BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS
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Nucleus The largest and most easily seen of all the organelleles within a eukaryotic cell is the nucleus. A cell may be mononucleate, binucleate or multinucleate. In animal cells the nucleus is typically located in the central region. It controls all the activities of the cell. Nucleus consists of nuclear membrance, nucleoplasm, nucleolus and Fig: 1.25 Nucleus chromosomes. A double membrane called nuclear envelope or nuclear membrane, bounds the surface of the nucleus. The two membranes are separated by a fluid-filled perinuclear space. These are connected at nuclear pores. It consists of protein and lipid bilayer. The outer membrane is covered with ribosomes and is connected with the membranes of ER. The perinuclear space is continuous with the lumen of ER. The inner membrane is smooth. The nuclear membrane is perforated by numerous pores. The pores regulate the nucleo-cytoplasmic exchange of materials.
Science Titbits The structures and molecules which can cross the nuclear envelops are ions, micromolecules, macromolecules, tRNA, mRNA, ribosomal RNA, proteins, nucleotides, and some hormones for regulation of DNA. Nucleoplasm is the transparent semifluid ground substance formed of a mixture of proteins, enzymes (DNA and RNA polymerase), phosphorus, nucleotide, some nucleic acids and metal ions (Mg) for the synthesis of DNA and RNAs. It also contains histone and non-histone protein. Whereas cytoplasm contains amino acids, carbohydrates, proteins, enzymes, vitamins, nucleotides, tRNA etc. Proteins occur as colloidal particles. The dark staining region in the nucleus is called nucleolus, (plural: nucleoli). A cell may have one or more nucleoli. Nucleolus appears during