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GENE EXPRESSION CH 17 http://www.youtube.com/watch?v=88kMwpC7CCg
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I. Basic principles of gene expression
A. General characteristics
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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
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B. The genetic code
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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
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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
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Promotor: where transcription starts.
Coding sequence: what is transcribed to RNA Terminator: where transcription stops
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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
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Three stages: initiation, elongation, termination
Initiation: RNA polymerase binds to promoter with the help of various transcription factors and unzips DNA.
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Elongation: RNA polymerase reads template stand of DNA making RNA
Termination: transcription stops
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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
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snRNPs binds to intron/exon junction
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
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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
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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
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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
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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
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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
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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
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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
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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
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3. Termination At the stop codon:
At the stop codon: Release factor binds to stop codon in the A site Translation stops and protein leaves
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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
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Many ribosomes can translate mRNA at the same time forming polyribosome. Can make a lot of protein quickly
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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
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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
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Two types of small scale mutations: substitution mutations and frameshift mutation
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Silent: no effect on protein due to redundancy in genetic code
Base substitution mutation: a single nucleotide is changed. Can be silent, missense, nonsense Silent: no effect on protein due to redundancy in genetic code
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Missense: mutation results in a different amino acid
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Nonsense: amino acid changed to stop codon
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Insertion /deletion mutations: loss or addition of nucleotides and are most often disastrous
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http://highered. mcgraw-hill
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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.
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