Additional genes are located in mitochondria and chloroplasts The nucleus

Additional genes are located in mitochondria and

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Additional genes are located in mitochondria and chloroplasts. The nucleus averages about 5 microns in diameter. The nucleus is separated from the cytoplasm by a double membrane called the nuclear envelope. The two membranes of the nuclear envelope are separated by 20–40 nm. The envelope is perforated by pores that are about 100 nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are fused to form a continuous membrane. A protein structure called a pore complex lines each pore, regulating the passage of certain large macromolecules and particles. The nuclear side of the envelope is lined by the nuclear lamina, a network of protein filaments that maintains the shape of the nucleus. There is evidence that a framework of fibers called the nuclear matrix extends through the nuclear interior. Within the nucleus, the DNA and associated proteins are organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome is made up of fibrous material called chromatin, a complex of proteins and DNA. Stained chromatin appears through light microscopes and electron microscopes as a diffuse mass. As the cell prepares to divide, the chromatin fibers coil up and condense, becoming thick enough to be recognized as the familiar chromosomes. Each eukaryotic species has a characteristic number of chromosomes. A typical human cell has 46 chromosomes. A human sex cell (egg or sperm) has only 23 chromosomes. In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus. In the nucleolus, ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits. The subunits pass through the nuclear pores to the cytoplasm, where they combine to form ribosomes. The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA). The mRNA travels to the cytoplasm through the nuclear pores and combines with ribosomes to translate its genetic message into the primary structure of a specific polypeptide.
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Ribosomes build a cell’s proteins. Ribosomes, containing rRNA and protein, are the organelles that carry out protein synthesis. Cell types that synthesize large quantities of proteins (e.g., pancreas cells) have large numbers of ribosomes and prominent nucleoli. Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol. Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic reticulum or nuclear envelope. These synthesize proteins that are either included in membranes or exported from the cell.
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