Prokaryotic and Eukaryotic Genomes

7/27/2019 Prokaryotic and Eukaryotic Genomes

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PROKARYOTIC ANDEUKARYOTIC GENOMES

Sara Kevorkian


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What is a genome?

The haploid set of an organisms hereditary

material.

Haploid = one set of chromosomes

Humans are diploid, we have 46 chromosomes in

two sets of 23

Includes both coding and non-coding sequences


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Prokaryotes and Eukaryotes

Prokaryotes and Eukaryotes organize their DNA

differently

Prokaryotes = without a nucleus

Eukaryotes = with a nucleus

Archaea and Bacteria are prokaryotes

Eukaryotes include some unicellular organisms and

all multicellular organisms


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Differences between Prokaryotic and

Eukaryotic Genomes

DNA all over cell

One circularchromosome

Very small and compact

Related genes clustered

together Little non-coding DNA

No introns

Generally haploid

DNA inside nucleus

Multiple linearchromosomes

Genes widely spreadout

Each gene regulated

separately Lots of non-coding DNA

Lots of introns

Generally diploid

Prokaryotes Eukaryotes


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Lac Operon (Bacterial Gene Structure)

An operon is a cluster of related genes all

transcribed at once

LacZ, LacY, LacA all required for lactose metabolism

The promoter is where RNA polymerase binds

The operator is the regulatory site for the Lac

operon

The terminator is where transcription ends


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Lac mRNA

When the Lac operon is transcribed, all three genes

are on the same mRNA

Each gene is translated separately

The Shine-Dalgarno sequence exists in prokaryotes

for the ribosome to find the start codon


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Bacterial Sex!!!

Plasmids are small circularpieces of DNA found inprokaryotes

They can be transferredbetween organisms

They can contain adaptivegenes, such as antibacterial

resistance Scientists use them for

cloning and makingtransgenic organisms


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Plasmid Compared to Chromosome


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Eukaryotic Genomes


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Structure of a Eukaryotic Gene


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Lets start with Exons and Introns

RNA polymerase binds atthe promotor

A pre-mRNA is made

Introns are spliced out ofthe pre-mRNA

Only exons are left in themRNA

Splice sites mark the

boundaries between exonsand introns

Transcription will continueuntil RNA polymerase hitsthe polyadenylation site


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mRNA

Exons contain both coding and non-coding regions

The coding regions are recognized by the ribosome andtranslated into proteins

The non-coding regions are located at the 5 and 3

ends The 5 UTR contains the ribosome binding site

The 3 UTR is important

for mRNA stability,

localization

The polyA tail is

important for nuclear

export


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Does this make sense?


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Open Reading Frames

Since codons are read in triplets, there are three

possible reading frames on a strand of DNA or RNA

5-ATG|TCG|ATC|CAT|GGC|TGC|TAA-3

Met Ser Ile His Gly Cys Stop

5-A|TGT|CGA|TCC|ATG|GCT|GCT|AA-3

Cys Arg Ser Met Ala Ala

5-AT|GTC|GAT|CCA|TGG|CTG|CTA|A-3

Val Asp Pro Trp Leu Leu


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Open Reading Frames

When you add in the fact that DNA is double

stranded, for any region a gene is located, there

are 6 possible reading frames



3-TAG|AGC|TAG|GTA|CCG|ACG|ATT-5Stop Ser Stop Val Pro Thr Ile


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Now we will move on to regulatory

regions

Regulatory regions are made of binding sites foractivators or repressors(transcription factors)

The right combination of transcription factors will turn a

gene on or off


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Regulatory Elements

Regulatory elements are regions of DNA whereprotein binds DNA to turn on or off genes

Activators promote RNA polymerase binding the

promoter Repressors block RNA polymerase binding the

promoter

They work similarly to prokaryotes, as in the lac

operon example, but are often more complex In some cases multiple proteins may need to bind

before a gene is activated or repressed


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Regulatory Elements

Regulatory elements in Eukaryotes can be anywhere

in the genome

When they are close to the promoter they are

called proximal control elements

Farther from the gene of interest, they are called

enhancers.

Cis-Regulatory Elements (CRE) contain multiplebinding sites for regulatory proteins

Regulatory elements can occur within introns


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Pax 6 Expression

Pax 6 is important for the development of many

structures

There are different independently acting CREs

important for its expression in different tissues


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Pax 6 Expression

By adding the Retina CRE in front of a reporter

gene, expression of Pax 6 by that CRE can be

detected

Lac Z was used as a reporter gene


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Any questions?


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Mutations!!


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At the DNA Level

Point mutations

Substitution of one base pair for another.

5-ATG CGT TTA-3 becomes

5-ATG CGA TTA-3

Frameshift mutations

Insertion or deletion of one or more base pairs

5-ATG CGT TTA-3 becomes

5-ATG ACG TTT A-3


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At the Protein Level

Point mutations affect individual codons

A synonymous mutation does not change the amino acid

Ex) CAA(Gln) to CAG(Gln) still encodes Glutamine

A missense mutation changes an amino acid into a different

amino acid A conservative missesnse mutation is chemically similar

Ex) GGU(Gly) to GUU(Val) still encodes hydrophobic amino acids

A nonconservative missesnse mutation is chemically dissimilar

Ex) UCA (Ser) to CCA (Pro) changes from hydrophobic tohydrophilic

A nonsense mutation changes the amino acid to a stopcodon

Ex) UAU(Tyr) to UAA(stop) prematurely ends the amino acid chain


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At the Protein Level

Insertions or Deletions alter the reading frame



5-ATG|ATC|GAT|CCA|TGG|CTG|CTA|A-3

Met Ile Asp Pro Trp Leu Leu

A deletion will work similarly

Insertions or deletions of base pairs in multiples ofthree will retain the reading frame


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This is where you come in!!

Needle in a Hay Stack

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Prokaryotic and Eukaryotic Genomes