4 arn composicion y funcion

8/14/2019 4 ARN Composicion y Funcion

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Composicin, estructura yfuncin del ARN

HealthCare

GenticaEsto es un gen

GCGCG..TATATACCAA..ATGTGCTTCTTACAGGAAT..TAACGCGGG..AATAAAGCGCCTTT..

Amplificador

Represor

Promotor

Secuencia codificante

Secuencia de finalizacin

Secuencia transcrita (ARNm)

1 2 3

IntronExon

Composicin del ARN

1. Bases nitrogenadas: purcas [adenina (A) y guanina(G)] y pirimdicas [citosina (C) y Uracilo (U)].

2. Ribosa (azcar de 5 carbonos)

3. Grupo fosfato (acido fosfrico)

Estructuralmente una molcula de ARN es

parecida a una molcula de ADN, con las

siguientes diferencias:

1. El azcar del ARN es una ribosa y no una

de desoxirribosa.

2. En el ARN el Uracilo remplaza a la Timina,como base nitrogenada. complementaria

de la Adenina.

3. El ARN es de hebra simple.

4. El ARN es mucho ms pequeo que el

ADN.

Estructura del ARN

RNt

Transcripcin en eucariotas

(sntesis del ARN)


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Transcripcin en procariotas1. ADN conteniendo el gen a ser transcrito.2. Factor sigma (protena)

3. ARN polimerasa (una sola)

4. dNTPs: dATP, dUTP, dCTP, dGTP

5. Factor Rhro (proteina) -opcional

Principales elementos implicados

ARN Polymerasa (aloenzima compuesta de varias

sibunidades )(Mg++)

RNA polymerase lacks the exonuclease activ ities. As a result, RNA polymerase does not proof-read and is much moreerror prone than DNA polymerase. Accuracy: ~ 1 error in 105 bases

factor sigma

Etapas de la trascripcin

a) Iniciacin

b) Elongacin

c) Terminacin

1. Iniciacin

Promotor procariota

spacer

The -35 and -10 boxes contain consensus sequences

E. coliRNA polymerase + (factor sigma)

Strands melted from 10 to about +1 (TIS) producing a si ngle stranded DNA in the active site of RNAPolymerase.


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No necesita primers!

Una vez que se han aadido varios nucletidos, el factorsigma se disocia y la elongacin continua.

Elementos adicionales2. Elongacin

En E. coli, 50 nucletidos/segundo a 37 grados C

Los codones stop no detienen la ARN polimerasa..

3. Finalizacin

A) Terminacin directa

Repeticin inversa rica en G:C y residuos de U sirven parasoltar la ARN polimerasa.

Esta secuencia forma una horquilla


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Sitio rico en G/C

Pausa de la ARN Pol.

El factor Rho alcanza el

complejo

Se separa el complejo de elongacin

B) Terminacin dependiente del factor Rho(Helicasa dependiente del ATP)

Transcripcin en eucariotas

1. ADN conteniendo el gen a ser transcrito.

2. Protena que interacta con la caja TATA (TPB: TATA binding protein).

3. Factores de transcripcin (TF: trancription factors) TFIID, TFIIB, TFIIA, TFIIF, TFIIE, TFIIH.

4. ARN polimerasa (I, II y III). La ARN polimerasa I transcribe los ARNr, la ARN polimerasa II

transcribe los ARNm y la ARN polimerasa III transcribe los pequeos ARNs (p.e. ARNt).

5. La ARN polimerasa II contiene un dominio llamado CTD (carboxy terminal domain)

compuesto de 52 repeticiones del hepta-pptido YSPTSPS) que es esencial para su

actividad.

6. dNTPs: dATP, dUTP, dCTP, dGTP

7. Amplificadores de la transcripcin (enhancers) tales como el ARC (activator recruited

complex), CBP (CREP binding protein), DRIP (vitamin D receptor interaction protein),

SP1 (specificity protein 1) SREBP (sterol response element binding protein), etc.

8. Enzimas enfundadoras 5 (5 capping enzymes): ARN trifosfatasa, guanililtransferasa,

ARN metiltransferasa.

9. Factor de ruptura y poliadenilacion (CPSF: Cleavage and Polyadenylation Specificity

Factor)

10. Factor estimulador de ruptura (CstF: Cleavage Stimulation Factor)11. Poliadenilasa polimersa o Poli A polimerasa (PAP: Polyadenylate Polymerase)

12. Protena nuclear protectora de la cola Poli-A (PABPN1: Nuclear Polyadenylate Binding

Protein).

13. Espliceosoma (complejo ribonucleoproteico)

Principales elementos implicados RNAs polimerasas en eucariotas

1. Iniciacin (ARNm)

Plants have AGGA instead of CAAT!!

Sitio de iniciacinTBP

Amplificadores


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2. Elongacin

Enzimas enfundadoras 5

1. ARN trifosfatasa2. guanililtransferasa

3. ARN metiltransferasa

CPSF

CstF

pol II

5 methylguanosine cap added, prior to transport out ofnucleu s.

Cleavage and

PolyadenylationSpecificity Factor

Cleavage

StimulationFactor

3. Finalizacin

CPSF

CstF

35b

pol II

pol II

Cleavage and Polyadenylation Specificity Factor

Cleavage Stimulation Factor

4. Modificacines post-transcripcionales

PABPN1

poly a tail ( several to 250) added after cap; degredation rapid if tailmissing

Nuclear Polyadenylate Binding Protein

Modificacines post-transcripcional

Small nuclear RNA (snRNA)

small nuclear RiboNucleoProteins

Small nuclear RNA(snRNA) Playsstructural andcatalytic roles inspliceosomes, thecomplexes of proteinsand RNA that splicepre-mRNA in theeukaryotic nucleus.

ARN maduro

UAA

UAGUGA


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Enhancers are DNA regions which are usuallyrich in transcription factor binding sites and/orrepeats. They enhance transcription of theresponsive promoter independent of orientationand position.

Some transcription factors ("Enhancer-bindingprotein") bind to regions of DNA that arethousands of base pairs away from the genethey control. Binding increases the rate oftranscription of the gene.

Enhancers can be located upstream,downstream, or even within the gene theycontrol

Enhancers Prokaryotic vs. Eukaryotic Transcription

Prokaryote All promoters upstream of functional gene

Main promoter consensus sequences TATAAT (-10) and TTGACA (-35)

One RNA polymerase with subunit makes mRNA, tRNA, rRNA No enhancers

mRNA is primary transcript ready to go short lifetime (just a fewminutes)

Eukaryote

Promoter positions differ for each polymerase- not all upstream

Main consensus sequence TATA box (-25) and CAAT box (-60 to -120)Plants have AGGA instead of CAAT

RNA POL I rRNA

RNA POL II mRNA

RNA POL III ss rRNA, tRNA DNA enhancer regions work with some promoters to increase transcription

Initial product of transcription is not usable mRNA. Primary transcript mustbe processed to form mRNA. Longer lifetime (hours/days)

Initially discovered by Temin and Baltimore copies RNA

into DNA. The HIV enzyme is a dimer of a p66 and a p51

cleaved subunit. The two protein differ in a region of the

crystal structure wich covers the RNase H region (p51

lacks the activity). The crystal structure reveals

resemblance to T7 pol and KF.

Transcriptasa inversa

Example Human Immunodeficiency Virus (HIV)


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The flow of genetic information1960

Protein

mRNA

DNA

transcription

translation

Moleculeswith

effectorfunction


Protein

hnRNA

DNA

t r a n s c r i p t i o n

translation

Moleculeswith

effectorfunction

tRNA rRNA

reversetranscription

mRNA

splicing

ribozymes


Protein

hnRNA

DNA

t r a n s c r i p t i o n

translation

Moleculeswith

effectorfunction

tRNA rRNA

reversetranscription

mRNA

s p l i c i n g

ribozymes

Other noncoding RNAsncRNAs

4 arn composicion y funcion

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