CHARACTERISATION OF MARKER CHROMOSOMES

 

About Authors:
Masani Jigneshkumar Naranbhai
School Of Science
Biomedical Technology
Jiwaji University, Gwalior
jigneshmasani11@gmail.com

ABSTRACT
A supernumerary marker chromosome is an extra structurally abnormal chromosome of unknown origin. Various factors contribute to the difficulty of clarifying the phenotypicrisks of super numerary marker chromosomes (SMC), including differences inthe size, structure, and origin of marker chromosomes.

All marker chromosomes do not contribute the same risk of recurrence. Therefore characterization of the marker chromosomes is of utmost importance for Genetic Counseling. Different molecular techniques can be used to identify the chromosome material present in the markers. In the present retrospective data analysis of 15,000 karyotypes of all patients referred to the Center of Medical Genetics, Sir Ganga Ram Hospital from 1998 to 2012. For constitution chromosome abnormality detection, we found fifteen (0.01%) patients had presence of the SMC. Of these fifteen, eight markers had been characterized after FISH was introduced in the year 2004.  This contributed to personalized Genetic Counseling and help for future pregnancies.


Reference Id: PHARMATUTOR-ART-1411

INTRODUCTION

WHAT IS CYTOGENETICS
Cytogenetics is the study of chromosomes structure, function, behavior and pathology. The study of human chromosome plays a role in the diagnosis, prognosis, and monitoring of treatment involving conditions seen not only by medical geneticist and genetics counselors but also by pediatricians obstetrician/gynecologist oncologist.

AIMS AND GOALS OF CYTOGENETICS STUDIES
1. Etiological studies
2. Assortment of morbidity and mortality in the affected
3. Genotype phenotype correlation
4. Recurrent risk and reproductive fitness studies
5. Break point studies in structural rearrangement

HISTORY
· The Cytogenetic era started when the first chromosomes were visualized through microscope 1880.
· The first evaluable human metaphase chromosomes spread for diagnostics was published almost six decades ago (Tijo and Levan 1956).
· This followed the detection of the first chromosomes abnormalities such as Down syndrome (Lejeune 1959), and the detection of the presence of sSMC (Ilberry et al. 1961).
· Nowadays, the G bands by trypsin- Giemsa stain (GTG) banding technique (Seabright 1971) is still the starting point and gold standard of all cytogenetic techniques. It is relatively cheap, easy to perform, and gives an overview of the whole human genome. However marker chromosomes are not always easy to identify even with G-banding.  Therefore Molecular cytogenetic techniques have to be used to characterize a marker chromosome.

 

COMMON INDICATION FOR CYTOGENETIC ANALYSIS
PRECONCEPTION
1.  Unexplained infertility or sterility
2.  Exposure to potential teratogenic agents
3.  Two or more pregnancy loss
4.  A previous child with a genetic disorder or birth defect (e.g. down        syndrome)

NEONATES AND CHILDREN
1.  A history of IUGR or failure to throne
2.  Abnormal growth patterns
3.  Ambiguous or abnormal genitalia early onset of puberty
4.  Microcephaly or Carniotenosis
5.  Hypotonia, hypertonic
6.  Dysmorphic feature
7.  Abnormal body and limb proportions
8.  Major or minor congenital abnormalities
9.  Unusual behavior

ADOLECENT AND ADULTS
1.  Abnormal sexual maturation
2.  Amenorrhea delayed puberty
3.  Growth retardation
4.  Excessive tall stature
5.  Mental retardation
6.  Hematological disorder instability syndrome

PRENATAL
1.  A women who will be 35 years of age or older at delivery.
2.  A women or her partner who carries a chromosomal rearrangement abnormality or a single gene disorder.
3.  Couples with a family history of neural tube defects.
4.  Women with an abnormal level of maternal serum.
5.  Women who exposed to infectious disease radiation drug or other environmental agents during pregnancy.
6.  Family history of a chromosomal syndrome maternal anxiety.

CHROMOSOMES
Chromosomes are thread-like structures located inside the nucleus. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

The term chromosome comes from the Greek words for color (chroma) and body (soma). Scientists gave this name to chromosomes because they are cell structures, or bodies, that are strongly stained by some colorful dyes used in research.

(genome.gov)

Chromosomes were first observed in plant cells by Karl Wilhelm von Nageli in 1842. Their behavior in animal (salamander) cells was described by Walther Flemming, the discoverer of mitosis, in 1882. The name was coined by another German anatomist, von Waldeyer in 1888.

SHAPE
Theyare basically rod shaped in outline.
The smaller larger chromosomes may appear dots with rounded or oval outline.
The larger chromosomes may be straight or bent to become arc like.

SIZE
Thesize of the normal chromosomes varies from 0.5 – 30 um in length and 0.2 – 0.3 um in breadth.

STRUCTURE
Basicallythe chromosomesconsist of centromere, secondary constrictions nucleolus organizer, telomere and satellites.

CHROMOSOME NUMBERS
·   All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number.
·   These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined.
·   The gametes contain the haploid number (n) of chromosomes.

Diploid numbers of some commonly studied organisms

Homo sapiens(human)

46

Mus musculus(house mouse)

40

Saccharomyces cerevisiae (budding yeast)

32

Zea mays (corn or maize)

20

Drosophila melanogaster (fruit fly)

8

CENTROMERE
Thechromosomes have constriction during division the centromere is functional,therefore the centromere may the locus of genes for mitotic and meiosis activity.

Dependingupon the position ofcentromere chromosomes are dividedinto-

Metacentric-centromere in the middle chromosomes appear V shaped

Submetacentic-centromere is situated some distance away from the middle.

Telomeres-The tip of the chromosomes is called telomere. It has a unique property in that it prevents the ends of chromosome from sticking together.

Telomeres are the special modified regions of chromosomes for attachment to nuclear envelope.

Inhuman Chromosomesare the structures in each of the body’s cells that carry the geneticinformation. Theycome in pairs, one from each parent, and are numbered 1 to 22 approximately from largest to smallest. The sex chromosomes are the last pair, called X & Y in males and X & X in females. Each chromosome has a short (p) arm and a long (q) arm. So we humansusually have 46 chromosomes.

Geneticistswrite this in a code (karyotype)format: 46, XX (for a female)
Or 46, XY (for a male)

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