nucleic acids: the molecules of life. dna and rna both are polymers. they are made up of monomers...

Nucleic Acids:

The Molecules of Life

DNA and RNA

Both are polymers.

They are made up of monomers called nucleotides

phosphate sugar base

CH2

OC

C C

C

O

O

O

OP

-

-

N1’

2’3’

4’

5’

OH OH

• 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 H2O molecule

The Bases

• Pyramidines: thyminecytosine

• Purines: adenineguanine

RNA has URACIL instead of thymine

Base pairing…

• Complementary base pairing

http://uk.youtube.com/watch?v=qy8dk5iS1f0

Watson and Crick Model (and Franklin…)

DNA

X-ray crystallograph

y

DNA REPLICATIONWhen 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

1

2

3

4

Semi - conservative replication – Meselson & Stahl

READING THE CODEDNA 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

http://www.youtube.com/watch?v=41_Ne5mS2ls

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