Overview: This unit is designed to explore chromosome structure and karyotypes. It will involve use of the Internet, video microscopy, and general laboratory techniques. This unit has been designed to be used with middle school students and assumes that basic cell structure and function have been addressed previously.

Teacher Background:

The cell is the basic unit of structure and function in living organisms. If eukaryotic, it contains organelles which carry out the regulatory functions within the cell. The nucleus of a eukaryotic cell contains the reproductive material (DNA) in the form of chromosomes, stored within the nuclear membrane.

The word chromosome comes from the Greek language and means "colored body". Unless the cell is in the process of dividing, the form in which the chromosomes appear is chromatin. Chromatin is made up of DNA and histones; these are arranged as nucleosomes. Chromosomes are tightly coiled and if uncoiled these DNA strands would stretch out several meters (e.g. the human chromosomes would stretch to about 2 meters of DNA.).

Chromosomes for any given species are distinguished by size and location of the centromere. They are studied under the microscope after being prepared and stained. Chromosomes are examined during the metaphase portion of mitotic division, since this is when they are most coiled and therefore most distinct. The chromosomes are then photographed,cut out, and arranged from the largest to smallest. By convention, the shorter arms of the chromosomes (p arms) are on top; the longer arms (q)below. This arrangement is referred to as a karyotype.

The number of chromosomes varies among species, but for each species there is a particular number of chromosomes (n) per haploid set. Since the organisms for this exercise are diploid, each cell has 2 sets (2n). For example, humans have 23 chromosomes per haploid set; 2n = 46. Drosophila have a haploid number of 4; 2n = 8. During mitosis, the 2n number is maintained in the daughter cells; in meiosis, the number is reduced to n.

In each species, chromosomes in a karyotype are numbered and paired so that each pair contains one set of homologues: one maternal and one paternal. From the staining procedures applied a pattern of banding appears which is unique to each chromosome. It is this banding which allows for specific genes to be located on a region of a chromosome. The arms are further subdivided into regions which are numbered beginning with the centromere. This numbering system allows each gene to have a specific "address".

Here is an example of how to use the chromosome "address". First you list the chromosome number (1)locater under the chromosome. You then list the arm (p). following the arm you list the region (3) followed by the band (5) . The address would then be written 1p3.5.

In research, karyotypes may be used to characterize and to compare species, and to investigate the evolutionary origins of related species. In medicine, physicians analyze chromosome numbers and physical integrity vs. rearrangements. Abnormal numbers or rearrangements of chromosomes represent mutations and indicate abnormalities of cells or of the whole individual.