Download Protein Synthesis I

GENE EXPRESSION:
THE USE OF INFORMATION
Protein Synthesis
Genes (DNA information) are used to synthesize enzymes
Mutation
number:
Neurospora crassa
(fungus)
The colinearity of DNA and proteins
•
•
•
•
DNA information determines enzyme/protein structure.
DNA information is stored in the base sequence.
Protein structure depends on the amino acid sequence.
Thus DNA base sequence must determine protein amino acid sequence
(through a “genetic code”).
amino acid 1
amino acid 2
amino acid 3
amino acid 4
How many DNA bases are needed to “code” for one amino acid?
•There are four kinds of bases (base pairs).
•There are 20 kinds of amino acids (plus “stop” signs).
•Thus, the cells must use permutations of multiple bases to code for one
amino acid.
amino acid 1
amino acid 2
amino acid 3
amino acid 4
Three bases make a “codon.”
(Degenerate, but not ambiguous)
Transcription: RNA synthesis
! DNA is not directly involved in protein synthesis
! Instead, DNA base sequence is “transcribed” to
an RNA base sequence.
! RNA is directly involved in protein synthesis
(“translated”).
transcription
translatio n
(This is Crick’s “Central Dogma”. Not as
dogmatic now, it has been modified in two
important ways.)
PROTEIN SYNTHESIS
! Use of DNA information to synthesize proteins
! First step: transcription of DNA information
Transcription: RNA synthesis
Several types of RNA are needed for protein
synthesis; each is made using information carried
by DNA
Structure of RNA
! Polynucleotide chain like DNA
! Sugar is ribose (-OH on C2’)
! U replaces T (no methyl)
! Molecule is generally single-stranded (though
there may be some helix in hairpins).
OH
I
O
OH
I
O
OH
I
O
OH
I
O
Synthesis of RNA
Players
! Nucleoside triphosphate substrates (ATP, CTP,
GTP, UTP)
! Enzyme (RNA polymerase)
! DNA “template” to direct the sequence of bases
polymerized.
Initiation
RNA
pppG
Template DNA….…CpApApTp…
Elongation
p
p
RNA
pppG pU
Template DNA….…CpApApTp…
RNA
pppGpU
Template DNA….…CpApApTp…
(Add an NTP)
p
p
p
p
RNA
pppGpU pU
Template DNA….…CpApApTp…
(Form the phosphodester bond)
(Add an NTP)
p
p
RNA
pppGpUpU
Template DNA….…CpApApTp…
(Form the phosphodester bond)
Result
! One RNA polynucleotide chain
! Base sequence determined by (complementary
to—not identical to) the base sequence of the
template strand
Summary
! DNA base sequence ("codons") provides
information for protein amino acid sequence
! DNA information transcribed onto RNAs:
ribosomal, messenger, transfer RNAs
! Next: use of RNAs to synthesize proteins