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Download GEN2MHG – MOLECULAR AND HUMAN GENETICS DNA is made
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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