Chromosome Structure, Banding & Karyotyping

Question 1

What is trypsin and what is it’s role?

Answer 1

An enzyme.
Partially digests some of the proteins in the chromosome and relaxes the chromatin, allowing the subsequent dye to access the DNA.

Question 2

How does staining with Giemsa produce a banding pattern?

Answer 2

Giemsa intercalates with DNA.

Attaches itself to regions where there are lots of A T bases. These tend to be late replicating regions.

Areas of heterochromatin stain darkly as they are AT-rich and gene poor. Dark regions are more condensed.

Less condensed areas incorporate less Giemsa stain and stain lightly. These areas are GC-rich and transcriptionally active.

Question 3

What are the different chromosome shapes?

Answer 3

Metacentric e.g 1, 3, 16
Sub-metacentric e.g. 4, 5, 6
Acrocentric e.g. 13, 14, 15

Question 4

What are the groups of chromosomes?

Answer 4

A (1 to 3)
B (4 and 5)
C (6 to 12)
D (13, 14, 15)
E (16, 17, 18)
F (19 and 20)
G (21 and 22)

Question 5

What is a chromosome made up of? (Basic)

Answer 5

Chromatin (DNA and protein).
Long strand of DNA which when tightly condensed forms a chromosome.
Shape comes from the DNA being wrapped round histone proteins. Every 8 histones with DNA wrapped is a nucleosome.
Histones are positively charged which attracts them to the negatively charged DNA.

Question 6

What is C-banding used for? Briefly how does it work?

Answer 6

Stains constitutive heterochromatin - C for constitutive. Mainly centromeres.
Acid treatment, hot saline incubation, alkali treatment.
C-bands are most resistant to this treatment and stain darkly when Giemsa dye is applied.