MICRO VS MACRO - A REVIEW OF STRUCTURE AN D FUNCTIONS OF AVIAN MICROCHROMOSOME AND MACROCHROMOSOME

Karyotypes of lower vertebrates mainly consist of microchromosomes. In higher vertebrates, microchromosomes are present in each class of the most primitive orders. Birds have more microchromosomes in their kary otype than other vertebrates. Accumulation of microchromosomes in the avian karyotype probably occurred after separation of birds from repti lians in Triassic, but prior to radiation of ancestors of the modem or ders (late Cretaceous-early Jurassic). In this review, the structural, molecular, and functional organization of avian macro- and microchrom osomes and their participation in genetic processes are discussed. The average size of an avian microchromosome is about 12.4 Mb, which is t en times less than the size of an average macrochromosome. In contrast to macrochromosomes, medium and small avian chromosomes lack the high est level of chromosomal organization: their chromonemes do not have s piral coiling. Microchromosomal euchromatin largely consists of GC-ric h R regions. More than half of the mapped avian genes are located on m icrochromosomes. Crossing-over frequency in microchromosomes is approx imately threefold higher than in macrochromosomes. This may be caused by high GC content and recombination hot spats, which are present on e ach microchromosome. High recombination frequency in microchromosomes increases the probability of their correct meiotic segregation.