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GENE EXPRESSION CH 17 http://www.youtube.com/watch?v=88kMwpC7CCg https://www.google.com/search?q=sheldon%27s+luminous+fish&rlz=1C2LENN_enUS503&tbm=isch&imgil=3XFQ79gU_S1qM%253A%253Bhttps%253A%252F%252Fencryptedtbn1.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcThpLda10oteUPqWIZLCTkBlP7bfCQqtXBOvoe7vQ4NJ Ri8IapDuQ%253B1280%253B720%253BKhHuDyDXYFiFkM%253Bhttp%25253A%25252F%25252Fbigbangtheory.wikia. com%25252Fwiki%25252FThe_Luminous_Fish_Effect&source=iu&usg=__l46dGqtGYUJv1fThfb4gVFtkYPk%3D&sa=X& ei=dAcJU8jmFemkyQHNjICACg&ved=0CCoQ9QEwAA&biw=1366&bih=643#facrc=_&imgrc=3XFQ79gU_S1qM%253A%3BKhHuDyDXYFiFkM%3Bhttp%253A%252F%252Fstatic2.wikia.nocookie.net%252F__cb20130 115214805%252Fbigbangtheory%252Fimages%252Fd%252Fd3%252FLumfish.jpg%3Bhttp%253A%252F%252Fbigbang theory.wikia.com%252Fwiki%252FThe_Luminous_Fish_Effect%3B1280%3B720 I. Basic principles of gene expression • A. General characteristics • process by which genetic info in DNA is converted to protein – DNA → RNA is transcription – RNA → protein is translation • RNA is the bridge between proteins and genes that code for them • The concept of gene is universal to all domains of life • The general process of gene expression is also universal • The genetic code is also universal B. The genetic code • Language of DNA and RNA are nucleotides • Language of proteins are amino acids • The nucleotide sequence must be translated into amino acid sequence • Nucleotide sequence is read in groups of 3 nucleotides called codons • The genetic code is redundant II. Transcription • Process by which the genetic info in DNA is copied into RNA • Occurs at specific regions of DNA called genes • the basic structure of genes are the same in all domains Promotor: where transcription starts. Coding sequence: what is transcribed to RNA Terminator: where transcription stops A. The process of transcription • Only one strand of the DNA is transcribed into RNA, the template strand • RNA strand is complementary to DNA strand copied • Enzyme is RNA polymerase Three stages: initiation, elongation, termination Initiation: RNA polymerase binds to promoter with the help of various transcription factors and unzips DNA. Elongation: RNA polymerase reads template stand of DNA making RNA Termination: transcription stops B. Post-transcriptional processing of mRNA in eukaryotes Before mRNA is usable it must be processed • 5’ CAP and 3’ poly A tail put on • Purpose of CAP and tail: help RNA leave nucleus, prevent its degradation, and help ribosome bind to 5’end • Splicing out of introns • Introns: segments of gene that are transcribed into mRNA but don’t code for protein • Must be cut out • snRNPs binds to intron/exon junction • snRNPs attract each other looping out the introns • introns are cut out and exons are glued together http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::53 5::535::/sites/dl/free/0072437316/120077/bi o30.swf::How%20Spliceosomes%20Process%2 0RNA III. Translation • Process by which genetic information carried in the mRNA is converted into protein • Requires the help of tRNA which transfers amino acids to growing protein in the ribosome A. The structure of a tRNA • Anticodon: a group of 3 nucleotides complementary to a codon in mRNA • CCA site: place where amino acid is attached • Accurate translation requires 2 steps: – There must be a correct match between tRNA and amino acid which is done by aminoacyl tRNA synthase – There must be a correct match between anticodon and codon B. Ribosomes • • • • Where translation occurs Facilitates interaction of tRNA and mRNA Made of 2 subunits of rRNA Overall structure of bacterial and eukaryotic ribosomes are similar but many antibiotics target bacterial ribosomes without affecting eukaryotic ribosomes Ribosomes have 3 binding sites for tRNA P site: holds the tRNA that carries growing polypeptide chain A site: holds the tRNA carrying the next amino acid to be added to growing chain E site: exit site where free tRNAs leave C. The Process of Translation • Occurs at the ribosome and is fundamentally the same in prokaryotes and eukaryotes but eukaryotic ribosomes are larger • occurs in 3 stages: initiation, elongation, termination 1. Initiation • mRNA interacts with a ribosome such that 1st AUG sits in the P site • initiator tRNA binds to the FIRST AUG codon in mRNA 2. Elongation and translocation • 2nd tRNA with the correct anticodon binds to the 2nd codon in mRNA in the A site • The 2 adjacent amino acids are linked via dehydration reaction • Ribosome moves down 3 nucleotides • 1st tRNA leaves thru the E site • This process continues 3. Termination https://highered.mcgrawhill.com/sites/0072507470/student_view0/ch apter3/animation__how_translation_works.h tml • At the stop codon: Release factor binds to stop codon in the A site Translation stops and protein leaves If a protein is destined for another location like the cell membrane or is to be secreted, it has a signal sequence that brings the ribosome to the RER Many ribosomes can translate mRNA at the same time forming polyribosome. Can make a lot of protein quickly • Protein can be modified after translation to make the functional protein: – 2 or more protein chains interact to form the functional protein (quaternary structure) – Small carbohydrate chains can be added to some proteins • IV. Mutations • Changes in the DNA sequence • Can be a product of mistakes made during replication, transcription, or DNA repair • Most are caused by mutagens: agents that damage DNA • Most change the way the protein folds, affecting its function • Two types of small scale mutations: substitution mutations and frameshift mutation • Base substitution mutation: a single nucleotide is changed. Can be silent, missense, nonsense Silent: no effect on protein due to redundancy in genetic code Missense: mutation results in a different amino acid Nonsense: amino acid changed to stop codon • Insertion /deletion mutations: loss or addition of nucleotides and are most often disastrous • • • • • • • • • • http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapter11/animation_quiz_3.html http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapter11/animation_quiz_4.html https://www.youtube.com/watch?v=kp0esidDr-c http://highered.mcgrawhill.com/sites/0070960526/student_view0/chapter18/animation_quiz_1.html https://www.youtube.com/watch?v=FgMKGIED4Yo V. Evolutionary significance of Mutations • Mutation rate is relatively low. Keeps genome constant from generation to generation – DNA repair mechanisms – DNA polymerase proofreads – Double strandedness and coiling of DNA protect it • However, mutations do occur. Mutations provide genetic variation for evolution to act on.