Download RNA

Things to do today?
• Double Helix Essay - bring to lab!
• Write your own exam question.
• Review material (including today)
for Friday’s exam.
Exam 2: Friday, March 15th.
Exam 2 (3/15) will include a metabolic
pathways handout
Handout Copy is
on Web Site
A competition between dorms to
reduce their energy usage by the
most percentage compared to the
month prior to the competition. Prizes
are a dinner with President Paxson,
bikes, and other prizes. Only those
who are registered can have the
opportunity to win the prizes.
buildingdashboard.net/brown
DNA Replication
All polymerases, including DNA polymerase,
add bases to the 3’ end of a growing strand.
3’
5’
DNA replication is
Continuous on one strand:
Replication is not as
simple as it once
seemed!
Direction of Fork
Movement
5’
And Discontinuous
on the other:
The 3’ and 5’ labels on this slide have been corrected!
3’
5’
3’
Every piece of DNA begins
with a short strand of RNA:
Direction of Fork
Movement
5’
Primase makes
an RNA primer
5’
3’
5’
The 3’ and 5’ labels on this slide have been corrected!
3’
Replication continues until
the RNA fragments are
replaced by DNA
5’
DNA Ligase joins
fragments together
Direction of Fork
Movement
3’
5’
The 3’ and 5’ labels on this slide have been corrected!
But it’s even more complex than that — there are at
least 2 distinct DNA polymerases on each replication
fork, and a DNA-unwinding protein called DNA helicase
Freeman 4/e (p. 267)
Transcription & Translation
The “Central Dogma”
Transcription
RNA polymerase
Promoters
Introns & Exons
The Genetic Code
The Ribosome
tRNA, rRNA, and mRNA
Translation
The“Central
Dogma”
The“Central
Dogma”
DNA
transcription
RNA
translation
Protein
replication
RNA:
1) Ribose sugar
2) Uracil in Place
of Thymine
RNA is synthesized against a DNA
template by RNA polymerase, a
complex multisubunit enzyme.
RNA
DNA
Movement of RNA polymerase along DNA
can actually be visualized in the EM:
3’
5’
5’
Base-pairing during
transcription follows
the Watson-Crick
pattern.
5’
3’
3’
RNA polymerase (like DNA
polymerase) produces a new
strand in the 5’ to 3’ direction.
As a result, RNA is complementary to one strand
of DNA (the “template”) and nearly identical to the
other (the “coding strand”)
How does RNA polymerase know “which” strand to copy?
DNA sequences known as “promoters” serve
as RNA polymerase binding sites.
And RNA polymerase binds to a promoter
oriented in a particular direction.
Promoters have general features that
(sometimes) enable them to be recognized in a
DNA sequence.
DNA
mRNA (messenger RNA)
tRNA (transfer RNA)
rRNA (ribosomal RNA)
Transcription produces three general classes* of
RNA, each of which plays a role in translation
(protein synthesis)
* actually, there are many more classes of small RNA
molecules that perform important functions in the cell, including
gene regulation and RNA splicing.
He thought mRNA would match
up perfectly to the DNA from
which it was transcribed:
In 1975, when Phillip
Sharp first compared
mRNA sequences to
the DNA template
strands that produced
them, he was
surprised.
Sometimes it did.
But sometimes it didn’t!
In eukaryotes (but not
in prokaryotes), most
mRNAs are made in a
pre-mRNA form, which
is then cut and spliced
to remove intervening
sequences (“introns”)
before leaving the
nucleus.
Exons = Expressed
sequences.
Introns = Intervening
sequences
Next question: How does the sequence of bases in RNA determine
the sequence of amino acids in a polypeptide?
??
George Gamow
(physicist & SciFi writer)
1) There are 20 common amino acids
2) The code must specify 20 different
possibilities in each "word"
3) If one nucleotide corresponded to one
amino acid, only 4 words could be
formed (only 4 bases in RNA).
4) If two bases specified an AA, that would only allow 16
(4x4) words. Not enough.
5) But three bases per AA would work (4x4x4=64, more
than enough to produce the minimum of 20 words needed).
6) Since nature always works in the most economical way,
the genetic code must be written in triplets - three bases at
a time.
5’
AUG
Translation
begins with a
“start” codon,
usually AUG...
UAA
… and ends with
1 of 3 “stop”
codons.
3’
But Translation requires a complex
“machine” to make it all work:
Ribosomes are the sites
of protein synthesis
Ribosomes bind to the
beginning of a mRNA
molecule (the AUG initiator
near the 5’ end), and catalyze
the polymerization of amino
acids to form a polypeptide.
Ribosomes (250Å
diameter) seen in
electron microscope.
3-D functional
model of ribosome
prepared from EM
images (Joachim
Frank, SUNY
Albany):
Ribosomes consist
of two subunits
(large & small)
Ribosomes are
composed of RNA
and protein.
(Eukaryotic: 4 rRNA
molecules and 83
proteins)
The detailed structure of the
ribosome is now known at the
atomic level. rRNA forms the
functional core of the ribosome.
Q: How does the ribosome pick the “right” amino acid
to match each codon in the message?
met
UAC
5’
his
CAC
AUGCACGGAUUUUCCAAC . . . . . . .
UAA
A: It doesn’t!
Amino acids are bound to transfer RNA (tRNA)
molecules, which base-pair to mRNA codons.
3’
tRNAs are short (75-90 bases) RNA molecules.
Enzymes attach the appropriate Amino Acid to each
tRNA based on the tRNA’s own base sequence.