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![Page 1: From Gene to Protein. Gene Expression Process by which DNA directs the synthesis of a protein 2 stages transcription translation All organisms One gene.](https://reader030.fdocuments.in/reader030/viewer/2022032802/56649e045503460f94af0075/html5/thumbnails/1.jpg)
From Gene to Protein
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Gene Expression
• Process by which DNA directs the synthesis of a protein
• 2 stages transcription translation
• All organismsOne gene one protein
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1. Transcription of DNA
• Gene– Composed of DNA– 1000s of base pairs long– Sequence of G, A, T, C is a code
A portion of the insulin gene
ACATACATGTAGCAACGAAACCTGAACATGCGAACATTGACACAACAACAAUGUAGCCA
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• A gene is transcribed into mRNA– One DNA strand is a template– Follow base complementation using G, A, U, C
Which strand is the template strand?
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• This mRNA is referred to as a “transcript”• Single stranded• Uracil nucleotide (not thymine)• ONLY gene of interest is transcribed• mRNA will leave the nucleus for translation
step
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• mRNA (transcript) leaves nucleus • Translated to protein in cytoplasm
• What organelle translates mRNA to protein?
• How many amino acids?• How many amino acids long is a
protein?• How could 10 identical protein
molecules be translated from 1 mRNA?• What happens to mRNA when the cell
has finished with it?• Can the cell transcribe more than one
gene at a time?
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2. Translation of mRNA
• Nucleotides (G, A, U, C) translated to amino acids• Ribosomes assemble amino acids to form polypeptide• What is the difference between a polypeptide and a protein?• What is the length difference between a chromosome and an
mRNA transcript?• Why do researchers use frog embryos to study translation?
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Differences between prokaryotes and eukaryotes
• Proks – No nucleus– Translation can begin while transcription is still in
progress
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• Euks– Nuclear envelope– Transcript is modified before leaving nucleus• Primary transcript mRNA mRNA
– Translation in cytoplasm
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Central Dogma
DNA mRNA protein
(F. Crick 1956)
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The genetic code
Triplet code 43 = – # combinations of 3 using G, A, T, C– Can code for all 20 aa
• Each triplet codes for a certain amino acid
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Codon table (1960’s Nirenberg and Khorana)
• 3 stop codons• 1 start codon• Redundant• Nearly universal
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Which strand of DNA is the template?Identify the codons (triplets)Code is non-overlappingCompare the mRNA to the non-template strandHow many codons are needed for a 20 amino acid
long protein?How many nucleotides?
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The reading frame
AUG UGG GUU GGC UCA Amino acids? met –trp- phe-gly-ser
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Transcription in more detail
1. Initiation of transcriptionA. RNA polymerase binds to template strand of DNA
upstream ----------------------------------------------downstream
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The promoter is upstream fromthe start site for transcription
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B. Transcription factors (proteins) are required for RNA pol to bind
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C. mRNA begins to be synthesized
(RNA pol does not need a primer)
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2. Elongation of transcript
– RNA pol synthesizes 5’ 3’– More than one
transcript made
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3. Termination RNA pol detaches
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Translation summary
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Translation in more detail
• Cytoplasm• The players: • Transfer RNA tRNA• Ribosomes• mRNA• Amino acids
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1. tRNA
2o structure is a cloverleaf
CONCEPT CHECKHow many bases is a tRNA?How many bases is an anticodon?Where is the amino acid site with respect to the anticodon?Why does the tRNA look to be double stranded?What does the “t” mean in tRNA?
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• A charged tRNA carries an amino acid
• Contains an anticodon– Complementary to a
codon
Note the CCA
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• rRNA + protein• Large and small subunits
2. Ribosome
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3 binding sites on ribosome
• P site holds tRNA• A site for tRNA with next amino acid• E site allows tRNA to leave• Note: large and small • subunits
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Ribosomes
adds each aa from tRNA to growing polypeptide chain
up to 100,000s per cell
Activity overview of protein synthesis
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Stages of translation
1. Initiation• Ribosome scans
mRNA for AUG start
• tRNA brings met
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2. ElongationAmino acids added to chainPeptide bonds between amino acids
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3. Terminationstop codon
Polypeptide released
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• Polyribosomes• One mRNA
translated by many ribosomes
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Wobble hypothesis
• How many sense codons?• Stop codons?• BUT……………only 45 tRNAs• Some tRNAs recognize more than one codon
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3rd position in mRNA codon “wobbles”Us and Cs can be matched with G in anticodon
Rules of base pairing relaxed in the 3rd position
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After translation ER Golgi
Post – translational modification