Chapter 17 From Gene to Protein
Transcription and Translation: Links Between Gene and Protein
 _______________________ (to make a written copy)
– Process of making an RNA copy of the DNA gene
– Both nucleic acids use the same “language”

4 nucleotide bases:
Guanine, Cytosine, Adenine, Thymine/Uracil

Genes are 100s to 1000s nucleotides long
 ________________________ (change from one language to another)
– Process of making protein using the RNA instructions
– RNA and protein use different languages

4 nucleotide bases vs 20 amino acids

Number of amino acids in a polypeptide determined by gene
Transcription


mRNA = _______________________ RNA
–
Faithful copy of DNA in gene
–
Copied nucleotide by nucleotide
–
Process similar to DNA synthesis
–
Uses __________ polymerase and ___________ nucleotides
Prokaryotes – translation is direct
–

No nucleus, so ribosome attaches to mRNA strand immediately
Eukaryotes – translation waits for __________________
–
“___________________ ________________” = “Pre-mRNA” made in
nucleus
–
Transcript processed before leaving nucleus
3 Stages of Transcription
 Initiation at _____________________
– Promoter is sequence where RNA polymerase binds – ______________ end
 Elongation
– mRNA synthesized from template strand of DNA
– Only ________ DNA strand copied
 Termination at _________________
– Transcript falls off the template – ________________ end
Initiation
 Transcription factors bind first (proteins)
– Bind to TATA sequence in promoter

“_______________ box”

~25 nucleotides upstream from transcription start
 RNA polymerase binds next
Elongation
 RNA polymerase untwists double helix
–
Adds nucleotides to _______ end of ___________________ strand
–
Gene ____________ 3’  5’
–
RNA strand ____________ 5’  3’
 Helix re-forms behind
 New RNA peels away
–
~60 nucleotides/second
 New transcription may start again before elongation is over
Termination
 Eukaryotes
– RNA polymerase transcribes _______________ terminator sequence
 AAUAAA in pre-mRNA
– 100s of nucleotides later, transcript released
– Cut free of polymerase ~10-35 nucleotides beyond AAUAA
 Prokaryotes
– Transcript released at end of termination sequence
Transcript Processing in Eukaryotes

Transcript modified after release – both ends and the middle

_______ cap


–
Guanine modification
–
Protects mRNA from degradation
–
Attachment signal for ribosomes
_______ poly-A tail
–
50-250 Adenines
–
Inhibits degradation
–
Appears to facilitate export from nucleus
RNA __________________
RNA Splicing
 Removal of _____________– intervening sequences
–
Non-coding regions
 Leaves ________________– expressed sequences
–
Coding regions
 Continuous coding segment when done
–
8,000 nucleotides before processing  1,200 after

1,200 coding nucleotides  400 amino acids in polypeptide
Translation: Cracking the Code
 Translating the nucleotide language into ______________ language
– If taken 1 at a time (41) – 4 possibilities
 Not enough for 20 amino acids!
– If taken 2 at a time (42) – 16 possibilities
 Not enough for 20 amino acids!
– If taken 3 at a time (43) – 64 possibilities
 More than enough for 20 amino acids!
Genetic Code
 Triplet code
– Instructions for making a polypeptide
– ____________________ = 3-nucleotide words
– By convention listed as ___RNA sequence
 Code is universal to _________ life
The Genetic Code Table
 First base of codon (5’ end) along left side
 Second base of codon along top
 Third base of codon along right side
–
UUU  Phenylalanine
–
AAG  Lysine
 _____________________________ in code
–
All except methionine

AUG
 AUG also “_____________”
 UAA, UGA, UAG, are all “_______________” codons
mRNA ________________ than the Message
 Transcription from DNA template strand
 mRNA includes nucleotides upstream and some downstream of coding region
 Start reading from 5’ end
 Message starts at AUG
 Ends at ____________ or _____________ or ______________
Codons Read 5’  3’ in Reading Frame
 Triplet codons read in sequence
– Read in __________________ ___________________

Never overlap
– Each codon has a meaning
 Translation results in polymer of amino acids specified by codons
tRNA
 ~ 80 nucleotides long, folded
–
Cloverleaf formation (if flattened to 2-dimensional)
 Different tRNAs
–
Each has ____________________
–
To pair with the various codons
–
Each has attachment site for specific ______________ ___________ at 3’
end
 Anticodon loop pairs with complementary codon of mRNA
–
e.g. AAG anticodon pairs with complementary codon __ __ __
–
UUC codes for ____________
Translation Occurs at Ribosome
 mRNA attaches to ___________________
–
Instructions on mRNA
 Message translated by tRNA
–
tRNA transfers amino acids to growing polypeptide

(Cell keeps amino acids stocked in cytoplasm)
 tRNA like a little “go-fer”
–
Goes into cytoplasm to find appropriate amino acid
 Ribosome receives tRNA with amino acid
–
Adds new amino acid to polypeptide
Polypeptide Formation
 Three stages of translation
– ________________
 Brings all parts together
–
mRNA, tRNA w/first amino acid, Both ribosomal subunits
– Elongation
 _________________ _______________ added one by one
– _______________________
 Polypeptide released
______________________________
 mRNA can be used repeatedly
–
Single ribosome can make polypeptide in less than a minute
 New ribosome may attach to mRNA
–
After 5’ cap is cleared by previous ribosome
 Occurs in both prokaryotic and eukaryotic cells
Overview:
Transcription and Translation
Link Between Genotype and Phenotype
 ____________________ (proteins) are the products of genes
– Enzymes make other products
– Key steps in metabolic pathways
 __________________ gene means either
– Defective protein or
– Nonexistent protein
 One Gene  One Enzyme Hypothesis
– Beadle and Tatum
 Now called “One Gene  One Polypeptide”
Mutations: Changes in Genetic Material
 ________________ mutations
(Base-pair substitutions)
– _____________________ mutations

No effect on amino acid
– _____________________ mutations

Codes for different amino acid

Changes protein
– ______________________ mutations

Codes for premature stop

Most make non-functional proteins
Mutations, cont’d
 ___________________
–
Addition of nucleotide pairs
 ______________________
–
Loss of nucleotide pairs
 If in threes
–
_____ shift in reading frame (“frameshift”)
–
Addition or loss of amino acid
 _____________________________ mutation
–
If in ones or twos
–
Massive misreading downstream from insertion or deletion
–
Premature termination or nonfunctional protein