Microchromosome

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A microchromosome is an extremely small chromosome. They are common in birds and are also found in some reptiles, fish, amphibians, and monotremes. They are kept separate from the larger “macrochromosomes,” which is why the name microchromosome was used.

Size and appearance:
- By size, microchromosomes are typically less than about 0.5 micrometers in chromosome spreads and usually carry fewer than about 20 million base pairs (Mb) of DNA. Chromosomes larger than about 40 Mb are called macrochromosomes, while those between 20 and 40 Mb are intermediate.
- In species with small genomes, many chromosomes would be called microchromosomes.
- They are very small and often look alike in a karyotype, making them hard to order or identify. They usually lack the clear banding patterns and distinctive centromere locations used to identify larger chromosomes.
- During cell division (metaphase) they appear as tiny specks about 0.5–1.5 μm long.

Gene content and importance:
- Microchromosomes tend to be rich in genes and have high GC content. In chickens, they are estimated to contain 50–75% of all genes, even though they are small.
- They also tend to replicate earlier in the cell cycle and have higher rates of genetic recombination than macrochromosomes, which may help explain their high gene diversity.

Birds and evolution:
- Birds typically have about 80 chromosomes in total (2n around 78–80). Most of these are microchromosomes.
- One idea is that microchromosomes are remnants of an ancient genome made mostly of small chromosomes; macrochromosomes may have formed later by fusion of microchromosomes.
- In many mammals, microchromosomes are absent, making mammals unusual among vertebrates in this respect. Comparisons with species like the Florida lancelet suggest that the ancestor of vertebrates had many microchromosomes, and macrochromosomes arose later through fusion.

Examples and specifics:
- Chickens and other birds have been key models for studying microchromosomes. For example, chicken chromosome 16 is a microchromosome that carries the major histocompatibility complex (MHC), contributing to high genetic diversity.
- Turkeys also have about 80 chromosomes, but their karyotype shows differences in macrochromosome structure and some rearrangements involving microchromosomes as well.
- Soft-shelled turtles use microchromosomes as part of their sex-determination system.
- The monotreme platypus has an intermediate karyotype with chromosomes that are not quite microchromosomes.

In humans:
- Microchromosomes are rare in humans. A few microscopic microchromosomes have been observed in some individuals, and researchers have explored possible links to certain genetic disorders. The smallest human chromosome is chromosome 21, which is about 47 Mb.

In short, microchromosomes are very small but gene-rich chromosomes that play an important role in the genetics of birds and many other vertebrates, and they offer clues about how genomes have evolved over time.