Nuclei and Chromosomes

Abstract

The nucleus is a double membrane-enclosed organelle in which the genome is packaged. DNA replication and RNA transcription occur in the nucleus. Plant nuclei, which have large genomes, have globular domains in the nucleoplasm. The most conspicuous structure in the nucleus is the nucleolus. The nucleolus is responsible for ribosomal RNA synthesis and ribosome assembly. The outermost structure is the nuclear membrane, which consists of inner and outer membranes. The inner membrane is backed by a lamina. Multinucleated cells are found in plants, as well as in animal skeletal muscle cells. DNA in the nucleus is coiled around histones to form chromatin, which is further packaged into chromosomes. Condensed chromosomes are clearly observed during mitosis and meiosis. Fluorescent in situ hybridization is a powerful technique for analysis of the distribution of DNA and chromosome organization. Plant sex chromosomes genetically determine the sex of the organism in dioecious plants. Accurate distribution of the genome is a direct consequence of chromosome dynamics. Chromosome alignment and segregation are regulated by the interaction of kinetochores and microtubules. Chromatin movement was recently reported in interphase nuclei, which suggests that chromatin is not stable but changes structurally. Plant growth and development processes are occasionally accompanied by endopolyploidy, which arises from genome replication without cell division. Endoreduplication and endomitosis are the two mechanisms of endopolyploidy. In endoreduplication, DNA replication during S phase is not followed by subsequent mitosis. Endomitosis lacks sister chromatid segregation or cytokinesis, resulting in a doubling of chromosome number.