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

7. What is transcription?
The DNA strand serves as a
template for the synthesis of a
complementary RNA strand

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

15. Quick Think
How does the process
of transcription begin?

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

20. Quick Write
Describe the process of
transcription, including the 3
stages, in prokaryotes

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

25. 3’ poly-A tail
50-250 adenines are added at the
3’ end

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.