Download DNA ends!

Gil Ast
Dep. of Human Molecular
Genetics and Biochemistry
th
Room 1009, 10 floor
640 6893 :Tel
Sylabus
The Cell by B. Alberts
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Books
Essential Cell Biology by Alberts
Genetics
Science of genes
Molecular Biology
Gene expression
Embryonic stem cells
Day 1
Day 2-3
Day 6-9
Day 14
Day 4
Day 5
Each cell in our body contains the same genetic information (DNA)
Gene - encodes for protein
nucleus
The pathway of gene expression
A nucleotide
H
deoxy
The sugar of RNA and DNA
RNA
DNA
The base bind position 1
O
The bases
single strand DNA
ssDNA
ssDNA
5’-to-3’ (1
Phosphodiester bond
direction )2
5’ end )3
3’end
upstream and )4
downstream
How to describe DNA
GAACCTGAGACCTACTGGTCCG
Base-paring
dsDNA
Base pair = bp
Double strand
DNA = dsDNA
Anti-paralel
dsDNA
ladder .3
Each basepair is called 1bp .4
Anti-parallel .1
Complementary strand .2
C:G and T:A
A = 10-10
Gene – a DNA region that is transcribed to RNA, and the
RNA with a biological function
Gene 3
Intron – a
region in the
DNA that is
transcribed
but removed
from the
mRNA
precursor and
is not part of
the mature
mRNA
Exons – part
of the mature
mRNA
Introns are
found only
in
eukaryotes
Human genome
2.91 billion base pairs
24,000 protein coding genes
(~32,000 non-coding genes)
1.5% exons (127 nucleotides)
24% introns (~3,000 nucleotides)
75% intergenic (no genes)
Average size of a gene is 27,894 bases
Contains an average of 8.8 exons
Titin contains 234 exons.
We humans are 99.9% identical
at the DNA sequence level
• There are still ~3 million nucleotide
differences among us called SNPs (60,000
within the exons)---that presumably
account for differences in disease
susceptibility, drug responses, etc.
• Polymorphic variation between and within
populations
• Implications for concepts of “race,”
“individuality”
24,000
Duchenne Muscular Dystrophy
MOLECULAR BIOLOGY OF THE DISEASE
Duchenne Muscular Dystrophy is one of more than twenty
different types of muscular dystrophy. The Duchenne type
affects only boys and is known to result from a defect in
a single important protein in muscle fibers called
Dystrophin. The muscle fiber will break down if the
Dystrophin is missing and is unable to function properly.
As a result, the reduction in the number of good muscle
.fibers and the whole muscle becomes weak
Summary
•
•
•
•
•
•
DNA, Chromosome
Centromere, telomere, replication origin
Nucleosome, Chromatin,
Histone: H1, H2A, H2B, H3, H4
Histone octamer, DNA packaging
DNA binding proteins, Histone
modifications
Histone proteins
HDAC
HDAC activity
HAT
‫מבנה הנוקלאוזום‬
‫סיכת ביטחון‬
H2A
H2B
H3
H4
Covalent Modification of core
histone tails
Acetylation of lysines (K)
Mythylation of lysines
Phosphorylation of serines (S)
Histone acetyl transferase
(HAT)
Histone deacetylase (HDAC)
epigenetics
Acetylation
Mythylation
Total number of chromosomes/somatic (body) cell
There is no
connection
between the
number of
chromosomes
and the genome
size, gene
content, or any
other feature of
genomes. It is
and essentially
independent
characteristic.
Species
Fruit fly
Planaria
Frog
Cat
Mouse
Macaque
Human
Dog
8
16
26
38
40
42
46
78
Species
Corn
Algae
Potato
Yeast
Penicillin mold
20
20
48
32
4
Cockayne syndrome group B (CSB) cells that fail to express CSB
protein which causes profound neurological and developmental defects
Blue – DNA
White - gene
Chromosomal fragile sites are loci that
are especially prone to forming gaps or
breaks on metaphase chromosomes
when cells are cultured under
conditions that inhibit DNA replication
or repair. The relationship of "rare"
folate sensitive fragile sites with
(CCG)n expansion and, in some cases,
genetic disease is well established.
Fragile X syndrome
What is Fragile X syndrome? Fragile X syndrome is the most common inherited
cause of mental impairment, affecting approximately 1 in 2,000 males and 1 in
4,000 females worldwide.
Cytogenetic analysis of metaphase spreads demonstrates the presence of the fragile
.)site in less than 60% of cells in most affected individuals.
In 1991, the fragile X gene (FMR1) was characterized and found to contain a tandem
repeated trinucleotide sequence (CGG) near its 5' end. The mutation responsible for
fragile X syndrome involves expansion of this repeat segment. The number of CGG
repeats in the FMR1 genes of the normal population varies from six to approximately 50.
There are two main categories of mutation, premutations of approximately 50 to 200
repeats and full mutations of more than approximately 200 repeats. There is no clear
In man 104 to 105 sites a replication rate of 2 kb/minute
Origins of replication
• In E. coli only one site OriC
• In man 104 to 105 sites
• The direction of replication is bi-directional
OriC
OriC
OriC
The problem – DNA ends!
If this shoelace were a
chromosome,
then these two protective
tips would be its
The solution: adding repetitive sequences to the ends
CHROMOSOME
TELOMERE
TTAGGGTTAGGGTTAGGGTTAGGGTTAGGG
AATCCCAATCCC
5’
3’
TnAmGo type of minisatellite repeat
TTAGGG – human
TTTAGGG – Arabidopsis
TTGGGG - Tetrahymena
TTAGG – Bombyx
TTTTAGGG – Chlamydomonas
TTTTGGGG – Oxytricha
TTAGGC - Ascaris
(TG)1-3 - Saccharomyces cereviceae
Telomere
•
•
•
•
senescent cells have shorter telomeres
length differs between species
in humans 8-14kb long
telomere replication occurs late in the cell
cycle
• Telomeres are shortened by 40-to-200 bases
between one cell division to the other.
Functions
• Provide protection from enzymatic
degradation and maintain chromosome
stability
• Organization of the cellular nucleus by
serving as attaching points to the nuclear
matrix
• Allows end of linear DNA to be replicated
completely
End-to-end fusion
Telomerase
1. Telomerase binds to the telomer
and the internal RNA component
aligns with the existing telomer
repeats.
2. Telomerase synthesizes new
repeats using its own RNA
component as a template
3. Telomerase repositions itself on
the chromosome and the RNA
template hybridizes with the DNA
once more.
Telomerase is not active in most
somatic cells
Cancer cells have telomerase
Dolly is aging too rapidly?….or was born 6 years old (telomers were
80% of normal sheep)
Dolly has developed pre-mature arthritis
Werner Patient
Teenager
Age 48
A Japanese-American Werner patient as a teenager (left), and at age 48 (Case #1 Epstein et al,
1966, Medicine 45:177). She had eight children, two of whom were also affected. At 48, she had
hair loss and greying, thin extremities, chronic ulcerations of the ankles, atrophy of the skin and her
the right eye had been enucleated several years earlier due to acute glaucoma resulting from bilateral cateract extraction at the age of 27. She lived longer than many Werner patients, dying at 57.