Cell Cycle and Cell Division class 11 notes
Cell Cycle and Cell Division
The cell cycle involves three major processes – Cell growth (time required by a cell for synthesis and duplication of various components of the cell), DNA replication (the time when the DNA replicates) and cell division (an adult mature cell finally divides into two daughter cells). A typical eukaryotic cell cycle is represented with a human cell in various culture methods. These cells divide once in approximately every 24 hours. The yeast cell has the ability to finish the cell cycle in about 90 minutes.
Cell Cycle and Its Phase
The cell cycle is divided into two basic phases. Howard and Pelc classified interphase into three sub-stages.
- M-phase (Mitosis phase)
The preparatory phase, resting phase, beginning phase and also a phase involving great metabolic activity. The intermediate stage between the two consecutive cell divisions where no cell division or even chromosomes division takes place. However, the nucleus and cytoplasm are metabolically and synthetically very active in order to get prepared for the division. The length of this phase is 90% – 95% of the total cell cycle. The series of events occurring in the cell in this particular phase are: replication of DNA, synthesis of nuclear histones, division of centrioles to form a new pair of centrioles, synthesis of energy-rich compounds, RNA and proteins. The nuclear envelope remains intact, however, the nucleolus shows genetic DNA as long, coiled, indistinctly visible chromatin fibres in the chromosomes. Also, there are rRNA and ribosomal proteins accumulated in the nucleolus which greatly increases its size.
Interphase is further divided into three phases:
(ii) S or Synthesis Phase
It occurs at the end of a mitotic division (pro-mitotic gap phase). The initiation of DNA replication is a major function. Following biochemical changes are common during this sub-stage. The cell grows until its maximum size as the normal metabolic activity occurs for the DNA replication preparation, and DNA contents of the cell remain unchanged. The new proteins are translation and RNA: rRNA, tRNA and mRNA transcription occurs during this phase. Also Nucleotides, amino acids and ATPs are formed. The most variable phase which differs in time affecting the cell division duration for each cell.
G1 under certain stimuli can be terminated. Once G1 is completed in a cell and ‘S’ phase has started with the replication of DNA, it cannot be terminated. There are cells which do not exhibit division usually in animal adults (e.g., heart cells) and also some which divide occasionally, as and when required to replace the lost or injured cells. Once the replacement is complete, these cells stop further division and exit G1 phase. Then they enter an inactive stage called as the quiescent stage (G0) in the cell cycle. The cells are metabolically active, however, do not proliferate til the requirement. Hence, this phase of G0 can be temporary or permanent in the organism. Antephase, the end of the G1 phase where the cell will divide in all the conditions even under stress conditions.
(ii) S or Synthesis Phase
The synthesis or replication of DNA on the template or the existing DNA takes place. The amount of DNA in a cell doubles (means the cell has twice the normal set of genes). However, the chromosome number remains the same (Ploidy level remains the same). Assume: the initial amount of DNA as 2C, then the DNA amount increases to 4C, and the cell has 2n number of chromosomes at G1, which remains the same even after S-phase. The replication occurs inside the nucleus along with centriole doubling in the cytoplasm. Histone proteins are also synthesized in S-phase. This phase is called as the invisible phase of the cell cycle as the replicated chromosomes are invisible.
The phase just before the mitosis (pre-mitotic gap phase). The cytoplasmic organelles multiply like mitochondria, chloroplast and Golgi complex. Transcription of RNA and then translation protein continues. Spindle tubulin synthesis and aster formation start. A cell contains double the number (4C) of DNA present in the original diploid (2N) cell. The cell is now prepared to enter into “M” or Mitotic phase.
The main part is the synthesis of some protein kinases used in the regulation of cell division. Kinases regulating the cell cycle are called as Cdks (cyclin-dependent kinases) because they get activated after combination with the key protein called as cyclin. The kinase enzyme along with cyclin moves the cell cycle in the forward direction. S-kinase is capable of the DNA replication initiation after it combines with S-cyclin. After some time S-cyclin gets destroyed and S-kinase loses its activeness. The cell cycle in the meristem cells is with a special protein “Cyclin and Cdks” (discovered by Nurse, T. Hunt & Hartmann 2001 during the experiment on yeast cell). The cyclin protein triggers the DNA replication.
The phase when the actual cell division or mitosis is initiated. The steps involve nuclear division, the separation of daughter chromosomes (Karyokinesis) and ends in the division of the cytoplasm (cytokinesis). The 24 hour is the average duration of the cell cycle in a human cell, where the cell division, i.e., M-phase lasts for about an hour.
Mitosis produces genetically identical cells. The chromosomes undergo division and replicate to form duplicates which are similar to mother cell chromosome number (equational division). The division is also called as somatic cell division or equational division or indirect division. Mitosis was coined by Fleming in 1882.
Establishment: Strasburger observed mitosis in plants. While Boveri and Fleming observed the same in animals.
Duration: Dependent on the type of cell involved and its species. It takes 30 minutes to 3 hours. The various factors affecting the duration are the type of the tissue, its location, temperature and species of the organism. The actual cell division is for one hour from the 24-hour average duration.
Occurrence: A common division method for both the somatic or body cells and the germ cells in the sex organs. There are phases and the specific location where it is common and a regular method. Plant meristematic tissues (root and shoot tips) and animal skin, bone marrow, even the embryonic developmental stages have the mitotic division.
Cause of mitosis:
Kern plasm theory: Hertwig proposed kern plasm theory. According to this theory, mitosis occurs due to the disturbance in the Karyoplasmic index (KI) or the nucleocytoplasmic ratio of the cell.
Vn = Volume of the nucleus
Vc = Volume of cell
Vc-Vn= Volume of cytoplasm
Karyoplasmic Index (KI) of small cell is high as they have less cytoplasm. Nucleus efficiently controls the activity of the cytoplasm in small cells, so these cells are metabolically more active
During cell growth, cytoplasm increases, thus K.I. decreases. In a large cell, nucleus fails to control the activity of the cytoplasm. To attain the control of nucleus on metabolism a large cell divides into two cells.
○ Surface-volume Ratio: A cell draws all the materials needed for its maintenance and growth from its surface. When a cell grows in size its volumes increases more than its surface. So a stage will reach when the surface area becomes insufficient to draw the material. At such a critical stage, the division of cell started.
Phases of Mitosis
The phases of mitosis are as follows:
- Interphase (as described earlier).
- Division phase or M-phase or mitotic phase (duration 1hr) is the most dramatic period of the cell cycle.
- Karyokinesis – Division of the nucleus; and Cytokinesis – Division of cytoplasm.
Division of the nucleus occurs by sequential changes (Indirect division) Karyokinesis has 4 stages:
(i) Prophase (longest stage)
Chromatin threads get condensed to form the chromosomes. Centrioles get aligned towards the opposite poles. Astral ray formation from the proteins gelatinized around the centrioles (initiation of the assembly of mitotic spindle). Cells do not show Golgi complexes, ER, Nucleolus and nuclear membrane at the end of the prophase.
The nuclear envelope is completely disintegrated which highlights the start of the second phase in mitosis. The chromosomes spread throughout the cytoplasm. Spindle fibres attach to the chromosomes at their kinetochores. The condensation of chromosomes is complete. This is the stage where the morphology of chromosomes is easily visible. The chromosome is compiled in two sister chromatids, held together with the centromere. Each chromosome splits as per length upto the centromere (the division of matrix of the chromosome). Thus, replicated chromatids are clearly visible at metaphase stage. Chromosomes spilt up and arrange themselves on the equator to form metaphase plate (equatorial plate). Spindle fibres are microtubules. Chromosomal fibres, (discontinuous and run from pole to centromere) and supporting fibres, (continuous and run from pole to pole), arranged in a cell. The centromere lies at the equator with arms facing the poles.
(iii) Anaphase (smallest stage)
The early anaphase has interzonal fibres appearing at the equator. Chromosome centromere splits lengthwise (division of centromere). Chromosomes double inside a cell during mitotic anaphase. Every chromosome has one chromatid. Expansion of Interzonal fibres and the chromosomes are pushed towards the opposite poles (pushing). Contraction of chromosomal fibres such that they pull them towards opposite poles (pulling).
(iv) Telophase (reverse of prophase)
Nuclear membrane, Nucleolus, Golgi complex and ER now surround each of the chromosomal poles. The chromatin net is formed after the chromosomes decondense. Chromosomes lose their individuality which means the individual chromosomes are not present.
Cytokinesis is initiated in late the anaphase. It is different for plants and animals.
(i) Cytokinesis in animals
It occurs through constriction and furrow formation in the cell membrane. A mid-body equator is formed when the microtubules arrange in the middle while the microfilaments arrange in the peripheral ring just below the plasma membrane. The cell organelles arrange themselves at either side of the equator. The contraction occurs as the attraction occurs between mid-body and peripheral ring, forming a furrow from the outside of the cell to inside. The furrow formed deepens continuously and finally, the cell divides into two daughter cells. The cytokinesis in the animal cell occurs in the centripetal order.
(ii) Cytokinesis in plants
The cell plate formation takes place because the constriction or even furrow is not possible as the cell wall is rigid. Many Golgi vesicles and spindle microtubules arrange themselves on the equator and the cell has a Phragmoplast. It may also have the deposits of fragments of ER. Golgi vesicles membranes fuse and form a plate like structure which is called as the cell plate. Golgi vesicles then secret pectates of calcium and magnesium. The cell plate modifies into the middle lamella. The cytokinesis of plant cells occurs in the centrifugal order (cell plate formation is from centre to periphery).
Significance of Mitosis
Mitosis: The equational division is a common division method for the diploid cells only. However, some lower plants and social insects which have haploid cells, also use mitosis for the division. The significance of this division is essential to understand in the life of an organism. Mitosis results in the production of diploid daughter cells which have identical genetic chromosome number. The multicellular organisms grow due to the mitosis. Cell growth often results in disturbing the usual ratio of the nucleus and the cytoplasm. Thus, the cell divides and restores the nucleo-cytoplasmic ratio. A very significant contribution is that a cell is repaired. Best examples are the cells of the upper epidermis layer, cells of the gut lining, and blood cells being replaced constantly.
Meiosis is a method where the division produces genetically different type of cells. All the four daughter cells produced with meiosis have genetic differences among each other and also are different from the mother cell. Gametogenesis the formation of gametes is a common factor for meiosis to occur.
Phases of Meiosis
There are two different phases in the division of cell:
Meiosis I: Heterotypic division or reduction division. It leads to the reduction in chromosome number to half in daughter cells. Division of chromosome does not occur in meiosis-I, only segregation of homologous chromosomes takes place.
Meiosis II: Homotypic division or equational division. It does not lead to any change in chromosome number. Division of the nucleus occurs twice, however, the DNA replication and chromosome division occur only once.
Stages of Meiosis I
(i) Prophase – I:
The longest and most complex stage of the meiosis. Prophase I is further divided into five sub-stages as:
(a) Leptotene –
Chromatin threads are condensed so that they form chromosomes which are longest and thinnest fibers. There are bead-like structures present on it called as chromomeres. All the chromosomes move towards centrioles in the nucleus, so the group of chromosomes in the nucleus appears like a bouquet in the animal cell. (Bouquet stage).
(b) Zygotene or Synaptotene –
There is pairing of homologous chromosomes (Synapsis). The pairs of homologous chromosomes which are formed here, are called as Bivalents or Tetrads and are clearly identified in the next stage. A structure is developed in between the homologous chromosomes, is called as synaptonemal complex. It has three thick lines made up of DNA and proteins. The complete set helps in the pairing of the DNA.
(c) Pachytene (thick thread) –
There is increased attraction which causes homologous chromosomes to coil tightly around each other. Both the chromatids in the chromosome are clear and distinct and now the pair or bivalent is found as a tetrad. Both the chromosome chromatids are called as sister chromatids. Non–sister chromatids present in the bivalent develop into recombination nodules and exchange their parts called as the crossing over. This is an enzyme-mediated process and the enzyme is recombinase.
(d) Diplotene –
The diplotene starts with the dissolution of the synaptonemal complex. There is also the tendency in the bivalent recombined homologous chromosomes to separate from each other while still joint at the cross-overs. These X-shaped structures formed are called as chiasmata. The diplotene can last for months or years, in some vertebral oocytes which are called as dictyotene.
(e) Diakinesis –
The meiotic prophase I end in diakinesis. There is markable terminalisation of the chiasmata. The chromosomes get fully condensed and then the meiotic spindle assembles to prepare the homologous chromosomes which separate. When diakinesis ends, the nucleolus disappears and the nuclear envelope breaks down. Diakinesis ends and metaphase starts.
(ii) Metaphase I:
Bivalents form metaphase plate after arranging on the equator of the cell such that the centromeres face the poles while arms face the equator. Spindle fibres now attach to the pair of homologous chromosomes. There are in all 3 types of spindle fibres in the cell:
1. Chromosomal / Kinetochore Spindle fibres
2. Supporting / Continuous Spindle fibres
3. Interzonal Spindle fibres.
(iii) Anaphase I:
There is a contraction of chromosomal fibres and expansion of interzonal fibres. The homologous chromosomes move towards the opposite poles after they segregate from each other. Anaphase I has segregation or disjunction of the homologous chromosomes. There is no division of centromere.
(iv) Telophase I:
The nuclear membrane and nucleolus reappear. This is followed by the cytoplasm division or the cytokinesis and two daughter cells together are called as diad of cells. The chromosomes in some situations undergo some dispersion, and are thus fail to reach the extremely extended state of the interphase nucleus.
The connecting stage of the two meiotic divisions is called as interkinesis which is short in duration. DNA does not replicate in this stage. Interkinesis ends with the start of prophase II, which is simpler than prophase I.
Stages of Meiosis – II
(i) Prophase II:
Meiosis II is an intermediate step which starts immediately after cytokinesis, and before the chromosomes have elongated fully. Meiosis II is similar to a normal mitosis, in contrast to meiosis I. The nuclear membrane disappears and chromosomes are compact again at the end of this stage.
(ii) Metaphase II:
The chromosomes get aligned at the equator while at the opposite poles the spindle microtubules are in close contact with the kinetochores of the sister chromatids.
(iii) Anaphase II:
The simultaneous splitting of the chromosome centromere occurs (which was holding the sister chromatids together), which moves the chromosomes toward the opposite poles of the cell.
(iv) Telophase II:
The two sets of chromosomes are again enclosed in a nuclear envelope and cytokinesis begins. There is a formation of tetrads (four haploid daughter cells).
Significance of Meiosis
Meiosis is the division in which a specific chromosome number in each species is conserved. This is achieved in sexually reproducing organisms across several generations, even though there is a reduction of chromosome number by half in the whole process. The genetic variation increases in the population of organisms over various generations. Evolution is due to variations which are a very important factor that is progressive with time.