Circular chloroplast chromosomes: The grand illusion
Circular chloroplast chromosomes: The grand illusion
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DOI：
10.1105/tpc.160771
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发表时间：
2004-07-01
期刊：
影响因子：
11.6
通讯作者：
Bendich, AJ
中科院分区：
文献类型：
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作者：
Bendich, AJ
如果我们能从叶绿体中提取、纯化并可视化完整的DNA分子，这些分子会是什么样子？大多数人预计会发现叶绿体基因组大小的环状DNA分子。然而，相比之下，从叶绿体中获得的DNA中只有一小部分是基因组大小的圆圈。这一深刻差异的原因是本文的主题。我将追溯叶绿体DNA(CpDNA)的研究历史，以阐明为什么花了30多年的时间才意识到环不是叶绿体中DNA的主要形式，并研究了叶绿体基因组与叶绿体中分离的遗传单位或染色体之间的关系。关键的发现是叶绿体染色体可以包含许多基因组等价物。我还将讨论染色体复制和叶绿体分离之间的耦合。为了避免混淆，我定义了一些术语。基因组是病毒、原核生物、线粒体或叶绿体中遗传物质的完整补充，或真核物种的单倍体核遗传补充。基因组当量是指基因组的单个拷贝中的DNA含量。染色体是分离的遗传单位，在分裂过程中将基因组的一个子集或全部携带到每个子细胞或细胞器中。例如，蚂蚁Myrmecia Pilosula在细胞核中有一对同源染色体(Crosland和Crozier，1986)。单个染色体中的DNA数量等于基因组中的DNA数量。然而，在大多数蚂蚁和大多数真核生物中，单个核染色体的DNA含量低于基因组的DNA含量，并且分离的遗传单位的DNA含量永远不会超过基因组的DNA含量。大多数细菌的基因组，如大肠埃希氏菌，都包含在一个被称为类核的含有DNA的体内。当类核在分裂时分离到子细胞时，该类核代表细菌染色体。然而，与在真核细胞中发现的相反，在快速分裂的细胞中，每条细菌染色体的DNA数量通常比一个相当于一个基因组的DNA数量大几到几倍。正如我将要展示的，叶绿体中的情况类似于细菌中的情况，叶绿体染色体的大小很少像一个基因组那么小。
If we could extract, purify, and visualize the intact DNA molecules from chloroplasts, what would those molecules look like? Most would expect to find circular DNA molecules the size of the chloroplast genome. By contrast, however, only a small fraction of the DNA obtained from chloroplasts is found as genome-sized circles. The reasons for this profound discrepancy are the subject of this article. I will trace the history of research on chloroplast DNA (cpDNA) to elucidate why it has taken more than 30 years to realize that the circle is not the principal form of DNA in chloroplasts and to examine the relationship between the chloroplast genome and the segregating genetic unit or chromosome in chloroplasts. The critical finding is that the chloroplast chromosome can contain many genome equivalents. I will also discuss the coupling between chromosomal replication and segregation in chloroplasts. To avoid confusion, I define some terms. The genome is the entire complement of genetic material in a virus, prokaryote, mitochondrion, or chloroplast or the haploid nuclear genetic complement of a eukaryotic species. A genome equivalent is the amount of DNA in a single copy of a genome. A chromosome is the segregating genetic unit that carries either a subset or all of the genome into each daughter cell or organelle during division. For example, the ant Myrmecia pilosula has a single pair of homologous chromosomes in the nucleus (Crosland and Crozier, 1986). The amount of DNA in a single chromosome equals the amount of DNA in the genome. In most ants and most eukaryotes, however, the DNA content of a single nuclear chromosome is less than that of the genome, and the DNA content of a segregating genetic unit never exceeds that of the genome. The genome in most bacteria, such as Escherichia coli, is contained on a single DNA-containing body known as the nucleoid. As a nucleoid segregates to daughter cells upon division, that nucleoid represents a bacterial chromosome. In contrast with what is found in the eukaryotic nucleus, however, the amount of DNA per bacterial chromosome in rapidly dividing cells is typically several to many times larger than one genome equivalent. As I will show, the situation in chloroplasts is similar to that in bacteria, and the size of the chloroplast chromosome is rarely as small as one genome equivalent.