Chromosomes are thread-like structures of DNA and protein found in the nucleus of eukaryotic cells. Their morphology (shape and structure) is most clearly visible during metaphase of mitosis, when they are maximally condensed.
A typical eukaryotic chromosome consists of three essential components:
The centromere is a constricted region that divides the chromosome into two arms. It is the point where:
The centromere divides each chromosome into two arms:
The relative lengths of these arms determine the chromosome type (see below).
Telomeres are repetitive DNA sequences (e.g., TTAGGG in humans) located at the tips of each chromosome arm. Their functions include:
Some chromosomes possess a secondary constriction in addition to the primary constriction (centromere). This region is called the Nucleolar Organizer Region (NOR) because:
When a secondary constriction is present, it separates a small terminal segment of the chromosome called the satellite. Chromosomes bearing a satellite are called SAT chromosomes (satellite chromosomes). In humans, chromosomes 13, 14, 15, 21, and 22 are SAT chromosomes.
Chromosomes are classified into four types based on the position of the centromere:
| Type | Centromere Position | Arm Ratio | Anaphase Shape |
|---|---|---|---|
| Metacentric | Exactly in the middle | p = q (equal arms) | V-shaped |
| Sub-metacentric | Slightly off-centre | p slightly < q | L-shaped |
| Acrocentric | Near one end | p very short, q very long | J-shaped |
| Telocentric | At the very tip | Only q arm visible | I-shaped |
Note: Telocentric chromosomes are not found in the normal human karyotype but are present in mice.
A karyotype is an organized profile of all chromosomes in a cell, arranged by size and centromere position. Karyotypes are prepared from cells arrested at metaphase, when chromosomes are most condensed and easiest to photograph and analyse. Karyotype analysis is used to: