TY - JOUR

T1 - Micro versus Macro

T2 - A Review of Structure and Functions of Avian Micro- and Macrochromosomes

AU - Rodionov, A. V.

PY - 1996/5/1

Y1 - 1996/5/1

N2 - 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 karyotype than other vertebrates. Accumulation of microchromosomes in the avian karyotype probably occurred after separation of birds from reptilians in Triassic, but prior to radiation of ancestors of the modern orders (late Cretaceous-early Jurassic). In this review, the structural, molecular, and functional organization of avian macro- and microchromosomes and their participation in genetic processes are discussed. The average size of an avian microchromosome is about 12.4 Mb, which is ten times less than the size of an average macrochromosome. In contrast to macrochromosomes, medium and small avian chromosomes lack the highest level of chromosomal organization: their chromonemes do not have spiral coiling. Microchromosomal euchromatin largely consists of GC-rich R regions. More than half of the mapped avian genes are located on microchromosomes. Crossing-over frequency in microchromosomes is approximately threefold higher than in macrochromosomes. This may be caused by high GC content and recombination hot spots, which are present on each microchromosome. High recombination frequency in microchromosomes increases the probability of their correct meiotic segregation.

AB - 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 karyotype than other vertebrates. Accumulation of microchromosomes in the avian karyotype probably occurred after separation of birds from reptilians in Triassic, but prior to radiation of ancestors of the modern orders (late Cretaceous-early Jurassic). In this review, the structural, molecular, and functional organization of avian macro- and microchromosomes and their participation in genetic processes are discussed. The average size of an avian microchromosome is about 12.4 Mb, which is ten times less than the size of an average macrochromosome. In contrast to macrochromosomes, medium and small avian chromosomes lack the highest level of chromosomal organization: their chromonemes do not have spiral coiling. Microchromosomal euchromatin largely consists of GC-rich R regions. More than half of the mapped avian genes are located on microchromosomes. Crossing-over frequency in microchromosomes is approximately threefold higher than in macrochromosomes. This may be caused by high GC content and recombination hot spots, which are present on each microchromosome. High recombination frequency in microchromosomes increases the probability of their correct meiotic segregation.

UR - http://www.scopus.com/inward/record.url?scp=0001659126&partnerID=8YFLogxK

M3 - Review article

AN - SCOPUS:0001659126

VL - 32

SP - 517

EP - 527

JO - Russian Journal of Genetics

JF - Russian Journal of Genetics

SN - 1022-7954

IS - 5

ER -