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Cell Nucleolus: Within most types of eukaryotic Cell nucleus, a distinct region, not delimited by a membrane, in which some species of rRNA (RNA, Ribosomal) are synthesized and assembled into ribonucleoprotein subunits of ribosomes. In the nucleolus rRNA is transcribed from a nucleolar organizer, i.e., a group of tandemly repeated chromosomal genes which encode rRNA and which are transcribed by RNA polymerase I. (Singleton & Sainsbury, Dictionary of Microbiology & Molecular Biology, 2d ed)

The Nucleus

JoVE 10691

The nucleus is a membrane-bound organelle that contains a eukaryotic organism’s genetic instructions in the form of chromosomal DNA. This is distinct from the DNA in mitochondria or chloroplasts that carry out functions specific to those organelles. While some cells—such as red blood cells—do not have a nucleus, and others—such as skeletal muscle cells—have multiple nuclei, most eukaryotic cells have a single nucleus. The DNA in the nucleus is wrapped around proteins such as histones, creating a DNA-protein complex called chromatin. When cells are not dividing—that is, when they are in the interphase part of their cell cycle—the chromatin is organized diffusely. This allows easy access to the DNA during the transcription process when messenger RNA (mRNA) is synthesized based on the DNA code. When a eukaryotic cell is about to divide, the chromatin condenses tightly into distinct, linear chromosomes. Humans have 46 chromosomes in total. Chromatin is particularly concentrated in a region of the nucleus called the nucleolus. The nucleolus is important for the production of ribosomes, which translate mRNA into protein. In the nucleolus, ribosomal RNA is synthesized and combined with proteins to create ribosomal subunits, which later form functioning ribosomes in the cytoplasm of the cell. The interior of t

 Core: Biology

Ribosomes

JoVE 10692

Ribosomes translate genetic information encoded by messenger RNA (mRNA) into proteins. Both prokaryotic and eukaryotic cells have ribosomes. Cells that synthesize large quantities of protein—such as secretory cells in the human pancreas—can contain millions of ribosomes.

Ribosomes are composed of ribosomal RNA (rRNA) and proteins. Ribosomes are not surrounded by a membrane (i.e., despite their specific cell function, they are not an organelle). In eukaryotes, rRNA is transcribed from genes in the nucleolus—a part of the nucleus that specializes in ribosome production. Within the nucleolus, rRNA is combined with proteins that are imported from the cytoplasm. The assembly produces two subunits of a ribosome—the large and small subunits. These subunits then leave the nucleus through pores in the nuclear envelope. Each one large and small subunit bind to each other once mRNA binds to a site on the small subunit at the start of the translation process. This step forms a functional ribosome. Ribosomes may assemble in the cytosol—called free ribosomes—or while attached to the outside of the nuclear envelope or endoplasmic reticulum—called bound ribosomes. Generally, free ribosomes synthesize proteins used in the cytoplasm, while bound ribosomes synthesize proteins that are inserted into membranes, packaged into org

 Core: Biology

Mitosis and Cytokinesis

JoVE 10762

In eukaryotic cells, the cell's cycle—the division cycle—is divided into distinct, coordinated cellular processes that include cell growth, DNA replication/chromosome duplication, chromosome distribution to daughter cells, and finally, cell division. The cell cycle is tightly regulated by its regulatory systems as well as extracellular signals that affect cell proliferation. The processes of the cell cycle occur over approximately 24 hours (in typical human cells) and in two major distinguishable stages. The first stage is DNA replication, during the S phase of interphase. The second stage is the mitotic (M) phase, which involves the separation of the duplicated chromosomes into two new nuclei (mitosis) and cytoplasmic division (cytokinesis). The two phases are separated by intervals (G1 and G2 gaps), during which the cell prepares for replication and division. Mitosis can be divided into five distinct stages—prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis, which begins during anaphase or telophase (depending on the cell), is part of the M phase, but not part of mitosis. As the cell enters mitosis, its replicated chromosomes begin to condense and become visible as threadlike structures with the aid of proteins known as condensins. The mitotic spindle apparatus b

 Core: Biology

Identification of Nucleolar Factors During HIV-1 Replication Through Rev Immunoprecipitation and Mass Spectrometry

1Molecular and Cellular Biology Department, Beckman Research Institute at the City of Hope, 2Irell & Manella Graduate School of Biological Sciences, 3Department of Molecular Immunology, Beckman Research Institute and the City of Hope

JoVE 59329

 Immunology and Infection

In Situ Detection and Single Cell Quantification of Metal Oxide Nanoparticles Using Nuclear Microprobe Analysis

1Centre d'Etudes Nucléaires Bordeaux Gradignan (CENBG), Université de Bordeaux, 2Centre d'Etudes Nucléaires Bordeaux Gradignan (CENBG), CNRS, 3Institut de Chimie de la Matière Condens é e de Bordeaux (ICMCB), CNRS, 4Institut de Chimie de la Matière Condens é e de Bordeaux (ICMCB), Université de Bordeaux

JoVE 55041

 Chemistry
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