2_organisation and control of prokaryotic and eukaryotic genome - summary

8/12/2019 2_Organisation and Control of Prokaryotic and Eukaryotic Genome - Summary

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Organisation and Control of Prokaryotic

and Eukaryotic Genome

ORGANISATION

A. Prokaryotes:

Single DS circular DNA

o Associated with small amount of proteins

Located within nucleoid region

Smaller DNA rings Plasmids

Protein coding genes usually arranged in an operon.

Genes closely packed very few noncoding gaps

B. Eukaryotes:

N!N"!D#NG $%G#!NS&

#ntrons

o 'ithin genes

o Alternate $NA splicing codes for more than one polypeptide

(ransposons

o )etween genes

o Short inverted repeats that flank coding DNA

%.g. GAA Gene AAG

o *ove from one location on genome to another

DNA intermediate+ cutandpaste mechanism

$etrotransposons

Simple Se,uencing Genes -a.k.a. SA(%LL#(% DNA

o )etween genes

*ost mammals near centromeres

#n Drosophila in /oth centromeres and telomeres.

o *icrosatellites

01 /p2 030445


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relatively sta/le2 highly polymorphic DNA markers in linkage mapping.

o *inisatellites

64044 /p2 thousands of times

used in DNA fingerprinting

Pseudogenes

o Genes that have lost function

Due to random mutations

o %volutionary significance

C. Telomeres

*ultiple repetitions of short non-codinnucleotide se,uence

o #n humans2 ((AGGG repeated 04404445

7unction&

o Protects genes from erosion via successive rounds of DNA replication

Telomeres ser!e as "u##ers $sacri#icial %rotection&

To ensure t'at critical %roteins (ill still "e synt'esised in t'e dau'ter

cells des%ite t'e s'ortened c'romosomes.

Cells (ill also undero a%o%tosis a#ter a limited num"er o# cell

di!ision)mitosis $*+& i.e. ('en a critical lent' o# t'e telomere is

reac'ed.

T'is limits t'e e,tent o# accumulated mutations and %re!ents t'e

de!elo%ment o# cancer.

o Prevents fusion of ends with the ends of other chromosomes -chromosomal mutation

'ic' can disru%t reulatory control o# enes on t'e adoinedc'romosomes

o *aintains integrity of chromosomal ends

)roken chromosomes that lack telomeres recognised as defective /y cellular

DNA repair machinery2 which remedies situation /y putting /roken ends

together2 restoring the telomeres.

#nappropriate repair&

chromosome fusion !$

attracts en8ymes that degrade the chromosome entirely.


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/. Centromeres

Appear as constrictions in eukaryotic chromosomes.

%nsure proper segregation of chromosomes&

o 9old sister chromatids together

*itosis& up to /eginning of anaphase

*eiosis& up to anaphase 6

%la/oration of kinetochore -composed of DNA and proteins

o :inetochore site at which chromosomes attach to the spindle fi/res -/oth during

mitosis and meiosis

o *otor proteins in kinetochore assist movement of sister chromatids to opposite poles

-during anaphase2 after the centromere holding 6 sister chromatids divides

0. Gene Am%li#ication

Process of increasing num/er of copies of a gene

6 mechanisms&

o DNA replication

0. Single strand /roken

6. )roken strand replicated dou/le stranded /reak occurs

1. Leads to chromosomal rearrangement2 including gene amplification

o $ecom/ination and segregation

*isalignment and recom/ination /etween sister chromatids

!ne chromatid with a duplicated segment

Another with a deleted segment

7urther rounds of misalignment causes linear amplification of duplicated

segment.

Significance varies with organism

o #n developing South American clawed frog2 simultaneous transcription of amplified

r$NA genes allows ri/osome synthesis to /e completed in ;4


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"!N($!L

= levels&

(ranscriptional

Posttranscriptional (ranslational

Posttranslational

Stage Prokaryotes %ukaryotes

(ranscription Promoter

o 9as consensus se,uences

(A(AA( at 04

((GA"A at 13

Sima 0actor

O%eron

Promoter

o (A(A /o> at roughly 63

Transcri%tion 0actors

o ?? re,uired to recognise the (A(A /o> and

to recruit $NA polymerase

Control Elements

o = types&

Promoters

9igh level of transcription& /inding of

transcription factors to control elements

/eyond the promoter on DNA.

Promoter-%ro,imal elements $PPE&

En'ancers

are distal control elements

looping mechanism

Silencers

"ompetitive DNA /inding

*asking activation surface

Direct interaction with the general

transcription factors

Post

(ranscription

m$NA is immediately ready #or translation

*1 2-met'yluanosine $2-3G& ca%%in 41 PolyA tail

RNA S%licin

(ranslation mRNA deradation

o degraded /y nucleases after only a

few minutes

initiation o# translation

o antisense $NA

mRNA deradation

o halflives from minutes to months

initiation o# translation

o initiation factors

Post

(ranslation

Co!alent 3odi#ication $In Goli A%%aratus&

P'os%'orylation)/e%'os%'orylation

0eed"ack control

Cyto%lasm Goli A%%aratus


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A. Prokaryotes

(ranscription

Promoter

o (wo short se,uence elements within the promoter

Located appro>imately at 13 and 04o "onsensus se,uences

(A(AA( at 04

((GA"A at 13

Sigma 7actor

o Su/unit of $NA polymerase

o $ecogni8es promoter elements -at /oth 04 and 13

o Disassociates from polymerase once transcription /egins

o Different sigma factor for different genes

??Specificity ensures only certain genes are transcri/ed only when correct

sigma factor /ecomes availa/le

!peron

o $epressi/le trp operon

o #nduci/le lac operon

Post(ranscription

m$NA is immediately ready for translation

(ranslation

m$NA degradation

o short lifespan

o degraded /y nucleases after only a few minutes

initiation of translation

o antisense $NA

o /inds to m$NA to downregulate its translation

Post(ranslation

"ovalent *odification

o !ccurs in cytoplasm

Pri/now )o>

7or $NA polymerase to /ind to


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Phosphorylation@Dephosphorylation

7eed/ack control

B. Eukaryotes

(ranscription

Promoter

o (A(A /o> at roughly 63

(ranscription 7actors

o ?? ($ANS"$#P(#!N 7A"(!$S re,uired to recognise the (A(A /o> and to recruit

$NA polymerase

o vs. prokaryotic sigma factor

o specific in /inding -to proteins2 other transcription factors and control elements

"ontrol %lements

o a.k.a. cisacting elements

o noncoding DNA se,uences

/ound /y transcription factors to help regulate the transcription of a gene

o = types&

Promoters

/asal level of transcription& transcription factors interacting with $NA

polymerase with promoter.

9igh level of transcription& /inding of transcription factors to control

elements /eyond the promoter on DNA.

Promoterpro>imal elements -PP%

lies within 044644 /p upstream from start site of transcription and

(A(A /o>

"AA( /o> and G" /o> found within 34 and 044 region

%nhancers

-compared to PP% are distal control elements

positive regulatory elements+ upregulation of transcription

thousands of nucleotides upstream2 downstream of transcription start or

even within an intron

/ound /y transcription factors known as activators.

7or $NA polymerase to /ind to


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looping mechanism

o direct interaction of the activators with the $NA polymerase or

transcription factors at the (A( site upregulates@stimulates

transcription of a gene.

Silencers

DNA elements

#nhi/it gene e>pression

)ound /y transcription factors known as repressors

*echanism&

o "ompetitive DNA /inding

o *asking activation surface

o Direct interaction with the general transcription factors

Post(ranscription -still prem$NA

3 Bmethylguanosine -B*G capping

o prevents degradation /y cellular nucleases

o recognition of resultant m$NA /y initiation factors -promoting ri/osome /inding to

initiate translation

1 PolyA tail

o DNA 04 13 nucleotides /eyond AACAAA se,uence cleaved en8ymatically.

o PolyA polymerase then adds a/out 644 adenine nucleotides to 1 end.

o %nhances sta/ility of m$NA2 impeding degradation /y nucleases

o Direct the transport of m$NA from nucleus to cytoplasm

$NA Splicing

o Spliceosome removes introns2 oins e>ons

o Earious permutations of splicing

(ranslation

m$NA degradation

o halflives from minutes to months

initiation of translation

o initiation factors

needed to ena/le ri/osomes to attach to the m$NA for initiation of translation


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Post(ranslation

"ovalent *odification

o !ccurs at golgi apparatus

Phosphorylation@Dephosphorylation

7eed/ack control

2_organisation and control of prokaryotic and eukaryotic genome - summary

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