pestle to break down cell walls.
- Mix sample with detergent to break down cell membranes and release cell contents.
- Add salt to break hydrogen bonds between DNA and water molecules.
- Add protease to break down histone proteins to dissociate the DNA.
- Add ethanol to sample to cause DNA to form a white precipitate.