Download GEN2MHG – MOLECULAR AND HUMAN GENETICS DNA is made

GEN2MHG – MOLECULAR AND HUMAN GENETICS
DNA is made up of nucleotides composed of;
- a phosphate group attached to the 5’ carbon
- a free hydroxyl group (OH) attached to the 3’ carbon
- a nitrogenous base attached to the 1’ carbon
▪ phosphodiester bonds form between the 5’ position of one
nucleotide and the 3’ end of the next
▪ nitrogenous bases may be;
- purines (2 ringed structures) -> adenine and guanine
- pyrimidines (1 ringed structure) -> thymine (& uracil) and cytosine
Chargaff’s Rules;
A=T
C=G
A + G = C + T or purines = pyrimidines
DNA Structure;
▪ double helix
▪ diameter of 2nm
▪ complete turn is 3.5nm, 10 base pairs per complete turn
▪ strands run anti-parallel
▪ generally right-handed helix
▪ two grooves, major (22A wide) and minor (12A) of different sizes are present -> major groove is
more accessible to transcription factors
Alternative DNA structures;
▪ B-DNA – most common and biologically significant -> right handed and 10 base pairs per turn
▪ A-DNA – occurs in dehydrated laboratory conditions, right handed, 11bp per turn
▪ Z-DNA – short lived, left handed structure, 12bp per turn, may regulate chromosome structure
and gene expression
Cell Cycle;
Key Features of DNA replication;
▪ occurs in 5’ to 3’ direction
▪ require a template and a primer
▪ semi conservative (made up of one old and one newly synthesised strands) -> each parental
strand acts as a template -> generates two daughter strands identical to parental strand
▪ DNA replication is carried out by the enzyme DNA polymerase
▪ each parental strand acts as a template strand
▪ daughter nucleotides made up of one old (parental)
strand and one newly synthesised strand
▪ two daughter strands identical to parental strand
produced
▪ Meselsohn & Stahl labelled DNA with radioactive
isotopes
▪ DNA and nucleotides have polarity
▪ DNA synthesis proceeds in a 5’ to 3’ direction, nucleotides are added to the 3’ end
▪ DNA synthesis requires a 3’ OH to make the next phosphodiester bond during synthesis
▪ a free OH acts as a primer for DNA replication
▪ new nucleotides are added to 3’ OH
DNA polymerase;
▪ DNA polymerase III catalyses the addition of deoxyribonucleotides to the 3’ OH group
▪ occurs in 5’ to 3’ direction and is driven by the release of pyrophosphate and subsequent
hydrolysis
▪ DNA polymerase has a 3’ to 5’ exonuclease activity -> is capable of removing incorrectly
incorporated nucleotides = PROOFREADING function
▪ DNA polymerase has two catalytic sites -> one for DNA synthesis, one for proofreading -> when
an incorrect base is added a conformational change occurs allowing for removal of incorrect base
* a need for proofreading explains why DNA synthesis occurs in 5’ to 3’ direction
Replication Origins;
▪ DNA synthesis requires single stranded DNA template
▪ a protein complex origins of replication are rich in A/T (only two hydrogen bonds, therefore
easier to separate than G/C rich areas)
▪ multiple replication origins
Replication occurs in two directions but is semi-discontinuous due to both strands being
synthesised in a 5’ to 3’ direction (continuous replication on leading strand, discontinuous on
lagging strand)
* primers provide a free 3’ OH group which allows incorporation of the next base
▪ DNA polymerase cannot initiate synthesis on bare template
▪ leading strand requires only 1 primer, lagging strand requires a primer at the start of each
Okazaki fragment
▪ an enzyme called a primase (RNA polymerase) synthesises RNA primers in the 5’ to 3’ direction
complementary to the lagging strand
▪ the primer is later removed by the DNA repair enzyme RNase H that recognises RNA/DNA
hybrids, the gaps are filled in by a DNA polymerase and fragments joined with a DNA ligase