Download Lecture 16

The Central Dogma
and
Transcription
Chapter 17:
Sections 17.1-17.3
Today’s Exit Ticket
The bonds creating the primary structure of a protein are called 1)
peptide bonds and form between a 2) C atom in one amino acid and a
3) N atom in another amino acid.
The bonds creating the secondary structure of a protein are called 4)
hydrogen bonds and form between 5) the backbone molecules of
amino acids (NOT R-groups).
The bonds creating the tertiary structure of a protein can be covalent,
ionic, or hydrogen bonds, and form between 6) R-groups.
7) Describe the quaternary structure of a protein.
Quaternary structure is the interaction of different
subunits to make a larger molecule.
polypeptide
Unit 4
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Proteins
Transcription (DNA to mRNA)
Translation (mRNA to tRNA to proteins)
Gene expression/regulation (turning genes on and off)
Viruses
3
The Central Dogma
and
Transcription
Chapter 17:
Sections 17.1-17.3
How do we get from DNA to traits?
• Gene expression = DNA directs the synthesis
of proteins
TWO STEPS:
(1) transcription
(2) translation
All organisms do this!
Outline
1.
2.
3.
4.
Basic principles of transcription & translation
Transcription in detail
RNA processing in Eukaryotes
The genetic code
DNA
1 Synthesis of
mRNA in the
nucleus
Information flow
from genetic
information
encoded as DNA
blueprint (genes)
mRNA
to RNA copies
NUCLEUS
CYTOPLASM
mRNA
2 Movement of
mRNA into cytoplasm
via nuclear pore
Fig. 5.26
DNA & RNA provide information to make proteins
• DNA and RNA = both nucleic acids
• Both are made of nucleotide monomers
What’s the difference between DNA and RNA??
• 3 Major Differences:
DNA
RNA
– Different sugars:
deoxyribose
– Different bases:
C & G, A & T
C & G, A & U
– Structure:
double-stranded
single-stranded
ribose
(usually)
DNA
1 Synthesis of
mRNA in the
nucleus
Information flow
from genetic
information
encoded as DNA
blueprint (genes)
mRNA
NUCLEUS
CYTOPLASM
to RNA copies
mRNA
2 Movement of
mRNA into cytoplasm
via nuclear pore
Ribosome
and on to synthesis of
proteins.
3 Synthesis
of protein
Polypeptide
Amino
acids
Fig. 5.26
Transcription vs. Translation
DNA
RNA
Proteins
สวัสดีเพื่อน
Transcription:
• Like copying info from a
book in the reserved section
of the library
• Using the same language
Translation:
• Literally translating between
two different languages
• Take the copied info from
the library and translate it
into French/Spanish/Mandarin
Videos of Gene Expression:
Hank’s Transcription and Translation Crash Course
http://www.youtube.com/watch?v=itsb2SqR-R0
From DNA to Protein
http://www.youtube.com/watch?v=D3fOXt4MrOM
1. Overview of transcription and translation
Genes are nucleotide sequences, hundreds or
thousands of nucleotides long
1. Overview of transcription and translation
THE CENTRAL DOGMA:
DNA  RNA  PROTEIN
1. Overview of transcription and translation
PROTEIN
1. Overview of transcription and translation
Outline
1. Easing in: basic principles of transcription and
translation
2. Transcription in detail
3. RNA processing in Eukaryotes
4. The genetic “code”
Transcription vs. Translation
DNA
RNA
Proteins
สวัสดีครับ
Transcription:
• Like copying info from a
book in the reserved section
of the library
• Using the same language
Translation:
• Literally translating between
two different languages
• Take the copied info from
the library and translate it
into French/Spanish/Mandarin
17-4
2.Fig.Transcription
in detail
Successful transcription requires 3 basic processes:
1. Initiation
2. Elongation
3. Termination
17-4
2.Fig.Transcription
in detail
Successful transcription requires 3 basic processes:
1. Initiation
• Find the location where we start reading DNA
• Actually begin making mRNA
 To achieve this, we need some kind of signal on or
in the DNA that says “START TRANSCRIBING HERE”
the “start here”
signal
2. Transcription in detail
a) Initiation
Transcription
Unit:
where the
gene is
“Upstream” of the gene is a promoter
• whole promoter = several dozen nucleotides
 example of DNA that is essential but is not transcribed
Now we know WHERE to initiate, but HOW do we initiate?
HOW: With an enzyme, as usual!
2. Transcription in detail
a) Initiation
RNA polymerase
• Reads one strand of DNA and builds the mRNA
• Can’t bind to the promoter on its own (in eukaryotes)
• Only binds when specific transcription factors are
present
Promoter sequence
Promoter sequence
Once RNA polymerase binds, it can only synthesize RNA in a 5’ to
3’ direction. Which of the two DNA strands shown here will it
“read” as it makes RNA?
a) Top one
b) Bottom one
c) Both strands
2. Transcription in detail
a) Initiation
With transcription factors in place, RNA polymerase
can now bind DNA at the right place to begin
transcription of the gene
17-4
2.Fig.Transcription
in detail
Successful transcription requires 3 basic processes:
 Initiation
• Bind transcription factors, then RNA
polymerase to promoter region
2) Elongation
• Make the full length mRNA transcript
2. Transcription in detail
b) Elongation
RNA
Polymerase
untwists
DNA,
makes
mRNA
2. Transcription in detail
b) Elongation
Summary of elongation in transcription:
1.RNA polymerase untwists and separates 10-20 base
pairs of DNA at a time
2.RNA nucleotides enter and pair with the DNA template
(U, not T, pairs with A)
3.RNA polymerase bonds nucleotides onto the 3’ end of
the RNA molecule
4.RNA polymerase moves along, the new RNA molecule
peels away from the DNA, and the helix re-twists
17-4
2.Fig.Transcription
in detail
Successful transcription requires 3 basic processes:
 Initiation
 Elongation  make the full length mRNA
transcript
• Termination  stop transcribing; mRNA completed
2. Transcription in detail
c) Termination
But how does it stop?
Bacteria: termination sequence in the DNA
Eukaryotes: a bit more complicated
• enzymes cut the transcript free…among other things!
Outline
1. Easing in: basic principles of transcription and
translation
2. Transcription in detail
3. RNA processing in Eukaryotes
4. The genetic code
3. RNA processing in eukaryotes
Observations:
• Average human pre-mRNA transcript
length: 27,000 nucleotides
• Each amino acid is coded by 3 nucleotides
• Average human protein: 400 amino
acids  requires only 1200 nucleotides
How does that work?
3. RNA processing in eukaryotes
Before RNA transcripts leave the nucleus, they are modified.
3. RNA processing in eukaryotes
Before RNA transcripts leave the nucleus, they are modified.
Modified how?
1.Alteration of ends
2.Cutting out some of the middle
 offers cell a way of controlling when and where proteins
are produced
3. RNA processing in eukaryotes
1. Alteration of ends
3. RNA processing in eukaryotes
2. Cutting out some of the middle: RNA splicing
3. RNA processing in eukaryotes
The sequence
of DNA that
codes for a
eukaryotic
protein is NOT
a continuous
sequence
Some introns are
“self-splicing” 
catalyze their own
excision!
Ribozymes!
Thomas Cech
• CU Professor
• 1989 Nobel Prize
winner, along with
Sidney Altman
• Discovered that
RNA can sometimes
splice itself!
3. RNA processing in eukaryotes
1. Cutting out some of the middle: RNA splicing
Why do introns exist?
• Alternative splicing  alternative mRNA
 multiple proteins from a single DNA sequence
Outline
1. Easing in: basic principles of transcription and
translation
2. Transcription in detail
3. RNA processing in Eukaryotes
4. The genetic code
4. The genetic code
Nucleotides: A, T, G, and C in DNA
(A, U, G, and C in RNA)
Amino Acids  20 are commonly used by most organisms
The genetic code consists of 3-letter codons:
• Sequence of 3 nucleotides = specification of amino acid
• Each triplet of mRNA nucleotides is called a codon
17-4
4.Fig.The
genetic code
DNA
molecule
Gene 1
Gene 2
Gene 3
DNA template
strand
TRANSCRIPTION
U G G U U U G G C U C A
mRNA
Codons
TRANSLATION
Protein
Amino acid
note: either
strand may
serve as the
template
depending
upon the
particular
gene
Math check:
WAIT a second!
• 4 nucleotides...in sets of 3...
• Shouldn’t there be 43 codons??
YES!
• Using just 4 nucleotides, DNA can make 64 different
codons
BUT... You just said there are only 20 amino acids!?!
• Yes, friends, there are only 20.
U
C
A
G
U
U
C
A
G
U
C
C
A
G
U
A
C
A
G
U
G
C
A
G
17-4
4.Fig.The
genetic code
Some notes on codons:
1.When we say “codon”, we are referring to RNA triplets
2.Codons are read in the 5’ to 3’ direction, because that is how they are
read by the translation machinery
1.Codons don’t overlap (300 nucleotides encode 100 codons)
1
2
3
ACUUCCAAG
Today’s Exit Ticket
The final product of transcription is _(1)_. The template used for transcription is _(2)_. The first step of the process is called _(3)_ and
involves the _(4)_ binding to the _(5)_ region. This allows _(6)_ to bind to the DNA and begin transcribing, in a process called _(7)_.
During that process, the enzyme reads from the _(8)_’ to _(9)_’ direction and builds the new strand from _(10)_’ to _(11)_’. The last
step of transcription is called _(12)_. In _(13)_, there is another step before translation. This is called _(14)_, and involves removing
_(15)_ and adding a 5’ cap and 3’ poly-A tail.
WORD BANK (not all will be used, some are used more than once):
3
5
DNA
elongation
eukaryotes
exons
initiation
introns
mRNA
prokaryotes
promoter
RNA polymerase
RNA processing
termination
transcription factors