Chapter 16 The Molecular Basis of Inheritance

Chapter 16 The Molecular Basis of Inheritance - Chapter 16:...

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Unformatted text preview: Chapter 16: The Molecular Basis of Inheritance 16.1: DNA is the genetic material The Search for the Genetic Material - Once T.H. Morgan demonstrated that genes are located along chromosomes, the two chemical components of chromosomes became the candidates for the genetic material o DNA and protein - In the beginning, proteins seemed like a likelier candidate because there were more varied and had specific functions; also, little was known about nucleic acids, which seemed far too uniform to create the variety that we see in nature Evidence that DNA can transform bacteria - Frederick Griffith was studying streptococcus pneumonia, a bacteria that causes pneumonia o He had one pathogenic and one nonpathogenic strain of the virus When he killed the pathogenic bacteria with heat and mixed them with living nonpathogenic bacteria, some of the living bacteria became pathogenic Also, this newfound trait was found in subsequent generations of bacteria • He concluded that these cells underwent transformation: the assimilation of external DNA by a cell - Avery, McCar t y, a n d MacLe o d p e rfor m e d si mil ar e x p e ri m e n t s b u t wit h t hr e e isol a t e d s u b s t a RNA, a n d pr ot ei n s o In s e p a r a t e d s a m pl e s , t h e y kille d t w o of t h e t hr e e s u b s t a n c e s a n d t h e n mix e d t h e m wit h n o n p a t h o g e nic b a c t e ri a Only w h e n DNA w a s allow e d t o r e m ai n a c tiv e di d tr a n sfor m a tio n o c c u r Pe o pl e w er e s till u n c o n vi n c e d b e c a u s e n o t e n o u g h w a s k n o w n a b o u t b a c t e ri a Evidence that viral DNA can program cells - Sci e n tis t s b e g a n t o s t u d y viru s e s in or d e r t o u n d e r s t a n d m o r e a b o u t DNA a n d its p o t e n ti al a m a t e ri al o Th e y s t u di e d b a c t e rio p h a g e s , or p h a g e s , w hic h a r e m u c h si m pl er t h a n c ells Oft e n, t h e y a r e jus t DNA wit h a pr o t e c tiv e c o a t of pr o t ei n In or d e r t o r e pr o d u c e , t h e y m u s t inf e c t a h o s t c ell - Alfre d Her s h e y a n d Mart h a Ch a s e p e rfor m e d a n e x p e ri m e n t wit h t h e T2 p h a g e s o Th e y la b el e d t w o s e p a r a t e s a m pl e s; o n e wit h r a dio a c tiv e p h o s p h o r u s a n d t h e o t h e r wit h s ulfur Sulfur is o nly fou n d in pr o t ei n s; Ph o s p h o r u s is fou n d o nly in DNA o Th e T2 p h a g e s w er e allow e d t o inf e c t t h e b a c t e ri al c ells o Th e n, t h e e n tir e s olu tio n w a s pl a c e d int o a c e n t rifu g e a n d t h e b a c t e ri al c ells a n d t h eir c o f or m a pr e ci pit a t e a t t h e b o t t o m Wh e n t h e pr e ci pit a t e w a s fro m t h e s olu tio n la b el e d wit h s ulfur, it w a s n o t r a dio a c tiv e, t h a t t h e pr o t ei n di d n o t e n t e r t h e c ell Ins t e a d , DNA di d, fin ally d e t e r mi nin g t h a t DNA w a s t h e g e n e tic m a t e ri al, a t le a s t for v Additional Evidence that DNA is the Genetic Material - Erwin Ch ar g aff e x a mi n e d t h e DNA fro m a n u m b e r of diff er e n t or g a ni s m s o He fou n d t h a t or g a ni s m s h a v e diff er e n t b a s e c o m p o sitio n s (diff er e n t a m o u n t s of e a c h n u w hic h w a s e vi d e n c e of m ol e c ul ar div e r sity a m o n g s p e ci e s o He al s o n o tic e d t h a t t h e n u m b e r of a d e ni n e s a p p roxi m a t ely e q u al e d t h e n u m b e r of t h y m n u m b e r of cy t o si n e s a p proxi m a t ely e q u al e d t h e n u m b e r of g u a ni n e s Building a structural model of DNA - Wat s o n visit e d t h e la b of Ma uric e Wilkin s a n d Ros alin d Fra n klin, w h o w er e workin g o n X-r a y c ry s t allogr a p h y o He s a w a n X-r a y diffr a c tio n im a g e of DNA t h a t Fra n klin h a d p r o d u c e d He s a w t h a t t h e s h a p e of DNA w a s h elic al a n d it allow e d hi m t o a p p roxi m a t e t h e wid t h elix a n d t h e s p a ci n g of t h e nitro g e n o u s b a s e s al o n g it Th e widt h s u g g e s t e d t h a t it w a s m a d e u p of t w o s t r a n d s r a t h e r t h a n t h e 3 t h a t Linu s suggested - Watson and Crick experimented with different models of the double helix, placing the sugar backbone on t he outside of the helix (the nitrogenous bases are hydrophobic) o The sugar­phosphate backbones run antiparallel to each other o There are 10 nitrogenous bases with ever y t urn of the helix o Adenine was paired with t hy mine and cytosine was paired with guanine A and G are purines (two organic rings) C and T are pyri midines (single rings) • Pairing purines would make the double helix too wide and t wo pyri midines was too o A for ms 2 bonds with T, C for ms 3 bonds with G This explained Chargaff’s rule ...
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