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.