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Eukaryotes vs.
Prokaryotes
Prokaryotes
 small size of genome
 circular molecule of naked DNA called a PLASMID
 DNA is readily available to RNA polymerase
 control of transcription by regulatory proteins (operon)
 most of DNA codes for protein or RNA
 no introns, small amount of non-coding DNA
 regulatory sequences: promoters, operators
Plasmid
Prokaryote
Eukaryotes
 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
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
DNA packing/unpacking
2. RNA processing (premRNA  mRNA)
3. Degredation of mRNA
1.
DNA Packing
How 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
2005-2006 7
Nucleosomes
8 histone
molecules
 “Beads on a string”
 1st level of DNA packing
 histone 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
2005-2006 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 Packing
Euchromatin
Heterochromatin
 eu – true
 hetero – different
 loosely packed DNA
 tightly packed DNA
regions which allows
transcription to readily
occur
regions with little
transcription
DNA packing and transcription
 Degree of packing of DNA regulates transcription
 tightly packed

= no transcription
= genes turned off
darker DNA (Heterochromatin) = tightly packed
lighter DNA (Euchromatin) = loosely packed
2005-2006
DNA Methylation
 attachment of methyl groups (–CH3) to
cytosine
 Methylation of DNA blocks transcription factors
 no transcription = genes turned off
 nearly permanent inactivation of genes
12
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
2005-2006
13
RNA processing
 Alternative RNA splicing
 variable processing of exons creates a family of
proteins
2005-2006
Regulation of mRNA degradation
 Life span of mRNA determines pattern of protein
synthesis
 Eukaryotic mRNA can last from hours to weeks
 Prokaryotic 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 Proteosomes
 Protein degradation
 ubiquitin tagging
 proteosome degradation
Chromosomal Sections
centromere
 region where sister chromatids are connected
 made up of repetitive sequences
telomere
 ends of chromosomes
 made up of repetitive sequences
Chromosome Structure
 centromeres 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
Repeats
Disease
< 27
-
27 – 35
-
36 – 39
+/-
> 39
+
Huntington’s disease
Huntington’s Disease
 Mutation on chromosome 4
 CAG repeats



40-100+ copies
normal = 11-30 CAG repeats
CAG codes for glutamine
amino acid
 Abnormal (huntingtin) protein
produced
 chain of charged glutamines in
protein
 bonds tightly to brain protein,
HAP-1
21
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/Homework
 Section 5.7 Pg. 265 #2
 Section 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 #1-8
 Section 5.5 (control mechanisms) pg. 258 #1-6
 Section 5.7 (Prokaryotes vs. Eukaryotes) pg. 265 #2
 Section 5.8 (Genome organization) pg. 267 #1,3-5
 Chapter 5 Quest Date: Thursday, March 1