Table of Contents
Cell Cycle, Cell Division, Phases of Cell Cycle: Interphase, Mitosis – Prophase, Prometaphase, Metaphase, Anaphase, Telophase. Cytokinesis. Significance of Mitosis. NCERT Science Textbooks Class 6-12.
Cell Cycle and Cell Division
- During the division of a cell, DNA replication and cell growth takes place.
- All these processes, i.e., cell division, DNA replication, and cell growth have to take place in a coordinated way to ensure correct division and formation of progeny (offspring) cells containing intact genomes (the complete set of genetic material of an organism).
- The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.
- Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle.
- The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control [DNA].
Cell Cycle – Phases of Cell Cycle
- A typical eukaryotic cell divides once in approximately every 24 hours.
Eukaryotic vs Prokaryotic Cells: Eukaryotic vs. Prokaryotic Cells|Plant Cell vs. Animal Cell
- However, this duration of cell cycle can vary from organism to organism and also from cell type to cell type.
- Yeast for example, can progress through the cell cycle in only about 90 minutes.
Basic Phases of Cell Cycle – Interphase and M Phase or Mitosis
Interphase == Phase between two successive M phases.
M Phase [Mitosis phase] == Actual cell division or Mitosis.
- In the 24 hour average duration of cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than 95% of the duration of cell cycle.
- The M Phase or Mitosis starts with the nuclear division or karyokinesis [separation of daughter chromosomes].
- It usually ends with division of cytoplasm [cytokinesis].
- Interphase is called the resting phase.
- It is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication.
- The interphase is divided into three further phases.
- G1 phase (Gap 1)
- S phase (Synthesis)
- G2 phase (Gap 2)
- G1 phase == interval between mitosis and beginning of DNA replication [initiation of DNA replication].
- During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
S or synthesis phase
- S or synthesis phase == DNA synthesis or replication takes place.
- During this time the amount of DNA per cell doubles.
- If the initial amount of DNA is denoted as 2C then it increases to 4C.
- However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, even after s phase the number of chromosomes remains the same, i.e., 2n.
- In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
- During the G2 phase, proteins are synthesized in preparation for mitosis while cell growth continues.
- In the S and G2 phases the new DNA molecules formed are not distinct but intertwined.
Quiescent stage (G0)
- Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death.
- These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle.
- Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
Mitosis Phase or M Phase
- This is the most dramatic period of the cell cycle, involving a major reorganization of virtually all components of the cell.
- Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division.
- Though for convenience mitosis has been divided into four stages of nuclear division, it is very essential to understand that cell division is a progressive process and very clear-cut lines cannot be drawn between various stages.
- Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells.
- The word “mitosis” means “threads,” and it refers to the threadlike appearance of chromosomes as the cell prepares to divide.
- Early microscopists were the first to observe these structures, and they also noted the appearance of a specialized network of microtubules during mitosis.
- These tubules, collectively known as the spindle fibres, extend from structures called centrosomes — with one centrosome located at each of the opposite ends, or poles, of a cell.
- As mitosis progresses, the microtubules [spindle fibres] attach to the chromosomes, which have already duplicated their DNA and aligned across the center of the cell.
- The spindle tubules then shorten and move toward the poles of the cell. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell.
- This process ensures that each daughter cell will contain one exact copy of the parent cell DNA.
Mitosis consists of five morphologically distinct phases:
- anaphase, and
- Each phase involves characteristic steps in the process of chromosome alignment and separation.
- Once mitosis is complete, the entire cell divides in two by way of the process called
- In animals, mitotic cell division is only seen in the diploid somatic cells.
- But plants can show mitotic divisions in both haploid and diploid cells.
[Diploid == containing two complete sets of chromosomes, one from each parent].
[Haploid == only one set of chromosomes from one of the parent].
[Somatic == the parts of an organism other than the reproductive cells].
- Prophase is the first stage in mitosis, occurring after the conclusion of the G2 portion of interphase [see cyclic image above].
- During prophase, the parent cell chromosomes — which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase.
- The chromosomal material becomes untangled during the process of chromatin condensation.
- Because each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere, these structures now appear as X-shaped bodies when viewed under a microscope.
- The mitotic spindle also begins to develop during prophase.
- As the cell’s two centrosomes move toward opposite poles, microtubules [spindle fibres] gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart.
|Centriole == each of a pair of minute cylindrical structures near the nucleus in eukaryotic cells, involved in the development of spindle fibres in cell division.|
- The centriole, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell.
The completion of prophase can thus be marked by the following characteristic events:
- Chromosomal material condenses to form compact mitotic chromosomes.
- Chromosomes are seen to be composed of two chromatids attached together at the centromere (the point on a chromosome by which it is attached to a spindle fibre during cell division.).
- Initiation of the assembly of mitotic spindle, the microtubules, the proteinaceous components of the cell cytoplasm help in the process.
- Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
- When prophase is complete, the cell enters prometaphase — the second stage of mitosis.
- During prometaphase, the nuclear membrane breaks down into numerous small vesicles [a small fluid-filled sac]. As a result, the spindle microtubules now have direct access to the genetic material of the cell.
- Each microtubule is highly dynamic, growing outward from the centrosome and collapsing backward as it tries to locate a chromosome.
- Eventually, the microtubules find their targets and connect to each chromosome at its kinetochore, a complex of proteins positioned at the centromere.
- A tug-of-war then ensues as the chromosomes move back and forth toward the two poles.
- As prometaphase ends and metaphase begins, the chromosomes align along the cell equator.
- Every chromosome has at least two microtubules extending from its kinetochore — with at least one microtubule connected to each pole.
- At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth.
- The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell.
- By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This then, is the stage at which morphology of chromosomes is most easily studied.
- At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores.
- These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the center of the cell.
- Hence, the metaphase is characterized by all the chromosomes coming 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 by its kinetochore to spindle fibres from the opposite pole.
- The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
The key features of metaphase are:
- Spindle fibres attach to kinetochores of chromosomes.
- Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.
- Metaphase leads to anaphase, during which each chromosome’s sister chromatids separate and move to opposite poles of the cell.
- Upon separation, every chromatid becomes an independent
- At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles.
- As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole.
Thus, anaphase stage is characterized by the following key events:
- Centromeres split and chromatids separate.
- Chromatids move to opposite poles.
- During telophase, the chromosomes arrive at the cell poles, the mitotic spindle disassembles, and the vesicles that contain fragments of the original nuclear membrane assemble around the two sets of chromosomes.
- Climax results in the formation of a new nuclear membrane around each group of chromosomes.
- At the beginning of the final stage of mitosis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality.
- The individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles.
This is the stage which shows the following key events:
- Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
- Nuclear envelope assembles around the chromosome clusters.
- Nucleolus, golgi complex and ER reform.
Cell Organelles: Cell Organelles
Cytokinesis – Actual Cell Division
- Cytokinesis is the physical process that finally splits the parent cell into two identical daughter cells.
- Mitosis is the process of nuclear division, which occurs just prior to cell division, or cytokinesis.
- Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by a separate process called cytokinesis at the end of which cell division is complete.
- In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the center dividing the cell cytoplasm into two.
- Plant cells however, are enclosed by a relatively inextensible cell wall, therefore they undergo cytokinesis by a different mechanism.
- In plant cells, wall formation starts in the center of the cell and grows outward to meet the existing lateral walls.
- The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate.
- At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.
- In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).
Significance of Mitosis
- Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis.
- Mitosis usually results in the production of diploid daughter cells with identical genetic complement.
- The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
- A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
- Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.
Onion root tip cell has 16 chromosomes in each cell. Can you tell how many chromosomes will the cell have at G1 phase, after S phase, and after M phase?
Also, what will be the DNA content of the cells at G1, after S and at G2, if the content after M phase is 2C?