chapter 6 expression

8/3/2019 Chapter 6 Expression

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EXPRESSION OF BIOLOGICAL

INFORMATION

6.1DNA and genetic information

6.2DNA replication

6.3Protein Synthesis: Transcription and Translation

6.4Gene regulation and expressionLac operon


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DNA and genetic Information

1.DNA as a carrier of genetic information

Frederick Griffith(1931),

Avery et.al. (1944) andHershey-Chase (1952)

2.Gene concept : One gene one polypeptide

Beadle and Tatum (1941)

DNA Replication

1.Semi-conservative replication of DNA

Meselson and Stahl (1950).

Protein Synthesis1.Transcription

2.Translation

Gene regulation and expression-Lactose Operon

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Fig. 4.1 (b) The double helixThe helix isright-handed, curving up to right. The two strands are held together by hydrogen

bonds (dotted lines) between the nitrogenous bases, which are paired in the interior of the

double helix.11


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12

DNA

DNA molecule structure

5`

oP

oP

oP

oP

P

o

Po

P

o

P

o

C

T

G

A

A

T

G

C

5` 3`

3` Hydrogen bond

phosphodiester

linkageHydrogen

bond


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DNA AS THE CARRIER OF GENETICINFORMATION

DNA can be able to:

store information

replicate, in order to be in each cell of growing organism

control expression of traits

Encode the sequence of proteins

Change in a controlled away, in order to ensure survival of aspecies in a changing environment

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There are experiments that were conducted to prove that DNA is the genetic

material.

1. Frederick Griffith Experiment (1931)Hereditary Information Can Pass between Organisms

2. Avery, MacLeod & McCarty Experiment (Avery et.al.(1944)

Find out that the transforming agent is the DNA

3. Beadle and Tatum Experiment (1941)

Gene Concept : One gene One polypeptide

4.Hershey and Chase experiment (1952)

to confirm that DNA was the genetic material by using bacteriophages.

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http://../wiki/DNAhttp://../wiki/Genetic_materialhttp://../wiki/Genetic_materialhttp://../wiki/DNA


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1. The F.Griffith Experiment

Discovery ofTransformation

Griffith studied two strains of the bacterium Streptococcus

pneumoniae (known as pneumococcus)


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Background:

Streptococcus pneumoniae or pneumococcus, is a nasty little bacteria which,when injected into mice, will cause pneumonia and death in the mouse.

The bacteria contains a capsular polysaccharide on its surface whichprotects the bacteria from host defences.

Occasionally, variants (mutants) of the bacteria arise which have a defect inthe production of the capsular polysaccharide.

The mutants have two characteristics:

1) They are a virulent, meaning that without proper capsularpolysaccharide they are unable to mount an infection in the host

( they are destroyed by the host defences) and

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2) Due to the lack of capsular polysaccharide the surface of themutant bacteria appears rough under the microscope and can be

distinguished from the wild type bacteria ( whose surface appears

smooth).

Wild type, S strain

(Smooth, virulent)

Mutant type, R strain

(Rough, virulent)

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Observation :


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Conclusion:

He concluded that some chemical component from the dead Scells had genetically transformed living R bacteria into S bacteria,

although the identity of the substance was not known.

SOTransformation occurred !!!

Question:

Was the transforming agent protein or DNA, or

what?


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O.T.Avery (1944)

Background:

The experiment of Griffith could not be taken further untilmethods were developed to separate and purify DNA and proteincellular components. Avery utilized methods to extract relatively

pure DNA from pneumococus to determine whether it was thetransforming agent observed in Griffiths experiments.

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-PROVE THAT THE TRANSFORMING AGENT

IS DNA

- USE PURIFIED DNA FROM TYPES S

(SMOOTH) AND WAS TREATED WITH:

* DNASE : BREAK DOWN DNA

* RNASE : DEGRADES RNA* PROTEASE : DEGRADES PROTEIN

The Avery et. al. Experiment


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A more sophisticated experiment:

Purified type I DNA was divided into three aliquots.

1.Type I DNA + RNAase + mutant (rough) type II + mouse = deadmouse.

1.Type I DNA + Protease + mutant (rough) type II + mouse =

dead mouse

1.Type I DNA + DNAse + mutant (rough) type II + mouse =live mouse

Conclusion:

The work of Avery provided strong evidence that thetransforming agent was in fact DNA (and NOT protein or

RNA).

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BEADLE AND TATUM EXPERIMENT

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George Beadle and Edward Tatum during the late 1930sand early 1940s established the connection between

genes and metabolism.


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Gene concept;

ONE GENE ONE POLYPEPTIDE

Beadle and Tatum proposed the One Gene One Enzymehypothesis" for which they won the Nobel Prize in 1958.

Since the chemical reactions occurring in the body are mediatedby enzymes, and since enzymes are proteins and thus heritable

traits, there must be a relationship between the gene and proteins.

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They used Neurospora crassa as an experimental organism. It

had a short life-cycle and was easily grown.

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Neurospora can be grown on a minimal medium, and it's

nutrition could be studied by its ability to metabolize sugars and

other chemicals.

It was able to synthesize all of the amino acids and other

chemicals needed for it to grow, thus mutants in synthetic

pathways would easily show up.31


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Experiment :

Procedures

1.They used X-rays to induce mutations in Neurospora,2.The mutated spores were placed on growth medium

enriched with all essential amino acids.

3.Crossing the mutated fungi with non-mutated forms

produced spores which were then grown on mediumsupplying only one of the 20 essential amino acids.

Result :If a spore lacked the ability to synthesize a particular amino

acid, such as arg (arginine), it would only grow if the argininewas in the growth medium.

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Biosynthesis of amino acids (the building blocks of proteins) is acomplex process with many chemical reactions mediated by

enzymes, which if mutated would shut down the pathway, resulting

in NO-growth.

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Beadle and Tatum proposed the "one gene one

enzyme" theory.

One genecodes for the production ofone protein.

"One gene one enzyme" has since been modified to "onegene one polypeptide" since many proteins (such ashemoglobin) are made of more than one polypeptide

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The mold uses a multi step pathway to synthesize the amino acid

arginine from a precursor.

Beadle and Tatum identified three classes of mutants unable to

synthesize arginine.

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Each mutant class had a metabolic block (X in this diagram)

at a different step in the pathway

For example class II mutants failed to grow on minimal medium or minimal


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For example, class II mutants failed to grow on minimal medium or minimalmedium supplemented with ornithine.

Adding either citrulline or arginine to the nutritional medium enabled thesemutants to grow.

Beadle and Tatum deduced that class II mutants lacked the enzyme thatconverts ornithine to citrulline.

Adding citrulline to the medium bypasses the metabolic block and allows the

mold to survive

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The other classes of mutants lacked different enzymes


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The other classes of mutants lacked different enzymes.

Beadle and Tatum concluded that various mutations were

abnormal variations of different genes, each gene dictating theproduction of one enzyme; hence, the one gene-one enzyme

hypothesis.

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CONCLUSION

Beadle and Tatum concluded that various mutations were abnormalvariations of different genes.

Each gene dictating the production of one enzyme; hence, the one

gene-one enzyme hypothesis.

One gene codes for the production of one protein. "One gene one

enzyme" has been modified to "one gene one polypeptide" sincemany proteins (such as hemoglobin) are made of more than one

polypeptide

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DNA REPLICATION

1.DNA replication is Semiconservative.

2.Meselson and Stahl Experiment.

3.Unwound of DNA Strands

4.DNA Synthesis always proceeds in a 5---- 3

direction

(ii) DNA replication process.

(i) About DNA

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1. DNA Replication is Semiconservative

DNA could be precisely copied, a process known as DNAreplication

Watson and Crick Model of DNA suggest that each strand of DNA

molecule could serve as a template, for the synthesis of the opposite

strandEach half-helix could pair with their complementary nucleotides

to replace its missing partner

The result would be two DNA double helices, each indentical to the

original

This type of information copying is known as Semiconservative

Replication mechanism50


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Other kinds of information copying are 1.Conservative

replication and

2.Dispersive replication

Conservative replication mechanisme, both parent (or

old) strand remain together and would form second

double helix

Dispersive replication mechanisme, the parental and

newly synthesized strands might become randomlymixed during replication process, which form

intermediate DNA strand52


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2. Meselson and Stahl Experiment

In 1957, Meselson and Stahl grew cells ofEscheria coli on amedium that contained heavy nitrogen-15 (15N) in the form ofammonium chloride (NH4Cl)

The cell used the 15N (parent) to synthesize bases, whichthen became incorporated into DNA

The resulting 15N-containing DNA molecules were extractedfrom some of the cells

When they were subjected to density gradient centrifugation,they accumulated in the high-density region of the gradient

(generation 0)54


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The bacteria contained nitrogen-15 DNA were transferred to thelighter nitrogen-14 isotope, and

allowed to grow

DNA from cells of generation 1had an intermediate density called

hybrid DNA , indicating thatthey contained half as many

nitrogen-15 as the parent DNA

This finding support thesemiconservative model

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Intermediatedensity

parent

New strand


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L/H

L/HL/L

L/LL/H

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After another cycle of cell division in the lighter 14N, generation 2produce two type of DNA strand.

1.hybrid DNA helices (15N and 14N strand),

2. lighter DNA strands (14N)

This finding refused the Dispersive model, which predicted that allstrand should have intermediate density

In generation 2, with further division in 14N, more lighter DNAwere produced but the hybrid DNA remain the same.

This indicates that, replication of DNA strands by mean ofsemiconservative mechanisme.

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DNA REPLICATION

1.DNA replication is Semiconservative.

2.Meselson and Stahl Experiment.

3.Unwound of DNA Strands

4.DNA Synthesis always proceeds in a 5---- 3

direction

(ii) DNA replication process.

(i) About DNA

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3. Unwound of DNA Strands

Separating the two strand of DNA is accomplished byDNA helicase enzymes that travel along the helix.

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O th h li t d H li d t bili i


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Once the helix are separated, Helix-destabilizingproteins bind to single DNA strand, preventing re-

formation of the double helix until strands are copied.

Enzymes called Topoisomeraseproduce to prevent theformation of knots during replication


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4. DNA Synthesis always proceeds in a 5 ----- 3

direction

The enzymes that catalyze the linking together of the nucleotides

subunits are called DNA polymerase.

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They are able to add nucleotides only to the 3 end of the growing

polynucleotides strand.

This strand must be paired with the strand being copied.67


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Nucleotides with three phosphate (Nucleosidetriphosphate) groups are used as substrates

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(II) DNA REPLICATIONPROCESS

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The replication process involve;

1.Opening up the DNA double helix.

2.Building a primer

3.Assembling complementary strands.

4.Removing the primer

5.Joining the Okazaki fragments.

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1. OPENING UP THE DNA DOUBLE HELIX.

unwinding enzyme called helicase unwinds the DNA double

helix.

The strands separate as hydrogen bonds are broken.

Two replication forks form and proceed to separate the strands in

both directions.

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2. Building a primer

RNA primer is first synthesised at the point of initiation ofreplication

RNA primer is synthesized by specialized RNA polymerase

calledprimase in a multisubunit complex known as primosome

After a few nucleotides have been added, the primosome is

displaced by DNA polymerase (III), which add subunit to the 3

end of the short RNA primer.

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III


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3. Assembling complementary strands

DNA polymerases (III) can only add nucleotides to the free 3end of a growing DNA strand.

A new DNA strand can only elongate in the 5->3 direction.

DNA replication is continous (leading strand) in one strand and

discontinous (lagging strand) in the other.

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III


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At the replication fork, one parental strand (3-> 5), the

leading strand, can be used by polymerases as a template for acontinuous complimentary strand.

The other parental strand (5-> 3), the lagging strand, iscopied away from the fork in short segments (Okazaki

fragments)

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To summarize at the replication fork the leading stand


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To summarize, at the replication fork, the leading standis copied continuously into the fork from a singleprimer.

The lagging strand is copied awayfrom the fork in short segments,each requiring a new primer.

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4. Removing the primer

Another DNA polymerase (I) later replaces the primerribonucleotides with deoxyribonucleotides complimentary to the

template.

DNA polymerase (I) removes the RNA primer and fills in thegap, as well as any gaps between Okazaki fragment.

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5. Joining the Okazaki fragments

Finally, the gaps in the sugar-phosphate backbone are sealed

with DNA ligase forms the lacking phosphaseester bond.

Now, there are 2 identical double helices of DNA.

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PROTEIN SYNTHESIS

1.TRANSCRIPTIONDNA mRNA (nucleus)

2.TRANSLATIONmRNA Protein (cytoplasm)

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Protein synthesis is the process in which cells build proteins.

Importance in production of hormones and enzymes.


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1 TRANSCRIPTION


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1. TRANSCRIPTION

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1 TRANSCRIPTION


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1. TRANSCRIPTION

Definition of transcription : DNAdirected synthesis ofRNA.

Transcription also the process by which genetic informationcontained in DNA is transcribed or copied to an RNA molecule.

The information that has been transcribed to mRNA can then betranslated and thereby expressed by the formation of specificprotein.

mRNA (messenger RNA), the carrier of information from DNA

to protein synthesizing machinery, is transcribed from templatestrand of gene.

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RNA l l l i i 5 3 di i


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RNA molecule elongate in its 53 direction.

Specific sequences of nucleotides along the DNA mark the

initiation (transcription of gene begin) and termination site

(transcription of gene ends).

The entire stretch of DNA that is transcribed into a single RNAmolecule is called a transcription unit.

There are three key steps in transcription.

1.Initiation2.Elongation

3.Termination

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1 Initiation


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1.Initiation

RNA polymerasebind to regions of DNA called promoters.

It includes the initiation site, where the transcription begins.

Certain regions within the promoter are important for

recognition by RNA polymerase.

Once active RNA polymerase is bound to a promoter region,the enzyme begins to separate the two DNA strands at the

initiation site and transcription is under way.

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1.Elongation


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g

As RNA polymerase moves along the

DNA, it untwists one turn of the doublehelix at a time, separating the strands and

exposing about ten DNA bases for pairing

with RNA nucleotides.

The enzyme adds nucleotides to the 3end of the growing RNA molecule as it

continues along the double helix.

Transcription progresses at the rate of

about 60 nucleotides per second.

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A single gene can be transcribed simultaneously by severalmolecules of RNA polymerase.

The congregation of many polymerase moleculessimultaneously transcribing a single gene increases the number

of mRNA molecules and allows a cell to produce a particular

protein in large amounts.

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1.Termination

Transcription process proceeds until the RNA polymerasereaches a termination siteon the DNA.

The sequence of nitrogenous bases that marks this site signals RNA

polymerase to stop adding nucleotides to the RNA strand and release

the RNA molecule.

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Extra notes


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RNA processing (Pre-mRNA process)

- Occur in eukaryotes only.- mRNA is modified before move out tothe cytoplasm.

- RNA splicing : - Removal of Introns,

- Joining theExons.-Addition of 5 cap at the 5end, andpoly (A) tail at the 3end.

Gene has exon & intron :-Exon = Coding sequence.

= Sequence bases that encodesa protein,

Intron = Non-coding sequence.

Genetic code


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Genetic code : Base triplet in DNA provides a template forordering the complementary triplet in mRNA molecule. Every

base triplet is amino acid.

Three bases of an mRNA codon are designated as first,second and third bases.

There are only four nucleotide to specify 20 amino acids;A-adenine, C-cytosine, G-guanine, T-thymine (unique to DNA),

U-uracil (unique to RNA) [pyrimidine, very similar to thymine].

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Fl f i f ti


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Flow of informationfrom gene to protein

is based on triplet

code.

A cell cannot

directly translate agenes base tripletsinto amino acids.

An mRNA molecule is complementary rather than


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identical to its DNA template according to base-pairingrules.

A-U, T-A, C-G, G-C

mRNA base triplets are called codons.

Noticed that U only can be found on mRNA strand,substitute for T (only on DNA strand).U on mRNA pairs with A ,

T on DNA strand pairs with A .

Two important codons in protein synthesis are1.initiation codon (start signal) and

2.termiation codon (stop signal).102

Initiation codon (start codon)


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Initiation codon (start codon)

Codon AUG is a starter to the process of translation.

Codon AUG has dual function,

as a start signal or also called initiation codon and

it also code for amino acid methionine.

Since AUG code for methionine, polypeptide chainsbegin with methionine when they are synthesized.

However, an enzyme may subsequently remove starteramino acid from chain.

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Termination codon (stop codon)


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Termination codon (stop codon)

Three triplet bases ofstop signal: UAA, UAG, UGA.

Stop signal marking the end of a genetic code.

Any one of these termination codons marks the end of a

genetic message, and the completed polypeptide chain is

released from the ribosome

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2 TRANSLATION


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2. TRANSLATION

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2 TRANSLATION


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2. TRANSLATION

Definition: RNAdirected synthesis of polypeptide.

In translation process, a cell interprets a genetic massage andbuilds a protein accordingly.

The massage is a series ofcodons along an mRNA molecule andinterpreter is transfer RNA

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tRNA(transfer RNA) = It function is to transfer amino acidf l i id l ib


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from cytoplasms amino acid pool to ribosome

Molecule of tRNA are not all identical.

Each type of tRNA molecule associates a particular mRNAcodon with a particular amino acid.

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As a tRNA molecule arrives at a ribosome, it bears a specific

amino acid at one of its ends. At the other end is a base triplet

called the anticodon.

For example : mRNA codon AUG has UAC as its anticodon.

U A C

A U G

mRNA

tRNA

Codon

Anticodon

MET

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The structure and functions of RNA


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tRNA molecules are transcribed from

DNA templates within the nucleus of a

eukaryotic cell.

tRNA travel from nucleus to cytoplasm

(where translation occurs).

tRNA can be used repeatedly.

tRNA molecule consists of a single RNA

strand (about 80 nucleotides long)

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F ti i ki d i t d


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Function : picking up designatedamino acid in cytosol, depositing it at

ribosome and then leaving theribosome to pick another load.

Structure of tRNA molecule fits itsfunction as a shuttle for specific

amino acids.

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2 Aminoacyl-tRNA synthetase


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2. Aminoacyl-tRNA synthetase

Codon-anticodon bonding is actually the second of two recognitionsteps required for the translation of a genetic massage.

tRNA that binds to an mRNA codon specifying a particular amino

acid must carry only that amino acid to the ribosome.

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Each amino acid is matched


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with the correct tRNA by aspecific enzyme called

aminoacyl-tRNA synthetase.

The active site of each type ofamino acyl-tRNA synthetase

fits only a specific combination

of amino acid and tRNA.

The resulting amino acid-tRNA complex is released from

the enzyme and delivers itsamino acid to a growing

polypeptide chain on aribosome. 112

3. RIBOSOMES


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Facilitate the specific coupling of tRNAanticodons with mRNA codons during protein

synthesis.

Ribosome is made up of two subunits; thelarge and small subunits.

In eukaryotes, ribosomal subunits areconstructed in the nucleolus and exported via

nuclear pores to the cytoplasm.

A large and small subunit join to form a

functional ribosome only when they attach tomRNA molecule.

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Each ribosomal subunit is an aggregate of numerous proteins and


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Each ribosomal subunit is an aggregate of numerous proteins andanother form of specialized RNA called ribosomal RNA (rRNA).

Ribosome of prokaryotes and eukaryotes are very similar instructure and function.

The structure of a ribosome reflects its function of bringing

mRNA together with amino acid, bearing tRNAs.

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RIBOSOME HAS TWO BINDING SITE FORMRNA:


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P siteHolds the tRNA carrying the growing

polypeptide chain.

A siteHolds the tRNA carrying the next amino acid

to be added to the chain.

Ribosome hold the tRNA and mRNA

molecule close together and catalyzes theaddition of amino acid to the carboxy end ofthe growing polypeptide chain.

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3 steps of translation process; synthesis of a polypeptide chain :


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1.Initiation

Bring together mRNA, a tRNA bearing the first amino acid ofpolypeptide and two subunits of ribosome.

Firstly, small ribosomal subunit binds to both mRNA and special

initiator tRNA.

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119

Small ribosomal subunitattaches to a specific

sequence of nucleotides at5end (upstream) of mRNA.

Downstream from thisloading site is the initiationcodon, AUG (start codon)


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Initiator tRNA which carries the


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Initiator tRNA, which carries the

amino acid methionine, attaches to the

initiation codon.

The union of mRNA, initiator tRNA

and small ribosomal subunit is

followed by the attachment of a large

ribosomal subunit to form a functional

ribosome.

Initiator tRNA site in the P site of

ribosome and vacant A site is ready for

next tRNA.

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2. Elongation


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g

Amino acid are added one by one to the initial amino acid. Each

addition, occurs in 3 steps cycle.

i. Codon recognition

ii. Peptide bond formationiii. Translocation

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i Codon recognition


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i. Codon recognition

mRNA codon inA

site forms hydrogen bonds with theanticodon of an incoming molecule of tRNA carrying itsappropriate amino acid.

Elongation factors (which involves the participation of

several protein) ushers the tRNA into A site.

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12

ii.Peptide bond formation


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A component of the large ribosomal subunit catalyzes theformation of peptide bond between polypeptide extending from

P site and newly arrived amino acid in Asite.

Polypeptide separate from tRNA to which it was bond and is

transferred to amino acid carried by the tRNA in A site.

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1 21 2

Peptide bond

iii. Translocation


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tRNA in P site dissociates from ribosome.

tRNA in A site now attached to the growing polypeptide,is translocated to the P site

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2


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Peptide bond

1

1

1

12

2

2

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as the tRNA changes sites, its anticodon remains hydrogenb d d t th RNA d ll i th RNA d tRNA


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bonded to the mRNA codon, allowing the mRNA and tRNAmolecules to move as a unit.

This movement, in turn, brings the next codon to be translatedinto Asite.

mRNA is moved through ribosome in 5 to 3 direction only or

perhaps it is the ribosome that moves.

Ribosome and mRNA move relatively to each other,unidirectionally, codon by codon.

Elongation cycle takes only about 60 miliseconds and is repeatedas each amino acid is added to the chain until the polypeptide is

completed.127

3. Termination


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Elongation continues until a termination codon reaches A site.

Termination codons do not code for amino acids but instead act assignals to stop translation.

Termination base triplets also called stop signals are UAA,UAG, UGA

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Protein called release factor binds directly to the termination codon in A site.


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Release factor causes the ribosome to add a water molecule instead of an aminoacid to the polypeptide chain.

This reaction hydrolyzes the completed polypeptide from the tRNA that is in P site,thereby freeing the polypeptide from the ribosome.

The ribosome then separates into its small and large subunits.

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Typically a single mRNA is used to make many copies of a polypeptide

simultaneously.


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simultaneously.

Multiple ribosomes, polyribosomes, may trail along the same mRNA.

A ribosome requires less than a minute to translate an average-sized mRNA

into a polypeptide.

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LACTOSE OPERON


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Introduction..

The first control system for enzyme production worked out at themolecular level described the control of enzymes that are produced

in response to the presence of the sugar lactose inE. coli cell.

The work was performed byJacob and Monodfor which theywere awarded the Nobel Prize.

E.coli cell living in the erratic environment of a human colon,dependent for its nutrients on the whimsical eating habits of its host.

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The bacteria can absorb the lactose and break it downf i f i b f


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for energy or use it as a source of organic carbon forsynthesizing other compounds.

Lactose metabolism begins with hydrolysis of thedisaccharide into its two component monosaccharides,

glucose and galactose.

The enzyme that catalyzes this reaction is called -galactosidase.

Lactose -----------------> Glucose + Galactose-galactosidase

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Operon- a cluster ofstructural genes that ared d th i i t d t d


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expressed as a group and their associated promoter andoperator

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Control Circuit for the lac Operon

I P O | Z | Y | A


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Controlling || Stuctural genesRegion

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This entire transcription unit is under the command of a singleoperator and promoter


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operator and promoter.

The regulatory (lac I) gene, located outside the operon, codes foran allosteric repressor protein that can switch off the lac operon by

binding to the operator.

The lac repressoris binding to the operator and switching the lacoperon off.

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Several proteins involved in lactose metabolism in theE.coli cell They are:


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coli cell. They are:

a) -galactosidase - converts lactose into glucoseand galactose

b) -galactoside permease- transports lactose intothe cell

c) -galactoside transacetylase- function unknown

The gene for B-galactosidase is part of an operon, the lac

operon ( lac for lactose metabolism), that includes twoother genes coding for proteins that function in lactosemetabolism.

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In this case, a specific small molecule, called an


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inducer, inactivates the repressor.

For the lac operon, the inducer is allolactose, an

isomer of lactose formed in small amounts from

lactose that enters the cell

In the absence of lactose (and hence allolactose), the

lac repressor is in its active configuration, and the

genes of the lac operon are silent.

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1

23

4

138

Wi h l i h ll h


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Without lactose in the cell, the repressor

protein binds to the operator and prevents theread through of RNA polymerase into the three

structural genes.

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If lactose is added to the cells nutrient medium


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If lactose is added to the cell s nutrient medium,

allolactose binds to the lac repressor and alters its

confirmation, nullifying the repressors ability to

attach to the operator.

Now, on demand, the lac operon produces mRNA for

the enzymes of the lactose pathway.

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In the context of gene regulation, these enzymes are referred to as

inducible enzymes because their synthesis is induced by a chemical


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inducible enzymes, because their synthesis is induced by a chemical

signal (allolactose).

Inducible enzymes usually function in catabolic pathways, which

break a nutrient down to simpler molecules. By producing the

appropriate enzymes only when the nutrient is available, the cell

avoids making proteins that have nothing to do.

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With lactose in the cell, lactose binds to the repressor.


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With lactose in the cell, lactose binds to the repressor.This causes a structural change in the repressor and it

loses its affinity for the operator.

Thus RNA polymerase can then bind to the promoterand transcribe the structural genes. In this system, lactose

acts as an effector molecule.

Effector molecule - a molecule that interacts with therepressor and affects the affinity of the repressor for the

operator

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REMOVAL OF INTRONS DURING RNA PROCESSINGIN EUKARYOTES


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Intron (non-coding region)Exon (coding region)

-Eukaryotic cells process mRNA within nucleus bycutting out introns (non coding portions of DNA).

- such alternative splicing of transcripts enables asingle gene to code for various polypeptides.

exon exon exon

exon exon exon exon

intron intron

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EXON EXON EXON

chapter 6 expression

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