Microchromosomes

Microchromosomes are very tiny chromosomes that are present in varying numbers in the karyotypes of all vertebrates except mammals. For avian genomes, any chromosomes that are smaller than 20 mega base pairs are characterized as microchromosomes. For many other species, classification of microchromosomes is based upon the presence of a large size demarcation between groups of chromosomes and smaller chromosomes, which are nonoverlapping. For example, the largest microchromosome pair is always smaller in size than the smallest macrochromosome pair in the complement. Because of their small size (often <1.5 µM), microchromosomes are very difficult to characterize using standard chromosome banding techniques. Microchromosome number also varies from species to species ranging from a few pairs (e.g., Falconiformes) up to 30–35 pairs (e.g., in most birds and in many fish). Through comparative genomic analysis it has been shown that microchromosomes have evolved (at least in birds) through chromosome fusion. Microchromosomes are typically guanine and cytosine (GC)-rich and gene-rich, hypermethylated, and hyperacetylated and are hot spots for recombination. Microchromosomes also contain fewer repetitive sequences than macrochromosomes and replicate early during cell division.