专业外语Unit 5.ppt

Unit 5 lesson 3 The Generalized Eukaryotic Cell (2) The Generalized Eukaryotic Cell Within the nucleus, either suspended in the interior or lying close to the nuclear envelope, is a body of irregular outline and variable size, the nucleolus. Although no membranes separate the nucleolus from the rest of the nucleus, this structure is morphologically distinct and can be readily identified as a concentrated mass of dense granules and fibrils in the nucleoplasm. The granules of the nucleolus, often numbering in the thousands, are about 150 ? in diameter and somewhat indistinct or fuzzy in outline. Nucleolus 核仁 Concentrated 浓缩的 indistinct 轮廓不清晰 Fuzzy 模模糊糊 fuzzy maths 模糊数学 The Generalized Eukaryotic Cell These granular zones alternate with the masses of fibrils and spaces containing parts of the chromatin and nucleoplasm that extend into the substance of the nucleolus. The nucleolus contains large quantities of RNA and protein; by various indirect means this structure has been shown to be the site of the synthesis and assembly of the subunits of cytoplasmic ribosomes. The Generalized Eukaryotic Cell The nucleolus is directly connected to a segment of the chromosomes called the nucleolar organizer. While this connection is not apparent in interphase cells, the attachment becomes visible as the chromatin condenses into distinct chromosomes during division of the nucleus. In most cell types, a nucleolar organizer site is present on both members of at least one set of homologous chromosomes. These either remain separate at interphase to form multiple nucleoli or fuse to form a single large nucleolus. Interphase 间期 The membranous envelope separating the nucleus from the cytoplasm is a double system consisting of two concentric membranes. The two membranes of the envelope are separated from each other by a narrow enclosed space 200- 300 ? in diameter, the perinuclear space. A