05_01_Chromosomes'jpg

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The Structure and Function of DNA A DNA molecule
--- two polynucleotide chains known as DNA chains
or DNA strandsdouble helix
--- four types of nucleotide subunits.
--- held together by
hydrogen bonds between the base portions of the
nucleotides. ---
nucleotides five-carbon sugar nitrogen
containing base one or more phosphate groups.

the sugar is deoxyribose in case of DNA.

bases --- adenine (A), cytosine (C), guanine (G)
or thymine (T) ---
nucleotides are covalently linked together
through the sugars and phosphates --- backbone
--- phosphate groups give
the polarity
http//science.nhmccd.edu/biol/bio1int.htm
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Griffith demonstrates that heat-killed bactera
can transform living cells.
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Avery, MacLeod and McCarty demonstrate that DNA
is the genetic material
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Hershey and chase demonstrate definitively that
genes are made of DNA
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Double helix --- held together by hydrogen bonds
between the bases on diffrent strands.
--- bases inside of the helix,
sugar-phosphate backbones on the outside.
--- A pairs with T, G with C.
(purines with pyrimidinescomplementary base
pairing) purines two
ring bases, pyrimidine single-ring base.
enables the base pairs to be
packe in the energitically most favorable
arrangement in the helix.
sugar-phospahate backbones an equal
distance apart.
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The nucleotides are linked together covalently by
phosphodiester bonds through the 3- hydroxyl
(-OH) group of one sugar and the 5- phosphate
(PO4)of the next. 3 end carries unlinked
hydroxyl group attached to the 3 position on the
sugar ring. 5 end carries a free phosphate group
attached to the 5 position on the sugar
ring. The members of the base pair can fit
together within the double helix only if the two
strands of the helix are antiparellel, that is
only if the polarity of the one strand is
oriented opposite to that of the other strand.
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Double helix --- 10 bases per helical turn
--- coiling of two strands around
each other creates two grooves minor groove and
major groove (suitable for protein
binding) Major groove - deeper groove winding
round the DNA used by DNA-binding motifs to read
and bind to the DNA molecule as part of gene
regulation Minor groove  -  In a helix, refers
to the smaller of the unequal grooves that are
formed as a result of the double-helical
structure
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Genome the complete set of information in an
organisms DNa is called genome. The human genome
contains 3 billion (3 x 109) base pairs
(information of hundered books, human DNA 2 m)
DNA sequence of a DNA fragment
How only 4 base pairs can produce different
protein? How genes are read and gene expression
is regulated? How such a long DNa and is stored
in a very small cell? Many other questions...
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The structure of Eucaryotic Chromosomes Human
DNA is 2 m length and cell nucleus is only 5-8 µm
(like 40 km fine thread in a tennis ball) How it
is stored in such a small place? Eucaryotic DNA
is packed into chromosomes Each chromosome
consists of a single, enourmosly long DNA
molecule associated proteins that hold and pack
the DNA. The complex of DNA and protein is called
chromatin. Chromosomes --- associated with other
proteins involved in gene expression,replication,
DNA repair. --- except
germ cells and highly specialized cells that lack
DNa (such as red blood cells), all cells
have two copies of chr.
--- maternal and paternal
chromosome pairs --- homolog chromosomes.
--- the only nonhomologous chr.
are sex chromosomes, X and Y chromosomes
--- can be distinguished by
DNA hybrydization method (flourecent staining of
each chr. with a
diffrent color) or stainchromosomes with dyes
Karyotype a display of the full set of 46
human chromosomes is called human karyotype.
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Another way to distinguish one chromosome from
another is to stain the the chromosomes with dyes
that bind to certain types of the DNA sequences.
Giemsa staining Giemsa is a complex of stains
specific for the phosphate groups of DNA. --- It
is sed to stain chromosomes and often used to
create a karyotype. --- It is used to identify
chromosomal aberrations such as translocations
and interchanges. --- Giemsa attaches itself to
regions of DNA where there are high amounts of
A-T bonding.
Human karyotype
Normal chromosome 12
Abnormal chromosome 12 (a segment of ch 4 is
added) in inherited ataxia disease
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Fluorescence in situ hybridization (FISH)
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Fluorescence in situ hybridization (FISH) probes
are strands of DNA that have been labeled with
fluorescent dye molecules. The probes bind
uniquely to complementary strands of chromosomal
DNA, thus pinpointing the positions of target DNA
sequences.
human karyotype
Genetic defects can be visualized by FISh method
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In general, more complex an organism, the larger
its genome. But this relation is not always true
For example human genome 200 times gt S.
Cerevisiae but 30 times lt some plants. Even
similar genomic sizes may differ in chromosome
sizes and number. Exmaple
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05_17_cell cycle.jpg
Chromosomes in interphase and mitosis
chromatin condensed
chromosome chromatin
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Specialized DNA sequences found in all eucaryotes
ensure that the interphase chromosomes replicate
efficiently --- Telomere, --- replication
origins Another specialized DN sequence
(centromere) which ensures that duplicated
chromosomes can be easily separated when cell
divides.
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Interphase chromosomes are organized within the
nucleus The interior of the nucleus is not a
random jumble of its many DNA, RNA nad protein
components. Each interphase chromosome probably
occupies a paticular region of the nucleus so
that different chromosomes do not become
entangled with each other.
An electron micrograph showing an enourmous
tangle of chromosomal DNA spilling out of a lysed
interphase nucleus
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The DNA in Chromosomes is highly Condensed DNa
of interphase chromosomes, although less condesed
than that of mitotic chromosomes, is still packed
tightly, with a compaction ratio about
100-fold. Nucleosomes first and most
fundemental level of chromatin packing. Nucleosom
es--- are seen like beads on a string string is
DNA, bead is a nucleosome core particle.
--- consists of DNA that wound aroun
a core of proteins formed from histones.
Histone octamer- two of each
histone, H2A, H2B, H3, H4.
lenth of the double stranded DNA that wraps
around the histone octamer is 146 nucleotide
bp.. Linker DNA DNA between nucleosome core
particles. Length is 80 bp (may vary a few bp)

Electron microscopy of isolated chromatin, 30
nm thick
Nucleosomes, experimentally unpacked (
decondensed) after isolation of chromatin
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Coiling of double helix into nucleosomes
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Octameric histone core All four of the histones
that make up the nucleosome core are relatively
small proteins with a high proportion of
positively charged amino acids (lysine and
arginine). The positive charges help the
histones bing tightly to the negatively charegd
sugar-phosphate backbone of the DNA.
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Nucleosomes are assembled into 30 nm chromatin
Packing of nucleosomes into the chromatin
fiber depends on a fifth histone called H1 which
is thought to pull the the nusleosome together
into a regular repeating array. Chromatin
structure of DNA is found during interphase stage
of cell cycle.
Zigzag model of chromatin
Zigzag model of chromatin
Electron microscopy image
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Nucleosomes are packed into a higher level of
packing by H1 protein.
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Coiling og of DNA double helix into nucleosome,
chromatin and chromosome
DNA double helix
Nucleosomes
Chromatin
Chromosome
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Scaffold attachment proteins (SARs) and
chromosome coiling
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Chromatin in interphase contain both condensed
and more extended forms of chromatin The mitotic
chromosomes unfold into more extended form the
interphase chromosomes However, the chromatin in
an interphase chromosome is not in the same
packing state. Heterochromatin the most highly
condensed (compact) form of interphase
chromatin. ---
those that contain quiescent genes because
chromatin is highly condensed.
--- makes up about 10 of an
interphase chromosome.
--- typically concentrated around the
centromere region and in the telomeresat the ends
of the
chromosomes. Euchromatin the rest of the
interphase chromatin, which is in a variety of
more extended state.

--- contains the genes that are being
expressed. Expression of a gene can be altered
by moving it to another location in the
genome. Exmaples
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An individual X bchromosome can be inactivated by
heterochromatinformation
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Chromosome remodeling complexes, protein machines
that use the ATP hydrolysis to change the
structure of nucleosomes. Make DNA more accecible
other proteins for DNA replication, transcription
and repair. .
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Another approach to the changing of histone
tails. N-terminal tails perform functions in
regulating chromatin structure. Attracts specific
proteins which facilitate acces to the DNA
orcondense DNA.