Nucleic Acids: The Molecules of Life

DNA and RNA Both are polymers. They are made up of monomers called nucleotides

Nucleotides have three parts to them: a phosphate group a pentose sugar a nitrogenous base.

DNA: deoxyribonucleic acid RNA: ribonucleic acid

Phosphodiester bonding… Joins nucleotides together Condensation reaction Eliminates H 2 O molecule

The Bases Pyramidines:thymine cytosine Purines:adenine guanine RNA has URACIL instead of thymine

Base pairing… Complementary base pairing

Watson and Crick Model (and Franklin…) DNA X-ray crystallography

DNA REPLICATION When cells divide, you need to have 2 copies of the DNA in the cell

DNA Replication Most important feature of DNA is its ability to replicate itself, (almost) exactly The double helix is ‘unzipped’ during replication New nucleotides are aligned along the DNA strands (built by DNA polymerase) The new helix contains one original strand and one new strand

Semi - conservative replication – Meselson & Stahl

READING THE CODE DNA carries the information to allow protein synthesis

How is DNA organised? You have roughly 1m of DNA in a cell! It is stored in sections: chromosomes Each chromosome is coiled up around proteins (histones) so it fits in a cell A section of a chromosome that codes for a protein is called a gene Different versions of genes (allelles) arise from mutations of the DNA code

DNA – genetic code Strings of base pairs make up a code (GENE) The 4 base pairs are arranged in triplets called CODONS Codons each encode a unique AMINO ACID Codons are copied (transcribed) from the DNA molecule into a molecule of mRNA The mRNA containing the codons is ‘translated’ into amino acids at the ribosomes (outside the nucleus)

Triplet code – genetic translation table Triplet codes are degenerate

Protein synthesis The central dogma: DNA is transcribed to mRNA which is translated into proteins 5mS2ls

RNA Uses uracil instead of thymidine It travels out of the nucleus to the cytoplasm where it is translated into amino acids by the ribosomes

Mutation….

Genetic mutations Single codons or base pairs can be mis-read or mismatched during replication/transcription These changes are known as MUTATIONS These may have no effect OR can alter the structure/function of proteins Mutations can be passed to offspring and can result in genetic diseases Chances of mutation happening in DNA replication is 2-30x10 -7

Effect of mutation on whole organisms A source of variation Occasionally results in new protein/function Could give reproductive advantage Basis of evolution by natural selection Most likely to occur during replication Mutagen increase rate of mutagenesis