The growth and developmental patterns observed in eukaryotic cells are different and complex as compared with prokaryotic cells. Also, the genetic material, the cellular compartmentalization, and mechanisms differ from that of prokaryotes. Eukaryotic cells are studied more than the prokaryotic cells because we humans are eukaryotes. The capacity of the cells to reproduce independently is tremendous. Hence, the study of the cell cycle helps to know exactly how a cell undergoes growth and division. The concept of two cells forming from a single cell gave rise to tissue and organ systems in the body. Thus it depends on how and when the cell undergoes a cyclical pattern. Every part of the cell has a specific role to play. Not only division but also the cells undergo differentiation. Thus there exist different types of cells such as immune cells, blood cells, sperm cells, and egg cells. The cells may be haploid or diploid containing one or two sets of chromosomes.
The genetic material is packed artistically in the cells. The chromosomes appear in different shapes when visualized under a microscope. We may describe a cell cycle as a process of cells in undergoing growth, replication of the genetic material and division. Before starting the discussion on the way cells undergo cyclical patterns, we must know the basic components of the cells. A eukaryotic cell structure consists of a nucleus and a cytoplasm. The nucleus is a house for the genetic material consists of DNA-protein interactive structures known as chromosomes. It is a place where DNA replication and transcription occurs. The extranuclear or outer part of the nucleus is known as the cytoplasm. It is a site for organelles and protein synthesis.
INFO BOX: Terminologies
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1. DNA: The genetic material in the nucleus. DNA is the blueprint of life.
2. Chromosome: It is a linear structure composed of DNA-protein interaction.
3. Chromatid: It is a subunit of the chromosome. The two chromatids constitute a chromosome. (Symmetrical halves running parallel to each other)
4. Centromere: Chromatids are attached to each other through centromere. It is a constricted region.
5. Centriole: It is a self-reproducing material involved in cellular processes.
6. Nucleolus: It is RNA enriched spherical body associated with a chromosome segment.
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Although a great variation exists in eukaryotic organisms and their cells, a typical eukaryotic somatic cell cycle duration is 24 hours. Four main stages of the cell cycle include the G1 phase, S phase, G2 phase, and M phase. The G1, S and G2 phases are clubbed and termed as interphase. The M phase or the mitotic phase is the division phase of the cell. The G1 phase is an initial period of growth or a pro synthetic gap with duration of ten hours. The S phase is the period of DNA synthesis. It lasts for nine hours. The G2 phase is a post-synthetic gap phase. It lasts for four hours. The duration of the mitotic phase is one hour. During the G1 phase, the chromosomes become thin and extended. The cell is responsive to growth signals. The G2 phase accompanies chromosome condensation. The cell cycle checkpoints regulate the transition from one phase to another. They monitor the integrity of the genome. The mitosis occurs in the somatic cells. Another process occurring in gametes is known as meiosis.
Mitosis:
The process of mitosis occurs in both haploid and diploid cells. Prophase, metaphase, anaphase, and telophase are stages of mitosis.
a. Prophase: This is the initial stage of mitosis. It exhibits chromosome condensation and visibility. The spindle apparatus forms. A spindle is a set of tubulin fibers that move the eukaryotic chromosome during division. The spindle assembles outside the nucleus during the prophase. Early prophase, middle prophase, and late prophase are three subphases of prophase. In early prophase, the centrioles move apart. The chromosomes in early prophase reduce in their sizes. However, after some time they start getting visible. The nucleolus begins to disappear. The middle prophase involves the movement of the centrioles apart from each other. This phase is just a beginning of the mitotic spindle formation. The chromosomes are clearly visible in this phase. The late prophase involves the movement of centrioles towards the opposite sides. The spindle finally begins to form. The prophase chromosomes coil to produce a series of compact gyres. The kinetochore is a specialized protein that binds to the centromere.
a. Metaphase: It starts with the disappearance of the nuclear envelope. In this phase, the kinetochores are well attached to the centromeres so that the chromosomes align on the equator of the spindle. Hence their position is fixed on the equatorial plane. It is one plane halfway between the two spindle poles. The long axis of the chromosomes forms an angle of 900 to the spindle axis. A metaphase plate is an equatorial plane to which the chromosomes are aligned. During metaphase, the alignment of the chromosomes is perfectly on the spindle. The processed cells get arrested in the metaphase for visualizing under an electron microscope. Visualization under an electron microscope reveals a dense framework of proteins (scaffold) surrounded by an uncoiled DNA. Hence it reveals a double-stranded DNA.
b. Anaphase: Here, the centromeres of the sister chromatids separate forming two daughter chromosomes. The two chromatids separate by moving under the action of the traction fibers towards the spindle pole. Along with the chromatids, the kinetochores also separate. Due to disjunction, the sister chromatids get converted to independent chromosomes and move towards the poles by shortening the microtubules. Thus, the chromosomes acquire specific shapes such as V, J or rod-shaped. The position of the centromere decides the shapes of the chromosomes. A submetacentric chromosome is J-shaped. A metacentric chromosome is V-shaped. Anaphase is very crucial for the cell due to centromere splitting. However, incorrect centromere splitting could be catastrophic for the cell.
c. Telophase: The grouping of the daughter chromosomes occurs in such a way that the chromosomes align in two groups at the opposite ends of the cell. The chromosomes begin uncoiling and form elongated shapes. The spindle disappears. Reconstruction of the nuclear envelope begins. The nucleoli reappear. The nuclear division completes at this point. The cells get two nuclei.
d. Cytokinesis: Division of the cytoplasm and the compartmentalization of the two nuclei occur into separate cells.
Image 1: Phases of Mitosis
Meiosis:
It is a process occurring in the gametes. It results in the doubling of the gametic chromosomes resulting in the zygotic chromosome number. The process of meiosis involves a single chromosomal duplication followed by two successive nuclear divisions. It occurs after one DNA replication cycle. The original diploid nucleus undergoes two successive divisions. It consists of one haploid set of chromosomes from the father and the other from the mother. Meiotic division and differentiation lead to the formation of the gametes (the sperm and the egg). In the first meiotic division, the chromosome number gets reduced from diploid to haploid. The second meiotic division is equivalent to mitotic division. Four stages of meiosis I include prophase I, metaphase I, anaphase I and telophase I.
a. Prophase I: It is somewhat similar to mitotic prophase. However, the difference lies in the behavior of the chromosome and the crossing over process. Five subphases of the prophase I include leptotene, zygotene, pachytene, diplotene, and diakinesis. The coiling of chromosomes occurs in the leptotene stage. Through this phase, the cells begin its commitment to meiosis. Zygonema is the early mid-prophase I. In this phase the shortening of the chromosomes takes place. Chromosomes align themselves roughly and are known as homologous chromosomes. They are similar to each other and synapse during meiosis. The homologous chromosomes retain their genes from the common ancestors. The process of synapsis leads to the formation of zipper-like structures along the length of the chromatids. The synaptonemal complex aligns the two homologs precisely. The telomeres initiate the process of synapsis. They get clustered on the nuclear envelope forming a bouquet like arrangements. Telomeres assist the chromosomes to undergo synapsis by moving them around. The mid-prophase or pachynema stage starts after the completion of the synapsis. Since the homologous chromosomes consist of four chromatids, they are known as bivalents or tetrad. Here, the crossing over or a physical exchange takes place. If there are genetic differences in the homologous chromosomes, crossing over results into formation of new genetic material. This crucial step leads to genetic variation and recombination. It just involves reciprocal exchanges. At the end of the pachynema, the synaptonemal complex disassembles and enters into diplonema stage. The homologous chromosomes begin to move apart. The crossing over, as the name suggests, begins to look like a cross. It is known as chiasmata where the homologous chromosomes associate tightly. In the diakinesis, the nuclear envelope breaks down with spindle assembly.
b. Metaphase I: There is a complete breakdown of the nuclear envelope with the alignment of bivalents on an equatorial plane. Spindle formation and microtubule attachment take place.
c. Anaphase I: The chromosomes disjoin and migrate to the opposite poles. Hence maternally and paternally derived centromeres move towards each pole. The segregated sister chromatids remain attached at their respective centromeres.
d. Telophase I: Formation of the new nuclear envelope occurs. The cell proceeds for cytokinesis.
Meiosis II with prophase II, metaphase II, anaphase II and telophase II results in the formation of four haploid cells from two haploid cells of meiosis I. The results of meiosis II in males are known as spermatids. They are haploids giving rise to spermatozoa or sperms. The result of meiosis II in females is called ootid and a polar body.
[1] Medical genetics, G.P. Pal
[2] Human Genetics, 3/e, Gangane
[3] Vogel and Motulsky's Human Genetics: Problems and Approaches, Friedrich Vogel, Gunter Vogel, Arno G. Motulsky
[4] Biology for the IB Diploma: Standard and Higher Level, Andrew Allott
[5] Principles of Medical Genetics, Thomas D. Gelehrter
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