Biology: Class 11 — Chapter 10: Cell Cycle and Cell Division

A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide once in approximately every 24 hours. Yeast, by contrast, can progress through the cell cycle in only about 90 minutes. The cell cycle is divided into two basic phases:

• Interphase: Represents the phase between two successive M phases. It lasts more than 95% of the duration of the cell cycle. It is also called the resting phase.
• M Phase (Mitosis phase): Represents the phase when the actual cell division or mitosis occurs. It lasts for only about an hour in human cells.

The Interphase is divided into three further phases:

1. G₁ phase (Gap 1): Corresponds to the interval between mitosis and initiation of DNA replication. The cell is metabolically active and continuously grows but does not replicate its DNA.
2. S phase (Synthesis): Marks the period during which DNA synthesis or replication takes place. The amount of DNA per cell doubles (if initial DNA is $2\text{C}$, it increases to $4\text{C}$), but there is no increase in the chromosome number ($2n$ remains $2n$). In animal cells, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
3. G₂ phase (Gap 2): Proteins are synthesised in preparation for mitosis while cell growth continues.

Quiescent Stage (G₀): Some cells in adult animals do not exhibit division (e.g., heart cells) and many others divide only occasionally to replace lost cells. These cells exit the $G_1$ phase to enter an inactive stage called the quiescent stage ($G_0$) of the cell cycle. They remain metabolically active but no longer proliferate unless called on to do so.

Prophase is the first stage of karyokinesis of mitosis, following the S and $G_2$ phases of interphase.

• Initiation of condensation of chromosomal material. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
• The centrosome (duplicated during S phase) begins to move towards opposite poles of the cell.
• Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres form the mitotic apparatus.
• Cells at the end of prophase do not show Golgi complexes, endoplasmic reticulum, nucleolus, and the nuclear envelope.

The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis.

• Chromosomes are spread through the cytoplasm of the cell. Condensation of chromosomes is completed, making this the stage at which morphology of chromosomes is most easily studied.
• Metaphase chromosome is made up of two sister chromatids held together by the centromere.
• Small disc-shaped structures at the surface of the centromeres are called kinetochores, which serve as sites of attachment of spindle fibres.
• All chromosomes come to lie at the equator, with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected to the opposite pole. The plane of alignment is referred to as the metaphase plate.

• At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously.
• The two daughter chromatids, now referred to as daughter chromosomes, begin their migration towards the two opposite poles.
• The centromere of each chromosome remains directed towards the pole (at the leading edge) with the arms trailing behind.

This is the final stage of karyokinesis.

• Chromosomes that have reached their respective poles decondense and lose their individuality. Individual chromosomes can no longer be seen.
• Nuclear envelope develops around the chromosome clusters at each pole, forming two daughter nuclei.
• Nucleolus, Golgi complex, and ER reform.

The division of cytoplasm itself into two daughter cells, completing cell division.

• Animal Cells: Achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre, dividing the cytoplasm into two.
• Plant Cells: Plant cells are enclosed by a relatively inextensible cell wall. Wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with a simple precursor called the cell-plate, which represents the middle lamella.
• In some organisms, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition called syncytium (e.g., liquid endosperm in coconut).

Prophase I: Typically longer and more complex when compared to prophase of mitosis. It is subdivided into five phases based on chromosomal behaviour:

1. Leptotene: Chromosomes become gradually visible under the light microscope. Compaction of chromosomes continues.
2. Zygotene: Chromosomes start pairing together, a process called synapsis. The paired chromosomes are called homologous chromosomes. This is accompanied by the formation of a complex structure called the synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
3. Pachytene: The four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads. Characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes, mediated by the enzyme recombinase.
4. Diplotene: Recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.
5. Diakinesis: Marked by terminalisation of chiasmata. Chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope breaks down.

Metaphase I: The bivalent chromosomes align on the equatorial plate. Microtubules from opposite poles of the spindle attach to the kinetochore of homologous chromosomes.

Anaphase I: Homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows, and this is called a dyad of cells. The stage between the two meiotic divisions is called interkinesis and is generally short-lived. There is no replication of DNA during interkinesis.

• Prophase II: Initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears and chromosomes become compact.
• Metaphase II: Chromosomes align at the equator and microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
• Anaphase II: Begins with the simultaneous splitting of the centromere of each chromosome, allowing sister chromatids to move toward opposite poles of the cell by shortening of microtubules.
• Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows, resulting in the formation of a tetrad of cells, i.e., four haploid daughter cells.