Both layers of the nuclear envelope are lipid bilayers like the plasma membrane - The outer membrane of the nuclear envelope is continuous with the rough ER and resembles it in structures - Many openings called nuclear pores extend through the nuclear envelope - Each nuclear pore consists of a circular arrangement of proteins surroundings a large central opening that is about 10x wider than the pore of a channel protein in the plasma membrane - Nuclear pores control the movement of substances between the nucleus and the cytoplasm - Small molecules and ions move through the pores passively by diffusion - Most large molecules, such as RNAs and proteins, cannot pas though the nuclear pores by diffusion - Instead their passage involves the active transport process in which the molecules are recognized and selectively transported through the nuclear pore into or out of the nucleus - Ex. Proteins needed for nuclear functions move from the nucleus into the cytosol into the nucleus; newly formed RNA molecules move from the nucleus into the cytosol in this matter
- Inside the nucleus are one or more spherical bodies called nucleoli that function in producing ribosomes - Each nucleolus is simply a cluster of protein, DNA and RNA; it is not enclosed by a membrane - Nucleoli are the sites of synthesis of Rrna and assembly assembly of Rrna and proteins into ribosomal subunits - Nucleoli are quite prominent in cells that synthesize large amounts of proteins, such as muscle and liver cells - Nucleoli disperse and disappear during cell division and reorganize once new cells are formed - Within the nucleus are most of the cells hereditary units, called genes, which control cellular structure and direct cellular activities - Genes are arranged along chromosomes - Human somatic (body) cells have 46 chromosomes, 23 inherited from each parent - Each chromosome is a long molecule of DNA that is coiled together with several proteins - This complex of DNA, proteins, and some RNA is called chromatin - The total genetic information carried in a cell or an organism is its genome - In cells that are not dividing, the chromatin appears as a diffuse, granular mass - Electron micrographs reveal that chromatin has a beads-on-a-string structure - Each bead is a nucleosome that consists of a double-stranded DNA wrapped twice around a core of eight proteins called histones, which help organize the coiling and foiling of DNA - The string between the beads is called linker DNA, which holds adjacent nucleosomes together - In cells that are not dividing, another histone promotes coiling of nucleosomes into a larger diameter chromatin fiber, which then folds into large loops - Just before cell division takes place, however, the DNA replicates as the loops condense even more, forming a pair of chromatids - During cell division, a pair of chromosomes constitutes a chromosome Protein Synthesis - Although cells synthesize many chemicals to maintain homeostasis, much of the cellular machinery is devoted to synthesizing large numbers of diverse proteins -
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- Spring '17