لخّصلي

The nucleus contains the cell’s genome. It is bounded by a membrane that consists of a pair of unit membranes sepa￾rated by a space of variable thickness. The inner membrane is usually a simple sac, but the outermost membrane is, in many places, continuous with the endoplasmic reticulum (ER). The nuclear membrane exhibits selective permeability because of pores, which consist of a complex of several proteins whose function is to import substances into and export substances out of the nucleus. The chromosomes of eukaryotic cells con￾tain linear DNA macromolecules arranged as a double helix. They are only visible with a light microscope when the cell is undergoing division and the DNA is in a highly condensed form; at other times, the chromosomes are not condensed and appear as in Figure 2-2. Eukaryotic DNA macromol￾ecules are associated with basic proteins called histones that bind to the DNA by ionic interactions. A structure often visible within the nucleus is the nucle￾olus, an area rich in RNA that is the site of ribosomal RNA synthesis (see Figure 2-2). Ribosomal proteins synthesized in the cytoplasm are transported into the nucleolus and com￾bine with ribosomal RNA to form the small and large sub￾units of the eukaryotic ribosome. These are then exported to the cytoplasm, where they associate to form an intact ribo￾some that can function in protein synthesis

The nucleus contains the cell’s genome. It is bounded by a
membrane that consists of a pair of unit membranes sepa￾rated by a space of variable thickness. The inner membrane is
usually a simple sac, but the outermost membrane is, in many
places, continuous with the endoplasmic reticulum (ER). The
nuclear membrane exhibits selective permeability because of
pores, which consist of a complex of several proteins whose
function is to import substances into and export substances
out of the nucleus. The chromosomes of eukaryotic cells con￾tain linear DNA macromolecules arranged as a double helix.
They are only visible with a light microscope when the cell is
undergoing division and the DNA is in a highly condensed
form; at other times, the chromosomes are not condensed
and appear as in Figure 2-2. Eukaryotic DNA macromol￾ecules are associated with basic proteins called histones that
bind to the DNA by ionic interactions.
A structure often visible within the nucleus is the nucle￾olus, an area rich in RNA that is the site of ribosomal RNA
synthesis (see Figure 2-2). Ribosomal proteins synthesized in
the cytoplasm are transported into the nucleolus and com￾bine with ribosomal RNA to form the small and large sub￾units of the eukaryotic ribosome. These are then exported to
the cytoplasm, where they associate to form an intact ribo￾some that can function in protein synthesis