Molecular Genetics and Cytogenesis

What is a karyotype?

An organized display of all chromosomes from a cell arranged in homologous pairs according to size, centromere position and banding pattern

Which stage of mitosis is best used for karyotyping?

Metaphase because chromosomes are maximally condensed and visible

What is the first step in karyotyping?

Collection of cells capable of division such as peripheral blood lymphocytes

Why are lymphocytes cultured during karyotyping?

To stimulate cell division and increase the number of metaphase cells

What substance is added to arrest cells in metaphase during karyotyping?

Colchicine (or colcemid)

What is the purpose of colchicine in karyotyping?

Prevents spindle formation and arrests cells in metaphase

Why are chromosomes stained during karyotyping?

To visualize characteristic banding patterns for chromosome identification

What three chromosome features are assessed during karyotype analysis?

Length, centromere position and banding pattern

What is the normal male karyotype?

46,XY

What is the normal female karyotype?

46,XX

What is the short arm of a chromosome called?

p arm

What is the long arm of a chromosome called?

q arm

What chromosomal abnormality is seen in Down syndrome?

Trisomy 21 (47,+21)

What chromosomal abnormality is seen in Turner syndrome?

Monosomy X (45,X)

What chromosomal abnormality is seen in Klinefelter syndrome?

47,XXY

What type of chromosomal abnormalities can karyotyping detect?

Numerical abnormalities and large structural abnormalities

What are examples of structural chromosomal abnormalities?

Deletion, duplication, inversion, insertion and translocation

What is G-banding?

Chromosome staining using Giemsa stain producing characteristic dark and light bands

Which chromosome banding technique is most commonly used in clinical cytogenetics?

G-banding

What type of chromatin is represented by dark G-bands?

AT-rich heterochromatin

What type of chromatin is represented by light G-bands?

GC-rich euchromatin

What is R-banding?

Reverse banding pattern of G-banding highlighting GC-rich regions

What is the key feature of R-banding?

Produces a banding pattern opposite to G-banding

What stain is used in Q-banding?

Quinacrine fluorescent dye

What is the major characteristic of Q-banding?

Fluorescent banding pattern similar to G-banding

Which banding technique primarily stains centromeres?

C-banding

What type of chromatin is highlighted by C-banding?

Constitutive heterochromatin around centromeres

Which chromosome banding method is associated with centromere identification in exams?

C-banding

What does FISH stand for?

Fluorescence In Situ Hybridization

What is the principle of FISH?

A fluorescent DNA probe hybridizes to a complementary DNA sequence on a chromosome

What is the main advantage of FISH over conventional karyotyping?

Higher resolution for detecting small chromosomal abnormalities

What type of abnormalities is FISH particularly useful for detecting?

Microdeletions, microduplications and specific translocations

Which molecular cytogenetic technique is commonly used to confirm DiGeorge syndrome?

FISH

What chromosomal abnormality causes DiGeorge syndrome?

22q11 microdeletion

What does CGH stand for?

Comparative Genomic Hybridization

What is the principle of CGH?

Comparison of fluorescently labelled patient DNA with normal reference DNA to detect copy number changes

What abnormalities can CGH detect?

Chromosomal gains, losses, deletions and duplications

What major limitation does CGH have?

Cannot detect balanced translocations or balanced inversions

Which molecular cytogenetic technique provides genome-wide screening for copy number changes?

CGH

Compare karyotyping, FISH and CGH

Karyotyping detects large chromosomal abnormalities, FISH detects targeted small abnormalities, and CGH detects genome-wide copy number gains and losses with high resolution