We can visualize our entire DNA by constructing a chromosome

karyotype

Karyotyping Procedure

• Sample collection and cell culture –

– White cells from a blood sample or amniotic cells from a developing fetus would be ideal.

Chorionic villus sampling (CVS)

• CVS is a prenatal test that involves taking a sample of some of the placental tissue.

• This tissue contains the

same genetic material as the fetus and can be tested for chromosomal abnormalities and some other genetic problems.

Karyotyping Procedure

• Inhibition of Mitosis with Drugs

– The cultures are treated with drugs which arrest the mitotic process at metaphase

– Chromosomes become condensed and microscopically visible during mitosis metaphase

• Certain drugs (like colchicines) interfere with the spindle fiber apparatus and the chromosomes will remain paired at the metaphase plate; hence, these drugs are used to `freeze' the chromosomes in place at a time when they are the most visible.

• To be noted: these types of drugs are commonly used as `chemotherapeutic agents' to inhibit rapidly-dividing tissues characteristic of most cancers

Karyotyping Procedure

• Separate the Chromosomes – – A hypotonic salt solution is added to the

cultured cells which causes the cells to take in water and in effect swell up

– This process causes the chromosomes which are tightly paired at the metaphase plate to separate from one another (hence each chromosome can be individually seen).

Karyotyping Procedure

• Staining – – Chemicals are used to stain the DNA which

makes up most of the chromosome composition; in this way the chromosomes are highly visible under the microscope.

Karyotyping Procedure

• Photography – – With a camera fitted to a

microscope, photographs are taken of several `mitotic plates' in which the chromosomes are visible and individually separated.

– Enlargements are made of the prints, and the chromosomes are individually cut out.

Karyotyping Procedure

• Karyotype Preparation – – The individual chromosomes are paired up according to

size and banding pattern.

– The larger chromosomes (# 1-5) are arranged on the top row, and in order of descending size, four more rows are constructed with the paired chromosomes.

– Once the 22 pairs of autosomes have been paired up, the remaining sex chromosomes (XX or XY) are positioned at the bottom right of the karyotype.

Assessment Statements • 4.2.5State that, in karyotyping, chromosomes

are arranged in pairs according to their size and structure.

• 4.2.6State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

• 4.2.7Analyze a human karyotype to determine gender and whether non-disjunction has occurred.