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Function Of Nucleus In Eukaryotic Cell
Nucleus, in biology, a specialized structure that occurs in most cells (except bacteria and blue-green algae) and separated from the rest of the cell by a double layer, the nuclear membrane. This membrane appears to be continuous with the endoplasmic reticulum (membrane network) of the cell and has pores that probably allow the entry of large molecules. The nucleus controls and regulates the activities of the cell (for example growth and metabolism) and carries genes, structures that contain hereditary information. Nuclei are small bodies often seen in the nucleus. The gel-like matrix in which the nuclear components are suspended is the nucleoplasm.
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Because the nucleus contains the organism’s genetic code, which determines the amino acid sequence of proteins that are critical for daily function, it serves primarily as the cell’s information center. The information in the DNA is transcribed, or copied, into a series of messenger ribonucleic acid (mRNA) molecules that encode all the information for one protein (and in some cases more than one protein, such as bacteria). The mRNA molecules are then transported across the nuclear envelope to the cytoplasm, where they are translated, serving as a template for the synthesis of specific proteins. For more information about these processes,
A cell usually contains only one nucleus. However, under some conditions, the nucleus does not divide the cytoplasm. This produces a multinucleated cell (syncytium) as occurs in skeletal muscle fibers. Some cells – for example human red blood cells – lose their nuclei after maturation. Eukaryotic organisms include protozoa, algae, fungi, plants and animals. Some eukaryotic cells are independent, single microorganisms, while others are part of multicellular organisms. The cells of eukaryotic organisms have several distinct features. First, eukaryotic cells are defined by the presence of a nucleus surrounded by a complex nuclear membrane. Even eukaryotic cells are characterized by the presence of membrane-bound organelles in the cytoplasm. Organelles such as mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes and peroxisomes are held in place by the cytoskeleton, an internal network that supports the transport of intracellular components and helps maintain cell shape (Figure (PageIndex)). The genome of eukaryotic cells is packaged into multiple rod-shaped chromosomes as opposed to the single circular chromosome that characterizes most prokaryotic cells. Table (PageIndex) compares the characteristics of the structures of eukaryotic cells with those of bacteria and archaea.
Figure (PageIndex): Illustration of a generalized, unicellular eukaryotic organism. Note that the cells of eukaryotic organisms vary greatly in structure and function, and a given cell may not have all the structures shown here.
After coming home from school, Sarah, age 7, complains that a large spot on her arm won’t stop itching. She constantly scratches him, attracting the attention of her parents. Looking more closely, they see that it is a red circular dot with a raised red border (Image (PageIndex)).
Important Points About Nucleus Of Cell
Image (PageIndex): Ringworm presents as a raised, red ring on the skin. (Credit: Centers for Disease Control and Prevention)
The next day, Sarah’s parents take her to her doctor, who examines the area with a wooden lamp. The wood’s lamp produces ultraviolet light that makes the spot on Sarah’s hand fluoresce, confirming what the doctor already suspected: Sarah has a case of mycosis. Sarah’s mother is horrified to hear that her daughter has a “worm”. How can this happen?
Unlike prokaryotic cells, in which DNA is loosely contained within the nucleoid region, eukaryotic cells possess a nucleus (plural = nuclei) surrounded by a complex nuclear membrane that holds the DNA genome (Figure (PageIndex)). Containing the DNA of the cell, the nucleus ultimately controls all the activities of the cell and also plays an essential role in reproduction and heredity. Eukaryotic cells usually have their DNA organized into multiple linear chromosomes. The DNA in the nucleus is highly organized and condensed to fit into the nucleus, which is achieved by wrapping the DNA around proteins called histones.
Figure (PageIndex): Eukaryotic cells have a well-defined nucleus. The nucleus of this mammalian lung cell is the large, dark, oval structure in the lower half of the image.
What’s The Difference Between Prokaryotic And Eukaryotic Cells?
Usually have two complete nuclei: a small nucleus used for reproduction (micronucleus) and a large nucleus that directs cell metabolism (macronucleus). In addition, some fungi temporarily form cells with two nuclei, called heterokaryotic cells, during sexual reproduction. Cells whose nuclei divide but whose cytoplasm does not divide are called coenocytes.
The nucleus is bound by a complex nuclear membrane, often called the nuclear envelope, which consists of two distinct lipid bilayers next to each other (Figure (PageIndex)). Despite these connections between the inner and outer membranes, each membrane contains unique lipids and proteins on its inner and outer surfaces. The nuclear envelope contains nuclear pores, which are large rosette-shaped protein complexes that control the movement of materials in and out of the nucleus. The overall shape of the nucleus is determined by the nuclear lamina, a network of intermediate filaments located just inside the nuclear envelope membranes. Outside the nucleus, additional intermediate filaments form a looser network and serve to anchor the nucleus in position in the cell.
Image (PageIndex): In this fluorescence microscope image, all intermediate filaments are stained with a bright green fluorescent stain. The nuclear lamina is the intense light green ring surrounding the dark red nuclei.
The nucleolus is a dense region in the nucleus where the synthesis of ribosomal RNA (rRNA) occurs. Additionally, the nucleus is also where ribosome assembly begins. How does he do that? Some chromosomes have stretches of DNA that code for ribosomal RNA. Preribosomal complexes are assembled from rRNA and proteins and ribosomal subunits in the nucleus; they are then transported to the cytoplasm, where ribosome assembly is completed (Figure (PageIndex)).
Chapter 27. Eukaryotic Cells: Origins And Diversity
Figure (PageIndex): (a) The nucleus is the dark, dense area inside the nucleus. It is the site of rRNA synthesis and preribosomal assembly. (b) Electron micrograph showing the nucleus.
Between the nucleus and the nuclear envelope is the chromatin. To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures in the nucleus that consist of DNA, the hereditary material, and carry information (genes) that is needed. You may recall that in prokaryotes DNA is organized into a single circular chromosome. Eukaryotic chromosomes are typically linear, and eukaryotic cells contain many different chromosomes. Many eukaryotic cells contain two copies of each chromosome and are therefore diploid. The length of even a single chromosome is much greater than the length of the cell, so the chromosome must be packed into a very small space to fit into the cell. For example, the combined length of all the DNA of the human genome would be about 2 meters when fully stretched, and some eukaryotic genomes are many times larger than the human genome. Each eukaryotic species has a specific number of chromosomes in the nuclei of its body cells. For example, in humans the number of chromosomes is 46, while the fruit fly has eight. Chromosomes are visible and distinguishable from each other only when the cell is preparing to divide. When a cell is in the growth and maintenance phase of its life cycle, the proteins attach to chromosomes and look like an unwanted, tangled bunch of threads. These unfolded protein-chromosomal complexes are called chromatin (Figure (PageIndex)); Chromatin describes the material that makes up chromosomes both when condensed and decondensed.
(a) This image shows different levels of chromatin organization (DNA and proteins). (b) This image shows paired chromosomes. (Credit b: Modification of work by NIH; Scale data by Matt Russell)
Cytoplasmic ribosomes in eukaryotic cells are 80S ribosomes, consisting of a 40S small subunit and a 60S large subunit. In terms of size and composition, this makes them different from the ribosomes of prokaryotic cells. The two types of cytoplasmic eukaryotic ribosomes are defined by their location in the cell: free ribosomes and membrane-bound ribosomes. Free ribosomes are found floating in the cytoplasm and serve to synthesize water-soluble proteins; Membrane-bound ribosomes are found attached to the cytoplasmic side of the rough endoplasmic reticulum and make proteins to be inserted into the cell membrane or proteins to be exported from the cell.
Biology 2e, The Cell, Cell Structure, Eukaryotic Cells
In contrast, ribosomes found in eukaryotic organelles such as mitochondria or chloroplasts have 70S ribosomes – the same size as prokaryotic ribosomes. The differences between eukaryotic and prokaryotic ribosomes are clinically relevant because different anticancer drugs are designed to target one or the other. For example, cycloheximide targets eukaryotic action, while chloramphenicol targets prokaryotic ribosomes.
Because human cells are eukaryotic, they are generally not damaged by antibodies that destroy prokaryotic ribosomes in bacteria. However, negative side effects can sometimes occur due to mitochondria in humans
