Translocation

Translocation refers to an exchange of chromosome
segments. A translocation can arise by
centric fusion of two acrocentric chromosomes
(Robertsonian translocation) or by exchange
between two chromosomes (reciprocal translocation).
With centric fusion, two complete chromosomes
are involved; with reciprocal translocation,
only a part of each of the two chromosomes
is exchanged. In a translocation it is important
to determine the breakpoints in each of
the chromosomes involved.

Centric fusion of acrocentric chromosomes

Chromosome 14 and chromosome 21 (1) are the
most frequently involved in fusions (about 1 in
1000 newborns). By fusion of the long arm of
chromosome 21 (21q) and the long arm of chromosome
14 (14q), a chromosome t (14q21q) is
formed (2). The satellite-carrying short arms of
both chromosomes are lost, but this is insignificant.
When germ cells (gametes) are formed,
deviation from the normal chromosome number
may result (3). Since chromosome 14 and
chromosome 21 pair during meiosis, the following
possible gametes may result: chromosome
14 alone (no chromosome 21), one chromosome
14 and one chromosome 21 (normal),
the chromosome 14 fused to chromosome 21
(balanced), or the fused chromosome and one
chromosome 21

After fertilization

After fertilization, the corresponding zygotes
contain either only one chromosome 21 (unviable
monosomy 21), a normal chromosome complement,
a balanced chromosome complement
with the fused chromosome, or three chromosomes
21 (trisomy 21). In the latter case, the
clinical disorder Down syndrome (formerly
called mongolism) results

Reciprocal translocation

A reciprocal translocation is an exchange of
chromosomal material between two chromosomes.
Since usually no chromosomal material
is lost or added with a reciprocal translocation,
it does not cause clinical signs (i. e., it is
balanced). However, carriers of a reciprocal
translocation may form gametes with unbalanced
chromosome complements. During
meiosis, the chromosomes involved in the reciprocal
translocation take part as usual in the
homologous pairing of meiosis I. Each of the
chromosomes not involved in the translocation
pairs with its homologous partner that is involved
in the translocation. This leads to the formation
of a characteristic quadriradial configuration
of the involved chromosomes. When
these four chromosomes separate (segregation)
during anaphase of meiosis (see p. 116), one of
three possibilities may occur: With alternate
segregation, one gamete receives the two normal
chromosomes, and the other gamete the
chromosomes involved in the translocation,
i. e., it is balanced.

.......

With nonalternate segregation (neighboring or
adjacent chromosomes), the two chromosomes
on the left go into one gamete and the two chromosomes
on the right into the other (adjacent-
2). With the other possibility, the upper chromosomes
go into one gamete, and the lower
two into the other (adjacent-1). In each of the
last two cases, an unbalanced distribution of
the involved chromosome segments results. For
example, after adjacent-2 segregation, gametes
receive a partial duplication of the chromosome
segment marked with red and a partial deficiency
of the segment marked with blue (left
pair of chromosomes) or a partial duplication of
the blue segment and a partial deficiency of the
red (duplication/deficiency). Different types of
disorders result depending on the chromosome
segments involved.

Different Types of Structural Chromosomal Aberrations

Structural changes in chromosomes can be
classified according to cytological types and
their effect on the phenotype. The main cytological
types are translocation (exchange) (see
p. 198), deletion (loss, see p. 182), inversion, insertion,
isochromosome, dicentric chromosome,
and ring chromosome (see below). According
to their effects, they can be differentiated
as balanced or unbalanced. With a
balanced rearrangement, no chromosomal
material has been lost or gained. In this case,
there is no effect on the phenotype. In unbalanced
aberrations, chromosomal material
has either been added (partial duplication) or
lost (partial deficiency).

Inversion

An inversion is a 180-degree change in direction
of a chromosomal segment. Prerequisite for
every inversion is a break at two different sites,
followed by reunion of the inverted segment.
Depending on whether the centromere is involved,
a pericentric inversion (when the
centromere lies within the inverted segment)
and a paracentric inversion can be differentiated.

The consequences of crossing-over in the inverted region

With homologous pairing during meiosis, an inversion
loop is formed in the region of the inversion
(1). When the inverted segment is relatively
large, crossing-over may occur in this region
(2). In the daughter cells, one chromosome
may showa duplication (e. g., of segments A and
B) and a deficiency (of segment F) (3), while the
other chromosome shows deficiency of segments
A and B and duplication of segment F (4).
These chromosome segments are not balanced
(aneusomy by recombination).

Isochromosome

An isochromosome arises when a normal chromosome
(1) divides transversely instead of
longitudinally, so that it is composed of two
long arms (2) or of two short arms (3). In each
case, the other arm is missing.

Dicentric chromosome

A dicentric chromosome contains two centromeres.
It is unstable because it is usually torn
apart during mitosis and its parts are divided
between the two daughter cells.

Ring chromosome

A ring chromosome arises after two breaks followed
by a joining of the two opposite ends. The
distal segments are lost. Therefore, a ring chromosome
is unbalanced.

Consequences of a ring chromosome

A ring chromosome is unstable because a break
with reattachment (”crossing-over”) of the
chromatids during the prophase of mitosis usually
leads to difficulties. In this case, a large ringshaped
chromosome with two centromeres
arises during metaphase and telophase. Since
the centromeres migrate in different directions
during anaphase, the ring becomes disrupted. If
this does not occur strictly symmetrically, two
daughter cells will result with certain segments
either missing (deficiency) or duplicated (duplication).
In the example, one daughter cell
with a deficiency of segment 4 and one
daughter cell with a duplication of segment 4
are formed. Not infrequently, ring chromosomes
are lost completely and a monosomy results.

Detection of Structural Chromosomal Aberrations by Molecular Methods

Structural chromosomal rearrangements occur
with a frequency of about 0.7–2.4 per 1000
mentally retarded individuals. Small supernumerary
chromosomes are observed about
once in 2500 prenatal diagnoses. In both situations
it is mandatory to identify the chromosome
or chromosomes involved. Many small
changes cannot be identified with conventional
light-microscopic chromosome analysis, even
in the best preparations using one of the banding
techniques. The proportion of identifiable
aberrations can be greatly enhanced by
molecular cytogenetics

great variety of methods

A great variety of methods is available for identifying
small rearrangements. Single-copy
probes hybridizing to specific sites on individual
chromosomes can be used to identify
specific locations on a chromosome. Numerous
probes from one chromosome can be applied to
identify a whole chromosome (chromosome
painting). In comparative genomic hybridization,
the genomic DNA of a cell population, for
example tumor cells, is hybridized to normal
metaphase chromosomes. DNA segments that
are overrepresented or underrepresented in the
tumor tissue owing to duplication or deletion
will appear as increased or decreased signals.
The preparation of extended DNA fibers (fiber
FISH) increases the resolution. The following
are selected examples of the use of multiplex
fluorescence in situ hybridization.

Derivative chromosome 1 with extra material:

46,XX,der(1)t(1 :12)(q43;p13.3)
In the conventional G-banded karyogram
(shown on the left) a small amount of additional
chromosomal material is not visible. The
karyogram by multiplex FISH on the right
shows a small extra band at the end of one chromosome
(1), shown by an arrow. The FISH
analysis reveals that the extra band at the end of
the long arm of a chromosome 1 (1 q) is derived
from a chromosome 12. The breakpoints could
be determined to be in chromosome 1 in region
4, band 3 (1q43) and chromosome 12 in region
1, band 3.3 (12p13.3).

Additional isodicentric chromosome 15: 47,XY,+psu idic (15)(q11)

A small additional chromosome is present in
the metaphase on the left (arrow) and in the
corresponding karyotype on the right. The FISH
karyotype reveals that the extra chromosome
belongs to chromosome 15. It consists of a small
isodicentric chromosome with a duplication of
the proximal long arm 15q11

Additional derivative chromosome 21

The metaphase on the left and the karyotype on
the right show a small additional chromosome
(arrow) with fluorescence in two colors.
Detailed analysis of many metaphases revealed
a compound chromosome consisting of material
from a chromosome 18 and a centromere
from a chromosome 21. Thus, the karyotype is
unbalanced for additional material from a chromosome
18 (partial trisomy 18), which is a
cause of developmental retardation. The origin
of such a compound chromosome usually remains
obscure.

Regulation and Expression of Genes

The Cell Nucleus and Ribosomal
RNA
The cell nucleus is the main center from which
cell functions are regulated. A highly important
function of cells is to produce endogenous proteins
(protein synthesis). The proteins, in turn,
are required for innumerable vital processes,
such as the catalyzation of complex biochemical
reactions, production of energy, transport of
molecules, etc. Cells from different tissues
differ with respect to the genes that are expressed.

Cell nucleus and protein synthesis

Transcription and processing of the primary
transcripts (RNA splicing) occur in the cell nucleus.
RNA in the nucleus is bound to nuclear
RNA-binding proteins for stabilization. The mature
RNA is then released from the nucleus into
the cytoplasm. For translation the mRNA must
be bound to ribosomes. Ribosomes are complex
protein structures made up of numerous subunits,
which in turn are the products of individual
genes (ribosomal genes).

Nucleolus and the synthesis of ribosomes

The nucleolus is a morphologically and
functionally specific region in the cell nucleus
in which ribosomes are synthesized. In man,
the rRNA genes (200 copies per haploid
genome) are transcribed by RNA polymerase I
to form 45 S rRNA molecules. After the 45 S
rRNA precursors have been produced, they are
quickly packaged with ribosomal proteins
(from the cytoplasm). Before they are transferred
from the nucleus to the cytoplasm, they
are cleaved to form three of the four rRNA subunits.
These are released into the cytoplasm
with the separately synthesized 5 S subunit.
Here they form functional ribosomes. The sizes
of ribosomes, their subunits, and different types
of ribosomal RNAs (rRNA) are given in Svedberg
units (S). This is the rate at which a molecule
sediments in a solvent. The S values are not additive.
A functional ribosome consists of a small
and a large subunit.

Overview of the structure and components of ribosomes

Ribosomes are the centers of protein synthesis.
They provide the workplace and the tools. The
70 S ribosome in prokaryotes consists of two
subunits of 30 S and 50 S. The 50 S subunit consists
of a large (23 S) and a small (5 S) rRNA of
~2900 and 120 nucleotides, respectively, and
33–35 different proteins; the 30 S subunit contains
a large 16 S rRNA and 21 proteins. The 50 S
subunit provides peptidyltransferase activity,
while the 30 S subunit is the site where genetic
information is decoded. The 30 S subunit also
has a proofreading mechanism to minimize errors
in translation. The whole ribosome has a
molecular weight of 2.5 million daltons (MDa)
and a sedimentation coefficient of 70 S. The
eukaryotic ribosome is much larger (4.2 MDa
and 80 S), with 60 S and 40 S subunits, which
contain an array of rRNAs and proteins as
shown in the figure. Recent observations of
bacterial 30 S and 50 S ribosomal subunit structures
at 5 Å resolution have helped to elucidate
the details of ribosomal structure and function.