DNA and protein synthesis

• Deoxyribonucleic and ribonucleic acid are both nucleic acids, which are polymers of nucleotides.
• Nucleotides consist of pentose which is a 5 carbon sugar, a nitrogen-containing organic base and a phosphate group.
• The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine.
• Adenine and guanine both have two nitrogen-containing rings and are classified as purine bases.
• The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil.
• Thymine, uracil and cytosine all have a single ring structure and are classified as pyrimidines.
• Pyrimidines are smaller than purines as they only contain one nitrogen-containing ring.
• Nucleotides join together via phosphodiester bonds formed in condensation reactions.
• RNA is single-stranded and comes in multiple different forms, such as mRNA (messenger RNA), tRNA (transfer RNA) and rRNA (ribosomal RNA), which are involved in protein synthesis.
• A DNA molecule is a double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases - there are two hydrogen bonds between adenine and thymine, and three hydrogen bonds between cytosine and guanine.
• The genetic code consists of triplets of bases called codons.
• Each codon codes for an amino acid.
• The amino acids are then joined together by peptide bonds and form a polypeptide chain.
• A gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.
• Not all of the genome codes for proteins - the non-coding regions of DNA are called introns and the coding regions are called exons.
• The genetic code is non-overlapping, meaning that each triplet is only read once and triplets don't share any bases.
• The genetic code is universal, meaning it is the same in all organisms and species.
• The genetic code is degenerate, meaning that more than one triplet codes for the same amino acid.
• The genetic code reduces the effect of mutations which are changes to the base sequence such as base deletions, insertions or substitutions.
• A deletion or insertion is more likely to be harmful because it causes a 'frameshift', in which all codons 'downstream' of the mutation are read differently.
• Some mutations which do change the base sequence are harmful, such as the mutation which leads to sickle cell anemia, in which a mutated form of haemoglobin distorts the shape of red blood cells.
• The genetic codes contains start and stop codons which either start or stop protein synthesis.
• A change in the base sequence of DNA may not affect the amino acid coded for as the new triplet may still code for the same amino acid.