The Central Dogma and Transcription

The Central Dogma and

Transcription

Chapter 17: Sections 17.1-17.3

Today’s Exit TicketThe 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 polypeptide

subunits to make a larger molecule.

Unit 4• 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!All organisms do this!

Outline

1. Basic principles of transcription & translation2. Transcription in detail3. RNA processing in Eukaryotes4. The genetic code

Fig. 5.26

Information flowfrom geneticinformation

encoded as DNAblueprint (genes)

to RNA copies

mRNA

Synthesis ofmRNA in thenucleus

DNA

NUCLEUS

mRNA

CYTOPLASM

Movement ofmRNA into cytoplasmvia nuclear pore

1

2

What’s the difference between DNA and RNA??

• 3 Major Differences: DNA RNA– Different sugars: deoxyribose ribose– Different bases: C & G, A & T C & G, A & U– Structure: double-stranded single-stranded

(usually)

DNA & RNA provide information to make proteins• DNA and RNA = both nucleic acids• Both are made of nucleotide monomers

and on to synthesis of proteins.

Fig. 5.26

Information flowfrom geneticinformation

encoded as DNAblueprint (genes)

to RNA copies

mRNA

Synthesis ofmRNA in thenucleus

DNA

NUCLEUS

mRNA

CYTOPLASM

Movement ofmRNA into cytoplasmvia nuclear pore

Ribosome

AminoacidsPolypeptide

Synthesisof protein

1

2

3

Transcription vs. Translation

DNA RNA Proteins

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

สวั�สดี�เพื่�อนHello, friend

Hullo,

mate

Videos of Gene Expression:

Hank’s Transcription and Translation Crash Course

http://www.youtube.com/watch?v=itsb2SqR-R0

http://www.youtube.com/watch?v=D3fOXt4MrOM

From DNA to Protein

1. Overview of transcription and translation1. Overview of transcription and translation

Genes are nucleotide sequences, hundreds or thousands of nucleotides long

THE CENTRAL DOGMA:

DNA RNA PROTEIN



PROTEINPROTEIN


Outline

1. Easing in: basic principles of transcription and translation

2. Transcription in detail3. RNA processing in Eukaryotes4. The genetic “code”

Transcription vs. Translation

DNA RNA Proteins

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

สวั�สดี�ครั�บHello Hello

Fig. 17-4 2. Transcription in detail2. Transcription in detail

Successful transcription requires 3 basic processes:

1. Initiation2. Elongation3. Termination



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”

2. Transcription in detail2. Transcription in detail

a) Initiationa) Initiation

“Upstream” of the gene is a promoter• whole promoter = several dozen nucleotides

example of DNA that is essential but is not transcribed

where the gene is

the “start here” signal

Now we know WHERE to initiate, but HOW do we initiate?Now we know WHERE to initiate, but HOW do we initiate?

Transcription Unit:


a) Initiationa) InitiationHOW: With an enzyme, as usual!HOW: With an enzyme, as usual!

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

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 oneb) Bottom onec) Both strands

Promoter sequence


a) Initiationa) Initiation

With transcription factors in place, RNA polymerase can now bind DNA at the right place to begin

transcription of the gene

With transcription factors in place, RNA polymerase can now bind DNA at the right place to begin

transcription of the gene



Initiation • Bind transcription factors, then RNA

polymerase to promoter region

2) Elongation• Make the full length mRNA transcript


b) Elongationb) ElongationRN

A Polym

erase

untwists

DN

A, m

akes m

RNA

RNA

Polymera

se untw

ists D

NA,

makes

mRN

A


b) Elongationb) Elongation

Summary of elongation in transcription:

1.RNA polymerase untwists and separates 10-20 base pairs of DNA at a time2.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 molecule4.RNA polymerase moves along, the new RNA molecule peels away from the DNA, and the helix re-twists



Initiation

Elongation make the full length mRNA transcript

• Termination stop transcribing; mRNA completed


c) Terminationc) 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


2. Transcription in detail3. RNA processing in Eukaryotes4. 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?How does that work?


Before RNA transcripts leave the nucleus, they are modified.


Before RNA transcripts leave the nucleus, they are modified.

Modified how?1.Alteration of ends2.Cutting out some of the middle

offers cell a way of controlling when and where proteins are produced


1. Alteration of ends


2. Cutting out some of the middle: RNA splicing


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!


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


2. Transcription in detail3. RNA processing in Eukaryotes4. The genetic code

4. The genetic code4. 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

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

Fig. 17-4

DNAmolecule

Gene 1Gene 2

Gene 3DNA template strand

TRANSCRIPTION

TRANSLATION

mRNACodons

Protein

Amino acid

4. The genetic code4. The genetic code

note: either strand may serve as the template depending upon the particular gene

U U U U UG G G G C C A

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.

C

C

C

C

C

C

U

U

U

U

U

U

A

A

A

A

A

A

G

G

G

G

G

G

Fig. 17-4 4. The genetic code4. 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)

ACUUCCAAG

1 2 3

Today’s Exit TicketThe 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

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The Central Dogma and Transcription