This document provides an overview of transcription in prokaryotes and eukaryotes. It discusses how DNA serves as a template for RNA production. The three main stages of transcription are initiation, elongation, and termination. In eukaryotes, the pre-mRNA undergoes processing including 5' capping, poly-A tail addition, and splicing of introns before becoming a functional mRNA that can be translated into a protein.
1. From Genes to Proteins Transcription
Ch. 17
Sections 17.1, 17.2, & 17.3
2. To aid in your notetaking…
Key vocabulary terms are in
orange, bold font and underlined
3. Overview of Concepts
1. The information in DNA is in the
sequence of nucleotides
2. Transcription is the production of RNA
1. There are several types of RNA
2. mRNA is the transcribed code for a protein
based on the DNA
3. mRNA is modified in eukaryotic cells
before it is functional
4. “One gene-one polypeptide”
hypothesis
The work of Beadle &
Tatum in the early 1900s
w/ Neurospora supported
the hypothesis that each
gene codes for a
particular enzyme
“one gene-one enzyme
revised to “one gene-one
protein”, then “one geneone polypeptide”
5. RNA is the bridge between DNA
& proteins
There are some key differences between DNA & RNA
DNA
Sugar deoxyribose
RNA
Sugar - ribose
Bases - A, T, C, G Bases - A, U, C, G
Double stranded
& very long
Single stranded &
shorter
6. To go from the DNA to proteins
requires 2 major steps:
Transcription & Translation
Transcription - DNA
to RNA
Translation - RNA to
protein
We will focus on
Transcription today
8. Why can’t we just go straight
from DNA to proteins?
Having a “middleman” in the form of
RNA protects the all
important DNA
It is more efficient many copies of that
gene can be made &
used simultaneously
9. Quick Think
What is transcription and why
is it important in the
synthesis of proteins?
10. Prokaryotes vs. Eukaryotes
Prokaryotes - both
transcription & translation
occur in the cytosol, since
there is no nucleus
These processes can happen
simultaneously
Eukaryotes - transcription
in nucleus, translation in
cytosol
11. The synthesis of mRNA
Messenger RNA is made
in much the same way
that DNA is replicated
The DNA strand serves as
a template for the linking
of complementary base
pairs
12. RNA polymerase
This enzyme separates the DNA strands
It also bonds the RNA nucleotides
together
It attaches at a special sequence of
bases on the DNA called the promoter
region
13. Types of RNA polymerase
Only 1 type in
prokaryotes
Three types in
eukaryotes
RNA pol II is used
in the synthesis of
mRNA
RNA
polymerase
14. The stretch of DNA that gets
transcribed is called the
transcription unit
This is the region of the DNA that
contains the information for
making the protein
16. The 3 stages of
Transcription
1. Initiation
2. Elongation
3. Termination
start
build
break-off
17. Initiation
In prokaryotes - RNA
pol recognizes the
promoter region and
binds directly to it
In eukaryotes - proteins
called transcription
factors attach to
promoter 1st, then RNA
pol II attaches
This whole thing is called a
transcription initiation
complex
An important promoter
sequence in eukaryotes is
called a TATA box
It contains the bases
TATAAAA
18. Elongation
RNA pol untwists the
DNA 10-20 bases at
a time
RNA strand is made
in the 5’ to 3’
direction, with new
bases added to the 3’
end
As it gets longer, it
peels away from the
DNA chain & the DNA
double helix reforms
Several RNA pol can be working on
the same gene at the same time,
increasing the rate of transcription
19. Termination in Prokaryotes
In prokaryotes RNA pol goes
through a
termination
sequence,
detaches, &
releases the
transcript.
The transcript is
available for
immediate use by
the cell
21. Termination in Eukaryotes
The pre-mRNA
strand is cut off
from the growing
RNA chain
RNA pol is still
attached to the DNA
and continues to
transcribe it
RNA pol continues
much further down
the DNA and
eventually falls off
22. Quick Write
How is termination of transcription
different in prokaryotes versus
eukaryotes?
eukaryotes
23. Modification of the mRNA in
eukaryotes
Enzymes in eukaryotic cells
modify the mRNA before it
becomes functional
This is why it is called premRNA
In general, both ends of the
pre-mRNA are altered
Some of the middle parts
may be cut out and the
remains sliced together
24. 5’ cap
The 5’ end is the end that was
transcribed first
A modified guanine nucleotide is
added
This is called the 5’ cap
26. Why???
The 5’ cap and the 3’ tail
help to:
Export mRNA from the
nucleus
Protect mRNA from
hydrolytic enzymes
Allow ribosomes to attach
at the 5’ end
27. In eukaryotic genes
and the transcribed
mRNA, there are long
non-coding regions
between coding regions
We call
regions
We call
regions
the non-coding
introns
the coding
exons
RNA splicing removes
the introns & joins the
exons to make a
continuous coding
sequence in the mRNA
RNA
splicing
28. How does it know what to cut
out?
There are regions at
the ends of introns
that are recognized by
molecules called
spliceosomes (an
assembly of RNA and
proteins)
The spliceosomes
cut out the introns and
fuse the remaining
exons
29. Quick Write
What are some ways the RNA is
modified before it is translated in
eukaryotic cells?
30. Ribozymes
Ribozymes are RNA
molecules that function
as enzymes in the
splicing of RNA
Their discovery
eliminated the
hypothesis that all
enzymes were proteins
31. Why introns?
Allows for alternative RNA splicing to occur
Genes can code for more than one polypeptide
depending on which segments are treated as exons
during RNA splicing
Introns increase the likelihood of crossing over
(more places for it to occur if gene is longer)
Exons from different genes may get combined
Exon shuffling can lead to new proteins,
increasing genetic variation
32. Quick Write
How is the functional mRNA
different from the DNA template
that was used to produce it?
Think of as many differences as
you can. I can think of at least 7
differences.