微染色體

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^[1]；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^[2][3]；哺乳類^{[註 1]}、鱷與蛙類則無微染色體^[6]。有研究發現有些龜鱉類（刺鱉（英語：Spiny softshell turtle）與中華鱉）決定性別的ZW/ZZ染色體為微染色體^[7]。

雞的染色體核型圖，箭頭所指為大型染色體上經染色的基因條帶，點狀結構則為微染色體

微染色體的長度通常小於20Mb^[8] ，這些染色體在核型分析中較難鑑別，在有絲分裂的中期（英語：metaphase）大小僅有約1微米，且難以觀察到中節與染色條帶^[6]。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^[9]。微染色體可能是由大型染色體形成的碎片演化而來^[10]。

註腳

1. 少數唐氏症與X染色體易裂症等染色體異常疾病患者細胞中多出的小型染色體有時也被稱為微染色體^[4][5]。

參考文獻

• 分子與細胞生物學主題

1. O'Connor RE, Kiazim L, Skinner B, Fonseka G, Joseph S, Jennings R; et al. Patterns of microchromosome organization remain highly conserved throughout avian evolution.. Chromosoma. 2019, 128 (1): 21–29. PMC 6394684 . PMID 30448925. doi:10.1007/s00412-018-0685-6.
2. Nishida-Umehara C, Tsuda Y, Ishijima J, Ando J, Fujiwara A, Matsuda Y; et al. The molecular basis of chromosome orthologies and sex chromosomal differentiation in palaeognathous birds.. Chromosome Res. 2007, 15 (6): 721–34. PMID 17605112. doi:10.1007/s10577-007-1157-7.
3. Nishida C, Ishijima J, Kosaka A, Tanabe H, Habermann FA, Griffin DK; et al. Characterization of chromosome structures of Falconinae (Falconidae, Falconiformes, Aves) by chromosome painting and delineation of chromosome rearrangements during their differentiation.. Chromosome Res. 2008, 16 (1): 171–81. PMID 18293111. doi:10.1007/s10577-007-1210-6.
4. Ramos C, Rivera L, Benitez J, Tejedor E, Sanchez-Cascos A. Recurrence of Down's syndrome associated with microchromosome.. Hum Genet. 1979, 49 (1): 7–10. PMID 157321. doi:10.1007/BF00277682.
5. López-Pajares I, Delicado A, Pascual-Castroviejo I, López-Martin V, Moreno F, Garcia-Marcos JA. Fragile X syndrome with extra microchromosome.. Clin Genet. 1994, 45 (4): 186–9. PMID 8062436. doi:10.1111/j.1399-0004.1994.tb04020.x.
6. Fillon, Valérie. The chicken as a model to study microchromosomes in birds: a review. Genetics Selection Evolution. 1998, 30 (3): 209. ISSN 1297-9686. doi:10.1186/1297-9686-30-3-209.
7. Badenhorst D, Stanyon R, Engstrom T, Valenzuela N. A ZZ/ZW microchromosome system in the spiny softshell turtle, Apalone spinifera, reveals an intriguing sex chromosome conservation in Trionychidae.. Chromosome Res. 2013, 21 (2): 137–47. PMID 23512312. doi:10.1007/s10577-013-9343-2.
8. Axelsson, Erik; Webster, Matthew T.; Smith, Nick G. C.; Burt, David W.; Ellegren, Hans. Comparison of the chicken and turkey genomes reveals a higher rate of nucleotide divergence on microchromosomes than macrochromosomes. Genome Research. 2005, 15 (1): 120–5. PMC 540272 . PMID 15590944. doi:10.1101/gr.3021305.
9. McQueen, Heather A.; Siriaco, Giorgia; Bird, Adrian P. Chicken microchromosomes are hyperacetylated, early replicating, and gene rich. Genome Research. 1998, 8 (6): 621–30. PMC 310741 . PMID 9647637. doi:10.1101/gr.8.6.621.
10. Burt, D.W. Origin and evolution of avian microchromosomes. Cytogenetic and Genome Research. 2002, 96 (1–4): 97–112. PMID 12438785. doi:10.1159/000063018.