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The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope, space between these two membrane is called perinuclear space. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing.
Flagella may have hairs (mastigonemes), as in many stramenopiles. Their interior is continuous with the cell's cytoplasm. [42] [43] Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots.
The cell nucleus (from Latin nucleus or nuculeus 'kernel, seed'; pl.: nuclei) is a membrane-bound organelle found in eukaryotic cells.Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many.
The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles [1] and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell and contained within the nuclear membrane is termed the nucleoplasm .
Prokaryotes lack membrane-bound organelles, such as mitochondria or a nucleus. [6] Instead, most prokaryotes have an irregular region that contains DNA, known as the nucleoid. [7] Most prokaryotes have a single, circular chromosome, which is in contrast to eukaryotes, which typically have linear chromosomes. [8]
The distinction is that eukaryotic cells have a "true" nucleus containing their DNA, whereas prokaryotic cells do not have a nucleus. [55] Eukaryotic cells are some 10,000 times larger than prokaryotic cells by volume, and contain membrane-bound organelles .
[3] [4] In the three-domain model, the first two are prokaryotes, single-celled microorganisms without a membrane-bound nucleus. All organisms that have a cell nucleus and other membrane-bound organelles are included in Eukarya and called eukaryotes. Non-cellular life, most notably the viruses, is not included in this system.
The nuclear pore complex (NPC), is a large protein complex giving rise to the nuclear pore. Nuclear pores are found in the nuclear envelope that surrounds the cell nucleus in eukaryotic cells . The nuclear envelope is studded by a great number of nuclear pores that give access to various molecules, to and from the nucleoplasm and the cytoplasm.