1. DNA Organization

2. Eukaryotes vs. Prokaryotes

3. Prokaryotes small size of genome
circular molecule of naked DNA called the PROKARYOTIC CHROMOSOME
Contains very small amounts of protein
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

4. Prokaryotes
Also have small, circular molecules called PLASMIDS that carry a few “accessory” genes
Provides resistance to antibiotics
Direct metabolism of rarely encountered nutrients
In most environments, prokaryotes can exist without plasmids, but the presence of plasmids can significantly increase chances of survival
Chromosome
Plasmid

5. Prokaryote

6. 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

7. 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
RNA processing (pre-mRNA  mRNA)
Degredation of mRNA
Degredation of protein

8. 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
nucleosomes
“beads on a string”
1st level of DNA packing
histone proteins have high proportion of positively charged amino acids (arginine & lysine)
bind tightly to negatively charged DNA
from DNA double helix to condensed chromosome 8 8

9. Nucleosomes “Beads on a string” 1st level of DNA packing
8 histone molecules
Nucleosomes
“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 9 9

10. 30 nm fibre (Solenoid Fibre)
nucleosomes are organized in a stacked spiral structure
the solenoid fibre is known as the 30 nm fibre

12. Chromatin Packing Euchromatin Heterochromatin eu – true
loosely packed DNA regions which allows transcription to readily occur
hetero – different
tightly packed DNA regions with little transcription

13. 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 13

14. 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 14 14

15. 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
Acetylation – neutralizes positive charge so it can no longer bind to neighbouring nucleosomes 15 15

16. RNA processing Alternative RNA splicing
variable processing of exons creates a family of proteins 16

17. 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
Prokaryote mRNA is usually degraded within a few minutes of their synthesis, so they quickly respond to environmental changes. 17

18. Protein Degradation by Proteosomes
ubiquitin tagging
proteosome degradation

19. Other DNA Regions

20. Chromosome Structure centromeres split chromosomes p arm – petit arm
q arm – long arm

21. Chromosomal Sections centromere telomere
region where sister chromatids are connected
made up of repetitive sequences that protect the chromosome during cell division
telomere
ends of chromosomes
made up of repetitive sequences that protect the chromosome from degradation during DNA replication

22. VNTRs (microsatellites)
variable number tandem repeats (VNTRs) – repetitive DNA sequences in noncoding regions and regulatory regions. These VNTRs are also call mircrosatellites.
repeating sequences can be of any length
usually 2 – 6 NTs
sequence repeated a different amount of times
Huntington’s Disease show as
VNTRs within the coding
regions (genes).
Repeats
Disease
< 27 -
27 – 35
36 – 39
+ / -
> 39 +

23. Huntington’s disease Huntington’s Disease Mutation on chromosome 4
CAG repeats
copies
normal = 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 23 23

24. 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.

25. Classwork/Homework Homework
Section 5.7 (Prokaryotes vs. Eukaryotes) pg. 265 #2
Section 5.8 (Genome organization) pg. 267 #1,3-5