We will start talking about human genetics at the point of chromosomes.We need to know the basics of chromosome organization and identification to understand the mechanisms and tests for common chromosome disorders.
A chromosome is a piece of DNA that carries many genes. In humans, we have 46 chromosomes (23 pairs) and they are found in the protected nucleus of cells.
The basic structure of the human chromosome:
Centromeres: a specialized region of the chromosome that condenses during cell division (mitosis). I like to think about the centromere as the navigator of the cell (more about this during nondisjuntion discussions).
The centromere separates the chromosomes into two arms:p arm: shorter. The p stands for ‘petit’ or small in French. q arm: longer. Some say that q stands for ‘queue’ (tail) and others think it is just the letter after p.
The placement of the centromere can help describe the chromosome:metacentric: centromere in the middle submetacentric: centromere is off-centre, leading to unequal chromosome arms acrocentric: centromere at one end so the p (short) arm is small so it’s hard to see on a karyotype
Note: human chromosomes are always shown with the q (longer) arm on the bottom.
Humans have 46 chromosomes = 23 pairs. We get one set from our mom and one from our dad. Most of the chromosomes are numbered 1 to 22 and arranged in size from largest to smallest. Chromosome 1 is the largest chromosome and 21 and 22 are the smallest. You see, cytogeneticists aren’t perfect and chromosome 21 is actually the smallest – but it is close enough to 22 so the numbering stands as first described.
The last pair of chromosomes are the sex chromosomes: X and Y. In general, women have two X chromosomes and men have one X and one Y. (We’ll talk about exceptions later).
You can see how the chromosomes are arranged by size here:
The chromosomes are visualized under a light microscope and arranged by cytogeneticists to line up by size. The resulting image is a karyotype. Sadly, the chromosomes do not normally follow the science convention of lining up and usually just look like scribbles of short lines – so scientists have to rearrange photos into the usual karyotype image. The chromosomes are often folded up so the size differences aren’t always obvious.
You can see dark and light sections of the chromosomes = banding. Along with size, these bands can further distinguish the chromosomes. We will cover how to follow the bands next week. For now, it is important to note that one band does not equal one gene.