3 gene expression

8/8/2019 3 Gene Expression

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Instructional objective :

After attending this course,students will be able to describe

the Gene Expression.

3Subject:

Gene Expression


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Sub Subject:

3.1. Regulation of Expression

3.2. Protein Synthesis


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The effect of a gene can bevisually observed for instance in

grape fruit color from green, lilac

to dark purple in color. Theappearance of fruit bunches from

poor, dense to very dense.

All the differences are caused by

different gene expression.

3.1. Regulation of Expression


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dark purple

very dense

bunch

lilac

poor

bunch

green

dense

bunch


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T

he effect of a gene can also bedistinguished from the level

expression intensity. For example

the differences in expression

intensity ofoxgene of rice thatcontrol dwarf characters, stunted,

causing differences in plant height.

The higher the intensity ofoxgene

expression will resulted causinglower rice growth.


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Expression

intensity


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DNA

hnRNA

mRNA

Ribosome

Protein

Transcription

Translation

Phenotypic Expression

nuclear

cytoplasm

RNA processing

Expression take place in nuclear and

cytoplasm


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1. Class I Gene:

Genes of this class makes a largeprecursor to the major rRNA

(ribosome RNA) of 5.8S,and18S,

or 28S rRNA in vertebrates).

Based on their end result, the genes

are grouped into 3 class of:


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2. Class II Gene :

Genes of this class involved in

synthesis of protein that work as an

enzyme in physiological pathway

and responsible to phenotype ofindividual organism. The

transcription product of the gene

named as mRNA (messenger RNA).

It also make most small nuclear

RNAs (snRNAs).


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3. Class III Gene:

Genes of this class involved in

synthesis of protein that bring

amino acid in cytoplasm

ribosome in translation processfor polypeptide chain synthesis.

The transcription product of the

gene named as tRNA (transfer

RNA). Also produce other small

RNAs (5s rRNA, snRNAs)


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Based on expression pattern, the

class II genes could be divided

into two groups of :

1. Constitutive expression genes

2. Specific expression genes


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1. Constitutive expression genes.The genes expressed constitutively on

most tissues or organelles, since the

protein products are necessary for

basic living mechanism, such asrespiration to generate energy for cell

metabolism. The genes also named as

housekeeping genes, and shared

about 10% of entire gene of anorganism.


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2. Specific expression genes.The genes are expressed in specific

tissue or organelle in response to

specific growth and development stage

(germination, flowering, fruiting, andseed development) as well as certain

environment circumstance (biotic and

abiotic stresses). The genes also

named as inducible genes, and sharedabout 90% of entire gene of an

organism.


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Process Expression of a gene that occurs

in the cell nucleus is two stages of:

1.The transcription.

At this stage, the coding strand / positive

strand of DNA was replicated usingcomplementation mechanism with other

strand as a template (negative strand).

The result of this process is hetero-

nuclear RNA (hnRNA)


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2. RNA Processing.

At this stage hnRNA experienced three

processes of, capping at 5 ends;

discarding intron fragment , shielding 3with polyA fragment. The result is RNA

fragment that is ready translated into

amino acid chain, called as messenger

RNA

(mRNA).


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Transcription Steps

a. Template recognition)

b. Initiation

c. Elongation

d.Termination


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Promoter Terminator

Coding Strand

Template Strand+1

Upstream Downstream

a

b

c

d


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a. Template recognition

At this stage the enzyme RNA

polymerase II bind to the double

stranded DNA of the target gene,

and recognizes a promoter, which

lies upstream of the gene.


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1. TATA box (TATAAA, 10 bp);2. CAAT box (GGCCAATCT, 22 bp);

3. GC box ( GGGCGG, 20 bp);

4. Octamer (ATTTGCAT, 20 bp);

5. kB (GGGACTTTCC, 10 bp);

6. ATF (GTGACGT, 20 bp).

There are many types of promoters,

but the most common is a TATA boxlocated 10 bases before the initiation

site(10 base pairs upstream)


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b. Initiation

The polymerase binding causes theunwinding of the DNA double helix.

This is followed by initiation of RNA

synthesis at the start site.After the first nucleotide is in place,

the polymerase joins a second

nucleotide to the first, forming theinitial phosphodiester bond in the

RNA chain


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AT G C

C GTA

T G C ATA

CG

T G

Coding strand

Template strandRNA


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c. RNA Elongation

RNA polymerase directs the sequential

binding of ribonucleotides to the growing

RNA chain in the 5`-3` direction.

Each ribonucleotide is inserted into the

growing RNA strand following the rules of

base complementation. This process is

repeated till the desired RNA

length issynthesized


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d. Termination

Other regions at the end of genes; calledterminators, signal termination. These work in

conjunction with RNA polymerase to loosen the

association between RNA product and DNA

template. The result is that the RNA dissociatefrom RNA polymerase and DNA and so stop

transcription. The product is hnRNA

Termination site

TATA EXON AAUAAINTRON EXON

Transcribed regionStart sitepromoterregulator

ORF UTRUTR

AUG UGA,UAA,UAG

Poly A tailing site


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RNA processing

a. Capping at 5 end of hnRNA

fragment

b. Splicing to remove intron from

the fragmentc. Adding Poly-A fragment to 3 end

(Polyadenylation)

a bc


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a. Capping

Special structure called a cap added to the 5`end. The cap consist of a 7- methylguanosine(m7G) residue linked to transcript by three

phosphate groups. The cap protects the

mRNA from being degraded by enzymes;enhancement of mRNA translatability.


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b. SplicingStep-by-step removal of introns present

in the pre-mRNA and joining of the

remaining exons. The removal of introns

and joining of exons takes place on aspecial structures called spliceosomes.


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c. Polyadenylation

Adding of the poly (A) tail involvescleavage of its 3' end and then theaddition of about 200 adenine residues

to form a poly (A) tail; This completesthe mRNA molecule (mature mRNA),

which is now ready for export to thecytosol for protein synthesis.

.AA AAAA


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b. Splicing

mGppp

Coding Strand

Template Strand

hnRNA Strand

a. Capping

mGppp

Intron ExonExon

c. Polyadenylation

mGppp AAAAAA

mRNA Strand


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Protein Synthesis consist of 4 steps:

1. mRNA transport from nucleus

2. mRN

A attachment to ribosome3. mRNA Translation to make

polypeptide chain with tRNAparticipation

4. Development polypeptide chaininto active protein

3.2. Protein Synthesis


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1. mRNA transport

to initiate translation process the

processed mRNA should be

brought from nucleus to cytoplasm

which protein machine ribosome

located.

nucleus ribosome


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2. mRNA attachment to ribosome

mRNA yang sudah berada dalam

sitoplasma selanjutnya berikatan

dengan ribosome sebagai tahap awal

dari proses sintesis protein (translasi)


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one amino acid encoded by 3

bases, since only 4 bases available

will be resulted 64 (43). However,due to the 3 combinations encode

stop codons, then the remaining 61

combination.

3. mRNA Translation


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There were 20 amino acid thats

composed the protein is 20, and

61 codons available, thereforeseveral amino acid should be

encoded by more than1codon.


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No. 3 letters 1 letter Codon No. 3 letters 1 letter Codon

1 Met M AUG 11 Tyr Y UA C/U

2 Trp W UGG 12 Ile I AU A/C/U

3 Asn N AA C/U 13 Ala A GCI

4 Asp D GA C/U 14 Gly G GCI5 Cys C UG C/U 15 Pro P GCI

6 Glu E GA A/G 16 Thr T ACI

7 Gln Q CA A/G 17 Val V GUI

8 His H CA C/U 18 Arg R A/C GI

9 Lys K AA A/G 19 Leu L C/U UI

10 Phe F UU C/U 20 Ser S A/U G/C I


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Ribosome as protein biosynthesismachine using the mRNA as a

template, the ribosome traverses

each codon of the mRNA, pairing it

with the appropriate amino acid.This is done using molecules of

transfer RNA (tRNA) containing a

complementary anticodon on one

end and the appropriate amino acidon the other.


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The protein synthesis started by polypeptide

chain synthesis that occur in 3 phases:1. Accurate and efficient initiation occurs, the

ribosome binds to the mRNA, and the first amino

acid of Methionine attached to its tRNA. To make

M-tRNA for initiating polypeptide chain synthesis

2. Chain elongation, the ribosome adds one amino

acid at a time to the growing polypeptide chain .

3. Accurate and efficient termination, the ribosomereleases the mRNA and the polypeptide after

reach the first stop codon in the mRNA

fragment.


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M-tRNA

UACAUG UGA, UAA, UAG

UTR UTRORF


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MWNDCEQHKFYIAGPTVRLS

Since start codon is Methionine

(M), the polypeptide chainalways started by Methionine,

and called as Nitrogen end. The

end of polypeptide chain namedas carboxyl end


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4. Protein Development

The produced polypeptide chain, willbe used as precursor to construct anenzyme or together with other chain

composed active enzyme.Starting from enzyme synthesis, thesubsequent physiological pathwaywill occurs and responsible to

determine plant phenotype

3 gene expression

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