Transcription Notes PDF

CHAPTER 15: Genes  15.1 The Genetic Code In 1956, Francis Crick gave name central dogma to flow of information from DNA → RNA → protein Transcription is mechanism by which information encoded in DNA template strand is copied into a complementary RNA strand Translation uses information...

CHAPTER 15: Genes  15.1 The Genetic Code In 1956, Francis Crick gave name central dogma to flow of information from DNA → RNA → protein Transcription is mechanism by which information encoded in DNA template strand is copied into a complementary RNA strand Translation uses information encoded in RNA copy to assemble amino acids into a polypeptide  Overview: Transcription and Translation Transcription: RNA polymerase copies DNA sequence of a gene into an RNA sequence [protein-coding gene is transcribed into messenger RNA (mRNA)] Translation: mRNA associates with a ribosome, on which amino acids specified by mRNA are joined one by one to form polypeptide  Prokaryotes and Eukaryotes In eukaryotes, transcription in nucleus produces a precursor-mRNA Pre-mRNA ends are modified and extra segments are removed by RNA processing In prokaryotes, transcription in cytoplasm produces a functional mRNA directly Prokaryotes and Eukaryotes DN A Transcrip tion Pre- RNA mRNA processin g mRN A Translat ion Polypepti Riboso de me  DNA and RNA Nucleotides RNA “alphabet” consists of A, U, G, and C – base uracil (U) acts in place of thymine (T) Sequence of RNA nucleotides in mRNA is translated into a polypeptide containing 20 different types of amino acids  Genetic Code To code for 20 different amino acids, four bases in an mRNA (A, U, G, and C) are used in combinations of three base pairs Codon: each three-letter “word” (triplet) of code Three-letter codons in DNA are transcribed into complementary three-letter RNA codons  Gene Gene 5’ a b 3’ 3’ 5’ Nontemplat e strand 5’ 3’ Gene RNA is made in 5′→3′ direction 3’ 5’ using 3′→5′ DNA Transcripti Template on 5’ strand 3’ strand as mRN template A Codo n Translati on Polypepti de Amino acid  Think/Pair/Share For the two DNA templates below, what would be the sequences of the RNA transcribed from them? 1. 3-CAAATTGGCT-5 2. 5-GCCGATTCAT-3  Start and Stop Codons Four mRNA nucleotides combine to form 64 different 3-letter combinations (43) Of 64 codons, 61 sense codons specify amino acids AUG (methionine) is always first codon read in an mRNA translation (start codon) UAA, UAG, and UGA are stop codons that do not code for amino acids and stop polypeptide synthesis  Features of Genetic Code Only two amino acids, methionine and tryptophan, are specified by a single codon – rest are represented by more than one codon (degeneracy) Genetic code is commaless, with no indicators to mark end of one codon and beginning of next Genetic code is universal – essentially the same in all living organisms and viruses (how cool is that!) Genetic Code Table  Think/Pair/Share How does the degeneracy of the genetic code make cells more robust to mutations?  15.3 Eukaryotic Transcription Gene consists of two main parts: Promoter (control sequence for transcription) Transcription unit (section of gene that is copied into an RNA molecule) Transcription takes place in three stages: 1. Initiation 2. Elongation 3. Termination  Transcription Initiation Initiation Molecular machinery assembles at promoter and begins synthesizing an RNA copy of gene Molecular machinery includes: Transcription factors (TFs) that bind to promoter in area of a special sequence known as TATA box RNA polymerase: enzyme that catalyzes assembly of RNA nucleotides into an RNA strand  Transcription Initiation Process DNA is unwound to expose template strand, RNA polymerase II begins RNA synthesis RNA is made in 5′→3′ direction using 3′→5′ DNA strand as template  Transcription Elongation and Termination Elongation RNA polymerase II moves along gene extending RNA chain DNA continues to unwind ahead of enzyme Termination RNA transcript and RNA polymerase II are released from DNA template when a stop codon is reached Transcrip Gen tion e Promot Transcription er unit Transcript Transcript ion start ion stop point point Transcription factors RNA polymerase II Transcription initiation complex  RNA RNA transcri pt DNA template strand RNA polymerase Growing II RNA transcript Hybrid RNA- RNA DNA double + helix RNA copy + transcri pt  Differences in Replication and Transcription Only one of two DNA nucleotide strands acts as a template for synthesis of a complementary copy Only sequence encoding a single gene is copied RNA polymerases catalyze assembly of RNA nucleotides into an RNA strand, no primer needed  Think/Pair/Share The figure below shows amino acid changes that occurred at a particular position in a polypeptide as a result of mutations in the gene encoding the polypeptide. The amino acids connected by a line are specified by codons that differ in a single base. Use the genetic code to deduce the codons that specify the amino acids. Write the codons next to the amino acids.  15.4 RNA Processing in Eukaryotes mRNAs contain regions that code for proteins, along with noncoding regions that are important in protein synthesis Eukaryotic protein-coding gene is transcribed into a precursor-mRNA (premRNA) that must be processed in nucleus to produce translatable mRNA  Introns and Exons Pre-mRNA for a eukaryotic protein-coding gene non–protein-coding sequences called introns are removed during processing in nucleus Amino acid-coding sequences that are retained in finished mRNAs are called exons  mRNA Splicing mRNA splicing: occurs in nucleus, removes introns from pre-mRNAs and joins exons together mRNA splicing takes place in a spliceosome formed between pre- mRNA and several small ribonucleoprotein particles (snRNPs) More than 70 individual introns can be present in transcript, and each has to undergo process of  Exo Intro Exo Intro Exo n n n n n 5' 3' UTR UTR Ca Poly( A) p Exo Intro Exo Intro Exo tail Pre- n n n n n AAAAAAA mRNA …3 ' 5' 3' UTR UTR mRNA splicing Protein-coding sequence mRN AAAAAAA A …3 ' Translation Translation  Alternative Splicing Pre-mRNAs are often processed by reactions that join exons in different combinations (alternative splicing) to produce different mRNAs from a single gene Increases number and variety of proteins encoded in cell nucleus without increasing size of genome  Think/Pair/Share Why is transcription necessary? Why don’t cells use their DNA as a direct model for protein synthesis?