Terminology
DNA occurs as a single strand in the form of a double helix that runs the length of the chromosome. The DNA molecule is split into genes.
Chromosomes only become visible at the onset of cell division.
Shortly before cell division, the chromosome makes a copy of itself. The single thread becomes two identical threads.
The two identical threads of DNA are known as chromatids and they lie parallel along most of their length but are joined only in a specialised region called the centromere.
- Chromatid. One of the two identical parts of the chromosome after the S phase of interphase.
- Centromere. The point where the two chromatids touch, and where the microtubules attach.
- Short (p) arm
- Long (q) arm.
Ploidy
The number of chromosomes in a set is the haploid number (n).
However, in humans, chromosomes are found in matching pairs, called homologous pairs.
In a human body cell, there are 23 pairs of homologous chromosomes (i.e. 46 chromosomes).
One set comes from your mum and the other from your dad.
Having two full sets of chromosomes is known as the diploid number (2n = 46).
The number of complete sets of chromosomes in an organism is its ploidy level.
Organisms with more than two full sets of chromosomes are known as polyploid.
Aneuploidy is the presence of an abnormal number of chromosomes in a cell.