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Meiosis Meiosis creates 4 genetically different daughter cells (n) Each cell theoretically has one complete set of chromosomes. Theoretically!!

Nondisjunction ·Error in meiosis in which a pair of homologous chromosomes fails to separate ·Results in one gamete with no copies of that particular chromosome and another gamete with an extra copy

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.