Microchromosome
Encyclopedia
A microchromosome is a type of very small chromosome
which is a typical component of the karyotype
of bird
s, some reptile
s, fish
, and amphibian
s; they tend to be absent in mammals. They are less than 20 Mb in size; chromosomes which are greater than 40 Mb in size are known as macrochromosomes, while those between 20 and 40 Mb are classified as intermediate chromosomes. Microchromosomes are characteristically very small and often cytogenetically indistinguishable in a karyotype
. While originally thought to be insignificant fragments of chromosomes, in species where they have been studied they have been found to be rich in gene
s. In chicken
s, microchromosomes have been estimated to contain between 50 and 75% of all genes. The presence of microchromosomes makes ordering and identifying chromosomes into a coherent karyotype
particularly difficult. During metaphase
, they appear merely as 0.5-1.5 μm long specks. Their small size and poor condensation into heterochromatin
means they generally lack the diagnostic banding patterns and distinct centromere
locations used for chromosome identification.
) usually have karyotypes of approximately 80 chromosomes (2n = 80), with only a few being distinguishable macrochromosomes and an average of 60 being microchromosomes. They are more abundant in avians than any other group of animals. Chickens are an important model organism for studying microchromosomes. Examination of microchromosomes in birds has led to the hypotheses that they may have originated as conserved fragments of ancestral macrochromosomes, and conversely that macrochromosomes could have arisen as aggregates of microchromosomes. Comparative genomic
analysis shows that microchromosomes contain genetic information which has been conserved across multiple classes of chromosomes. This indicates that at least ten chicken microchromosomes arose from fission of larger chromosomes and that the typical bird karyotype arose 100-250 mya.
Replication timing
and recombination rates have been found to differ between microchromosomes and macrochromosomes in chickens. Microchromomes replicate earlier in the S phase
of interphase
than macrochromosomes. Recombination rates have also been found to be higher on microchromosomes. Possibly due to the high recombination rates, chicken chromosome 16 (a microchromosome) has been found to contain the most genetic diversity of any chromosome in certain chicken breeds. This is likely due to the presence on this chromosome of the major histocompatibility complex (MHC).
For the many small linkage groups in the chicken genome which have not been placed on chromosomes, the assumption has been made that they are located on the microchromosomes. Interestingly, groups of these correspond almost exactly with large sections of certain human chromosomes. For example linkage groups E29C09W09, E21E31C25W12, E48C28W13W27, E41W17, E54 and E49C20W21 correspond with chromosome 7
.
s, crocodilians, and frog
s.
In rare cases, microchromosomes have been observed in the karotypes of individual humans. A link has been suggested between microchromosome presence and certain genetic disorders like Down syndrome
and fragile X syndrome
. The smallest chromosome in humans is normally chromosome 21
, which is 47 Mb.
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
which is a typical component of the karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
of bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, some reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, and amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s; they tend to be absent in mammals. They are less than 20 Mb in size; chromosomes which are greater than 40 Mb in size are known as macrochromosomes, while those between 20 and 40 Mb are classified as intermediate chromosomes. Microchromosomes are characteristically very small and often cytogenetically indistinguishable in a karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
. While originally thought to be insignificant fragments of chromosomes, in species where they have been studied they have been found to be rich in gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s. In chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...
s, microchromosomes have been estimated to contain between 50 and 75% of all genes. The presence of microchromosomes makes ordering and identifying chromosomes into a coherent karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
particularly difficult. During metaphase
Metaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
, they appear merely as 0.5-1.5 μm long specks. Their small size and poor condensation into heterochromatin
Heterochromatin
Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...
means they generally lack the diagnostic banding patterns and distinct centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
locations used for chromosome identification.
In avians
Birds (except falconiformesFalconiformes
The order Falconiformes is a group of about 290 species of birds that comprises the diurnal birds of prey. Raptor classification is difficult and the order is treated in several ways.- Classification problems :...
) usually have karyotypes of approximately 80 chromosomes (2n = 80), with only a few being distinguishable macrochromosomes and an average of 60 being microchromosomes. They are more abundant in avians than any other group of animals. Chickens are an important model organism for studying microchromosomes. Examination of microchromosomes in birds has led to the hypotheses that they may have originated as conserved fragments of ancestral macrochromosomes, and conversely that macrochromosomes could have arisen as aggregates of microchromosomes. Comparative genomic
Genomics
Genomics is a discipline in genetics concerning the study of the genomes of organisms. The field includes intensive efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts. The field also includes studies of intragenomic phenomena such as heterosis,...
analysis shows that microchromosomes contain genetic information which has been conserved across multiple classes of chromosomes. This indicates that at least ten chicken microchromosomes arose from fission of larger chromosomes and that the typical bird karyotype arose 100-250 mya.
Chickens
Chickens have a diploid number of 78 (2n = 78) chromosomes, and as is usual in birds, the majority are microchromosomes. Classification of chicken chromosomes varies by author. Some classify them as 8 pairs of macrochromosomes, one pair of sex chromosomes, with the remaining 32 pairs being intermediate or microchromosomes. Other arrangements such as that used by the International Chicken Genome Sequencing Consortium include five pairs of macrochromosomes, five pairs of intermediate chromosomes, and twenty-eight pairs of microchromosomes. Microchromosomses represent approximately one third of the total genome size, and have been found to have a much higher gene density than macrochromosomes. Because of this, it is estimated that the majority of genes are located on microchromosomes, though due to the difficulty in physically identifying microchromosomes and the lack of microsatellite markers, it has been difficult to place genes on specific microchromosomes.Replication timing
Replication timing
Replication Timing refers to the order in which segments of DNA along the length of a chromosome are duplicated.-DNA Replication:In eukaryotic cells , chromosomes consist of very long linear double-stranded DNA molecules...
and recombination rates have been found to differ between microchromosomes and macrochromosomes in chickens. Microchromomes replicate earlier in the S phase
S phase
S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...
of interphase
Interphase
Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for cell division. In preparation for cell division, it increases its size and makes a copy of its DNA...
than macrochromosomes. Recombination rates have also been found to be higher on microchromosomes. Possibly due to the high recombination rates, chicken chromosome 16 (a microchromosome) has been found to contain the most genetic diversity of any chromosome in certain chicken breeds. This is likely due to the presence on this chromosome of the major histocompatibility complex (MHC).
For the many small linkage groups in the chicken genome which have not been placed on chromosomes, the assumption has been made that they are located on the microchromosomes. Interestingly, groups of these correspond almost exactly with large sections of certain human chromosomes. For example linkage groups E29C09W09, E21E31C25W12, E48C28W13W27, E41W17, E54 and E49C20W21 correspond with chromosome 7
Chromosome 7 (human)
Chromosome 7 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 7 spans more than 158 million base pairs and represents between 5 and 5.5 percent of the total DNA in cells.Identifying genes on each chromosome is an active area of genetic...
.
Turkey
The turkey has a diploid number of 80 (2n = 80) chromosomes. The karyotype contains an additional chromosomal pair relative to the chicken due to the presence of at least two fission/fusion differences (GGA2 = MGA3 and MGA6 and GGA4 = MGA4 and MGA9). Given these differences involving the macrochromosomes, an additional fission/fusion must also exist between the species involving the microchromosomes if the diploid numbers are valid. Other rearrangements have been identified through comparative genetic maps, physical maps and whole genome sequencing.In humans and other animals
Microchromosomes are absent from the karyotypes of mammalMammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, crocodilians, and frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...
s.
In rare cases, microchromosomes have been observed in the karotypes of individual humans. A link has been suggested between microchromosome presence and certain genetic disorders like Down syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
and fragile X syndrome
Fragile X syndrome
Fragile X syndrome , Martin–Bell syndrome, or Escalante's syndrome , is a genetic syndrome that is the most commonly known single-gene cause of autism and the most common inherited cause of intellectual disability...
. The smallest chromosome in humans is normally chromosome 21
Chromosome 21 (human)
Chromosome 21 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. The trisomy of the 21st chromosome causes Down Syndrome...
, which is 47 Mb.