H2 Biology 7 - DNA Replication(i)

1. Nitrogenous base -

purine; double-ringed, Adenine, Guanine

pyrimidine; single-ringed, Cytosine, Thymine, Uracil(in RNA)

2. Pentose sugar -

ribose in RNA, deoxyribose in DNA(O atom removed from C2)

3. Phosphate group -

confers negatively-charged/acidic property

nitrogenous base linked to C₁ of pentose sugar by condensation rxn, give nucleoside >

phosphate group joins nucleoside @ C₅ of pentose sugar in condensation rxn >

nucleotide w/ deoxyribose = DNA nucleotide/deoxyribonucleotide,

nucleotide w/ ribose = RNA nucleotide/ribonucleotide

1. monomers of DNA/RNA

2. source of chemical energy(ATP)

3. chemically modified to be used as signalling molecules(cAMP)

4. combine w/ other chemical groups to form coenzymes

phosphodiester bond formed b/w -OH group on C₃ of pentose sugar of 1 nucleotide, phosphate group on C₅ of pentose sugar of adjacent nucleotide in condensation rxn w/ loss of H₂O molecule catalysed by DNA pol >

many nucleotides link tgt form polynucleotide >

each polynucleotide strand has SP backbone w/ nitrogenous bases projecting outwards from sugars >

has 2 distinct ends, 3'(-OH on C₃) & 5'(-OH on C₅)

double helix of 2 comp polyNT strands >

2 strands coil around EO, form right-handed double helix >

2 strands antiparallel, 1 runs 5'>3', comp strand 3'>5' >

each strand consists long chain of DNA NT >

DNA NT comprises doxyribose sugar, phosphate group & 1/4 nitrogenous bases(ATCG) >

each strand contains SP backbone maintained by PDE bonds b/w -OH group on C₃ of 1 NT, P group on C₅ of adjacent NT >

bases arranged as side groups of strands, width b/w 2 SP backbones = 2nm = 1bp >

1 complete turn of double helix = 3.4nm(10bp) >

surface of molecule has major, minor grooves >

bases of 1 strand pairs w/ opposite strand bases by H bonding >

AT 2H bonds, CG 3H bonds >

base pairing specific & complementary, A>T, C>G >

molecule stabilised by hydrophobic interactions b/w stacked bases

1. storage of genetic information

2. passing down genetic material from 1 generation to the next

3. direct protein synthesis via transcription->translation in cell

-vely-charged DNA wounds around +vely-charged histone octamer(2xH₂A, 2xH₂B, 2xH₃, 2xH₄ histones), form nucleosome >

individual nucleosome connected by linker DNA & H₁ histones form 10nm beads-on-a-string/nucleohistone complex >

w/ aid of H₁ histones, complex coils, form 30nm chromatin fibre >

fibre folds, form looped domains attached to base of scaffolding proteins form 300nm chromatin fibre >

fibre further coiled, compacted form chromosome >

compaction protects DNA from breaking when pulled apart during anaphase

2 parental DNA strands separate via breaking of H bonds b/w comp bases >

both strands act as template for synthesis of new comp DNA strands >

each new DNA molecule consists 1 original DNA strand, 1 newly synthesised DNA strand

SCDR begins @ ORI, helicase unwinds, unzips DNA molecule, H bonds b/w comp bases break, 2 parental DNA strands separate >

ssDNABP bind 2 separated strands, stabilise ssDNA so each strand @ unwound region can serve as template for synthesis of new comp strand >

primase catalyses formation of short RNA primer, DNA pol binds to RNA primer, adds free DNA NT to free 3'-OH end; DNA pol only works from 5'>3', can only add NT to free 3'-OH end of existing strand >

free DNA NT attach by CBP via H bonds w/ comp bases, DNA pol catalyses formation of PDE bonds b/w adjacent NT, synthesis of newly synthesised strand occurs 5'>3', DNA template read 3'>5' as DNA strands antiparallel >

leading strand synthesised continuously as DNA pol moves in same direction as unwinding, unzipping of DNA

lagging strand synthesised discontinuously, results in Okazaki fragments as 2 parental DNA strands anti-parallel >

conf of AS of DNA pol & its enzyme specificity allows it to only work 5'>3' >

DNA pol can only add DNA NT to free 3'-OH end of strand >

RNA NT of all RNA primers replaced w/ DNA NT w/ another DNA pol >

DNA ligase seals gaps b/w Okazaki fragments by catalysing formation of PDE bonds b/w adjacent NT, form continuous strand >

@ end of each round of repn, original DNA strand & newly synthesised strand rewinds into double helix >

each new dNA molecule consists 1 original DNA strand, 1 newly synthesised DNA strand

conf of AS of DNA pol & its enzyme specificity allows enzyme to only work in 5'>3' >

DNA pol can only add DNA NT to free 3'-OH end of existing strand >

2 parental strands anti-parallel

DNA pol requires free 3'-OH end of existing strand to add nucleotides >

RNA primer synthesised to provide free 3'-OH end, replaced by DNA NT >

RNA primer removed from 5' end of newly synthesised DNA strand, unable to be replaced w/ DNA NT as no free 3'-OH end >

DNA pol unable complete repn @ 5' end of lagging strands, creates 3' overhang @ end of chromosome