A microchromosome (μChr) is a type of very small chromosome which is a typical component of the karyotype of birds, some reptiles, fish, and amphibians; they have yet to be found in mammals. They are less than 20 Mb in size; chromosomes which are greater than 40 Mb in size are known as macrochromosomes (MChrs), while those between 20 and 40 Mb are classified as intermediate chromosomes. Microchromosomes are characteristically very small and often cytogenetically indistinguishable in a karyotype.
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| - Microchromosome (en)
- Микрохромосомы (ru)
- 微染色體 (zh)
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| - Микрохромосо́мы — очень маленькие хромосомы, типичные для кариотипов птиц, некоторых рептилий, рыб и земноводных; у млекопитающих они, судя по всему, отсутствуют. Их размер составляет меньше 20 мегабаз; хромосомы, чей размер превышает 40 мегабаз, называются макрохромосомами, а хромосомы размером от 20 до 40 мегабаз — промежуточными хромосомами. (ru)
- 微染色體（Microchromosome）是鳥類與部分爬行類、兩生類及魚類細胞中一類較小的染色體，其中以鳥類的微染色體數量最多，鳥類通常具有約2n = 80條（40對）染色體，其中僅7至10對染色體為易於鑑別的大型染色體（包括性染色體），其他均為微染色體；隼形目和鸚形目鳥類的一些微染色體彼此融合，因此染色體的總數較少；哺乳類、鱷與蛙類則無微染色體。有研究發現有些龜鱉類（與中華鱉）決定性別的ZW/ZZ染色體為微染色體。 微染色體的長度通常小於20Mb ，這些染色體在核型分析中較難鑑別，在有絲分裂的大小僅有約1微米，且難以觀察到中節與染色條帶。微染色體上帶有許多基因，例如雞基因組中有50%至75%的基因位於微染色體上。微染色體可能是由大型染色體形成的碎片演化而來。 (zh)
- A microchromosome (μChr) is a type of very small chromosome which is a typical component of the karyotype of birds, some reptiles, fish, and amphibians; they have yet to be found in mammals. They are less than 20 Mb in size; chromosomes which are greater than 40 Mb in size are known as macrochromosomes (MChrs), while those between 20 and 40 Mb are classified as intermediate chromosomes. Microchromosomes are characteristically very small and often cytogenetically indistinguishable in a karyotype. (en)
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| - PNAS 2021 article suggests clade-specific thresholds -- maybe it's not that clearly defined (en)
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| - Relationship among mentioned organisms (en)
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| - A microchromosome (μChr) is a type of very small chromosome which is a typical component of the karyotype of birds, some reptiles, fish, and amphibians; they have yet to be found in mammals. They are less than 20 Mb in size; chromosomes which are greater than 40 Mb in size are known as macrochromosomes (MChrs), 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 genes and high in GC content. In chickens, 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. μChrs, where they are found, interact closely with each other and are well-conserved. The genomic organization of Florida lancelet – part of a sister group to all vertebrates – paints a picture of a possible ancestral amniote (and vertebrates in general) genome consisting entirely of μChrs. Comparison between lancelet and modern vertebrate chromosomes shows that the MChrs are a result of fusion between μChrs. In addition, retention of μChrs is shown to be the norm; the complete loss of them in mammals is the outlier instead. (en)
- Микрохромосо́мы — очень маленькие хромосомы, типичные для кариотипов птиц, некоторых рептилий, рыб и земноводных; у млекопитающих они, судя по всему, отсутствуют. Их размер составляет меньше 20 мегабаз; хромосомы, чей размер превышает 40 мегабаз, называются макрохромосомами, а хромосомы размером от 20 до 40 мегабаз — промежуточными хромосомами. (ru)
- 微染色體（Microchromosome）是鳥類與部分爬行類、兩生類及魚類細胞中一類較小的染色體，其中以鳥類的微染色體數量最多，鳥類通常具有約2n = 80條（40對）染色體，其中僅7至10對染色體為易於鑑別的大型染色體（包括性染色體），其他均為微染色體；隼形目和鸚形目鳥類的一些微染色體彼此融合，因此染色體的總數較少；哺乳類、鱷與蛙類則無微染色體。有研究發現有些龜鱉類（與中華鱉）決定性別的ZW/ZZ染色體為微染色體。 微染色體的長度通常小於20Mb ，這些染色體在核型分析中較難鑑別，在有絲分裂的大小僅有約1微米，且難以觀察到中節與染色條帶。微染色體上帶有許多基因，例如雞基因組中有50%至75%的基因位於微染色體上。微染色體可能是由大型染色體形成的碎片演化而來。 (zh)
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