Genetic Code, Transcription, and Translation Notes PDF

CENTRAL DOGMA 1  In prokaryotes, translation of mRNA can begin before transcription has finished In a eukaryotic cell, the nuclear envelope separates transcription from translation © 2011...

CENTRAL DOGMA 1  In prokaryotes, translation of mRNA can begin before transcription has finished In a eukaryotic cell, the nuclear envelope separates transcription from translation © 2011 Pearson Education, Inc.  Overview: The Flow of Genetic Information The information content of DNA is in the form of specific sequences of nucleotides The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins Proteins are the links between genotype and phenotype The process by which DNA directs protein synthesis, includes two stages: transcription and translation © 2011 Pearson Education, Inc.  Basic Principles of Transcription and Translation RNA is the bridge between genes and the proteins for which they code Transcription is the synthesis of RNA under the direction of DNA Transcription produces messenger RNA (mRNA) Translation is the synthesis of a polypeptide, using information in the mRNA Ribosomes are the sites of translation © 2011 Pearson Education, Inc.  Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA A primary transcript is the initial RNA transcript from any gene prior to processing © 2011 Pearson Education, Inc.  The Genetic Code How are the instructions for assembling amino acids into proteins encoded into DNA? There are 20 amino acids, but there are only four nucleotide bases in DNA How many nucleotides correspond to an amino acid? © 2011 Pearson Education, Inc.  Codons: Triplets of Nucleotides The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words The words of a gene are transcribed into complementary nonoverlaping three- nucleotide words of mRNA These words are then translated into a chain of amino acids, forming a polypeptide © 2011 Pearson Education, Inc.  Figure 17.4 DNA template 3 5 DNA strand A C C A A A C C G A G T molecule T G G T T T G G C T C A 3 Gene 1 5 TRANSCRIPTION Gene 2 U G G U U U G G C U C A mRNA 5 3 Codon TRANSLATION Protein Trp Phe Gly Ser Gene 3 Amino acid  During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide © 2011 Pearson Education, Inc.  Cracking the Code All 64 codons were deciphered by the mid-1960s Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation The genetic code is redundant (more than one codon may specify a particular amino acid) but not ambiguous; no codon specifies more than one amino acid Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced © 2011 Pearson Education, Inc. Figure 17.5 Second mRNA base U C A G UUU UCU UAU UGU U Phe Tyr Cys Third mRNA base (3 end of codon) First mRNA base (5 end of codon) UUC UCC UAC UGC C U Ser UUA UCA UAA Stop UGA Stop A Leu UUG UCG UAG StopUGG Trp G CUU CCU CAU CGU U His CUC CCC CAC CGC C C Leu Pro Arg CUA CCA CAA CGA A Gln CUG CCG CAG CGG G AUU ACU AAU AGU U Asn Ser AUC Ile ACC AAC AGC C A Thr AUA ACA AAA AGA A Lys Arg AUG Met start or ACG AAG AGG G GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G  Evolution of the Genetic Code The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals Genes can be transcribed and translated after being transplanted from one (b) Pig expressing a jellyfi (a) Tobacco plant expressing species to a firefly gene gene another Figure 17.6 Khatam 13  Concept 17.2: Transcription is the DNA-directed synthesis of RNA: a closer look Transcription is the first stage of gene expression © 2011 Pearson Education, Inc.  Molecular Components of Transcription RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides The RNA is complementary to the DNA template strand RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator The stretch of DNA that is transcribed is called a transcription unit © 2011 Pearson Education, Inc. Figure 17.7-1 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase Figure 17.7-2 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase 1 Initiation Nontemplate strand of DNA 5 3 3 5 Template strand of DNA RNA Unwound transcript DNA Figure 17.7-3 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase 1 Initiation Nontemplate strand of DNA 5 3 3 5 Template strand of DNA RNA Unwound transcript DNA 2 Elongation Rewound DNA 5 3 3 5 3 5 RNA transcript Figure 17.7-4 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase 1 Initiation Nontemplate strand of DNA 5 3 3 5 Template strand of DNA RNA Unwound transcript DNA 2 Elongation Rewound DNA 5 3 3 5 3 5 RNA transcript 3 Termination 5 3 3 5 5 3 Completed RNA transcript Direction of transcription (“downstream”)  Concept 17.3: Eukaryotic cells modify RNA after transcription Enzymes in the eukaryotic nucleus modify pre- mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm During RNA processing, both ends of the primary transcript are usually altered Also, usually some interior parts of the molecule are cut out, and the other parts spliced together © 2011 Pearson Education, Inc.  Alteration of mRNA Ends Each end of a pre-mRNA molecule is modified in a particular way – The 5 end receives a modified nucleotide 5 G-cap – The 3 end gets a poly-A tail These modifications share several functions – They seem to facilitate the export of mRNA – They protect mRNA from hydrolytic enzymes – They help ribosomes attach to the 5 end © 2011 Pearson Education, Inc.  Figure 17.10 Protein-coding Polyadenylation segment signal 5 3 G P P P AAUAAA AAA… AAA Start Stop 5 Cap 5 UTR 3 UTR Poly-A tail codon codon  Split Genes and RNA Splicing Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions These noncoding regions are called intervening sequences, or introns The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence © 2011 Pearson Education, Inc.  Figure 17.11 5 Exon IntronExon Intron Exon 3 Pre-mRNA5Cap Poly-A tail Codon 130 31104 105 numbers 146 Introns cut out and exons spliced together mRNA 5Cap Poly-A tail 1146 5 UTR 3 UTR Coding segment Khatam 25