Transcript Ch. 17: From Gene to Protein

Ch. 17: From Gene to
Protein
DNA
RNA
Protein
Protein Synthesis: overview
One gene-one enzyme
hypothesis (Beadle and
Tatum)
One gene-one polypeptide
(protein) hypothesis
Transcription:
synthesis of RNA under
the direction of DNA (mRNA)
Translation:
actual synthesis of a
polypeptide under the
direction of mRNA
The Triplet Code
The genetic instructions
for a polypeptide chain
are ‘written’ in the DNA
as a series of
3-nucleotide ‘words’
Reading Frame w/
Dictionary
Codons code for Amino
Acid
‘U’ (uracil) replaces ‘T’
in RNA
Comparison of DNA and
RNA
Types of
sugar
nitrogenous
bases
single vs.
double strand
function
DNA
Deoxy Ribose
G,C,T,A
DS
Replication
RNA
Ribose
G,C,A,U
SS
Proteins
Processes from Gene to
Protein
Transcription
Synthesis of mRNA
Translation
Synthesis of Protein
Transcription, I
Promoter region on DNA:
where RNA polymerase
attaches and where initiation
of RNA begins
RNA polymerase:
pries DNA apart and
hooks RNA nucleotides
together from the DNA code
Terminator region:
sequence that signals the
end of transcription
Transcription unit:
stretch of DNA transcribed
into an RNA molecule
Transcription
II
Initiation~ transcription
factors mediate the binding
of RNA polymerase to an
initiation sequence (TATA
box)
Elongation~ RNA
polymerase continues
unwinding DNA and adding
nucleotides to the 3’ end
Termination~ RNA
polymerase reaches
terminator sequence
Eukaryote Cells vs. Prokaryote
Initiation of transcription

RNA polymerase
 Prokaryotes bind to RNA Poly directly.
 Eukaryotes bind to transcription factors that
bind to DNA.
promoters (TATA Box)
 transcription factors
 Transcription initiation complex

mRNA modification
1) 5’ cap: modified guanine; protection; recognition
site for ribosomes
2) 3’ tail: poly(A) tail (adenine); protection;
recognition; transport
3) RNA splicing: exons (expressed sequences) kept
introns (intervening sequences) are spliced out
forming a spliceosome
What are they?????
Introns
Exons
RNA Splicing
•Introns: intervening sequence.
Non coding
•Exon: translates to Amino Acids
sequence
•Spliceosome/Ribozymes
Small nuclear
ribonucleoproteins
Evolutionary Importance??
Alternative RNA Splicing
 Gene gives rise to different proteins
depending on which segments are exons
during RNA processing
 Potentially new proteins w/ new functions
Increase chance of crossing over b/w genes,
 increase genetic recombination
Translation, I
RNA to Protein
mRNA from nucleus
is ‘read’ along its
codons by tRNA’s
anticodons at the
ribosome
tRNA
anticodon
(nucleotide triplet);
amino acid
Translation, II
rRNA
site of mRNA codon & tRNA
anticodon coupling
P site
holds the tRNA carrying the
growing polypeptide chain
A site
holds the tRNA carrying the
next amino acid to be added
to the chain
E site
discharged tRNA’s
Translation, III
Initiation~
union of mRNA, tRNA,
small ribosomal subunit;
followed by large subunit
Elongation~
•codon recognition
•peptide bond formation
•translocation
Termination~
‘stop’ codon reaches ‘A’ site
Polyribosomes:
translation of mRNA by
many ribosomes (many
copies of a polypeptide
very quickly)
Role of RNA
Coupling in Bacterial DNA
Transcription Factors
Compartmentalization
Bacteria couple
Transcription and
translation
Signals for Specific Destinations
Free Ribosomes

Proteins used in cytosol
Bound Ribosomes





Attached to ER
Endomembrane System
Secreted from Cell (Insulin)
Polypeptide marked by signal peptide (20a.a at
leading end, 5’ cap modified)
Recognized by Signal Recognition Particle (SRP)
Mutations: genetic material changes in a
cell
Point mutations….
Changes in 1 or a few base pairs in a
single gene
Base-pair substitutions:



silent mutations: no effect on protein
missense : ∆ in sequence, a different
amino acid (different protein)
Nonsense: ∆ to a stop codon causes a
nonfunctional protein
Base-pair insertions or deletions:


additions or losses of nucleotide pairs in
a gene;
alters the ‘reading frame’ of
triplets~frameshift mutation
Mutagens: physical and chemical agents
that change DNA
“What would happen
if…..”
A mutation changed a start codon
to some other codon?
A mutation changed a stop codon
to some other codon?
A mutation changed one amino
acid within a polypeptide chain?
Exploring a Gene