Chapter 10 From DNA to Proteins Transcription and RNA Processing A Life requires two basic functions 1 Living organisms must be able to store and faithfully transmit genetic information during 2 They must have the ability to catalyze chemical transformations to fire the reactions that B RNA can serve as a biological catalyst It can excise 400 nucleotides from its RNA in the absence reproduction drive life processes of any protein 1 Ribozymes catalytic RNA molecules can cut out parts of their own sequences connect some RNA molecules together replicate others and even catalyze the formation of peptide bonds between amino acids a This led to the discovery that RNA was the original genetic material 2 RNA molecules serves as both carriers of genetic information and as catalyzes that drove the chemical reactions needed to sustain and perpetuate life 3 Also RNA may have acquired the ability to synthesize protein based enzymes which are more efficient catalysts a Enzymes took over more of the catalytic functions leading too i ii RNA to become regulated to the role of information storage and transfer DNA with its chemical stability an faithfully replication eventually replaced RNA as the primary carrier of genetic information 10 1 RNA Consisting of a Single Strand of Ribonucleotides Participates in a Variety of Cellular Function A The Structure of RNA 1 RNA like DNA is a polymer consisting of nucleotides joined together by phosphodiester bonds 2 Difference between the structures a RNA nucleotides have ribose sugars unlike DNA dioxyribose sugars b RNA has a free hydroxyl group on the 2 carbon atom of the ribose sugar DNA deoxyribose sugar lacks this free hydroxyl group c Because of the free hydroxyl group RNA degrades rapidly under alkaline conditions DNA is more stable d RNA contains uracil instead of thymine in DNA e RNA is usually single stranded consisting of one polynucleotide chain DNA double stranded consisting of two polynucleotide strands joined by hydrogen bonding between complementary bases 3 Although RNA is single stranded short complementary regions within a nucleotide strand can pair and form secondary structures B Classes of RNA 1 Ribosomal RNA rRNA along with ribosomal protein subunits make up the ribosome 2 Messenger RNA mRNA carries the coding instructions for polypeptide chains from the site of protein assembly DNA to the ribosome a After attaching to a ribosome an mRNA molecule specifies the sequence of amino acids in a polypeptide chain and provides a template for joining amino acids 3 Pre messenger RNAs pre mRNAs precursor molecules are the immediate products of transcription in eukaryotic cells a They are modified extensively before becoming mRNA and exiting the nucleus b Bacterial cells do not possess pre mRNA in bacteria transcription takes place for translation into protein concurrently with translation 4 Transfer RNA tRNA serves as the link between the coding sequences of nucleotides in the mRNA and amino acid sequence of nucleotides in the mRNA and the amino acid sequence of a polypeptide chain a Each tRNA attaches to an amino acid and helps incorporate it into a polypeptide chain 5 Small nulear RNAs snRNAs combine with small protein subunits to form small nuclear ribonucleoproteins snRNPs participate in the processing of RNA converting pre mRNA into mRNA 6 Small nucleolar RNA snoRNAs take part in the processing of rRNA 7 microRNAs miRNAs and small interfering RNAs siRNAs are found in eukaryotic cells and carry out RNA interference RNAi a process in which these small RNA molecules help trigger the degradation of mRNA or inhibit their translation into protein 10 2 Transcription Is the Synthesis of an RNA Molecule from a DNA Template A All cellular RNAs are synthesized from DNA templates through the process of transcription 1 2 In many ways similar to replication the big difference is the length of the template In replication ALL nucleotides in the DNA template are copied In transcription only small parts of the DNA molecule usually a single gene are transcribed into DNA a Because not all the gene products are needed at the same time or in the same cell 3 Much of the DNA does not encode a functional product and transcription of such sequences would be pointless 4 Transcription is highly selective individual genes are transcribed only as their products are needed B Transcription requires three major components 1 A DNA template 2 The raw materials substrates needed to build a new RNA molecule 3 The transcription apparatus consisting of the proteins necessary to catalyze the synthesis of RNA C The Template for Transcription 1 The transcribed strand a The template for RNA synthesis is a single strand of the DNA double helix b Template strand the one nucleotide strand of DNA used for transcription c Nontemplate strand is not ordinarily transcribed i Although only one strand is transcribed different genes may be transcribed from different strands 2 During transcription an RNA molecule that is complementary and antiparallel to the DNA template strand is synthesized 3 Transcription unit is a stretch of DNA that encodes an RNA molecule and the sequences necessary for its transcription containing three critical regions a Promoter a DNA sequence that the transcription apparatus recognizes and binds i ii iii It indicates which of the two DNA strands is to be read as the template and the direction of transcription It also determines the transcription start site the first nucleotide that will be transcribed into RNA In most transcription units the promoter is located next to the transcription start site but it is not itself transcribed b RNA coding region a sequence of DNA nucleotides that is copied into an RNA molecule c Terminator a sequence of nucleotides that signals where transcription is to end Transcription stops only after the terminator has been copied into RNA i 4 The terms upstream and downstream to refer to the direction of transcription and the location of nucleotide sequences surrounding the RNA coding sequence a Downstream as transcription takes place it binds to the promoter and moves b The sequence on the nontemplate strand is written with the 5 end on the left and toward the terminator the 3 end on the right c Nucleotides downstream at the start site are assigned positive numbers and nucleotides upstream at the start site are assigned negative numbers D The Substrate for Transcription 1 RNA is synthesized from ribonucleoside triphosphates a Nucleotides are
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