In a chromosome, regions with special structures
and functions can be differentiated. The
centromere is the point of attachment of the
microtubuli of the mitotic spindle (kinetochore).
The telomeres at the ends contain no
genes and have a special structure.
Sunday, April 12, 2009
Heterochromatin and euchromatin
In 1928 Emil Heitz observed that certain parts
of the chromosomes of a moss (Pellia epiphylla)
remain thickened and deeply stained during interphase,
as chromosomes otherwise do only
during mitosis. He named these structures heterochromatin,
as opposed to euchromatin,
which becomes invisible during late telophase
and subsequent interphase. Functionally, heterochromatin
is defined as a region in which few
or no active genes lie and in which repetitive
DNA sequences occur. When active genes become
located close to the heterochromatin,
they usually become inactivated
of the chromosomes of a moss (Pellia epiphylla)
remain thickened and deeply stained during interphase,
as chromosomes otherwise do only
during mitosis. He named these structures heterochromatin,
as opposed to euchromatin,
which becomes invisible during late telophase
and subsequent interphase. Functionally, heterochromatin
is defined as a region in which few
or no active genes lie and in which repetitive
DNA sequences occur. When active genes become
located close to the heterochromatin,
they usually become inactivated
Characteristic regions of a chromosome
The centromere and telomeres contain repetitive
DNA sequences. They are evolutionarily
conserved because they are important for chromosome
stability. The segments located between
the telomeres and centromere consist of
trypsin-sensitive light and trypsin-resistant
dark G-bands. The light G-band areas of DNA
form loops in which the protein-coding genes
lie. The DNA loops are bound to a protein matrix
at special attachment sites.
DNA sequences. They are evolutionarily
conserved because they are important for chromosome
stability. The segments located between
the telomeres and centromere consist of
trypsin-sensitive light and trypsin-resistant
dark G-bands. The light G-band areas of DNA
form loops in which the protein-coding genes
lie. The DNA loops are bound to a protein matrix
at special attachment sites.
Model of a chromosome segment in interphase
A three-dimensional model of a chromosome
segment shows that the constitutive heterochromatin
(C-band) in the centromere region is
very tightly wound. In the light G-bands, the
euchromatin is relatively loosely packed, and in
the dark G-bands, somewhat more tightly
packed. (With kind permission of the author,
from Manuelidis, 1990, copyright 1990 by the
AAAS).
segment shows that the constitutive heterochromatin
(C-band) in the centromere region is
very tightly wound. In the light G-bands, the
euchromatin is relatively loosely packed, and in
the dark G-bands, somewhat more tightly
packed. (With kind permission of the author,
from Manuelidis, 1990, copyright 1990 by the
AAAS).
Constitutive heterochromatin (C-bands) in the centromeric region
The constitutive heterochromatin in the
centromeric region can be specifically stained
(C bands). The distal half of the long arm of the Y
chromosome is also C-band positive. The
centromeric heterochromatin in chromosomes
1, 9, and 16 and in the long arm of the Y chromosome
in humans is polymorphic. The lengths of
the heterochromatic segments in one or more
of these regions may vary among different individuals.
centromeric region can be specifically stained
(C bands). The distal half of the long arm of the Y
chromosome is also C-band positive. The
centromeric heterochromatin in chromosomes
1, 9, and 16 and in the long arm of the Y chromosome
in humans is polymorphic. The lengths of
the heterochromatic segments in one or more
of these regions may vary among different individuals.
Functional attributes of the euchromatin regions
The light and dark G-bands differ in functional
respects. An average G-band contains around
1.5 megabases
respects. An average G-band contains around
1.5 megabases
Special Structure at the Ends of a Chromosome: the Telomere
Unlike the circular chromosomes of bacteria,
bacteriophages, plasmids, and mitochondrial
DNA, the chromosomes of eukaryotes are linear.
Each end is ”sealed” by a specialized region, the
telomere. Telomeres stabilize chromosomes at
both ends.
bacteriophages, plasmids, and mitochondrial
DNA, the chromosomes of eukaryotes are linear.
Each end is ”sealed” by a specialized region, the
telomere. Telomeres stabilize chromosomes at
both ends.
Replication problem at the ends of linear DNA
Since DNA is synthesized in the 5' to 3' direction
only, the two templates of the parent molecule
differ with respect to the continuity of synthesis.
On the 5' to 3' template strand, synthesis occurs
in the reverse direction relative to the fork
movement (lagging strand synthesis). There,
DNA is synthesized in short fragments about
1000–2000 nucleotides long in bacteria and
about 200 nucleotides in eukaryotes (Okazaki
fragments, see DNA replication, p. 42). However,
8–12 bases at the end of the lagging strand
template cannot be synthesized by DNA polymerase
because the primer it requires cannot be
attached beyond the end of the template strand.
Hence, at each round of replication before cell
division, these 8–12 nucleotides will be lost at
the chromosome ends. Some organisms compensate
for this loss by adding telomeric repeats
to the ends of the chromosome during the
replication cycle.
only, the two templates of the parent molecule
differ with respect to the continuity of synthesis.
On the 5' to 3' template strand, synthesis occurs
in the reverse direction relative to the fork
movement (lagging strand synthesis). There,
DNA is synthesized in short fragments about
1000–2000 nucleotides long in bacteria and
about 200 nucleotides in eukaryotes (Okazaki
fragments, see DNA replication, p. 42). However,
8–12 bases at the end of the lagging strand
template cannot be synthesized by DNA polymerase
because the primer it requires cannot be
attached beyond the end of the template strand.
Hence, at each round of replication before cell
division, these 8–12 nucleotides will be lost at
the chromosome ends. Some organisms compensate
for this loss by adding telomeric repeats
to the ends of the chromosome during the
replication cycle.
G-rich repetitive sequences at the telomeric region
DNA at the telomeres consists of G-rich tandem
sequences (5'-TTAGGG-3' in vertebrates, 5'-
TGTGGG-3' in yeast, 5'-TTGGGG-3' in protozoa).
The G-strand overhangs are important for
telomeric protection by formation of a duplex
loop
sequences (5'-TTAGGG-3' in vertebrates, 5'-
TGTGGG-3' in yeast, 5'-TTGGGG-3' in protozoa).
The G-strand overhangs are important for
telomeric protection by formation of a duplex
loop
Telomerase activity and stabilization by a loop
Two features characterize the telomere: telomerase
activity to compensate for replication-related
loss of nucleotides at the chromosome
ends and telomeric DNA loop formation to stabilize
the chromosome ends. Telomerase is a
modified reverse transcriptase consisting of
protein and about 450 nucleotides of RNA. Near
the RNA 5' end are sequences complementary to
telomeric DNA repeat sequences. A short nucleotide
sequence of this RNA pairs with terminal
DNA sequences. The adjacent RNA nucleotides
provide the template for adding nucleotides
to the 3' end of the chromosome. After
telomerase has extended the 3' (G-rich) strand,
a new Okazaki fragment can be synthesized at
the 5' strand by DNA polymerase. Griffith et al.
(1999) have shown that telomeric duplex DNA
forms a loop (t-loop), thus avoiding the ”sticky
end” problem. The loop formation is mediated
by the two related proteins TRF1 (telomeric repeat-
binding factor) and TRF2, which bind to
mammalian telomere repeats, and the loop is
anchored by the insertion of the G-strand overhang
(see B) into a proximal segment of duplex
telomeric DNA.
activity to compensate for replication-related
loss of nucleotides at the chromosome
ends and telomeric DNA loop formation to stabilize
the chromosome ends. Telomerase is a
modified reverse transcriptase consisting of
protein and about 450 nucleotides of RNA. Near
the RNA 5' end are sequences complementary to
telomeric DNA repeat sequences. A short nucleotide
sequence of this RNA pairs with terminal
DNA sequences. The adjacent RNA nucleotides
provide the template for adding nucleotides
to the 3' end of the chromosome. After
telomerase has extended the 3' (G-rich) strand,
a new Okazaki fragment can be synthesized at
the 5' strand by DNA polymerase. Griffith et al.
(1999) have shown that telomeric duplex DNA
forms a loop (t-loop), thus avoiding the ”sticky
end” problem. The loop formation is mediated
by the two related proteins TRF1 (telomeric repeat-
binding factor) and TRF2, which bind to
mammalian telomere repeats, and the loop is
anchored by the insertion of the G-strand overhang
(see B) into a proximal segment of duplex
telomeric DNA.
General structure of a telomere
In the terminal 6–10 kb of a chromosome,
telomeric sequences and telomere-associated
sequences can be differentiated (1). The telomere-
associated sequences contain autonomously
replicating sequences (ARS). The telomere
sequences consist of about 250 to 1500 Grich
repeats (!9 kb). They are highly conserved
among different species (2). Telomerase activity
is essential for survival in protozoans and yeast.
In vertebrates it occurs mainly in germ cells,
and no telomerase activity is found in somatic
tissues. The cell division-dependent decrease of
telomere length is viewed as being related to
aging and death of cells because, ultimately,
functional DNAwill be lost. Unlike normal cells,
many tumors have telomerase activity.
telomeric sequences and telomere-associated
sequences can be differentiated (1). The telomere-
associated sequences contain autonomously
replicating sequences (ARS). The telomere
sequences consist of about 250 to 1500 Grich
repeats (!9 kb). They are highly conserved
among different species (2). Telomerase activity
is essential for survival in protozoans and yeast.
In vertebrates it occurs mainly in germ cells,
and no telomerase activity is found in somatic
tissues. The cell division-dependent decrease of
telomere length is viewed as being related to
aging and death of cells because, ultimately,
functional DNAwill be lost. Unlike normal cells,
many tumors have telomerase activity.
Metaphase Chromosomes
Chromosomes are visible as separate structures
only during mitosis. In interphase, the chromosomes
in chromatin cannot be individually
differentiated. In metaphase chromosomes,
DNA is packed about 10000 times more densely
than in interphase. Electron–microscopic studies
of metaphase chromosomes have yielded
some insight into chromosomal structure.
only during mitosis. In interphase, the chromosomes
in chromatin cannot be individually
differentiated. In metaphase chromosomes,
DNA is packed about 10000 times more densely
than in interphase. Electron–microscopic studies
of metaphase chromosomes have yielded
some insight into chromosomal structure.
A histone-free chromosome under the electron microscope
When certain proteins, especially histones, are
removed fromchromosomes, the chromosomal
skeleton becomes visible under the electron
microscope (1). Such a structure is surrounded
by numerous darkly stained threads. A higher
magnification (2) shows that this is a single
continuous thread. It corresponds to the DNA
double helix.
removed fromchromosomes, the chromosomal
skeleton becomes visible under the electron
microscope (1). Such a structure is surrounded
by numerous darkly stained threads. A higher
magnification (2) shows that this is a single
continuous thread. It corresponds to the DNA
double helix.
The microscopic appearance of metaphase chromosomes of man
With an approximately 1000-fold magnification,
the metaphase chromosomes of man and
other vertebrates can readily be recognized
under the light microscope as individual rodlike
structures. A metaphase is shown here at about
2800-fold magnification. The chromosomes
differ from each other in length, in the size and
arrangement of their transverse light and dark
bands (banding pattern), and in the point of attachment
of the spindle (centromere), which is
recognizable as a constriction. In prometaphase,
the chromosomes are longer than in
metaphase and showmore bands. Thus, for certain
purposes chromosomes are also studied in
prometaphase.
the metaphase chromosomes of man and
other vertebrates can readily be recognized
under the light microscope as individual rodlike
structures. A metaphase is shown here at about
2800-fold magnification. The chromosomes
differ from each other in length, in the size and
arrangement of their transverse light and dark
bands (banding pattern), and in the point of attachment
of the spindle (centromere), which is
recognizable as a constriction. In prometaphase,
the chromosomes are longer than in
metaphase and showmore bands. Thus, for certain
purposes chromosomes are also studied in
prometaphase.
Types of metaphase chromosomes
Depending on the location of its centromere
(point of attachment of the spindle during mitosis),
a chromosome can be distinguished as
submetacentric, metacentric, acrocentric, or
telocentric. The centromere divides a submetacentric
chromosome into a short arm (p arm)
and a long arm (q). In metacentric chromosomes,
the short and long arms are about the
same length. Acrocentric chromosomes show a
dense appendage called a satellite (not to be
confused with satellite DNA) at the end of the
short arm. Satellite size differs for each acrocentric
chromosome of an individual (chromosomal
polymorphism). Telocentric chromosomes
have neither a short arm nor a satellite.
None of the human chromosomes are telocentric,
whereas all chromosomes are telocentric in
the house mouse, Mus musculus. However, it is
debatable whether telocentric chromosomes
actually exist as defined.
(point of attachment of the spindle during mitosis),
a chromosome can be distinguished as
submetacentric, metacentric, acrocentric, or
telocentric. The centromere divides a submetacentric
chromosome into a short arm (p arm)
and a long arm (q). In metacentric chromosomes,
the short and long arms are about the
same length. Acrocentric chromosomes show a
dense appendage called a satellite (not to be
confused with satellite DNA) at the end of the
short arm. Satellite size differs for each acrocentric
chromosome of an individual (chromosomal
polymorphism). Telocentric chromosomes
have neither a short arm nor a satellite.
None of the human chromosomes are telocentric,
whereas all chromosomes are telocentric in
the house mouse, Mus musculus. However, it is
debatable whether telocentric chromosomes
actually exist as defined.
Simple structural aberrations
A functionally relevant deviation from the normal
structure is called a structural aberration.
This is to be differentiated from chromosomal
polymorphism. Loss (deletion) or doubling (duplication)
of a particular segment may occur. A
deletion may occur at the end of a chromosome
(terminal deletion) or within a chromosomal
segment (interstitial deletion). Prerequisite for
a terminal deletion is one break; for an interstitial
deletion, two breaks. A segment that has
been doubled is called a duplication. In
metaphase chromosomes, an aberration is seen
in both chromatids because as a rule it has occurred
before the S phase. Duplications and
deletions represent opposite, and in some respects
complementary, aberrations of chromosomal
structure
structure is called a structural aberration.
This is to be differentiated from chromosomal
polymorphism. Loss (deletion) or doubling (duplication)
of a particular segment may occur. A
deletion may occur at the end of a chromosome
(terminal deletion) or within a chromosomal
segment (interstitial deletion). Prerequisite for
a terminal deletion is one break; for an interstitial
deletion, two breaks. A segment that has
been doubled is called a duplication. In
metaphase chromosomes, an aberration is seen
in both chromatids because as a rule it has occurred
before the S phase. Duplications and
deletions represent opposite, and in some respects
complementary, aberrations of chromosomal
structure
Karyotype
Karyotype refers to the arrangement of chromosomes
in homologous pairs. They are arranged
and numbered according to a convention.
The basis for the arrangement is size of a
chromosome, position of the centromere, and
the chromosome-specific banding pattern. The
karyotype is characteristic for each species.
However, the term karyotype can also be applied
to an individual or to a single cell.
in homologous pairs. They are arranged
and numbered according to a convention.
The basis for the arrangement is size of a
chromosome, position of the centromere, and
the chromosome-specific banding pattern. The
karyotype is characteristic for each species.
However, the term karyotype can also be applied
to an individual or to a single cell.
The karyotype of man
Man (Homo sapiens) has 22 pairs of chromosomes
(autosomes) and in addition either two X
chromosomes, in females, or an X and a Y chromosome,
in males (karyotype resp. 46,XX or
46,XY). In front of the comma, the karyotype
formula gives the total number of chromosomes
present, and after the comma, the composition
of the sex chromosomes.
(autosomes) and in addition either two X
chromosomes, in females, or an X and a Y chromosome,
in males (karyotype resp. 46,XX or
46,XY). In front of the comma, the karyotype
formula gives the total number of chromosomes
present, and after the comma, the composition
of the sex chromosomes.
Karyotype of the mouse (Mus musculus)
The standard karyotype of the mouse consists
of 19 chromosome pairs in addition to the X and
Y chromosomes. All chromosomes except the X
and Y are telocentric and of similar size (1).
However, they differ in their banding patterns,
characteristic for each chromosome pair, and
therefore are individually distinct. Certain
strains of mice may show variants of the karyotype
(2). These variants arise from fusion of certain
of the chromosomes. In the example shown
here, only chromosome pairs 1, 15, 19, and X
correspond with those of the standard karyotype,
while the others consist of fused chromosomes,
e. g., chromosomes 4 and 2, chromosomes
8 and 3, etc. Structural rearrangements
of the karyotype occurred with the separation
of different species in evolution.
of 19 chromosome pairs in addition to the X and
Y chromosomes. All chromosomes except the X
and Y are telocentric and of similar size (1).
However, they differ in their banding patterns,
characteristic for each chromosome pair, and
therefore are individually distinct. Certain
strains of mice may show variants of the karyotype
(2). These variants arise from fusion of certain
of the chromosomes. In the example shown
here, only chromosome pairs 1, 15, 19, and X
correspond with those of the standard karyotype,
while the others consist of fused chromosomes,
e. g., chromosomes 4 and 2, chromosomes
8 and 3, etc. Structural rearrangements
of the karyotype occurred with the separation
of different species in evolution.
Flow cytometry karyotype in man
Because of their different lengths, metaphase
chromosomes can also be presented in a flowcytometry-
based karyotype. With this method,
individual chromosomes, stained and passed by
a laser light source, give signals corresponding
to their sizes. Although there are overlaps, e. g.,
between the similarly sized human chromosomes
9–12, or with chromosomes 1 and 2, a
distribution pattern of light impulses based on
individual chromosome sizes is obtained. The
size of the X chromosome lies between those of
chromosomes 8 and 7; the size of the Y chromosome
as a rule corresponds to that of a chromosome
22, although Y chromosome sizes may
differ considerably. Because of the technical expenditure
required and the unsharp resolution,
flow cytometry is not frequently used for a
practical diagnosis
chromosomes can also be presented in a flowcytometry-
based karyotype. With this method,
individual chromosomes, stained and passed by
a laser light source, give signals corresponding
to their sizes. Although there are overlaps, e. g.,
between the similarly sized human chromosomes
9–12, or with chromosomes 1 and 2, a
distribution pattern of light impulses based on
individual chromosome sizes is obtained. The
size of the X chromosome lies between those of
chromosomes 8 and 7; the size of the Y chromosome
as a rule corresponds to that of a chromosome
22, although Y chromosome sizes may
differ considerably. Because of the technical expenditure
required and the unsharp resolution,
flow cytometry is not frequently used for a
practical diagnosis
Subscribe to:
Comments (Atom)
