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Chapter 17
Replication- coping DNA
Transcription- creating mRNA from DNA
Translation- creating protein from mRNA
DNA to RNA to Protein
Transcription: 3 Stages
1) Initiation- transcription factors bind to a promoter such as the TATAA box.
This allows RNA polymerase 2 to bind, forming the transcription initiation
complex. This causes the DNA to unwind, and transcription begins.
2) Elongation- RNA synthesis progresses in the 5’ to 3’ direction.
3) Termination- transcription proceeds through the terminator then detaches
in prokaryotes. In eukaryotes, the pre-mRNA falls off after RNA
polymerase 2 transcribes a sequence called the polyadenylation signal
sequence.
Eukaryotes modify RNA after translation
5’ end is capped and a poly-A tail is added to the 3’ end. These facilitate export
from the nucleus and protect the RNA from the degradation. RNA is spliced by a
spliceosome made of snRNA, removing noncoding sections called introns, and
leaving exons. Some genes can produce multiple polypeptides depending on
what is spliced; this is called alternative RNA splicing. Exon shuffling during
cross-over may also be useful in evolution.
Translation also has 3 Stages:
1) Initiation- initiator tRNA attaches at AUG (start) codon. (prokaryotes also
have something called the Shine-Delgarno sequence about 10 base pairs
before AUG to distinguish start from other AUG combinations.) This is
followed by the attaching of the small and large ribosomal subunits.
2) Elongation- progresses in 5’-3’ direction
a) Anticodon in tRNA bonds with the matching codon on mRNA at A
site, using GTP for energy
b) Peptide bond forms between amino acids in A and P sites
c) Translocation- tRNA in A site moves to P site. Empty tRNA in P site
moves to E site and then is released.
3) Termination- stop codons (UAG, UAA, and UGA) cause that addition of
water instead of amino acid, hydrolyzing and releasing the polypeptide.
Multiple mRNA may attach to the same mRNA to make many copies at once.
All translation begins with free ribosomes. If a growing polypeptide includes a
signal peptide, a signal-recognition particle (SRP) helps attach it to the ER.
Because prokaryotes have no nucleus and no introns, transcription and
translation can be coupled, with translation beginning before transcription ends.
Point mutations are changes in the DNA that involve one base pair.
1) Substitution- nucleotides swapped
a) Missense- code for wrong amino acid
b) Nonsense- code for stop
2) Frameshift mutations- addition or deletion of a nucleotide. Messes up
whole protein.
Mutagens can cause mutations.
Comparing gene expression in prokaryotes and eukaryotes
Differences
Ribosomes
Promoter binding
Prokaryotic cells
Smaller than ribosomes
in eukaryotic cells differ
in molecular composition.
Difference is important
because certain
antibiotics that interfere
with protein synthesis in
prokaryotic cells do not
affect eukaryotic cells
RNA polymerase
recognizes and binds to
the promoter
The end of transcription
A terminator signals the
end transcription
Signal sequence
Prokaryotes use signal
sequences to target
proteins for secretion
after transcription.
Eukaryotic cells
Subunits of rRNA are
made in the nucleus.
Ribosomes are larger
that prokaryotic
ribosomes
Transcription factors
mediate the binding of
RNA polymerase to the
promoter
Pre-mRNA is cleaved
from the growing RNA
chain while RNA
polymerase 2 continues
transcription until the
polymerase eventually
falls off the DNA
Some proteins are target
for movement to
endomembrane system
organelles during
transcription