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Lecture 13

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Western University
Biology 1202B
Brenda Murphy

1 DNA to mRNA Transcription (Chapters 13) Similarity & Differences Between Transcription & Translation (Prokaryotic & Eukaryotic Cells) Central Dogma  Name given by Francis Crick  The flow of information from DNA to RNA to protein Transcription  The mechanism by which the information encoded in DNA is made into a complementary RNA copy o Information in the nucleic acid type is transferred to another nucleic acid type  The enzyme RNA polymerase creates an RNA sequence that is complementary to the DNA sequence of a given gene  The template strand (One of the DNA strands) is read by the RNA polymerase  The RNA transcribed from a gene encoding a polypeptide is called messenger RNA (mRNA) Translation  The use of information encoded in the RNA to assemble amino acids into a polypeptide o Information in a nucleic acid, in the form of nucleotides, is converted into a different kind of molecule—amino acids  An mRNA associates with a ribosome, a particle on which amino acids are linked into polypeptide chains  As the ribosome moves along the mRNA, the amino acids specified by the mRNA are joined one by one to form the polypeptide encoded by the gene Similarities  Processes are very similar Differences  Prokaryotic cells can transcribe and translate a given gene simultaneously  Eukaryotic cells transcribe & process mRNA in the nucleus before exporting it to cytoplasm for translation on ribosomes Genetic Code  DNA ‗alphabet‘ – A, T, G, C  RNA ‗alphabet‘ – A, U, G, C  The nucleotide information that specifies the amino acid sequence of a polypeptide is called the genetic code  Each genetic code triplet is read in three known as a codon 2 o mRNA is single stranded which make single stranded RNA, RNA is read in triplets which code for specific proteins  The template strand is always read from 3‘ to a 5‘ o In transcription, RNA polymerase reads the 3‘ to 5‘ nucleotide sequence of the DNA template strand o It then makes a complimentary RNA molecule o The sequence of the RNA from the 5‘ to 3‘ matches, in RNA bases, the 5‘ to 3‘ sequence of the DNA non-template strand Universal Genetic Code  The genetic code, written as codons, appear in mRNA being read as 5‘ to 3‘  Does not work for human mitochondria, yeast and plant chloroplast  There are 64 different combinations possible (4 ) which are specified into 20 amino acids  AUG – codes for methionine (start or initiator codon) o The first codon translated in any mRNA in both prokaryotic or eukaryotic cells  UAA, UAG, UGA – (Nonsense or termination codons) o Act as ―periods‖ indicating the end of a poly-peptide encoding sentence o When a ribosome reaches one of the stop codons, polypeptide synthesis stops and the new polypeptide chain is released from the ribosome  Only two amino acids, methionine and tryptophan, are specified by a single codon o The rest are represented by two – known as degeneracy or redundancy  Another feature of the genetic code is that it is commaless; the words of the nucleic acid are sequential, with no indicators to mark the end of one codon and the beginning of another  RNA and proteins are tissue specific and DNA is identical in every cell of the body Unique to Transcription  For a given gene, only one of the two DNA nucleotide strands act as a template for synthesis of a complimentary copy  Only a relatively small part of a RNA molecule—the sequence encoding a single gene—serves as a template  RNA polymerases catalyze the assembly of nucleotides into an RNA strand, rather than the RNA polymerases that catalyze the reaction  RNA molecules resulting from transcription as single polynucleotide chains Eukaryotic Protein-Coding Gene  The gene consists of two main parts: o Promoter – Which is a control sequence for transcription o Transcription Unit – The section of the gene that is copied into an RNA molecule 3  Once an RNA polymerase molecule has started transcription and progressed past the beginning of the gene, another molecule of RNA polymerase may start transcribing as soon as there is room at the promoter (continues until there are many RNA polymerase molecules spaced closely together) Similarities and Differences in Transcription of Eukaryotic and Prokaryotic Protein-Coding Genes  Similarities o Gene organization is the same o Elongation is identical  Differences Prokaryotes Eukaryotes - RNA polymerase binds directly to DNA; it is - RNA polymerase II, the enzyme that transcribes directed to the promoter by a protein factor that isprotein-coding genes, cannot bind directly to DNA; then released once transcription begins it is recruited to the promoter once proteins called transcription factors have bound - There are two types of specific DNA sequences - There are no equivalent ―transcription terminator‖ called terminators that signal the end of sequences transcription of the gene - Instead, the 3‘ end of the mRNA is specified by a - Both types of terminator sequences act after they different process are transcribed - In the first case, the terminator sequence on the mRNA uses complimentary base-pairing with itself to form a ―hairpin‖ - In the second case, a protein binds to a particular terminator sequence on the mRNA - Both of these mechanisms trigger the termination of transcription and the release of the RNA and RNA polymerase from the template Transcription of Non-Protein-Coding Genes  In Eukaryotes o RNA Polymerase II transcribes protein-coding genes o RNA Polymerase III transcribes tRNA genes and the gene for one of the four rRNAs o RNA Polymerase I transcribes the genes for the three other rRNAs o Promoters for these non-protein-coding genes are different from those of protein coding genes, being specialized for the assembly of the transcription machinery that involve the correct RNA polymerase type  In Prokaryotes o A single type of RNA polymerase transcribes all types of genes 4 o The promoters for bacterial non-protein-coding genes are essentially the same as those of protein-coding genes Transcription Takes Place in Three Steps  At one end of a gene is a sequence called the promoter that directs where transcription begins  The part of the gene copied into RNA is the transcription unit 1. Initiation – The molecular machinery that carries out transcription assembles at the promoter and begins synthesizing an RNA copy of the gene  To begin the initiation of transcription, proteins called transcription factors (TF) bind to the promoter in the area of the TATA box o TATA Box – 5‘…TATAAA…3‘  TATA box in the promoter is about 30 base pairs upstream of the transcription start point  Other TF‘s bind and together they recruit RNA polymerase II. The enzyme binds in an orientation for initiating transcription at the correct place o The combination of the TF‘s and RNA polymerase II is the transcription initiation complex  The DNA is unwound at the front of RNA polymerase II to expose the template strand. The enzyme begins synthesis at the transcription point and moves along the DNA of the gene. The TFs are released  During transcription, RNA nucleotides are base-paired one after another with the DNA template strand. RNA synthesis takes place in the 5‘ to 3‘ direction, making the 5‘ end of the mRNA the first part of the molecule to be synthesized 2. Elongation – The RNA polymerase moves along the gene extending the RNA chain  RNA polymerase II moves along the DNA, unwinding it an adding new RNA nucleotides to the transcript in the 5‘ to 3‘ direction  Behind the enzyme, the DNA strands reform into a double helix 3. Termination – Transcription ends and the RNA molecule—the transcript—and the RNA polymerase are released from the DNA template
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