Download PG1005 Lecture 14 Chromosomal Organisation and Composition

TR056/PG1005
Lecture 14
Chromosomal Organisation
(Composition and Packaging of DNA)
Dr Neil Docherty
Dr.
My Teaching Objectives
•Revise
R i th
the content
t t off th
the nucleus
l
and
d th
the
organisation of DNA into chromosomes
•Introduce
Introduce the structure of DNA and its
organisation into functional units known as genes
•Describe double helical and nucleosomal DNA
f ldi and
folding
d packaging
k i
The Nucleus Revision
Nucleus is the largest organelle (>6 micron diameter)
Surface
Nuclear envelope (double membrane), ribosome studded.
N l
Nucleoporin
i fformed
d channels.
h
l
Transmission Electron Microscopy
The Nucleus Inside
Discernable Features
-Heterochromatin
-Euchromatin
E h
i
-Nucleolus
Constituents
DNA, RNA
Nucleoprotein
p
((histone/non-histone))
Active cell
-Prominent
Prominent nucleoli
-euchromatin
-Dispersed basophilia
Resting cell
-Small absent nucleoli,
-Heterochromatin
H t
h
ti
-Dense basophilia
The Fundamental Role of
The Hereditary Material
Required
q
for
•Replication of cell
Tissue integrity/function
Required for
Function of cell
DNA
Required
for
•Gene transcription
Exception:
What about Red Blood Cells and Platelets??
Chromosomes
Human DNA-46 units of linear double stranded DNA polymer
-22
22 pairs of autosomes (Ch1
(Ch1-22)
22)
-2 sex chromosomes (XX / XY)
Total Length of Human DNA
Per chromosome=1.5cm-8.5cm
=Estimate of around 2m per cell total.
The Human Karyotype
N.B. DNA compaction of 10,000 fold in mitotic nuclei
Chromosomal Landmarks
Where are the Genes?
H
Human
Ch
Chromosome 22
Vital Statistics of Human Chromosome 22
and the Entire Human Genome
DNA length
g
Number of genes
Smallest protein-coding gene
Largest gene
M
Mean
gene size
i
Smallest number of exons per
gene
Largest number of exons per
gene
Mean number of exons per
gene
Smallest exon size
Largest exon size
Mean exon size
Number of pseudogenes**
Percentage of DNA sequence
in exons (protein coding
sequences)
Percentage of DNA in highcopy repetitive elements
Percentage of total human
genome
CHROMOSOME 22
pairs*
48 × 106 nucleotide p
approximately 700
1000 nucleotide pairs
583,000 nucleotide pairs
19 000 nucleotide
19,000
l tid pairs
i
HUMAN GENOME
3.2 × 109
approximately 30,000
not analyzed
2.4 × 106 nucleotide pairs
27 000 nucleotide
27,000
l tid pairs
i
1
1
54
178
5.4
8.8
8 nucleotide pairs
7600 nucleotide pairs
266 nucleotide pairs
more than 134
not analyzed
17,106 nucleotide pairs
145 nucleotide pairs
not analyzed
3%
1.5%
42%
approximately 50%
1.5%
100%
DNA Structure Basics
Deoxyribonucelic
y
acid duplex
p
Each strand is a linear polymer of nucleotides
1. Deoxyribose sugar unit
2. Phosphate
p
Purine (Adenine,Guanine)
3 Nitrogenous base
3.
Pyrimidine (Cytosine, Thymine)
Linkage in The Single Strand Polymer
N1-C1 link PYRMIDINE
C and T
Above plane
-glycosidic linkages
N9-C1
N9
C1 link
PURINE
A and G
Forming The Double Helix
Bases on two separate single strand polymers forming
specific base pairs which allow for the formation of a helix.
Watson-Crick Model
-Two
Two anti
anti-parallel
parallel helical chains coil around a common axis
axis.
-Sugar-phosphate
Sugar phosphate “backbone”
backbone runs on outside,bases inside.
-Bases lie perpendicular to common to axis.
-A regular double helix is formed due to the base pairs
occupying the same internal space
(A-T=G-C in space across helix).
Helical stacking promoted by intra-base pair hydrogen bonding
And stack-stack Van der Waal’s attraction.
The Double Helix
Hydrophobic interaction
Length equivalency
-Right handed helix
-10 bases per 360˚
base-base
hydrogen bonding
The Nucleosome
The double helix is packaged by proteins called histones.
Histones
H2A,H2B,H3,H4 (core histones)
H1 (linker histone)
25% arginine or lysine residues (basic amino acids)
Histones form a complex around which DNA can be;
-supercoiled
stac ed
-stacked
The Histone Octamer
Basic constituent of chromatin
-2
2 molecules of each H2 histone for unit
unit.
(H3)2(H4)2 tetramer (H2A-H2B dimer)2
-146bp
146bp wound on per unit in 1.8 turns (left hand supercoil).
-80bp linker attached to H1 (nucleosome seal).
-Repeat.
Solenoid
www.mun.ca/.../Histone_ Protein_Structure.htm
Solenoid Helices
Nuclesome formation compact DNA seven fold
Helical stacking of solenoids provides up to 104 fold condensation.
condensation
Now DNA resembles
packets=chromosomes.
Most prominent in
duplicated form during
mitotic metaphase.
p
Message
g On Packing
g
Clear implication is that packaging allows accommodation of
a large amount of genetic information
To access information (i.e. DNA, chromatin must be opened up)
See subsequent lectures for;
REPLICATION
GENE EXPRESSION
Your Learning Objectives
Your learning from today should focus on being able to;
•Define
D fi and
d articulate
ti l t what
h t genes and
d chromosomes
h
are as structural
t t l
and functional units of heritable material.
•Describe the basic chemical components of DNA
•Explain the biochemical basis for double helix formation
•Appreciate the necessityy for efficient packaging
g g of DNA and
comprehend how amino-acid sequence matches function in the
nucleosome.