Chapter 10From DNA to Proteins Transcription and RNA ProcessingA. Life requires two basic functions1. Living organisms must be able to store and faithfully transmit genetic information during reproduction2. They must have the ability to catalyze chemical transformations to fire the reactions that drive life processesB. RNA can serve as a biological catalyst. It can excise 400 nucleotides from its RNA in the absenceof any protein1. 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 acidsa. This led to the discovery that RNA was the original genetic material2. RNA molecules serves as both carriers of genetic information and as catalyzes that drove the chemical reactions needed to sustain and perpetuate life3. Also RNA may have acquired the ability to synthesize protein-based enzymes, which are more efficient catalystsa. Enzymes took over more of the catalytic functions leading too…i. RNA to become regulated to the role of information storage and transferii. DNA with its chemical stability an faithfully replication, eventually replaced RNA as the primary carrier of genetic information10.1 RNA, Consisting of a Single Strand of Ribonucleotides, Participates in a Variety of Cellular Function A. The Structure of RNA1. RNA, like DNA is a polymer consisting of nucleotides joined together by phosphodiester bonds2. Difference between the structuresa. 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 DNAe. 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 structuresB. Classes of RNA1. Ribosomal RNA(rRNA): along with ribosomal protein subunits make up the ribosome, the site of protein assembly 2. Messenger RNA(mRNA): carries the coding instructions for polypeptide chains from DNA to the ribosomea. 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 acids3. Pre-messenger RNAs(pre-mRNAs): precursor molecules are the immediate products oftranscription in eukaryotic cellsa. They are modified extensively before becoming mRNA and exiting the nucleus for translation into proteinb.Bacterial cells do not possess pre-mRNA in bacteria transcription takes place concurrently with translation4. 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 chaina. Each tRNA attaches to an amino acid and helps incorporate it into a polypeptide chain5. 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 mRNA6. Small nucleolar RNA(snoRNAs): take part in the processing of rRNA7. 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 TemplateA. All cellular RNAs are synthesized from DNA templates through the process of transcription1. In many ways similar to replication the big difference is the length of the template2. 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 DNAa. Because not all the gene products are needed at the same time or in the same cell3. Much of the DNA does not encode a functional product, and transcription of such sequences would be pointless4. Transcription is highly selective: individual genes are transcribed only as their products are neededB. Transcription requires three major components1. A DNA template2. The raw materials(substrates) needed to build a new RNA molecule3. The transcription apparatus, consisting of the proteins necessary to catalyze the synthesis of RNAC. The Template for Transcription1. The transcribed stranda. The template for RNA synthesis is a single strand of the DNA double helixb.Template strand: the one nucleotide strand of DNA used for transcriptionc. Nontemplate strand: is not ordinarily transcribedi. Although only one strand is transcribed, different genes may be transcribed from different strands2. During transcription, an RNA molecule that is complementary and antiparallel to the DNA template strand is synthesized3. Transcription unit: is a stretch of DNA that encodes an RNA molecule and the sequences necessary for its transcription containing three critical regionsa. Promoter: a DNA sequence that the transcription apparatus recognizes and bindsi. It indicates which of the two DNA strands is to be read as the template and the direction of transcriptionii. It also determines the transcription start site, the first nucleotide that will be transcribed into RNAiii. In most transcription units the promoter is located next to the transcription start site but it is not, itself, transcribedb.RNA-coding-region: a sequence of DNA nucleotides that is copied into an RNAmolecule c. Terminator: a sequence of nucleotides that signals where transcription is to endi. Transcription stops only after the terminator has been copied into RNA4. The terms upstream and downstream to refer to the direction of transcription and the location of nucleotide sequences surrounding the RNA-coding sequencea. Downstream as transcription takes place: it binds to the promoter and moves toward the terminatorb.The sequence on the nontemplate strand is written with the 5’ end on the left and the 3’ end on the rightc. Nucleotides downstream at the start site are assigned positive numbers, and nucleotides upstream at the start site are assigned negative numbersD. The Substrate for Transcription1. RNA is
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