Chapter 13

Chapter 13 - 13.1 How is RNA transcribed from DNA? Most...

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Unformatted text preview: 13.1 How is RNA transcribed from DNA? Most genes are transcribed into messenger RNA, or mRNA, the only kind that carries proteinbuilding instructions Others are transcribed into ribosomal RNA, or rRNA, a component of ribosomes large molecular structure upon which polypeptide chains are assembled Others transcribed to transfer RNA, or tRNA, which can deliver amino acids one at a time to a ribosome RNA has four kinds of ribonucleotides, each with 5-C sugar ribose, P-group and a base - Ade nin e - Cyt o si n e - Gu a ni n e - Ur a cil (like t h y mi n e , c a n p air wit h a d e ni n e) o Ne w RNA s t r a n d c a n b e b uilt a c c or di n g t o t h e s a m e b a s e-p airin g r ul e s a s DNA Tra n s criptio n diff er s fro m DNA r e plic a tio n - PART of a DNA s t r a n d is u s e d a s t h e t e m pl a t e - RNA p oly m e r a s e (no t DNA p oly m e r a s e ) a d d s rib o n u cl e o tid e s t o e n d of gr o win g s tr a n d of RN - Re s ult s in o n e fr e e s tr a n d of RNA ( n o t H-b o n d e d d o u bl e h elix) Pro m o t e r o n e s e q u e n c e of b a s e s in DNA t h a t si g n al s t h e s t a r t of a g e n e RNA s y n t h e si s b e gi n s a s s o o n a s RNA p oly m e r a s e s a n d o t h e r pr o t ei n s a t t a c h t o it - Ea c h p oly m e r a s e m o v e s al o n g DNA s t r a n d , joinin g o n e ribo n u cl e o tid e af t e r a n o t h e r o n t h e D a s a t e m pl a t e - Soo n r e a c h e s a n o t h e r b a s e s e q u e n c e in DNA t h a t si g n al s "t h e e n d " a n d n e w RNA is r el e a s e fr e e tr a n s cript Ne w m RNA m ol e c ul e n o t in fin al for m, " pr e-m RNA" - So m e e n z y m e s a t t a c h a m o difi e d g u a ni n e "c a p " t o s t a r t of e a c h pr e-m RNA tr a n s c rip t o Lat e r in c yt o pl a s m , c a p will h el p bi n d m RNA t o a rib o s o m e ; t ail's le n g t h d e t e r mi n e s h o w lon g a p a r tic ul ar m RNA m ol e c ul e will la s t - So m e a t t a c h 1 0 0-3 0 0 a d e ni n e rib o n u cl e o ti d e s a s a t ail t o t h e o t h e r e n d (p oly-A t ail) Exo n s pr ot ei n-c o din g p a r t s of e u k a r y o tic g e n e s Intro n s b e t w e e n e x o n s; s e q u e n c e s r e m o v e d b efor e a m a t u r e m RNA tr a n s crip t is tr a n sl a t e sit e s w h e r e pr ot ei n-b uildin g info c a n b e b r ok e n a n d r ef ur bis h e d in > 1 w a y ( alt er n a tiv e s plic w hic h allow s c ells t o u s e t h e s a m e g e n e t o m a k e pr o t ei n s t h a t diff er slig h tly in fu n c tio n a n d 13.2 Deciphering mRNA Transcripts After mRNA docks at a ribosome, its ribonucleotide bases are "read" three at a time, as triplets or codons Genetic code set of (64) different codons Each tRNA has a molecular "hook," an attachment site for an AA; it has an anticodon a ribonucleotide base triplet that can pair with an mRNA codon - Rul e s c a n loos e n for t hir d b a s e in c o d o n ("wo b bl e eff e c t") Int e r a c tio n s b e t w e e n tRNAs a n d m RNAs t ak e pl a c e a t rib o s o m e s - Ribo s o m e h a s ( 2) s u b u nit s b uilt fro m rRNA a n d s t r u c t u r al pr o t ei n s in t h e n u cl e u s, t h e n s hi p s e p a r a t ely t o c yt o pl a s m - Th er e , a lar g e a n d s m all s u b u nit c o n v e r g e int o a n int a c t, fu n c tio n al rib o s o m e o nly w h e n m R t o b e tr a n sl a t e d 13.3 Translating mRNA Into Protein Think of mRNA transcripts as intermediaries that deliver messages from DNA to ribosomes, which translate them into the polypeptide chains of proteins Translation has 3 stages: - - - Initi a tio n r e q uir e s a n initi a t or tRNA, t h e o nly o n e t h a t c a n s t a r t a tr a n sl a tio n; bi n d s t o s m all ribo s o m al s u b u nit t h e n m RNA's START c o d o n , AUG, join s wit h t h a t tRNA's a n tic o d o n. Lar g e ribo s o m al s u b u nit n o w joins wit h s m all o n e . Tog e t h e r, t h e rib o s o m e , m RNA, a n d initi a t or tR a r e a n initi a tio n c o m pl e x In el o n g a tio n, a p olyp e p ti d e c h ai n is s y n t h e siz e d a s t h e m RNA p a s s e s b/ w t h e t w o rib o s o m a s u b u nit s, like a t hr e a t p a s si n g t hr o u g h t h e e y e of a n e e dl e. tRNA m ol e c ul e s m o v e AA t o t h e ribo s o m e , w h e r e t h e y bi n d t o t h e m RNA in t h e or d e r s p e cifie d b y its c o d o n s . Part of a n rRN m ol e c ul e a t t h e c e n t e r of t h e lar g e rib o s o m al s u b u nit fu n c tio n s a s a n e n z y m e . It c a t alyz e s b o n d for m a tio n b/ w AA. Durin g t e r mi n a tio n, t h e ribo s o m e r e a c h e s t h e m RNA's STOP c o d o n. No tRNA h a s a c orr e s p o n di n g a n tic o d o n. Prot ei n s c all e d r el e a s e fac t or s bi n d t o t h e rib o s o m e , trig g e rin g e n a c tivity t h a t d e t a c h e s t h e m RNA a n d t h e p oly p e p ti d e c h ai n fro m t h e rib o s o m e . 13.4 Mutated Genes and Their Protein Products Gene mutations small-scale changes in the nucleotide sequence of a DNA molecule Base-pair substitutions one base is copied incorrectly during DNA replication, causing a protein to incorporate the wrong AA, or its synthesis may have been cut off too soon Frameshift mutations fall under insertions or deletions in which 1 base pairs become inserted/are deleted from DNA Other mutations arise from transposable elements, or transposons, that can jump around in genome, altering the timing/duration of its activity or blocking it completely Exposure to harmful radiation and chemicals in the environment can cause mutations in DNA * A protein product of a heritable mutation will have harmful, neutral, or beneficial effects on the individual's ability to function in the prevailing environment ...
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This note was uploaded on 05/01/2008 for the course BIOL 101 taught by Professor Hogan during the Spring '08 term at UNC.

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