Final Review 1 Transcription Chapter 17 pp 331 336 Transcription is the DNA directed synthesis of RNA Molecular Components of Transcription Messenger RNA the carrier of information from DNA to the cell s pro tein synthesizing machinery is transcribed from the template strand of a gene o RNA polymerase pries the strands apart and joins RNA nucleotides along the template in a 5 3 direction RNA polymerase has the ability to start chain without a primer o DNA sequence where RNA polymerase attaches and initiates tran scription is called the promoter Bacteria types of RNA have single type of polymerase that synthesizes all Eukaryotes have at least 3 types of polymerases RNA polymerase II is used for mRNA synthesis Other polymerases transcribe RNA molecules that are not translated into a protein Stretch of DNA that is transcribed into an RNA molecule is the transcrip tion unit Synthesis of an RNA Transcript Initiation Elongation Termina tion 1 Initiation o After RNA polymerase binds to the promoter the DNA strands unwind and the polymerase initiates RNA syn thesis at the start point on the template strand Promoter includes a start point and extends several nucleotide pairs upstream promoter also includes a TATA box o Promoter determines which of the 2 strands of the double helix is used as a template In bacteria polymerase recognizes and binds to the pro moter In eukaryotes transcription factors collection of proteins that must recognize the TATA box mediate the binding of RNA polymerase and initiation of tran scription The complex of transcription factors and RNA polymerase II bound to the promoter is called a transcription initiation complex A crucial promoter DNA sequence called a TATA box forms the initiation complex at a eukaryotic pro moter o It controls eukaryotic transcription 2 Elongation o The polymerase moves downstream unwinding the DNA and elongating the RNA transcript 5 3 In the wake of transcription the DNA strands reform a double helix RNA polymerase continues to untwist the double helix expos ing 10 20 DNA bases at a time to pair with RNA nucleotides Polymerase adds nucleotides to the 3 end The new RNA molecule peels away from its DNA tem plate and the helix reforms o This progresses at a 40 nucleotides per second rate The act of many polymerase molecules simultaneously tran scribing a single gene increases the amount of mRNA tran scribed This helps the cell make the encoded proteins in big quantities 3 Termination o Eventually the RNA transcript is released and the poly merase detaches from the DNA In bacteria transcription has a terminator sequence that sig nals the end of transcription Terminator causes polymerase to detach from DNA and release the transcript for the mRNA to use In eukaryotes RNA polymerase II transcribes a sequence on the DNA called the polyadenylation signal sequence which codes for a polyadenylation signal AAUAAA in the pre mRNA About 10 35 nucleotides down from this signal pro teins associated with the RNA transcript cut it free from the polymerase releasing the pre mRNA o Polymerase continues to transcribe DNA past the release of the pre mRNA Eukaryotic cells modify RNA after transcription Next RNA Processing is involved to modify pre mRNA before the genetic messages are dispatched by altering both ends of the primary transcript o 5 end is synthesized first and receives a 5 cap a modified form of guanine added on the 5 end after transcription of the first 20 40 nu cleotides o 3 end is modified before the mRNA exits the nucleus An enzyme adds 50 250 more adenine nucleotides to form a poly A tail to the 3 end o The 5 cap and the poly A tail share the following 1 They facilitate the export of the mature mRNA from the nu cleus enzymes 2 They help protect the mRNA from degradation by hydrolytic 3 They help ribosomes attach to the 5 end of the mRNA once the mRNA reaches the cytoplasm o Untranslated regions UTRs are parts of the mRNA at the 3 and 5 ends that will not be translated into protein Poly A tail and 5 cap are not translated either Removal of the large portions of the RNA molecule that is initially synthe sized is called RNA splicing o The sequence of DNA nucleotides that codes for a eukaryotic polypep tide is usually not continuous but is in segments The noncoding segments between the coding regions are called intervening sequences or introns The expressed segments are called exons sequences of RNA that exit the nucleus RNA polymerase II transcribes both introns and exons from the DNA o The introns are cut out o The exons are joined together forming an mRNA molecule with a continuous coding sequence This is the process of RNA splicing o The signal for RNA splicing is a short nucleotide sequence at each end of an intron Small nuclear ribonucleoproteins snRNPs recog nize these splice sites on the ends of the introns RNA in a snRNP is called a small nuclear RNA snRNA spliceosome Several snRNPs join with additional proteins to form a larger The spiceosome interacts with sites along an intron re leasing the intron and joining together 2 exons The catalytic role for snRNA arose from ribozymes RNA molecules that function as enzymes o The intron RNA functions as a ribozyme and catalyzes its own exci sion by removing its own introns o 3 properties enable this 1 Because RNA is single stranded a region of an RNA mole cule may base pair with a complementary region elsewhere on the same molecule 2 Some of the bases in RNA contain functional groups that may participate in catalysis 3 The ability of RNA to hydrogen bond with other nucleic acids adds specificity to its catalytic activity Splicing is necessary for the passage of mRNA from the nucleus to the cyto plasm Alternative RNA splicing is a type of gene regulation in which different mRNA molecules are produced from the primary transcript depending on which segments are considered to be exons and which are introns o For example sex differences in fruitflies are due to the differences in how males and females splice the RNA Result The number of protein products an organism produces can be much greater than its number of genes Proteins have a modular architecture consisting of discrete structural and func tional regions called domains o Different exons code for the different domains of a protein o Presence of introns may facilitate evolution of new proteins as a result of exon shuffling Introns increase probability of potentially beneficial crossing over between exons of alleles Terms that remain
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