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RNA Ribonucleic Acid A. RNA has ribonucleotides which have the sugar ribose and the nitrogenous bases A, C, G, and Uracil. RNA is single stranded. B. The 4 bases act like letters of the alphabet to communicate genetic information C. Transcription- the process of copying the genetic message on the DNA into a messenger RNA, mRNA. a. The sequence of nucleotides in a gene carries the information for the primary structure of a protein. ie, the linear sequence of a.a. on the protein b. In transcription, a gene on the DNA strand provides a template for the synthesis of a complimentary RNA strand. D. Translation- the process of synthesizing protein using the code (base sequences) on the mRNA. This process requires ribosomes, rRNA, and tRNA, transfer RNA. E. Transcription occurs in the nucleus and translation in the cytoplasm DNA RNA Protein F. Triplet Code- Nucleotide triplets (3 consecutive bases) specify each of the 20 amino acids. Therefore, There are 43 = 64 possible triplets G. 1. During transcription, one DNA strand, the template strand, provides a template for ordering the sequence of nucleotides of one gene, in an mRNA transcript. 2. blocks of three nucleotides base sequences on the mRNA are the codons 3. Then during translation, the codons, are decoded into a sequence of a.a. on a protein. 4. Because codons are base triplets, the number of nucleotides making up genetic messages must be three times the number of a.a. making up the protein product. 5. a. the genetic code is redundant (more than one triplet code for a particular a.a) b. the genetic code is not ambiguous (one triplet does not code for more than one a.a.) c. codons synonymous for the same a.a. usually differ in the third base position, only. d. the AUG codon indicates the start of translation e. UAA, UAG, and UGA codons indicate the termination of translation H. The genetic code is universal for all species it must have evolved early. I. Steps of Transcription: 3 stages: Initiation, Elongation, Termination 1.Initiation Specific sequences of the DNA template strand determine where gene transcription begins and ends. a. RNA polymerase attaches and initiates transcription of a gene at the promotor region on the DNA template strand. (1) transcription factors recognize the promotor region’s TATA box & bind (2) then RNA polymerase binds to the transcription factors to create a transcription initiation complex 2.Elongation- RNA polymerase moves along the DNA, it untwists the double helix about 10-20 base at a time. (1) RNA polymerase adds nucleotides to the 3 end of the growing RNA strand, elongating the transcript 5 3. (2) As RNA polymerase leaves behind the point of synthesis, the DNA double helix re-forms. 3. Termination – the transcription of one of the 3 stop codons: UAA, UAG, UGA, signals the end of transcription. (1) RNA Polymerase continues until the end of the terminator sequence: AAUAAA (2) RNA polymerase detaches and the (3) ‘pre mRNA’ is released J. The pre-mRNA is modified after transcription 1. 5 end of the pre-mRNA gets a guanine 5 cap. This serves as the attachment signal or ribosomes. 2. at the 3 end 50-250 adenine nucleotides are added- the poly-A-tail, this helps to export mRNA from the nucleus 3. the mRNA has nontranslated leader and trailer segments, which fringe the coding segment. 4. Introns are excised. Introns are non coding regions that lie between coding regions 5. Exons are spliced together. Exons are coding regions that are translated into a.a. sequences. 6. Spliceosome- has snRNP small nuclear ribonucloeproteins that act as ribozymes (RNA enzymes) that help remove the introns and join the exons to create mRNA K. Translation- Protein Synthesis 1. Materials needed: a. mRNA b. (1) tRNA – transfer RNA transfers aa to the ribosome’s growing polypeptide. (2) A tRNA has an anticodon that is complimentary to the codon on the mRNA. (3) The anticodon base-pairs with a complimentary codon on the mRNA and its a.a. is added to the growing polypeptide. (4) The codon sequence on the mRNA determines the sequence in which tRNAs come are placed (5) Wobble: the anticodons of tRNA recognize more than one codon, because it base pairs with only the first two bases of the mRNA, and wobbles for the third base c. ribosomes(1) each ribosome is made up of two subunits, composed of proteins and rRNA (2) each ribosome has a binding site for mRNA and three binding sites for tRNA (3) RNA acts as a catalyst for for peptide bond formation 2. Translation has three stages: a. Initiation- joins the mRNA, tRNA, and ribosome. Energy is provided by GTP b. Elongation- three steps to add amino acids (1) codon recognition hydrogen bonding between the mRNA codon under the A-site with the corresponding anticodon on the tRNA (2) peptide bond formation, the new a.a. on the A-site is bonded to the a.a. of the polypeptide on the P-site, this is catalyzed by rRNA. (3) Translocation-the ribosome moves the tRNA with the attached polypeptide from the A site to the P site. The mRNA moves along with it. Therefore mRNA is read5’ to 3’ codon by codon. c. Termination- occurs when one of the 3 stop codons reaches the A site. (1) the polypeptide is freed (2) the translation complex disassembles 3. Polyribosomes- many ribosomes translate a single mRNA simultaneously