Plus One Botany Notes Chapter 6 Cell Cycle and Cell Division is part of Plus One Botany Notes. Here we have given Kerala Plus One Botany Notes Chapter 6 Cell Cycle and Cell Division.
|Text Book||NCERT Based|
|Chapter Name||Cell Cycle and Cell Division|
|Category||Plus One Kerala|
Kerala Plus One Botany Notes Chapter 6 Cell Cycle and Cell Division
Growth and reproduction are characteristics of cells, indeed of all living organisms. All cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time they divide. These newly formed daughter cells can themselves grow and divide, giving rise to a new cell population that is formed by the growth and division of a single parental cell and its progeny.
Cell division is an essential process in all living organisms. The mode of cell division is fundamentally similar in all organisms. During the process of division of a cell, the process like DNA replication and cell growth must take place in a sequential and cordinated manner to ensure the correct division and formation of progeny cells with intact genomes. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.
Phases of cellcycle
The duration of cell cycle can vary from organ-ism to organism and also from cell type to cell type. Yeast for example, can progress through the cell cycle in only about 90 minutes. A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours.
It is the period between the end of one cell division to the beginning of the next cell division (between two successive M Phases). As no visible changes are observed at this stage, interphase is said to be resting stage. During this phase, cell prepares itself for .both cell growth and DNA replication in an orderly manner. So, it is also known as preparation phase. The interphase lasts more than 95% of the duration of cell cycle. Interphase is further divided into following three sub-stages on the basis of various synthetic activities.
i. G1 phase(Gapl).
This phase corresponds to the interval between mitosis and initiation of DNA replication. During G phase the cell is metaboiically active and continuously grows but does not replicate its DNA.
ii. S phase (Synthesis).
In this phase marks the period during which,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 (2n).
iii. G1 phase (Gap 2).
During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues.
G0 phase (Quiescent stage).
Some cells in the adult animals do not appear to exhibit division
(e.g., heart cells) and many other cells divine 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 G phase to enter an inactive stage called quiescent stage (G0) of the cell cycle.
2. M Phase (Mitosis phase)
Following the interphase, the cell enters the M-phase or mitotic phase. The M Phase starts karyokinesis and cytokinesis.
The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes is called karyokinesis. It is further divided into four main sub-stages, they are given below.
- Prophase which is the first stage of mitosis follows the S and G phases of interphase.
- Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation.
- Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
- Initiation of the assembly of mitotic spindle, the microtubules, the proteinaceous components of the cell cytoplasm help in the process. This phase is known as early prophase.
- Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
- During the late prophase the nucleolus disintegrates gradually and the nuclear envelope disappear.
- The chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed.
- Metaphase chromosome is made up of the centromere. Small disc shaped structures at the surface of the centromeres are called kinetochores.
- The metaphase is characterised 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.
- It is known to be the shortest duration phase and is also a simple stage.
- At the beginning of this phase, splitting of chromosomes takes place.
- The two daughter chrormatids now becomes the chromosomes of future daughter nuclei and start migrating towards the opposite poles along the path of their chromosome fibres.
- Centromeres split and chromatids separate. Chromatids move to opposite poles.<
- This is considered to be long and complex phase like prophase the final stage of mitosis.
- The chromosomes that have reached their respective poles decondense and lose their in-dividuality.
- The individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles.
- 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.
It is commonly known as the division of cytoplasm of the parent cell into two daughter cells after the division of nucleus or karyokinesis. It occurs, so that the each daughter cell can receive nucleus of its own.
Thus mitosis is not only the segregation of nucleus into two daughter nuclei, infact, the cell alsao divide itself into two daughter cells.
Cytokinesis in animal cells
- In animal cells, cytokinesis starts at metaphase. They typically divide by furrowing or by the appearance of furrow in the plasma membrane. This is also known as cleavage.
- Due to the contraction and development of micro-filaments, a constriction develops which further depends in a centripetal way known as cell furrow,
- The furrow starts deepening gradually during telophase and finally gets joined in the centre by dividing the cytoplasm into two.
Cytokinesis in plant cells
- Plant cells are enclosed by a relatively inextensible cell wall, thererfore they undergo cytokinesis by a different mechanism.
- In plant cells, 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 the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
- 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 multi nucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).
|It is known as division of daughter chromosomes M-phase||It is known as division of cytoplasm|
|Occurs or starts with karyokinesis itself||Occurs at the end of M-phase|
|It can occur independent of cytokinesis||It cannot occur without karyokinesis|
Significance of mitosis:
- It helps in the production of diploid daughter cells with equal and identical genetic comple ment.
- Mitosis helps in growth of multicellular organism.
- It also helps in maintaining a proper cell size by dividing an overgrown somatic cell.
- It is helpful in a cell repair mechanism. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
- 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.
- Mitotic divisions in the meristematic tissues the apical and the lateral cambium, result in a
continuous growth of plants throughout their life.
The production of offspring by sexual reproduction includes the fusion of two gametes,
each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by
half results in the production of haploid daughter cells. This kind of division is called meiosis.
- Meiosis involves two sequential cycles of nuclear and cell dMsion called meiosis I animeiosis II but only a single cycle of DNA replication.
- Meiosis is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
- Meiosis involves pairing of homologous chromosomes and recombination between them.
- Four haploid cells are formed at the end of meiosis II.
Meiotic events can be grouped under the following phases,
|Meiosis 1||Meiosis II|
|Prophase 1||Prophase Ii|
|Metaphase 1||Metaphase II|
|Anaphase 1||Anaphase II|
|Telophase 1||Telophase 11|
1. Meosis I
I. Prophase I.
Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behaviour.
In this chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.
During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called syn- aptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
During this stage recombination nodules appear. These are the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Thus results recombination of gene.
In this 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 cross overs. These X-shaped structures, are called chiasmata.
The final stage of meiotic prophase I is diakinesis. During this phase the 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 also breaks down. Diakinesis represents transition to metaphase.
ii. Metaphase I.
The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.
iii. Anaphase I.
The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
iv. Telophase I.
In this stage the nuclear membrane and nucleolus reappear, cytokine- Telophase I sis follows and this is called as diad of cells.
i. Prophase II.
Meiosis II is 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 by the end of prophase II. The chromosomes again become compact.
ii. Metaphase II.
At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
iii. Anaphase II.
It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell.
iv. 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 tetrad of cells i.e., four haploid daughter cells,
It stages are schematically given below.
Significance of meiosis
- It maintains the same chromosome number in the the sexually reproducing organisms. From a diploid cell, haploid gametes are produced which in turn fuse to form a diploid cell. Haploid gametes are formed due to reduction of chromosomes to its half.
- It restrict the multiplication of chromosome number and maintains the stability of the species.
- Maternal and paternal genes get exchanged during crossing over. It result in variations among the offspring.
- All the four chromatic of a homologous pair of chromosomes segregate and go over separately to four different daughter cells. This leads to variation in the daughter cell genetically.
- Paternal and maternal chromosomes assort independently. Thus, cause reshuffling of chromosomes and traits controlled by them.
Difference between mitosis and meiosis.
|The division occurs in somatic cells||It occurs in reproductive cells|
|The daughter cells resemble each other as well as their mother cell||The daughter cells neither resemble one another nor their mother cell|
|Respiration of chromosomes occurs before every mitotic division||Replication of chromosomes occurs only once though meiosis is a double division|
|Mitosis does not introduce variations||Meiosis introduce variations|
|It is a single division||It is a double division|
|Mitosis is required for growth, repair and healing||Meiosis is involved in sexual reproduction|
|Forms two daughter cells||Forms four daughter cells|
|Daughter cells have same number of chromosomes as the parent cell||Daughter cells have half the number of chromosomes of the parent cell|
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