gene expression [Autosaved].pptx

7/29/2019 gene expression [Autosaved].pptx

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Kevin White

Present

Moham


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Whats a gene?

gene (jn)n.

A hereditary unit consisting of a sequence of DNA

that occupies a specific location on a chromosome

and determines a particular characteristic in an

organism. Genes undergo mutation when their

DNA sequence changes.

[German Gen, from gen-, begetting, in Greek words

(such as genos, race, offspring).


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And so the gene is

A segment of DNA

It is inherited

It is transcribed

It may be translated

It has regulatory andother functional regions

it has certain coordinateand sequence

genes in different organimay be homologous

its sequence may beoptimized for transcrip

its sequence may beoptimized for translatio

the structure and sequeof these regions may havsomething in common


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Where are the genes located?

Genes are located on the

chromosomes.

Every species has a differentnumber of chromosomes.

There are two types ofchromosomes: autosomes and

sex chromosomes


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Genes are located on the chromosomes which arefound in the nucleus of a cell.

When a cell is undergoing cell reproduction, the

chromosomes are visible. Chromosomes appearwhen the chromatin condenses and become visible

Most of the time (90%) the genetic material in theform of chromatin.

A genome is the complete genetic information

contained in an individual.

(gene + chromosome)


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Gene Expression

Gene expressionis the process bywhich a genes information is converted intothe structures and functions of a cell by aprocess of producing a biologically functionalmolecule of either protein or RNA (geneproduct) is made.

Gene expression is assumed to becontrolled at various points in the sequenceleading to protein synthesis.


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Gene Structure

Eukaryotic gene structure: Mosteukaryotic genes in contrast to typicalbacterial genes, the coding sequence(exons) are interrupted by noncodingDNA (introns).

The gene must have

( Exon; start signals; stop sregulatory control elemen


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The Central Dogma

Proposed by Francis Crick 1958

DNA holds the coded hereditary information in thenucleus

This code is expressed at the ribosome during protesynthesis in the cytoplasm

The protein produced by the genetic information is

what is influenced by natural selection If a protein is modified it cannot influence the gene

that codes for it

Therefore there is one way flow of information:

DNA RNA Protein


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The central dogma of molecular biology


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Overview of Gene ExpressionAn organism may contain many types of somatic cells, each with distinct shape and function. How

they all have the same genome. The genes in a genome do not have any effect on cellular functio

they are "expressed". Different types of cells express different sets of genes, thereby exhibiting

shapes and functions.

Figure. Essential

steps involved in the

expression of protein

genes.

Gene expression" means the

production of a protein or a fun

RNA from its gene. Several ste

required:

Transcription: A DNA strand is

the template to synthesize a RN

strand, which is called the prim

transcript.

RNA processing: This step invo

modifications of the primary tra

to generate a mature mRNA (fo

protein genes) or a functional t

rRNA.

For RNA genes (tRNA and rRNA

expression is complete after a

functional tRNA or rRNA is

generated. However, protein g

require additional steps:

Nuclear transport: mRNA has t

transported from the nucleus to

cytoplasm for protein synthesis

Protein synthesis: In the cytop

mRNA binds to ribosomes, whic

synthesize a polypeptide based

sequence of mRNA.


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TRANSCRIPTION

TRANSLATION

DNA

mRNA

Ribosome

Polypeptide

(a) Bacterial cell

Nuclearenvelope

TRANSCRIPTION

RNA PROCESSINGPre-mRNA

DNA

mRNA

TRANSLATION Ribosome

Polypeptide

(b) Eukaryotic cell


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Transcription


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Transcription: The synthesis of a strand of mR(and other RNAs)

Uses an enzyme RNA polymerase Proceeds in the same direction as replication (5 to 3

Forms a complementary strand of mRNA

It begins at a promotor site which signals thebeginning of gene is not much further down the

molecule (about 20 to 30 nucleotides) After the end of the gene is reached there is a

terminator sequence that tells RNA polymerase tostop transcribing

NB Terminator sequence terminator codon.


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Process of Transcription

Transcription starts with RNA polymerase binding to the promoter.

This binding only occurs under some conditions: when the gene is on.Various other proteins (transcription factors) help RNA polymerase bind the promoter. Other DNA sequences further upstream from the promotare also involved.

Once it is bound to the promoter, RNA polymerase unwinds a small sectiof the DNA and uses it as a template to synthesize an exact RNA copy ofthe DNA strand.

The DNA strand used as a template is the coding strand; the other strais the non-coding strand. Notice that the RNA is made from 5 end to 3end, so the coding strand is actually read from 3 to 5.

RNA polymerase proceeds down the DNA, synthesizing the RNA copy.

In prokaryotes, each RNA ends at a specific terminator sequence. Ineukaryotes transcription doesnt have a definite end point; the RNA isgiven a definitive termination point during RNA processing.


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Transcription Bubble


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RNA Processing (Pre-mRNA mRNA)

Capping Splicing

Addition of poly A tail


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The primary product of RNA transcription; thehnRNAs contain both intronic and exonic sequences

These hnRNAs are processed in the nucleus to give

mature mRNAs that are transported to the cytoplasmwhere to participate in protein synthesis.


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RNA Processing

Capping

The cap structure is added to the 5' of the newly transcribed mRNAin the nucleus prior to processing and subsequent transport of the mmolecule to the cytoplasm.

Splicing:Step by step removal of pre mRNA and joining of remaining exons

place on a special structure called spliceosomes.


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RNA Processing

Addition of poly A tail:Synthesis of the poly (A) tail involves cleavage of its 3' end and th

addition of about 40- 200 adenine residues to form a poly (A) tail.


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Transcription plan

Transcription

DNA

messenger

RNA

Gene

Nucleus


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The Genetic Code

Translation of nucleic acids into am


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The words of the DNA language are triplets of bacalled codons

The codons in a gene specify the amino acid sequence of apolypeptide

Translation of nucleic acids into amacids

Codons: Triplets of Bases


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Codons: Triplets of Bases

The flow of information from gene to protein isbased on a triplet code: a series of nonoverlappin

three-nucleotide words These triplets are the smallest units of uniform

length that can code for all the amino acids

Example: AGT at a particular position on a DNAstrand results in the placement of the amino acidserine at the corresponding position of thepolypeptide to be produced


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During transcription, one of the two DNA strandscalled the template strand provides a template fo

ordering the sequence of nucleotides in an RNAtranscript

During translation, the mRNA base triplets, called

codons, are read in the 5 to 3 direction

Each codon specifies the addition of one of 20amino acids


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Features of the Code all 64 codons have been assigned

61 code for amino acids

3 (UAA, UAG, and UGA) serve as termination signals

AUG also serves as an initiation signal (also Met)

only Trp and Met have one codon each

more than one triplet can code for the same amino acid; Leuand Arg, for example, are each coded for by six triplets

(degenerate) the third base is irrelevant for Leu, Val, Ser, Pro, Thr, Ala, Gly,

Arg


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Features of the Code

for the 15 amino acids coded for by 2, 3, or 4 triplets, it is only the

of the codon that varies. Gly, for example, is coded for by GGA, GGand GGU

the code is almost universal: it the same in viruses, prokaryotes, aeukaryotes; the only exceptions are some codons in mitochondria


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DNA molecule

Gene 1

Gene 2

Gene 3

DNA strand

TRANSCRIPTION

RNA

Polypeptide

TRANSLATION

Codon

Amino acid

Virtually all organisms share the same genetic cod


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U C A G

U

C

A

G

UUU

UUC

UUA

UUG

CUU

CUC

CUA

CUG

AUU

AUC

AUAAUG

GUU

GUC

GUA

GUG

phe

leu

leu

ile

met (start)

val

UCU

UCC

UCA

UCG

CCU

CCC

CCA

CCG

ACU

ACC

ACAACG

GCU

GCC

GCA

GCG

ser

pro

thr

ala

UAU

UAC

UAA

UAG

CAU

CAC

CAA

CAG

AAU

AAC

AAGAAA

GAU

GAC

GAA

GAG

tyr

stop

stop

his

gln

asn

lys

asp

glu

UGU

UGC

UGA

UGG

CGU

CGC

CGA

CGG

AGU

AGC

AGAAGG

GGU

GGC

GGA

GGG

cys

stop

trp

arg

ser

arg

gly

FirstBase

Second Base

y g g

unity of life


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Translation


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Translation

Translation is the process by which ribosomes read the genetic

message in the mRNA and produce a protein product accordinthe message's instruction.

l tid


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polypeptides

Codons

tRNAmolecules

mRNA

Growingpolypeptide

Largesubunit

Smallsubunit

mRNA

mRNA

bindingsite

P site A site

P A

Growingpolypeptide

Next a

to be polype

An initiation codon marks th


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t at o codo a s tstart of an mRNA message

End

Start of genetic message

AUG = methionine

mRNA a specific tRNA and the ribosome subunits


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mRNA, a specific tRNA, and the ribosome subunitsassemble during initiation

1

Initiator tRNA

mRNA

Startcodon Small ribosomal

subunit

2

P site

Largeribososubun

A site

Amino acid

Polypeptide


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1 Codon recognition

Anticodon

AsiteP site

Polypeptide

2 Peptide bond

formation

3 Translocation

Newpeptide

bond

mRNAmovement

mRNA

Stopcodon

Summary of TRANSCRIPTION


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Summary oftranscription andtranslation 1Stage mRNA

transcribed from

DNA template.

Anticodon

DNA

mRNA

RNApolymerase

TRANSLATION

Enzyme

Amino acid

tRNA

Initiator

tRNA Largeribosomalsubunit

Smallribosomalsubunit

mRNA

StartCodon

2Stage Each am

acid attaches to itsproper tRNA with thelp of a specificenzyme and ATP.

3Stage Initiatiopolypeptide synt

The mRNA, the fi

tRNA, and theribosomal subuncome together.

TRANSCRIPTION


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4Stage ElongationGrowingpolypeptide

Codons

5Stage Termination

mRNA

Newpeptidebondforming

Stop Codon

The ribosome recognizesa stop codon. The poly-peptide is terminated andreleased.

A succession of tRNAs

add their amino acids tothe polypeptide chain asthe mRNA is movedthrough the ribosome,one codon at a time.

Polypeptide


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gene expression [Autosaved].pptx

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