chapter 10b: gene expression chapter 10b.pdfchapter 10b: gene expression 4. translation 3. the...
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Chapter 10B:Gene Expression
4. Translation
3. The Genetic Code
2. Transcription
1. Overview of Gene Expression
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1. Overview of Gene Expression
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Overview of Gene Expression
DNA (genetic info)
RNA (copy)
Protein
transcription(in nucleus)
translation(via ribosomesin cytoplasm)
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Gene ExpressionThe expression of a gene into an actual protein occurs by 2 basic processes:
1) Transcription of a gene into RNA
• this is essentially creating a “photocopy” of the gene
• occurs in the nucleus
2) Translation of the RNA transcript into protein• accomplished by ribosomes, in the cytoplasm
• RNA is a nucleic acid very similar to DNA (RNA uses “U” instead of “T”)
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Comparison of DNA & RNA
• sugar = Ribose
• single-stranded
• A, C, G & U (uracil)
RNA DNA• sugar = Deoxyribose
• double-stranded
• A, C, G & T (thymine)
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2. Transcription
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RNApolymerase
RNA nucleotides
Direction of transcription Template
Strand of DNA
Newly made RNA
TC
A T C C A A TT
GG
CC
AATTGGAT
G
U
C A U C C A AU
TranscriptionTranscription of gene to make an RNA copy is much like DNA replication except for the following:
• only 1 strand is used as a template
• catalyzed by RNApolymerase
• produces complementary RNA
• involves only 1 gene
• RNA is released, DNA “zips” back up!
*results in a copy of one DNA strand*
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The Process of Transcription
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Functions ofRNA made byTranscription
1) mRNA*• copy of gene
3) rRNA• part of ribosomes
2) tRNA• delivers AAsto ribosomes
*provides geneticinfo for translation
*
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3. The Genetic Code
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How are Genes related to DNA?Genes are segments of DNA that code for a
particular protein (or RNA molecule)
• the human genome contains ~3 billion basepairs (bps) and ~25,000 genes
• almost all genes encode proteins
( the “end products” for a small percentage of genes arespecial types of RNA molecules)
• when we talk about “genes” we will focus on thosethat express proteins
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What does DNA actually code for?In other words, “How do genes encode
proteins”?
• recall that proteins are linear polymers made ofthe 20 different amino acids
***genes need simply to encode the identity of each amino acid in a given protein***
• i.e., genes must be capable of encoding 20 differentamino acids and their order in a protein
• although DNA contains only 4 “letters” (i.e.,nucleotides), this is more than sufficient…
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The Genetic CodeEach amino acid in a protein is specified by3 nucleotide sequences called codons
• there are 64 possible “codon” triplets (4 x 4 x 4)• more than enough to encode 20 amino acids and the
signal to “stop” or end the protein (TGA, TAA or TAG)
• each of the 20 amino acids is coded for by aunique set of codons:
e.g. ATG = methionine (start codon)
GGN = glycineCAA or CAG = glutamine
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Table of the Genetic Code
If the DNA sequence is: CATGCCTGGGCAATAGThe RNA copy is: CAUGCCUGGGCAAUAG (transcription)The protein sequence is: *Met-Pro-Gly-Gln-“stop” (translation)
*always starts w/AUG (met)
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From DNA to RNA to Protein
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Normal gene
mRNA
Base substitution
Base deletion Missing
Met Lys Phe Gly Ala
Met Lys Phe Ser Ala
Met Lys Leu Ala His
A U G A A G U U U G G C G C A
A U G A A G U U U A G C G C A
A U G A A G U U G G C G C A U
U
Protein
• deletions, insertions cause a change in reading
The Effects of MutationMUTATION: any change in DNA sequence
frame(frameshift)
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C T T C A T
Normal hemoglobin
Mutant hemoglobin DNA
G A A G U A
Sickle-cell hemoglobin
Normal hemoglobin DNA
Glu Val
mRNA mRNA
Sickle Cell Anemia is due to aBase Substitution
1 nucleotide changes 1 amino acid resulting in a misfoldedhemoglobin protein (clump together in RBCs > sickle shape)
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4. Translation
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Ribosomes translate mRNA to Protein
• with the help of tRNAs
• in the cytoplasm
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tRNA Structure & Function
mRNA codons will “base pair” with a complementary
anticodon in a tRNA…
…the amino acid*attached to the tRNAis then added to thegrowing polypeptide
*
*
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Translation (aka “protein synthesis”)The building of a polypeptide, 1 amino acid at a time, by ribosomes using info in mRNA:
• ribosomes bind directly to mRNA, “read” codon by codon
• translation also involves tRNAs, each of which isattached to 1 of the 20 amino acids (AAs)
• ribosomes match the right tRNA (via the anticodon)with the right codon in the mRNA, then add its AA to the growing protein
• ribosomes always start at AUG (methionine)
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Translation by multiple Ribosomes
• the same mRNAcan be translatedmany, many times
• a given mRNA canbe translated bymany ribosomesat the same time
mRNA
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2) ribosome binds 2 tRNA-AAs, 2 codons at a time• i.e., tRNAs with anti-codons complementary to the mRNA
codons
3) ribosome then catalyzes peptide bond formationbetween the amino acids attached to each tRNA
General Steps of Translation
4) ribosome shifts 3 nucleotides (1 codon) on mRNAand repeats the process
5) this continues until the ribosome reaches a“stop” codon which causes translation to end
1) ribosome begins translation at AUG of mRNA
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Key Terms for Chapter 10B
• transcription, RNA polymerase
• translation, ribosome
• mRNA, tRNA, rRNA
Relevant Review Questions: 1, 3-5, 7, 9
• codon, anti-codon
• genetic code
• mutation: substitution, deletion, insertion
• reading frame, frameshift