4.4.1 The Nucleus and Ribosomes

A. Nucleus

• The nucleus has a diameter of about 5 μm

• Nucleoplasm is the semifluid medium of the nucleus

• Chromosomes are rodlike structures formed during cell division; composed of coiled or folded chromatin

  • Chromatin is a threadlike material that coils into chromosomes just before cell division occurs

    • contains DNA, protein, and some RNA

• The nucleolus is a dark region of chromatin inside the nucleus

  • it is the site where ribosomal RNA (rRNA) joins with proteins to form ribosomes

• The nucleus is separated from the cytoplasm by the nuclear envelope

  • which contains nuclear pores to permit passage of substances (e.g., ribosomal subunits, messenger RNA, proteins, etc.) in and out of the nucleus

B. Ribosomes

• 1. Ribosomes are the site of protein synthesis in the cell

  • In eukaryotic cells, ribosomes may occur freely or in groups called polyribosomes. 

• 2. Some ribosomes are attached to the endoplasmic reticulum (ER) 

  • if the protein being synthesized by the ribosome begins with a sequence of amino acids called a signal peptide.

• 3. The central dogma of molecular biology refers to the universality of the sequence of DNA being transcribed into mRNA and then being translated into a protein.