Gene Expression Diana

7/27/2019 Gene Expression Diana

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GENE EXPRESSION


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GENE EXPRESSION Process by which information from a gene is used in the

synthesis of a functional gene product

Gene products include; proteins, rRNA, tRNA or SnRNAgenes

Gene expression is virtually the same in all eukaryotes

Differences with prokaryotes


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Termination

No strong termination sequences like prokaryotes

RNA polymerase II continues transcribing up to

1000 to 2000 nucleotides beyond where the 3'

end of the mature mRNA will be.


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iff b i i i


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Differences between transcription in

plants and animals Animal genes are large while plant genes tend to

be much smaller (1-2 Kb)

Animal genes have numerous large introns while

plants have fewer and smaller introns

Plant transcripts retain introns more often than doanimal transcripts (30% of all genes in the model

plant, Arabidopsis, compared to 10% in humans)


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DIFFERENCES WITH PROKARYOTES Occurs in the cytoplasm

RNA pol is made up of 5 subunits; 2, , 1&

Sigma subunit recognise the promoter seq

Termination involves formation of the hair pin loopstructure

Release of the mRNA is either dependent or independenton the rho protein

No modification of the new transcript


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TRANSLATION IN EUKARYOTES Conversion of the information of mRNA into proteins

mRNA produced by transcription is decoded by the

ribosome to produce a specific amino acid chain

Occurs in the ribosomes

Ribosome facilitates decoding by inducing the binding of

tRNAs with complementary anticodon sequences to that of

the mRNA

The tRNAs carry specific amino acids that are chained

together into a polypeptide as the mRNA passes through
http://en.wikipedia.org/wiki/Ribosome


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Activation

The correct amino acid is covalently bonded to thecorrect tRNA

The amino acid is joined by its COOH to the 3' OH ofthe tRNA by a peptide bond

Charged tRNA has an amino acid linked to it


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Structure of tRNA

Clover shaped

Has various arms

Aligns each amino acid with the

corresponding codon

3 end has the 5- CCA sequence to

which aa are linked

The opposite end contains theanticodon loop

Contains modified bases


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Initiation Requires hydrolysis of ATP and GTP

Involves the interaction of proteins with a special tag bound to the 5' capfor recognition

Results in formation of a complex containing the mRNA, the ribosome andthe initiator Met-tR

The initiation complex bound to the 5 cap structure scans in a 5 to 3direction until initiating AUG is encountered

Initiation complex binds upstream of initiation codon


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Enlogation


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Termination

Translation is terminated

at one of three stop codons(UAA, UAG & UGA).

Termination codon at the

A site is recognized by the

release factor instead of atRNA

The release factor binds

the termination codon

The peptide chain is then

released followed by

dissociation of the tRNA

and the ribosome


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Differences in prokaryotes Translation begins before transcription is complete

Translation in prokaryotes is polycistronic

No post translation modifications


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IN EUKARYOTES Involves chemical modification of a protein after

translation

Can be done through changing the chemical nature of an

amino acid or structural changes

Amino acids can be removed from the amino end of the

protein, or cut the peptide chain in the middle

This extends the range of functions through attachment

to other biochemical functional groups e.g acetate,

phosphate, various lipids & carbohydrates,

GENETIC CODE


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GENETIC CODE Consists of 64 triplets of nucleotides called codons

Codon; a seq of 3 bases that specify an amino acid,

a start or stop signal


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Xtics of the Genetic Codon Code word

Colinearity

Start codon (AUG) Stop signal(UAA, UAG, UGA)

Degeneracy

Universality

CODON BIAS Except 2 of the amino acids (Met and Trp) can be encoded by

from 2 to 6 diff codons

However certain codons are preferred over others.

In man, alanine is encoded by GCC four times as often as byGCG

This reflects great translation efficiency by the translationapparatus for certain codons over their synonyms


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Regulation of gene expression Refers to the control of the amnt & timing of

appearance of the functional product of a gene

Vital as it allows a cell to produce only the gene

prdts it requires

This gives the cell control over its structure &

function thus basis for cellular differentiation &morphogenesis


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Terms used to describe genes

House keeping gene

Is a gene that is transcribed continually also calledconstitutive gene, eg actin

Facultative geneIs a gene which is only transcribed when needed

Inducible geneIs a gene whose expression is either responsive toenvironmental change or dependent on the position in thecell cycle.

R l ti f i


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Regulation of gene expression

Control occurs on many levels of gene expression

R l ti f t i ti


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Regulation of transcriptionControls the no. of mRNA transcribed

Modulators of transcription; Activators -enhance the interaction between RNA pol & a promoter

Enhancers -sites on the DNA helix that are bound to by activators soas to loop the DNA bringing a specific promoter to the initiation

complex

Specificity factors - alter the specificity of RNA pol making it more orless likely to bind e.g sigma factor

Repressors (silencers) - bind to non-coding sequences to thepromoter region impeding RNA pol progress along the strand,

General transcription factors -position RNA pol at the start of a startsite & then release it to transcribe the mRNA

P t t i ti l ti


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Post transcription regulation Addition of a cap and tail to the RNA identifies the mRNA

by the ribosomes & prevents degradation

Splicing can determine whether mRNA gets translated, If

not processed it is not transported out of the nucleus thus

not translated

Exon shuffling where some exons are exchanged changing

the protein produced

Translation Control


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Translation Control Controlling the no. of ribosomes allowed to attach a single

mRNA

Controlling the rate at which each ribosome transcribes amessage.

Use of inhibitory proteins that prevent the translation of

mRNA

Regulation of RNA Longevity

Lifespan of mRNA determines no. of times it can be usedto create proteins

Eukaryotic mRNA more stable than for prokaryotes

AUUUA is a signal for early degradation


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SIGNAL TRANSLATIONPurpose;

Regulate cell growth & differentation Co-ordinate diff phjsiological processes

Maintain homoestasis

Forms of translation

Use of molecular messengers (hormones &

paracines)

Use of non-diffusible cell adhesion proteins


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Molecular messengersHormones

Are molecules released by endocrine glands intocirculation to act on specific target cells widely distributed

3 categories; steroids, Proteinaceous & amino acid related

Steroids; Insoluble bt can freely cross the plasma membrane

Binds to specific recptors forming a complex

Complex binds to speciic regions on DNA to regulate

transcription


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Proteinaceous & aminoacid related Are hydrophillic

Cannot cross the plasma membrane thus interactwith surface receptors

Induces a 2o messenger which results in reguation

of transcriptionParacrines

Hv local action & restricted to neighboring cells

4 categories; mitogens, trophic factors, chemo-attractants & pleiotrophic

Can be hydrophilic or hydrophobic


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Non-diffusible adhesion molecules

Activate physical cell to cell matrix interaction

regulating gene transcription

Si l T l ti i Pl t


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Signal Translation in Plants Plants have evolved different signaling mechanisms

as their hormones function differently from in

animals

Plants also cope with environmental changes

differently as they dont physically escape except

possibly through reproduction.

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