Cytogenetics

Objectives
Hands-on work at the Cytogenetics workstations
What is cytogenetics?
How is DNA condensed into a chromosome?
What are the parts of a chromosome?
How are chromosomes classified?
Mitosis
Why do we care?
How are these syndromes diagnosed?
Chromosome Preparation
Molecular Cytogenetics - Fluorescence in situ hybridization (FISH)
Cytogenetics Glossary
Interesting Web Sites
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Cytogenetics is the study of chromosomes, the structures inside a cell that carry DNA. There are two copies of each chromosome in every cell of an organism. One set of chromosomes originates from the mother and one set from the father. Many diseases and birth defects are a direct result of missing, broken, rearranged, or extra chromosomes. Scientists called cytogeneticists can recognize and identify many of these gross chromosomal abnormalities by examining chromosomes through a microscope.

Cytogeneticists use three things to tell chromosomes apart:

· chromosome size
· the position of the centromere
· characteristic banding patterns of alternating light and dark bands (caused by staining the chromosomes with dyes)

During your visit to the Cytogenetics workstation, you will learn how to prepare and recognize chromosomes, how cytogenetics is applied to patient care, and some future applications of cytogenetics.

Objectives: top

· Identify the parts of a chromosome
· Relate cytogenetics to patient care
· Experience cytogenetic technical methods
· Learn about future applications and techniques in cytogenetics

Activities at the Cytogenetics workstation I: top

· Video and discussion of clinical applications
· Drop cells onto cold glass slides
· Place on hot plate to dry

· Look at G-banded chromosomes under the microscope
· Attempt to arrange a partial karyotype
· Demonstration of computer karyotyping
· Take home G-banded karyotype

What is cytogenetics? top

Cytogenetics is the study of chromosomes, the structures that carry genes.

How is DNA condensed into a chromosome? top

By wrapping around histone proteins and then forming compressed loops of DNA.

What are the parts of a chromosome? top

Telomere – The ends of the chromosome
Centromere – The primary constriction of the chromosome.
                      The centromere also divides the chromosome into a short arm (p) and a long arm (q)
Chromatid – A single molecule of DNA


How are chromosomes classified? top

By size and position of centromere:

· Metacentric – centromere in the middle of the chromosome
· Submetacentric – centromere divides the chromosome into 1/3 and 2/3
· Acrocentric – centromere near the end of the chromosome

Mitosis top

Interphase – DNA replicates- 2 double-stranded DNA molecules
Prophase – DNA condenses into chromosomes
Metaphase – Chromosomes are maximally condensed and line up along the equatorial plate
Anaphase – Chromosomes are pulled apart into individual chromatids
Telophase – Two daughter cells are formed with exact copies of the original DNA

Why do we care? top

Many diseases and birth defects are a direct result of missing, broken, or extra chromosomes. For example:

· Down Syndrome: three copies of chromosome 21 (+21)
· Cri du chat Syndrome: missing part of the short arm of chromosome 5 (5p-)
· Turner Syndrome, 45,X: missing one of the sex chromosomes

How are these syndromes diagnosed? top

Cytogeneticists determine the chromosomal make up of an individual by using karyotypes.

Chromosome Preparation top

1. Culture blood in media and bovine calf serum
2. PHA - a mitogen is added to stimulate white blood cells to divide in culture
3. Centrifuge cells - white blood cells go to the bottom of the tube (red cells on top)
4. Hypotonic solution - causes white blood cells to swell and ruptures the red cells
5. Fixation of cells - stabilizes cells and chromosomes
6. Drop cells onto slides - cells burst open and chromosomes fall out
7. Band chromosomes by staining with Giemsa
8. Analyze under the microscope

Molecular Cytogenetics - Fluorescence in situ hybridization (FISH) top

Instead of staining chromosomes with a dye, molecular probes are detected with a fluorescent label to determine the presence or absence of chromosomes, specific DNA sequences or genes.

Cytogenetics Glossary top

Cytogenetics: The study of chromosomes.

Chromosomes: The 3-dimensional structures inside a cell that carry DNA.

G-banding: Characteristic banding patterns caused by staining the chromosomes with a dye called Geimsa.

FISH (Fluorescence in situ hybridization): A molecular cytogenetic technique in which fluorescent gene probes are used to determine the presence or absence of chromosomes, DNA specific sequences or genes.

M-FISH: Multicolor FISH, all chromosomes are distinguished by a specific color.

Histones: Proteins that are involved in compacting DNA to form a chromosome.

Nucleosome: A structure formed by the condensation of DNA with histones.

Solenoid: A structure formed by several nucleosomes to further condense DNA into the cell.

Mitosis: The means by which a cell divides and produces two daughter cells each with 46 chromosomes.

Down Syndrome: The most common single, known cause of mental retardation. Individuals have 47 chromosomes with an extra chromosome21.

Turner Syndrome: Females have 45 chromosomes and a single X chromosome. Clinical features include: short stature, webbing of the neck, and infertility.

Cri-du-chat Syndrome: Due to a deletion of the end of short arm of chromosome 5. The clinical features include: cat-like cry, profound mental retardation, and a small head.

Karyogram: Organization of the chromosomes of an individual, lined up from largest to smallest and according to location of centromere.

Centromere: The primary constriction of the chromosome, interacts with microtubule proteins to move chromosomes during mitosis.

PHA (Phytohemagglutinin): A mitogen used to stimulate white blood cells to divide in culture.

Hypotonic solution: A chemical solution that lyses red blood cells and causes white blood cells to become swollen.

Interesting Web Sites: top

Genetic Science Learning Center
Chromosomes
Karyotyping Activity Link 1
Karyotyping Activity Link 2
Karyotyping Activity Link 3

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