Introduction to Cell & Molecular Biology (BIOL121) - Dr. S.G. Saupe (ssaupe@csbsju.edu); Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321 |
Karyotypes
A karyotype is a pictorial representation of an individual's chromosomes. Click here for an example of a karyotype of a normal female and a normal male. Note that the chromosomes are arranged into groups according to size, centromere position (metacentric = middle of chromosome; acrocentric = end of chromosome; submetacentric = somewhere between end and middle) and the presence of satellites (small protrusions). The advantage of a karyotype is that it enables a scientist to quickly determine the number of autosomes and sex chromosomes and whether or not the individual is male or female.
Karyotypes are usually prepared from blood. The red blood cells (which lack a nucleus and hence, chromosomes) are removed and then the white blood cells (which do possess a nucleus) are treated with colchicine. Colchicine stops mitosis at metaphase (by preventing the formation of microtubules) when chromosomes are spread out for the easiest viewing. The treated cells are fixed, stained and examined microscopically. A photograph is taken and enlarged (now done by computer). The chromosomes are cut out and arranged according to a convention, called the Denver classification (see below), that was established by a group of cytogeneticists.
Table
of Denver Chromosome Classification |
||
Chromosome
Group |
Chromosome
number |
Description
of chromosomes |
A |
1
- 3 |
Large,
metacentric |
B |
4
- 5 |
Large,
submetacentric |
C |
6
- 12 |
Medium-sized,
submetacentric; X chromosome closely resembles pair number 6, longest in
group |
D |
13
- 15 |
Medium-sized,
acrocentric; all three pairs with satellites on short arm |
E |
16
- 18 |
Smallest
of the medium-sized; pair 16 metacentric; others submetacentric |
F |
19
- 20 |
Short,
metacentric |
G |
21
- 22 |
very
short; acrocentric; both with satellites |
Exercise 1.
Karyotype Analysis
Obtain a copy of the "Unknown
karyotype" and then answer the following questions:
How many total chromosomes does this individual possess?
How many autosomes?
How many sex chromosomes?
Is this individual male or female?
Is this individual normal or show some aneuploiod condition? (missing or with an extra sex chromosome or autosome)
Is this
individual missing a part of a chromosome?
Exercise 2. Karyotype Preparation
Obtain
a karyotype preparation sheet (Click here) and a
one of the chromosome karyotype
(either karyotype 3,
karyotype 6,
karyotype 7). Record the number
of the karyotype on the preparation sheet.
Cut
the chromosomes apart. Don't
cut along the detail of each chromosome, but cut out squares or rectangles
each containing one chromosome.
Before
cutting, count the chromosomes to ensure you don't loose any.
And, this will be important later.
Arrange
the chromosomes in order, from largest to smallest.
(Don't tape/glue down yet)
Match
up homologous chromosomes. Remember
they will be of similar length and shape (i.e., centromere position,
satellites) and banding patterns.
Once
the chromosomes have been arranged, tape or glue them down with the
centromere on the dotted (longer arms should point down) in their
appropriate groups on the sheet provided.
A table of the Denver classification scheme is provided to help you
place the chromosomes in their respective groups.
Analyze the karyotype. Answer the following questions:
How many total chromosomes does this individual possess?
How many autosomes?
How many sex chromosomes?
Is this individual male or female?
Is this individual normal or show some aneuploid condition? (missing or with an extra sex chromosome or autosome)
Is this individual missing a part of a chromosome?
Last updated: July 14, 2009 � Copyright by SG Saupe