A-Level Biology OCR Notes

DNA & RNA

	DNA	RNA
Number of Strands	Two antiparallel strands	One strand
Length	Very long	Relatively short
Pentose Sugar	Deoxyribose	Ribos
Nitrogenous Bases	Adenine, Cytosine, Guanine & Thymine	Adenine, Cytosine, Guanine & Uracil
Function	Store genetic information	Transfer genetic information & forms ribosomes with proteins

DNA Double Helix & Replication

Polynucleotides are polymers made up of many nucleotide monomers joined together by a series of condensation reactions, forming phosphodiester bonds.
The DNA double helix is held together by hydrogen (H) bonds between complementary base pairs.
- 2 H bonds between Adenine & Thymine
- 3 H bonds between Cysteine and Guanine

Semi conservative replication is the method in which DNA replicates, creating two molecules of DNA that consist of one original DNA strand and one newly synthesised DNA strand.
- DNA helicase breaks H bonds between the two strands
- Free nucleotides complementary base pair to the exposed strands
- DNA polymerase catalyses condensation reactions to join adjacent nucleotides, forming phosphodiester bonds.

ATP

The hydrolysis of ATP can be coupled to energy-requiring reaction and used to phosphorylate compounds.
The condensation of ADP to form ATP can occur during respiration and photosynthesis

Genetic Information

In prokaryotic cells, DNA molecules are short, circular and not associated with proteins.
In eukaryotes, the nucleus contains very long, linear DNA molecules associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.
The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.
The genome is the full set of DNA found in an organism.
The proteasome is the full range of proteins that can be synthesised from the genome.
A gene is a section of DNA that code for polypeptides and functional RNA and are located at a fixed locus on a DNA molecule.
A sequence of three DNA bases, called a codon, codes for a specific amino acid. The genetic code is universal, non-overlapping and degenerate.
In eukaryotes, sections of the nuclear DNA do not code for polypeptides (introns). Exons are sections of DNA that code for amino acid sequences.

Protein Synthesis

Transcription is the process of making messenger RNA from a DNA template.
DNA helicase breaks the hydrogen bonds between the DNA helix, free RNA nucleotides base pair with the exposed DNA template strand,
In prokaryotes, transcription results directly in the production of mRNA from DNA.
In eukaryotes, transcription results in the production of pre-mRNA; this is then spliced to form mRNA
Translation is the process of making proteins by forming a specific sequence of amino acids based on coded instructions in mRNA. RNA polymerase catalyses phosphodiester bonds between adjacent RNA nucleotides and the mRNA strand detaches, allowing the DNA helix to reform.
mRNA attaches to a ribosome on the rough endoplasmic reticulum, tRNA carries the corresponding amino acid to each codon on the mRNA one at a time, with an enzyme catalysing the formation of a peptide bond between amino acids using ATP, until a stop codon is reached and the peptide is released, folding into its tertiary structure.