Humans have 23 pairs of chromosomes and having too much or too little chromosomes or sections of chromosome can lead to disease. As mentioned in the introduction to karyotypes, size is one way to distinguish chromosomes but chromosome bands also help label the chromosomes.
One pet peeve is the way that non-experts say the location within a chromosome but you’ll be an expert by the end of this piece.
The karyotypes discussed last week describe chromosomes in the metaphase of the cell cycle. Wait! This isn’t going to be a flashback to high school science classes. Basically, this is a time period during the cell cycle where the chromosomes are condensed while getting ready for cell division. There are tricks to get more of the analyzed cells to have these types of chromosomes, including culturing cells with a drug that stops cell division at metaphase (colchicine).
Metaphase chromosomes can be stained leading to distinctive banding patterns. The traditional stain is Giemsa, leading to G-bands. Cool thing: bands are the same across species.
It is tempting to think that chromosome banding patterns will be different between people but these are NOT the different bands discussed in DNA forensics. If you do a search for ‘what are chromosome bands’ online, you can often find the answer: bands are genes. This is not true!
So what are chromosome bands?
Well, these bands are chunks of chromosome and depending on the size of band and area of the chromosome, one band can encompass many genes. I’ll be honest: the mechanism of the patterns is not known but it may be related to the way the chromosome is packaged/folds up – more on packaging and epigenetics later.
Example chromosome description: Xp11.2 First, Step 1 tells us that it is the X chromosome and that it is one the short arm.
The rest of the description should be read as X-P-one-one-point-two NOT X-P-eleven-point-two. Why does this matter? These labels are like maps to help pinpoint a spot on the chromosome.
Chromosome banding has improved over the years. At first, you could only see one to three bands per chromosome arm. In our example below, there are 3 bands on the long (q) arm and two bands on the short (p) arm. You can see that the numbering starts at the centromere and goes towards the outside.
As the technique got better, more bands were seen but not every lab or person could get the increase in bands. Therefore the original bands were kept and the other bands were seen as subsections of the first set. This allowed labs to compare results even if they had different resolutions.
For our example, the location is found in the second section of the 1st sub-band of band one on the short (p) arm of the X chromosome (see right)
And guess what? The techniques kept improving, leading to more sub-bands seen. Our example has three sets of bands but they can go even more. I worked on a gene found in the Xp11.23.1 area.
One way to think about comparing sub-bands is to think of the zoom on an interactive map. You can get close but you need the positioning of the outer information first. I used to think of the descriptions starting with a helicopter view:
- Chromosome (X) = country
- Chromosome arm (Xp) = province or state
- band (Xp1) = city
- sub-band (Xp11) = neighbourhood
- sub-sub-band (Xp11.2) = street
- sub-sub-sub-band (Xp11.23) = house number
- sub-sub-sub-sub-band (Xp11.23.1) = apartment or room
I know this system of banding seems confusing and ineffective but the comparison between labs is invaluable. I was able to do karyotypes in the field and compare the results to the expert analysis done in a high tech lab.
We’ll use this knowledge to describe chromosome imbalances.