From Genes to Proteins - Transcription
From Genes to Proteins - Transcription
Ch. 17Sections 17.1, 17.2, & 17.3
Ch. 17Sections 17.1, 17.2, & 17.3
To aid in your notetaking…To aid in your notetaking…
Key vocabulary terms are in orange, bold font and underlined
Key vocabulary terms are in orange, bold font and underlined
Overview of ConceptsOverview of Concepts
1.1. The information in DNA is in the The information in DNA is in the sequence of nucleotidessequence of nucleotides
2.2. Transcription is the production of RNA Transcription 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 protein mRNA is the transcribed code for a protein based on the DNAbased on the DNA
3.3. mRNA is modified in eukaryotic cells mRNA is modified in eukaryotic cells before it is functionalbefore it is functional
1.1. The information in DNA is in the The information in DNA is in the sequence of nucleotidessequence of nucleotides
2.2. Transcription is the production of RNA Transcription 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 protein mRNA is the transcribed code for a protein based on the DNAbased on the DNA
3.3. mRNA is modified in eukaryotic cells mRNA is modified in eukaryotic cells before it is functionalbefore it is functional
“One gene-one polypeptide” hypothesis
“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 gene-one polypeptide”
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 gene-one polypeptide”
RNA is the bridge between DNA & proteins
RNA is the bridge between DNA & proteins
There are some key differences between DNA & RNAThere are some key differences between DNA & RNA
DNA RNASugar - deoxyribose
Sugar - ribose
Bases - A, T, C, G Bases - A, U, C, G
Double stranded & very long
Single stranded & shorter
To go from the DNA to proteins requires 2 major steps:
Transcription & Translation
To go from the DNA to proteins requires 2 major steps:
Transcription & TranslationTranscription - DNA
to RNA
Translation - RNA to protein
We will focus on Transcription today
Transcription - DNA to RNA
Translation - RNA to protein
We will focus on Transcription today
What is transcription?What is transcription?
The DNA strand serves as a template for the synthesis of a complementary RNA strand
The DNA strand serves as a template for the synthesis of a complementary RNA strand
Why can’t we just go straight from DNA to proteins?
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
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
Quick ThinkQuick Think
What is transcription and why is it important in the synthesis
of proteins?
What is transcription and why is it important in the synthesis
of proteins?
Prokaryotes vs. EukaryotesProkaryotes vs. EukaryotesProkaryotes - both
transcription & translation occur in the cytosol, since there is no nucleusThese processes can happen
simultaneously
Eukaryotes - transcription in nucleus, translation in cytosol
Prokaryotes - both transcription & translation occur in the cytosol, since there is no nucleusThese processes can happen
simultaneously
Eukaryotes - transcription in nucleus, translation in cytosol
The synthesis of mRNAThe synthesis of mRNA
Messenger RNA is made in much the same way that DNA is replicatedThe DNA strand serves as
a template for the linking of complementary base pairs
Messenger RNA is made in much the same way that DNA is replicatedThe DNA strand serves as
a template for the linking of complementary base pairs
RNA polymeraseRNA 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
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
Types of RNA polymeraseTypes of RNA polymerase
Only 1 type in prokaryotes
Three types in eukaryotesRNA pol II is used
in the synthesis of mRNA
Only 1 type in prokaryotes
Three types in eukaryotesRNA pol II is used
in the synthesis of mRNA
RNA polymerase
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
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
Quick ThinkQuick Think
How does the process of transcription begin?
How does the process of transcription begin?
The 3 stages of Transcription
The 3 stages of Transcription
1. Initiation2. Elongation3. Termination
1. Initiation2. Elongation3. Termination
start
build
break-off
InitiationInitiation 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 II attachesThis whole thing is called a
transcription initiation complex
An important promoter sequence in eukaryotes is called a TATA box
It contains the bases TATAAAA
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 II attachesThis whole thing is called a
transcription initiation complex
An important promoter sequence in eukaryotes is called a TATA box
It contains the bases TATAAAA
ElongationElongationRNA 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
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 Several 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
Termination in ProkaryotesTermination in ProkaryotesIn prokaryotesprokaryotes -
RNA pol goes through a termination sequence, detaches, & releases the transcript.
The transcript is available for immediate use by the cell
In prokaryotesprokaryotes - RNA pol goes through a termination sequence, detaches, & releases the transcript.
The transcript is available for immediate use by the cell
Quick WriteQuick Write
Describe the process of transcription, including the 3 stages, in prokaryotes
Describe the process of transcription, including the 3 stages, in prokaryotes
Termination in EukaryotesEukaryotesTermination in EukaryotesEukaryotesThe pre-mRNA
strand is cut off from the growing RNA chainRNA pol is still
attached to the DNA and continues to transcribe it
RNA pol continues much further down the DNA and eventually falls off
The pre-mRNA strand is cut off from the growing RNA chainRNA pol is still
attached to the DNA and continues to transcribe it
RNA pol continues much further down the DNA and eventually falls off
Quick WriteQuick Write
How is termination of transcription different in prokaryotesprokaryotes versus eukaryoteseukaryotes?
How is termination of transcription different in prokaryotesprokaryotes versus eukaryoteseukaryotes?
Modification of the mRNA in eukaryotes
Modification of the mRNA in eukaryotes
Enzymes in eukaryotic cells modify the mRNA before it becomes functionalThis 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
Enzymes in eukaryotic cells modify the mRNA before it becomes functionalThis 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
5’ cap5’ capThe 5’ end is the end that was
transcribed firstA modified guanine nucleotide is
addedThis is called the 5’ cap
The 5’ end is the end that was transcribed first
A modified guanine nucleotide is added
This is called the 5’ cap
3’ poly-A tail3’ poly-A tail
50-250 adenines are added at the 3’ end
50-250 adenines are added at the 3’ end
Why???Why???The 5’ cap and the 3’ tail
help to:Export mRNA from the
nucleusProtect mRNA from
hydrolytic enzymesAllow ribosomes to attach
at the 5’ end
The 5’ cap and the 3’ tail help to:Export mRNA from the
nucleusProtect mRNA from
hydrolytic enzymesAllow ribosomes to attach
at the 5’ end
RNA splicing
RNA splicing
In eukaryotic genes and the transcribed mRNA, there are long non-coding regions between coding regionsWe call the non-coding
regions intronsWe call the coding
regions exons
RNA splicing removes the introns & joins the exons to make a continuous coding sequence in the mRNA
In eukaryotic genes and the transcribed mRNA, there are long non-coding regions between coding regionsWe call the non-coding
regions intronsWe call the coding
regions exons
RNA splicing removes the introns & joins the exons to make a continuous coding sequence in the mRNA
How does it know what to cut out?
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
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
Quick WriteQuick Write
What are some ways the RNA is modified before it is translated in eukaryotic cells?
What are some ways the RNA is modified before it is translated in eukaryotic cells?
RibozymesRibozymesRibozymes are RNA
molecules that function as enzymes in the splicing of RNA
Their discovery eliminated the hypothesis that all enzymes were proteins
Ribozymes are RNA molecules that function as enzymes in the splicing of RNA
Their discovery eliminated the hypothesis that all enzymes were proteins
Why introns?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 combinedExon shuffling can lead to new proteins, increasing genetic
variation
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 splicingIntrons increase the likelihood of crossing over (more places for it
to occur if gene is longer)Exons from different genes may get combinedExon shuffling can lead to new proteins, increasing genetic
variation
Quick WriteQuick 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.
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.