S e pt . 20, 2007 L ec t ur e 4 B GYA 01

I n t he l a s t c l a s s w e w e r e t a l ki ng a bout DNA r e pl i c a ti on.

C li c ke r que s ti on

A n or i gi n o f r e pl i ca ti on

a ) oc c ur s a t t he e nd o f t he doub l e s t r a nde d DNA

b) ha s t w o R NA pr i m e r s / or i gi n

c ) oc c ur s i n t he i nt e r i or o f t he doubl e s t r a nde d DNA

d) ha s one R NA pr im e r pe r or i gi n

e ) a ns w e r s b a nd c a r e bot h t r ue

W e know t ha t DNA r e pl i ca ti on i s i ni ti a t e d a t o r i gi ns of r e pl i ca ti on ( w hi c h a r e i n t he i nt e r i o r o f

t he doubl e s t r a nde d DNA m ol ec ul e ) a nd t ha t r e pl i c a ti on pr oc ee ds bi - di r ec ti ona ll y fr o m t he

or i gi n. F i gu re 11.13, p age 244 a nd hom e m a de f i g ur e a ll

B i di r ec ti ona l r e pl i ca ti on c r ea t e s a g r ow i ng r e pl i ca ti on bubbl e , but t he t e xt book i s l az y a nd onl y

s how s e ve nt s f or ha l f o f t he bub bl e – one r e pl i c a ti on f or k.

W e a l s o know t ha t t he f i r s t nuc l e i c ac i d s ynt he s i ze d a t t he o r i gi n i s a c t ua ll y R NA . A n R NA

pr i m e r i s m a de f or eac h f or k ( i e t w o R NA p r i m e r s pe r or i gi n ) . hom e m a de f i gu r e fr a m e d a nd

F i gu re 11.16, p age 246

O nc e t he R NA pr i m e r s a r e l a i d dow n a t t he o r i gi n , t hi ngs go s m oot hl y on one ha l f o f eac h f o r k.

F or eac h f o r k F i gu re 11 . 18, p age 247 - O ne ne w l y s ynt he s i ze d s t r a nd ha s t he c or r ec t pol a r it y t o

r ea di l y c ont i nue r e pl i ca ti on fr o m t he or i gi na l R NA pr im e r ; eac h ti m e a nuc l e ot i de i s a dde d on,

t he r e i s a fr ee 3’ OH gr oup on t ha t nuc l e ot i de t o a d d on t o.

W e ca ll s uc h a ne w l y c r ea t e d s t r a nd a l ea di ng s t r a n d.

W e a l s o s a y DNA i s c r ea t e 5 t o 3 (fr om t he 5 ’ e nd t o t he ( g r ow i ng) 3’ e nd .

F or t hi s t e r m i nol ogy t he r e f e r e nc e i s t he ne w l y s yn t he s i ze d DNA .

A s he li ca s e c ont i nua ll y ope ns up t he DNA he li x of t hi s f or k, r e pl i c a ti on oc c ur s c ont i nuous l y on

t he l ea di ng s t r a nds . W e s a y r e pl i ca ti on on t he l ea d i ng s t r a nds i s c ont i nuous .

T he ot he r por ti on of t he s a m e f or k ( t he one t ha t us e s t he a nt i pa r a ll e l s t r a nd o f DNA a s t he

t e m pl a t e ) ha s a m or e di f f i c ul t ti m e . W hy?

I t i s be ca us e t ha t r e gi on ha s a t e m pl a t e , but no fr ee 3’ OH g r oup ( t o a dd ont o ) i n t he di r ec ti on t he

f or k i s ope ni ng. ) . t e xt b ook f i gu re 11.18, p age 24 7 t op p an e l S o a t t he m om e nt no r e pl i ca ti on

ca n oc c ur t he r e ( s t udy gui de r e pl i ca ti on f i gu r e , pa r t D

I ns t ea d r e pl i ca ti on l a gs on t h i s s i de of t he f o r k unt il t he DN A he li x ge t s m or e ope ne d up. W e

ca ll t he s t r a nd t ha t w ill e ve nt ua ll y be m a de on t hi s por ti on of t he f or k , t he l aggi n g s t r an d .

If you l ook a t t he hom e m a de f i gur e you s houl d be a bl e t o s ee t ha t t he r e a r e ac t ua l l y t w o l ea di ng