![]() The genes located in the tightly packed form of DNA are less active in their transcription (ie. The darker region next to the nuclear envelope is heterochromatin, which is a tightly packed form of DNA. You can see some gaps between the nuclear envelope these are nuclear pores, like channels that allow transportation of molecules such as RNAs between the nucleus and the cytoplasm. The double membrane system of the nuclear envelope (outer and inner membranes) is discovered by the transmission electron microscopic image below. The DNAs of eukaryotic cells reside in the nucleus, while the DNAs of prokaryotic cells are freely distributed in the cytoplasm. In contrast, the prokaryotic cells’ DNA is located in the cytoplasm of bacteria, lacking any membrane-bound organelle. The nucleus (plural: nuclei) stores most of the cell’s genetic information in the form of DNA, although mitochondria also contain their own DNA in a very small percentage relative to the nucleus. The nucleus is a key feature that distinguishes eukaryotic cells, including all animals and plants, from prokaryotic cells (bacteria and archaea). The nucleus consists of the nuclear envelope like double-layer membranes with pores ( nuclear pores), DNA, nucleolus (a site for ribosome synthesis, plural: nucleoli), nucleoplasm (like cytoplasm of a cell), and the nuclear matrix, a supportive structure like the cytoskeleton supports cells. The cell nucleus diagram and its structure. This is why we call the nucleus the brain of the cells. The main function of the nucleus is to govern cell activities and to carry genetic information to pass to the next generation. ![]() The nucleus is a specialized organelle that contains double-layer membranes with pores. How to see the cell nucleus under a microscope. ![]() Are there human diseases associated with nucleus dysfunction.How does the nucleus divide during cell division?.Regulate gene expression and protein synthesis.What is the biological function of cell nucleus?.Where is the nucleus located in a cell, and how many cell nuclei can be found in one cell?.The electronic arrangements of the first 20 elementsĪfter the third level, the pattern is altered due to the transition series. 2 in the first level, 8 in the second, and 7 in the third). The arrangement of the electrons is 2, 8, 7 (i.e.Therefore, there are 17 protons and 17 electrons. The periodic table gives an atomic number of 17.This process is demonstrated using chlorine: Arrange the electrons in levels, always filling up an inner level before filling an outer one.This is the number of protons, and hence the number of electrons. Look up the atomic number in the periodic table.Electrons always occupy the lowest possible energy level (nearest the nucleus), provided there is space.ĭetermining the electronic arrangement of an atom These levels can be visualized as getting successively further from the nucleus. The first level (nearest the nucleus) holds two electrons, and the second and third levels each hold eight. Each energy level can only hold a certain number of electrons. Similarly, every chlorine atom (atomic number = 17) has 17 protons every uranium atom (atomic number = 92) has 92 protons.Įlectrons are found at considerable distances from the nucleus, arranged in successive energy levels. If an atom has 12 protons (atomic number = 12), it must be magnesium. If an atom has 8 protons (atomic number = 8), it must be oxygen. The atomic number is tied to the position of the element in the periodic table the number of protons therefore defines the element of interest. If there are 9 protons, there must be 10 neutrons adding up to a total of 19 nucleons in the atom. The atomic number is the number of protons (9) the mass number counts protons + neutrons (19). ![]() This information can be expressed in the following form: The mass number is also called the nucleon number. Number of protons + number of neutrons = MASS NUMBER of the atom The atomic number is also given the more descriptive name of proton number. Number of protons = ATOMIC NUMBER of the atom Virtually all the mass of the atom is concentrated in the nucleus because electrons weigh so little in comparison to the nucleons.ĭetermining numbers of protons and neutrons Protons and neutrons are collectively known as nucleons. The nucleus, located at the center of the atom, contains the protons and neutrons.
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