This chapter should help the student to:

• Know the names and functions of the nuclear components.
  
• Know the subunits of each nuclear component and their functions.
  
• Explain the process of cell division and its effects on cell structure.
  
• Identify the factors and activities controlling the transition from each cell-cycle phase to the next.
  
• Recognize a cell’s nuclear components in a light or electron photomicrograph and hence predict the cell’s relative activity.
  
• Predict a cell’s nuclear appearance from its functional characteristics.
  
• Predict the functional deficits accompanying specific nuclear or chromosomal aberrations.
  
• Predict the nuclear component(s) likely to be involved in a functional deficit.
  
• Explain the role of the nucleus in cell differentiation.

1. List the four major structural components of a nucleus (I.A).

2. Explain why the term “nuclear envelope” is more appropriate than “nuclear membrane” (II).

3. List the substances and structures associated with the nuclear envelope’s internal and external surfaces (II.A and B).

4. Name the components of a nuclear pore complex and indicate which anchors the complex in the envelope and which form its walls as it penetrates the envelope (II.C).

5. List several important macromolecules that must traverse the nuclear pores for basic cell functions to be carried out (II.C).

6. Compare euchromatin and heterochromatin (III.B and C) in terms of their appearance in light and electron microscopy, degree of coiling, and involvement in transcriptional activity.

7. List the components of a nucleosome (III.A).

8. List the parts of a nucleolus and relate them to its function (IV.A, B.1 and 2).

9. In which cell types would you expect to find particularly large or abundant nucleoli (IV)?

10. List in order, the phases of mitosis and sketch the appearance and location of the chromosomes during each phase (VI.A.2.a–d).

11. Describe what happens to each of the following during mitosis and indicate the phase(s) during which each change occurs (VI.A.2.a–d):

1. 
   

   Nucleolus

   
   
2. 
   

   Nuclear envelope

   
   
3. 
   

   Nuclear lamins

   
   
4. 
   

   Centrioles

   
   
5. 
   

   Spindle apparatus

   
   
6. 
   

   Golgi apparatus

   
   
7. 
   

   Chromatin

   
   
8. 
   

   Cytoplasmic microtubules

12. Give examples of tissues characterized by fast and slow mitotic rates (VI.B.1).

13. List in order, the phases of interphase (VI.B.1–3; Fig. 3–2) and indicate which is associated with:

1. 
   

   Most protein and RNA synthesis

   
   
2. 
   

   Restoration of cell volume

   
   
3. 
   

   Exit from the cell cycle and entry into G0

   
   
4. 
   

   DNA synthesis and replication

   
   
5. 
   

   Duplication of the centrioles

   
   
6. 
   

   Accumulating energy (ATP) for mitosis

14. Diagram the cell-cycle phases (Fig. 3–2). Include molecules that affect transitions from phase to phase at the points in the cycle where they act (VI.B.4 and 5.a–c).

15. Beginning with transcription, trace the steps in the synthesis and secretion of a glycoprotein and relate the steps to the organelles involved. (This exercise helps to integrate the information in Chapters 2 and 3).

I. General Features of Nuclei

A. Major Components

Each nucleus has a nuclear envelope, chromatin, one to several nucleoli, and a variable amount of nucleoplasm.

B. Nuclear Function

The nucleus contains in its chromatin a linear code (DNA) for the synthesis of cell components and products, which confers on the cell a range of adaptability to changing environmental conditions and to extrinsic signals such as hormones. It is also involved in cellular reproduction, which occurs through a succession of nuclear and cytoplasmic changes comprising the cell cycle. This cycle ends with mitosis (cell division), yielding two identical daughter cells from a single parent. Continuous cycling occurs only with the presence and activity of regulatory molecules that allow progress beyond particular checkpoints.

C. Nuclear Morphology

Nuclei vary in appearance from tissue to tissue, from cell to cell, and during different cell-cycle phases. Although some mature cell types (e.g., erythrocytes) lack nuclei, at least one nucleus is present during some stage in all eukaryotic cells. The microscopic appearance of the nucleus is important in identifying and classifying both normal and diseased cells and tissues. Nuclei display wide variations in:

1. 
   

   Nuclear size. The size of the nucleus varies, both in absolute terms and relative to the amount of cytoplasm in the cell (nucleocytoplasmic ratio).

   
   
2. 
   

   Number per cell. Cells may be enucleate, mononucleate, binucleate, or multinucleate.

   
   
3. 
   

   Chromatin pattern. The amount and distribution of heterochromatin varies according to cell type and cellular activity.

   
   
4. 
   

   Nuclear location. The position of a nucleus within a cell varies according to cell type and may be basal, central, or eccentric.

II. Nuclear Envelope