copy of proteins synthesis 3

7/28/2019 Copy of Proteins Synthesis 3

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Proteins Synthesis

Transcription and Translation

By: Ms. Reis


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Protein Synthesis: An Overview

Genetic information is contained within thenucleus of a cell

DNA in the nucleus directs protein

synthesis but protein synthesis occurs inribosomes located in the cytoplasm

How does a ribosome synthesize the

protein required if it does not have accessto DNA?


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THE CENTRAL DOGMA OF

PROTEIN SYNTHESIS


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Protein Synthesis: An Overview

The answer lies in an intermediate substanceknown as mRNA.

Information is copied from DNA into mRNA, this

is transcription mRNA leaves the nucleus and enters the

cytoplasm of the cell

Ribosomes use the mRNA as a blueprint to

synthesize proteins composed of aa, this istranslation.


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DNA

3 main components:

Deoxyribose sugar

Phosphate group Nitrogenous bases-adenine, guanine,

cytosine and thymine

A forms 2 hydrogen bonds to T, G forms 3hydrogen bonds to C


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DNA vs RNA

Deoxyribose sugar

Double stranded

A pairs with T

G pairs with C

Resides in nucleus

Ribose sugar

Single stranded

A pairs with U

G pairs with C

Resides in nucleus

and cytoplasm


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RNA

There are three types of RNA:

mRNA is the blueprint for construction of

a protein

rRNA is the construction site where the

proteins are made

tRNA is the truck delivering the proper aato the site of protein synthesis


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Genes and Proteins

Genes are a sequence of nucleotides in

DNA that code for a particular protein

Proteins drive cellular processes,

determine physical characteristics, and

manifest genetic disorders by their

absence or presence


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Genetic Code

Proteins are composed of20 different amino acids

A sequence of 3 nucleotides is used to code eachamino acid

Each triplet of nucleotides is called a codon Start codon AUG codes for amino acid methionine

3 stop codons

There are 64 codons in the genetic code 43=64

Several different codons can code for the same aa,but no codon ever has more than one amino acidcounterpart.

Codons are always written in the form of the RNA

transcript from the original DNA molecule.


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Characteristics of the Code

Continuity The genetic code reads as a long

series of three-letter codons that have no spaces

or punctuation and never overlap.

Redundancy Several different codons cancode for the same amino acid, but no codon

ever has more than one amino acid counterpart.

Universality the genetic code is the same in

almost all living organisms, from bacteria to

mammals


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Transcription: Initiation

RNA polymerase binds to a segment of

DNA and opens up the double helix

RNA polymerase recognizes the promoter

region which is a sequence of DNA rich in A

and T bases (TATA box) found only on one

strand of the DNA.


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An RNA polymerase cannot recognize the

TATA box and other landmarks of the

promoter region on its own. Another

protein, a transcription factorthatrecognizes the TATA box, binds to the

DNA before the RNA polymerase can do

so.


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For transcription to be initiated, both promoter sequencesmust be present in their correct locations. The nucleotide

sequences in the promoters are slightly different from one

another, which means the RNA polymerase will bind in only

1 orientation, thus RNA polymerase can only face 1 way

during transcription. This ensures transcription will proceed

in only 1 direction.


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Transcription : Elongation

The RNA polymerase uses only one of the strandsof DNA as a template for mRNA synthesis. This iscalled the template strand orsense strand. Thecoding strand or anti-sense strand contains the

complementary nucleotide sequence to the sensestrand.

RNA polymerases can add nucleotides only to the3 end of a DNA sequence. Thus, an RNA

molecule elongates in the 5 to 3 direction. Consider the following DNA sequence

3 TACTTACTCGTCTTG 5


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The Coding Strand

RNA polymeraseuses the template

strand to transcribe.

Thus the RNA iscomplimentary to the

template. The

coding strand is

exactly identical tothe mRNA, but

mRNA has uracil in

place of thymine.


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Transcription: Termination

As the RNA

polymerase molecule

passes, the DNA helix

re-forms. Synthesiscontinues until the end

of a gene is reached

where RNA

polymeraserecognizes a

terminator sequence.


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Transcription

Once the RNA polymerase leaves thepromoter region, a new RNA polymerasecan bind there to begin a new mRNAtranscript.

Since prokaryotes lack a membranebound nucleus translation can begin evenbefore the mRNA dissociates. However

the pre-mRNA from eukaryotic cells needssome modification before it leave thenucleus.


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Processing of mRNA transcript

In eukaryotes, the mRNA that is released atthe end of transcription is called pre-mRNA.

Pre-mRNA undergoes several changesbefore it is exported out of the nucleus toprotect it from the cytoplasmic environment.

The 5 end of the pre-mRNA is capped with amodified form of the G nucleotide. At the 3end, an enzyme in the nucleus adds the poly

A tail, a long series of A nucleotides.


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mRNA Splicing

The entire gene (introns andexons) are transcribed bythe RNA polymerase.

The initial pre-mRNAcontains introns that areremoved from the pre-mRNA by spliceosomeswhile the exons are splicedtogether.

INtrons are cut OUT.


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mRNA Splicing

The removal of intronsmay follow differentpatterns thus producingdifferent proteins.

This accounts for thefact that the bodyproduces over 100,000different proteins eventhought the humangenome only contains30,000 to 35,000 genes


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Translation

After transcription mRNA exits the nucleus vianuclear pores and ribosomes bind to mRNA

Ribosomes synthesize different proteins by

reading the coding sequence on mRNA The mRNA is read in triplets of nucleotides eachof which encodes an aa

Consider the following mRNA sequence:

5 AUGAAUGAGCUGAAC 3


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Transfer RNA

The ribosome alone cannot synthesize the

polypeptide chain

The correct amino acids must be delivered

to the polypeptide building site by tRNA


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Transfer RNA

tRNA look like three-

lobed cloverleaf due

to base pairing

betweencomplementary

nucleotides on

different regions of

each tRNA moleculecausing it to fold


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Transfer RNA

At the end of one lobe oftRNA, a sequence of threebases called the anticodonrecognizes and iscomplementary to the codonof the mRNA.

The anticodon sequence iswritten in the 3 to 5 direction.

At the 3 end of the strand isan attachment site for thecorresponding aa specifiedby the mRNA codon.


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Wobble in the Genetic Code

Although there are 64 possible codoncombinations, the cytoplasm only holds

about 35-45 different tRNAs. This leaves

some anti-codons pairing with more thanone codon creating a more lenient

compliment in the third position.

This is consistent with the redundancy ofamino acid codons in the wobble position

hypothesis


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Aminoacyl-tRNA synthetase

Aa-tRNA (tRNA moleculebound to its particularamino acid) has 2binding sites; one is for a

specific amino acid, theother is specific to aparticular anticodon

When both are in the

enzymes active site theenzyme catalyzes areaction that binds thetwo.


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Ribosomes

Ribosomes are the siteof protein synthesis. A

ribosome is a complex

that contains a cluster of

different kinds of proteinsand rRNA which are

linear strands of RNA

The ribosome has

binding sites for the

mRNA transcript and the

aa-tRNA molecules.


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Ribosomes

Each active ribosome has 3 different

binding sites for tRNA molecules: the P

(peptide) site, which holds one aa-tRNA

and the growing chain of amino acids; theA (acceptor) site, which holds the tRNA

bringing the next amino acid to be added

to the chain; and the E (exit) site, whichreleases the tRNA molecules back into the

cytoplasm.


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The anticodon of an aa-tRNAmolecule binds to the mRNAcodon exposed in the A site.

Enzymes catalyze the formationof a bond between the last aa onthe lengthening polypeptide andthe new aa. The polypeptidechain is transferred from the

tRNA in the P site to the tRNA inthe A site.

The ribosome moves down themRNA strand, shifting the bindingsite a distance of 3 nucleotides (1codon), this is calledtranslocation. A new A site isexposed as the tRNA that was inthe P site is moved to the E siteand released.


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T i i f P i


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Termination of Protein

Synthesis Translocation of the ribosome exposes a

stop codon in the A site. Stop codons donot code for an aa, there are no

corresponding tRNAs. A protein called a release factorbinds to

the exposed A site causing thepolypeptide to separate from theremaining tRNA molecule

Ribosome falls of the mRNA andtranslation stops


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Termination of Protein Synthesis


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Hyperlinks

translation narrated

transcripton & translation 12 protein synthesis overview
http://bcs.whfreeman.com/thelifewire/content/chp12/1202003.htmlhttp://207.207.4.198/pub/flash/26/transmenu_s.swfhttp://72.14.205.104/search?q=cache:g2dcguf4-FoJ:www.wisc-online.com/objects/index.asp?objID=AP1302+wisconsin+online+protein+synthesis&hl=en&ct=clnk&cd=1&gl=cahttp://72.14.205.104/search?q=cache:g2dcguf4-FoJ:www.wisc-online.com/objects/index.asp?objID=AP1302+wisconsin+online+protein+synthesis&hl=en&ct=clnk&cd=1&gl=cahttp://207.207.4.198/pub/flash/26/transmenu_s.swfhttp://bcs.whfreeman.com/thelifewire/content/chp12/1202003.html


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HOMEWORK1. Why do all cells need to perform protein synthesis?

2. Why is it important that DNA never leave the nucleus?3. Differentiate between the terms transcription and

translation. What is the end result of each of theseprocesses and where in the cell do they take place?

4. What amino acids are coded for by each of the followingcodons?

i) UUC ii) ACU iii)GCG iv) UAA

5. Each codon codes for how many amino acids?

6. What codons could code for the amino acid proline (pro) ?

For the amino acid arginine (arg)?


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7. What are the advantages of having 4 different

codons for the amino acid proline?8. A portion of an mRNA molecule has the sequence

CCUAGGCUA. What is the sequence of the

complementary strand of DNA?

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