Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Cytology
The Nucleus
Cell Cycle

The Cell Cycle (cyclus cellularis) refers to the lifespan of a cell, from its formation through division, or from birth to death.

The Cell cycle is divided into interphase and mitosis.

Interphase consists of the following periods:

— the presynthetic phase (G1), characterized by rapid growth of the young cell and protein accumulation;

— the synthetic phase (S), during which chromosome and METABOLISM/36.html">DNA Replication takes place;

— the postsynthetic phase (G2), or premitotic phase, characterized by the synthesis of mRNA, ribosomal rRNA, and tubulins—the Proteins that make up the mitotic spindle.

Mitosis is an indirect Cell Division that is universal and widespread.

Mitosis consists of four phases:

1. Prophase, during which Chromosomes undergo Condensation, making them visible. Each chromosome consists of two chromatid strands, the nucleoli shrink and disappear, the centrioles migrate apart as the mitotic spindle begins to form between them, and the nuclear envelope breaks down.

2. Metaphase, in which The formation of the mitotic spindle is completed and the chromosomes align along the equatorial plane of the cell. The mitotic spindle is composed of microtubules originating from the centrioles as organizing centers; some microtubules extend from pole to pole (from centriole to centriole), while others attach to the centromere (constriction) of a chromosome.

3. Anaphase, in which the centromeres separate, and the chromatids are pulled toward opposite poles of the cell by the mitotic spindle.

4. Telophase, which involves the division of the Cytoplasm, chromosome decondensation, reconstruction of the nuclear envelope, and the reappearance of nucleoli.

Meiosis is a specialized form of cell division characteristic of Germ Cells. It consists of two successive nuclear divisions without an intervening interphase. Meiosis results in the formation of cells with a haploid chromosome number. A key feature of meiotic prophase is Crossing Over—the exchange of homologous chromosomal segments—which is a major driver of biological diversity.

Slides for Study

Slide 6. Karyokinesis in onion ROOT tip cells (Fig. 17).

High magnification. Examine and sketch the slide. At this magnification, locate a cell in interphase, identifying its nuclear envelope, nucleolus, and Chromatin granules within the nucleus. In prophase, chromosomes are visible, forming a dense or loose skein (in late prophase). In metaphase, chromosomes are arranged along the cell equator. In anaphase, chromatids separate and migrate toward the poles, resulting in two star-shaped groups of chromosomes visible within the cell. Telophase continues until the complete reconstruction of the nucleus; however, it is most convenient to observe early telophase, when each daughter star begins to merge into a more compact figure while still retaining its star-like shape, and in the cytoplasm—by slightly lowering the condenser—the forming cell plate can be observed.

Label the following on the diagram: 1) interphase; 2) prophase; 3) metaphase; 4) anaphase; 5) telophase.

Carefully examine the micrographs (Figs. 18, 19) to gain a better understanding of nuclear ultrastructure.

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Fig. 17. Karyokinesis in onion root tip cells. Iron hematoxylin stain. × 400:

1 — interphase; 2 — prophase; 3 — prophase, loose skein; 4 — metaphase; 5 — achromatic spindle; 6 — anaphase; 7 — telophase

Fig. 18. Cell Nucleus. Electron micrograph. × 15,000:

1 — karyoplasm; 2 — nucleolus; 3 — nuclear envelope; 4 — intracellular reticular apparatus; 5 — Mitochondria (after Yu. I. Afanasyev)

Fig. 19. Nuclear envelope. Electron micrograph. x 125,000:

1 — nucleus; 2 — nuclear envelope (karyotheca): a — inner nuclear membrane; b — outer nuclear membrane; 3 — nuclear pores; 4 — endomembranes of The Endoplasmic reticulum with Ribosomes (according to Yu. S. Chentsov)

Review Questions

1. Structure OF THE nucleus. Main structural components.

2. Nuclear envelope, structure.

3. Structure of the nuclear pore.

4. Structure of the nucleolus.

5. Karyoplasm. Structural features.

6. Chromosomes. Structure.

7. Chromatin. Types of chromatin.

8. CHARACTERISTICS OF THE cell cycle.

9. Mitosis. Characteristics.

10. Endomitosis. Characteristics.

Sample Examination Questions

1. The nucleus. Structure and characteristics of its structural components.

2. The cell cycle. Characteristics.

3. Mitosis. Characteristics of mitotic phases.