from gene to protein
DESCRIPTION
From Gene to Protein . Gene Expression. Process by which DNA directs the synthesis of a protein 2 stages transcription translation All organisms One gene one protein. 1. Transcription of DNA. Gene Composed of DNA 1000s of base pairs long Sequence of G, A, T, C is a code. - PowerPoint PPT PresentationTRANSCRIPT
From Gene to Protein
Gene Expression
• Process by which DNA directs the synthesis of a protein
• 2 stages transcription translation
• All organismsOne gene one protein
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
• 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?
• 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
• 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?
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?
Differences between prokaryotes and eukaryotes
• Proks – No nucleus– Translation can begin while transcription is still in
progress
• Euks– Nuclear envelope– Transcript is modified before leaving nucleus• Primary transcript mRNA mRNA
– Translation in cytoplasm
Central Dogma
DNA mRNA protein
(F. Crick 1956)
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
Codon table (1960’s Nirenberg and Khorana)
• 3 stop codons• 1 start codon• Redundant• Nearly universal
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?
The reading frame
AUG UGG GUU GGC UCA Amino acids? met –trp- phe-gly-ser
Transcription in more detail
1. Initiation of transcriptionA. RNA polymerase binds to template strand of DNA
upstream ----------------------------------------------downstream
The promoter is upstream fromthe start site for transcription
B. Transcription factors (proteins) are required for RNA pol to bind
C. mRNA begins to be synthesized
(RNA pol does not need a primer)
2. Elongation of transcript
– RNA pol synthesizes 5’ 3’– More than one
transcript made
3. Termination RNA pol detaches
Translation summary
Translation in more detail
• Cytoplasm• The players: • Transfer RNA tRNA• Ribosomes• mRNA• Amino acids
1. tRNA2o 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?
• A charged tRNA carries an amino acid
• Contains an anticodon– Complementary to a
codon
Note the CCA
• rRNA + protein• Large and small subunits
2. Ribosome
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
Ribosomes
adds each aa from tRNA to growing polypeptide chain
up to 100,000s per cell
Activity overview of protein synthesis
Stages of translation
1. Initiation• Ribosome scans
mRNA for AUG start
• tRNA brings met
2. ElongationAmino acids added to chainPeptide bonds between amino acids
3. Terminationstop codon
Polypeptide released
• Polyribosomes• One mRNA
translated by many ribosomes
Wobble hypothesis
• How many sense codons?• Stop codons?• BUT……………only 45 tRNAs• Some tRNAs recognize more than one codon
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
After translation ER Golgi
Post – translational modification