Lecture 7 - DNA structure and replication

What is DNA?

• Basis of cellular inheritance

• Chromosome contains both DNA and protein

• DNA encodes protein sequence via RNA in both prokaryotic and eukaryotic cells

• DNA undergoes transcription into mRNA

Describe DNA and RNA polymers of nucleotides?

• DNA is a polymer of nucleotides with 3 parts

• Sugar - pentose

• Nitrous base, A, C, G, T bound to c1

• Phosphate group bound to c5

• Nucleotides connected by hydroxy group on c5 and c3

• RNA similar to DNA but has one more O at c2

Describe chargaffs rules?

• A% = T%

• G% = C%

• Sequence is TGCA

• Sugar phosphate backbone, with base paring between strands

Describe how double helix works?

• Phosphate attached to c3 of nucleotide, and c5 of next nucleotide

• Strands are parallel

How DNA double helix is stabilised?

• Two strands stabilised by non covalent bonds

• HB between each strand

• Bonds can be broken using heat

Describe the base paring and the HB?

• purines have double rings, A, G

• Pyrimidines have single rings, C, T

• For these to create HB, the bonds before that need to be HB

DNA vs RNA structure

• DNA is found in double stranded form

• RNA have complex secondary structure

• Double stranded regions involve anti parallel structures

Describe nuclear acid polymerisation?

• DNA replicates from template DNA strand, RNA is transcribed from template DNA strand

• New nucleotides that base pair with complementary strand are added to the 3’ end of nucleic acids

• 3’ OH group on sugar reacts with primary phosphate

• Triphosphate provides energy to drive reaction

• DNA replication proceeds in a 5’ to 3’

Describe DNA replication at replication forks?

• Addition occurs in 5 to 3 direction

• Leads to different problem at each fork

• Strands run parallel

• Leading strand - towards replication fork

• Lagging strand - away from replication fork

Describe the 4 proteins that maintain the replication fork

• Helicase - untwists to give single strand, but increases coiling ahead

• Topoisomerase - fixes the increased coiling ahead

• Single stranded binding protein stabilises the strand

• Primase allows to initiate DNA polymerisation

Describe leading strand synthesis?

• Primase used to start synthesising DNA

• DNA polymerase 3 adds DNA nucleotides from 3’

• RNA primers removed

• Leading strand will continue

Describe lagging strand synthesis?

• Same synthesis as leading strand

• RNA primers need to be made frequently resulting in DNA fragments

• DNA polymerase 1 removes RNA primers and replaced it with DNA

• DNA ligase allows DNA to go from fragments to continuous strands

Describe mistakes in RNA replication?

• Mismatched base pairs need to be removed before replication

• Errors can occur during synthesis, or can cause radiation or chemical damage

• Nuclear cuts out damages nucleartoids