Download chromosomes

Dr MOHAMED FAKHRY 2015
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PROKARYOTES
EUKARYOTES
single chromosome plus
plasmids
many chromosomes
circular chromosome
linear chromosomes
made only of DNA
made of chromatin, a
nucleoprotein (DNA coiled
around histone proteins)
found in cytoplasm
found in a nucleus
copies its chromosome and
divides immediately afterwards
copies chromosomes, then
the cell grows, then goes
through mitosis to organise
chromosomes in two equal
groups
Dr MOHAMED FAKHRY 2015
Chromosomes
They were given the name chromosome (Chromo = colour; Soma
= body) due to their marked affinity for basic dyes
One member of each chromosome pair is derived from each
parent.
Somatic cells have diploid complement (set)=
chromosomes i.e. 46
Germ cells (Gametes:
complement i.e. 23
sperm
&
ova)
have
‫ مجموعة‬of
haploid
The chromosomes of dividing cells are most readily analyzed at
the `metaphase' stage of mitosis .
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Chromosomes

Chromosomes are composed of thin chromatin threads called
Chromatin fibers.
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These fibers undergo folding, coiling and supercoiling during
prophase so that the chromosomes become progressively
thicker and smaller.

Therefore, chromosomes become readily observable= ‫يمكن‬
‫ مالحظته بسهولة‬under light microscope.

At the end of cell division, on the other hand, the fibers uncoil
and extend as fine chromatin threads, which are not visible at
light microscope
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Gene for cystic fibrosis
(chromosome 7)
Gene for sickle cell disease
(chromosome 11)
• Chromosomes are made of
DNA.
• Each contains genes in a linear
order.
• Human body cells contain 46
chromosomes in 23 pairs – one
of each pair inherited from each
parent
• Chromosome pairs 1 – 22 are
called autosomes.
• The 23rd pair are called sex
chromosomes:
XX is female, XY is male.
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Chromosomes can be identified by:
 Their size
 Their shape (the position of the
centromere)
NB Chromosomes are flexible
 Banding patterns produced by
specific stains (Giemsa)
Chromosomes are analysed by
organising them into a KARYOTYPE
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© 2007 Paul Billiet ODWS
Dr MOHAMED FAKHRY 2015
© Biologyreference.com
Female
Male
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Trysomy-21  Down’s syndrome
Trysomy-18  Edward’s syndrome
Images believed to be in the Public Domain
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The Normal Human Chromosomes
Normal human cells contain 23 pairs of
homologous chromosomes:

22 pairs of autosomes numbered as 1-22 in
decreasing order of size

1 pair of sex chromosomes
p
Autosomes are the same in males & females
Sex chromosomes are:
- XX in females
- XY in males
q
Both X are homologous
Y is much smaller than X and has only a few
genes
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Chromosome Structure
Telomere
•
At the metaphase stage:
Each chromosome consists of two chromatids
joined at the centromere or primary constriction
•
The centromere divides chromosomes into short
(p i.e. petit) and long (q i.e. g=grand) arms.
•
The tip of each chromosome is called telomere
•
The exact function of the centromere is not clear,
but it is known to be responsible for the
movement of the chromosomes at cell division.
p
Centromere
q
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Chromosomes may differ in the position of the Centromere, the
place on the chromosome where spindle fibers are attached
during cell division.
In general, if the centromere is near the middle, the chromosome
is metacentric
If the centromere is toward one end, the chromosome is
acrocentric or submetacentric
If the centromere is very near the end, the chromosome is
telocentric.
The centromere divides the chromosome into two arms, so that,
for example, an acrocentric chromosome has one short and one
long arm,
While, a metacentric chromosome has arms of equal length.
All house mouse chromosomes are telocentric, while human
chromosomes include both metacentric and acrocentric, but no
telocentric.
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Chromosomes as seen at metaphase during cell
division
Telomere
DNA and protein cap
Ensures replication to
tip
Tether to nuclear
membrane
Short arm
p (petit)
Long arm
q
Telomere
Light bands
Replicate early in S phase
Less condensed
chromatin
Transcriptionally active
Gene and GC rich
Centromere
Joins sister chromatids
Essential for chromosome segregation at cell
division
100s of kilobases of repetitive DNA: some
non-specific, some chromosome specific
Dark (G) bands
Replicate late
Contain condensed chromatin
AT rich
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A pair of homologous chromosomes (number 1)
as seen at metaphase
Locus (position of a gene
or DNA marker)
Allele (alternative form of
a gene/marker)
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
Centromeres and telomeres are two essential features of all
eukaryotic chromosomes.

Each provide a specific function i.e., absolutely necessary for
the stability of the chromosome.

Centromeres are required for the segregation of the
chromosomes during meiosis and mitosis.

Teleomeres provide terminal stability to the chromosome and
ensure its survival
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The region where two sister chromatids of a chromosome
appear to be joined or “held together” during mitotic
metaphase is called Centromere
When chromosomes are stained they typically show a darkstained region that is the centromere.
Also termed as Primary constriction
During mitosis, the centromere that is shared by the sister
chromatids must divide so that the chromatids can migrate
to opposite poles of the cell.
On the other hand, during the first meiotic division the
centromere of sister chromatids must remain intact
whereas during meiosis II they must act as they do during
mitosis.
Therefore the centromere is an important component
of chromosome structure and segregation.
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
The two ends of a chromosome are known as telomeres.

It required for the replication and stability of the
chromosome.

When telomeres are damaged or removed due to
chromosome breakage, the damaged chromosome ends
can readily fuse or unite with broken ends of other
chromosome.
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Species
Arabidopsis
Human
Oxytricha
Slime Mold
Tetrahymena
Trypanosome
Repeat Sequence
TTTAGGG
TTAGGG
TTTTGGGG
TAGGG
TTGGGG
TAGGG
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Tetrahymena - protozoa organism.
The telomeres of this
organism end in the
sequence 5'-TTGGGG-3'.
The telomerase adds a
series of 5'-TTGGGG-3'
repeats to the ends of the
lagging strand.
A hairpin occurs when
unusual base pairs
between guanine residues
in the repeat form.
Finally, the hairpin is
removed at the 5'TTGGGG-3' repeat.
Thus the end of the
RNA Primer - Short stretches of
chromosome is faithfully ribonucleotides (RNA substrates) found
replicated.
on the lagging strand during DNA
replication. Helps initiate lagging strand
replication
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Gene Map
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Classification Of Chromosomes
Chromosomes are classified (analyzed) according to:
I. Morphology (Shape)
II. Staining
Morphologically (shape)
According to the position of the centromere :
1- Metacentric
2- Sub-metacentric
3- Acrocentric
4- Telocentric: (with centromere at one end) occurs in other
species, but not in man
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Metacentric
Chromosomes
Sub-Metacentric
Chromosomes
Telomeres
p
Centromere
q
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Acrocentric chromosomes (13, 14, 15, 21 and 22)
They have a small mass of chromatin known as satellite attached to
their short arm by narrow stalks (secondary constrict).
The stakes contain genes for 18S and 28S rRNA.
Satellite
Stalk
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→Classification Of Chromosomes

There are several staining methods for cytogenetic analysis of
chromosomes.

Each stain produces specific banding patterns known as
"Chromosome Banding = ‫" تباين – نطاقات أوأاشرطة‬
G banding
Q banding
R banding
C banding
The pattern is specific for each chromosome and is the
characteristics utilized to identify each chromosome.
1)
2)
3)
4)
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Staining Methods for Cytogenetic Analysis
G Banding:
Treat with trypsin and then with Geimsa Stain.
R Banding:
Heat and then treat with Geimsa Stain.
Q Banding:
Treat with Quinicrine dye giving rise to fluorescent bands.
C Banding:
Staining of the Centromere.
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The G-Banding
Pattern of
Chromosomes
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Staining and Banding chromosome
Staining procedures have been developed in the past two decades
and these techniques help to study the karyotype in plants and
animals.
1. Q banding:
The Q bands are the fluorescent bands observed
after quinacrine mustard staining and observation with UV
light.
The distal ends of each chromatid are not stained by this
technique.
The Y chromosome become brightly fluorescent both in the
interphase and in metaphase.
2. R banding:
The R bands (from reverse) are those located in the
zones that do not fluoresce with the quinacrine mustard, that is
they are between the Q bands and can be visualized as green.
Staining and Banding chromosome
G banding:
The G bands (from Giemsa) have the same location as Q bands and
do not require fluorescent microscopy.
3.
C banding:
The C bands correspond to constitutive heterochromatin.
The heterochromatin regions in a chromosome distinctly differ in
their stainability from euchromatic region.
3.
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