Eukaryotes vs. Prokaryotes

Prokaryotessmall size of genomecircular molecule of naked DNA called a PLASMID

DNA is readily available to RNA polymerasecontrol of transcription by regulatory proteins (operon) most of DNA codes for protein or RNA

no introns, small amount of non-coding DNAregulatory sequences: promoters, operators

Plasmid

Prokaryote

much greater size of genome located in nucleus how does all that DNA fit into nucleus?

DNA packaged into chromatin fibers regulates access to DNA by RNA polymerase

most of DNA does not code for protein 97% “junk DNA” in humans

Eukaryotes

Remember… The control of gene

expression can occur at any step in the pathway from gene to functional protein

Today we will talk about regulation of gene expression

1. DNA packing/unpacking2. RNA processing (pre-

mRNA mRNA)3. Degredation of mRNA

2005-2006

DNA PackingHow do you fit all that DNA into nucleus of a eukaryotic cell?

DNA coiling & folding double helix nucleosomes chromatin fiber looped domains chromosome

from DNA double helix to condensed chromosome 7

2005-2006

Nucleosomes “Beads on a string”

1st level of DNA packinghistone proteins

8 protein molecules many positively charged amino acids

arginine & lysine DNA backbone has a negative charge

histones bind to DNA due to a positive charge

8 histone molecules

8

30 nm fibre (Solenoid Fibre)nucleosomes are organized in a stacked spiral

structure

the solenoid fibre is known as the 30 nm fibre

Chromatin PackingEuchromatin Heterochromatin

eu – true

loosely packed DNA regions which allows transcription to readily occur

hetero – different

tightly packed DNA regions with little transcription

2005-2006

DNA packing and transcriptionDegree of packing of DNA regulates transcription

tightly packed = no transcription = genes turned off

darker DNA (Heterochromatin) = tightly packedlighter DNA (Euchromatin) = loosely packed

DNA Methylationattachment of methyl groups (–CH3) to

cytosine Methylation of DNA blocks transcription factors

no transcription = genes turned offnearly permanent inactivation of genes

12

2005-2006

Histone Acetylation attachment of acetyl groups (–COCH3) to histones

Acetylation of histones unwinds DNA loosely packed = transcription = genes turned on

conformational change in histone proteins transcription factors have easier access to genes

13

2005-2006

RNA processingAlternative RNA splicing

variable processing of exons creates a family of proteins

Regulation of mRNA degradationLife span of mRNA determines pattern of protein

synthesisEukaryotic mRNA can last from hours to weeksProkaryotic mRNA is usually degraded within a few

minutes of their synthesis Prokaryotes are therefore better able to respond quickly to

environmental changes

Protein Degradation by ProteosomesProtein degradation

ubiquitin taggingproteosome degradation

Chromosomal Sectionscentromere

region where sister chromatids are connectedmade up of repetitive sequences

telomereends of chromosomesmade up of repetitive sequences

Chromosome Structurecentromeres split

chromosomes

p arm – petit arm

q arm – long arm

VNTRs (microsatellites)variable number tandem repeats (VNTRs) – repetitive

DNA sequences in coding and regulatory regions repeating sequences can be of any length

usually 2 – 6 NTs sequence repeated a different amount of times

Huntington’s Disease RepeatsRepeats DiseaseDisease

< 27< 27 --

27 – 3527 – 35 --

36 – 3936 – 39 + / -+ / -

> 39> 39 ++

Huntington’s diseaseHuntington’s DiseaseMutation on chromosome 4

CAG repeats 40-100+ copies normal = 11-30 CAG repeats CAG codes for glutamine

amino acid

Abnormal (huntingtin) protein producedchain of charged glutamines in

proteinbonds tightly to brain protein,

HAP-1

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Pseudogenes

pseudogenes – NT sequence similar to that of another functional gene

not transcribed to RNA or make protein

Thought to have been mRNA which were reverse transcribed to DNA and inserted into the genome.

Classwork/HomeworkSection 5.7 Pg. 265 #2Section 5.8 Pg. 267 #1,3-5

Homework is being checked and taken up next class… so have your questions ready!Section 5.6 (mutations) pg. 263 #1-8Section 5.5 (control mechanisms) pg. 258 #1-6Section 5.7 (Prokaryotes vs. Eukaryotes) pg. 265 #2Section 5.8 (Genome organization) pg. 267 #1,3-5

Chapter 5 Quest Date: Thursday, March 1