Transcription of DNA to mRNA

1. Ch. 13 & 14
Anti - opposite
Intro - within
Muta - change
-script - to write
Helic - a spiral
Semi - half
Trans - across
Feb 17?

2. AP Biology Plans for the week of January 25th
through January
29th
, 2016
• Monday 1-25-16: Ch. 14 part 1 notes
• Tuesday 1-26-16: Ch. 14 part 2 notes
• Wednesday 1-27-16: big group diagram of transcription and
translation
• Thursday & Friday 1-28 & 1-29: Review and practice questions
for Ch. 13 and 14
Homework: Study Fig 14.24 on page 287 for a diagram quiz on MONDAY 2-1-16

3. Objectives
1. Identify and describe the 3 stages
of transcription
2. Describe how pre-mRNA is
altered to become functional
mRNA in eukaryotes

4. From Genes to Proteins -
Transcription
Ch. 14

5. To aid in your notetaking…
Key vocabulary terms are in
orange, bold font and underlined

6. Overview of Concepts
1.1. The information in DNA is in theThe information in DNA is in the
sequence of nucleotidessequence of nucleotides
2.2. Transcription is the production of RNATranscription is the production of RNA
1.1. There are several types of RNAThere are several types of RNA
2.2. mRNA is the transcribed code for a proteinmRNA is the transcribed code for a protein
based on the DNAbased on the DNA
3.3. mRNA is modified in eukaryotic cellsmRNA is modified in eukaryotic cells
before it is functionalbefore it is functional
1.1. The information in DNA is in theThe information in DNA is in the
sequence of nucleotidessequence of nucleotides
2.2. Transcription is the production of RNATranscription is the production of RNA
1.1. There are several types of RNAThere are several types of RNA
2.2. mRNA is the transcribed code for a proteinmRNA is the transcribed code for a protein
based on the DNAbased on the DNA
3.3. mRNA is modified in eukaryotic cellsmRNA is modified in eukaryotic cells
before it is functionalbefore it is functional

7. prokaryotic
eukaryotic

8. RNA is the bridge between DNA
& proteins
There are some key differences between DNA & RNA
DNA RNA
Sugar -
deoxyribose
Sugar - ribose
Bases - A, T, C, G Bases - A, U, C, G
Double stranded
& very long
Single stranded &
shorter

9. To go from the DNA to proteins
requires 2 major steps:
Transcription & Translation
Transcription - DNA
to RNA
Translation - RNA to
protein
We will focus onWe will focus on
Transcription todayTranscription today

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

11. Why can’t we just go straight
from DNA to proteins?
Having a “middle-
man” in the form of
RNA protects the all
important DNA
It is more efficient -
many copies of that
gene can be made &
used simultaneously

12. Quick Think
What is transcription and why
is it important in the
synthesis of proteins?

13. 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

14. 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

15. Quick Think
If DNA polymeraseDNA polymerase adds DNA
nucleotides to make a strand of
DNA
then
What enzyme do you suppose
adds RNA nucleotides to make a
strand of RNA?

16. 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

17. 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

18. The 3 stages of
Transcription
1. Initiation
2. Elongation
3. Termination
start
build
break-off

19. Initiation
In prokaryotesprokaryotes - RNA
pol recognizes the
promoter region and
binds directly to it
In eukaryoteseukaryotes - proteins
called transcription
factors attach to
promoter 1st, then RNA
pol attaches
An important promoter
sequence in eukaryotes is
called a TATA box
It contains the bases
TATAAAA

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

21. 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 onSeveral RNA pol can be working on
the same gene at the same time,the same gene at the same time,
increasing the rate of transcriptionincreasing the rate of transcription

22. Termination
In prokaryotesprokaryotes -
RNA pol goes
through a
termination
sequence,
detaches, &
releases the
transcript.
The transcript is
available for
immediate use by
the cell

23. Question for ya
What are the 3 stages of
transcription
What happens at each stage?

24. Termination in EukaryotesEukaryotes
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
The mRNA is NOT
ready for use yet

25. Quick Think
What are some differences
between transcription in
prokaryotesprokaryotes versus eukaryoteseukaryotes?

26. Modification of the mRNA in
eukaryotes
 In eukaryotic cells the
mRNA is modified before it
becomes functional
This is why it is called pre-
mRNA
In general, both ends of the
pre-mRNA are altered
Some of the middle parts
may be cut out and the
remains sliced together

27. Modification #1
The 5’ cap
A modified guanine nucleotide is
added
This is called the 5’ cap

28. Modification # 2
The 3’ poly-A tail
50-250 adenines are added at the
3’ end

29. 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
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

30. Modification # 3
RNA splicing
In the DNA and RNA
there are long non-coding
regions between coding
regions
non-coding regions =
introns
coding regions = exons
RNA splicing removes the
introns & joins the exons

31. How does it know what to cut
out?
introns recognized by
spliceosomes (an type
of RNA + proteins)
spliceosomes cut out
the introns and fuse
the exons

32. Quick Think
What are some ways the RNA is
modified before it is translated in
eukaryotic cells?

33. Ribozymes
Ribozymes are RNA
molecules that function
as enzymes in the
splicing of RNA
Their discovery
eliminated the
hypothesis that all
enzymes were proteins

34. 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

35. Exit Ticket
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.

36. 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

37. Termination in EukaryotesEukaryotes
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