Cell-Cycle-and-Cell-Division.pdf

Cell Cycle and Cell Division © 2022, Aakash BYJU'S. All rights reserved Key takeaways Cell and cell division 1 2 Phases of cell cycle Cell cycle checkpoints: Interphase Cell cycle 3 4 Interphase 5 G1 phase G1 /S Checkpoint S phase G2/M Checkpoint G2 phase G0 phase © 2022, Aakash BYJU'S. All rights r...

Cell Cycle and Cell Division © 2022, Aakash BYJU'S. All rights reserved Key takeaways Cell and cell division 1 2 Phases of cell cycle Cell cycle checkpoints: Interphase Cell cycle 3 4 Interphase 5 G1 phase G1 /S Checkpoint S phase G2/M Checkpoint G2 phase G0 phase © 2022, Aakash BYJU'S. All rights reserved Key takeaways Mitosis phase 6 7 Cytokinesis 8 Stages of karyokinesis Prophase Metaphase Cell furrow formation Anaphase Cell plate formation Telophase 9 Regulation of cell cycle © 2022, Aakash BYJU'S. All rights reserved 10 Significance of mitosis Key takeaways Meiosis-I 11 Prophase-I 12 Metaphase-I Prophase-II Anaphase-I Metaphase-II Telophase-I Anaphase-II Significance of meiosis 13 14 Summary © 2022, Aakash BYJU'S. All rights reserved Meiosis-II Telophase-II Meiosis- II vs Mitosis Cell  Cell is the basic structural and functional unit of life.  All living organisms are made of cells. Endoplasmic reticulum Chloroplast Cell membrane Cytoplasm Cell wall Nucleolus Lysosomes Nucleus Mitochondria Nuclear membrane Vacuole Ribosomes Plant cell © 2022, Aakash BYJU'S. All rights reserved Centrioles Ribosomes Golgi complex Animal cell Cell Division  Cell division is a process by which a parent cell divides into two daughter cells.  Cell division is responsible for the following: o Wound healing o Regeneration o Growth and development Daughter cell Cell division Parent Cell © 2022, Aakash BYJU'S. All rights reserved Daughter cell Cell Cycle  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.  During the growth of a cell, the cell organelles duplicate, and DNA replication takes place.  Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.  Therefore, it becomes essential for the cell to divide and restore the nucleocytoplasmic ratio. © 2022, Aakash BYJU'S. All rights reserved Phases of Cell Cycle  Cell cycle consists of two basic phases o Interphase o M Phase (Mitosis phase)  Interphase is the phase between two successive M phases, where the cell prepares itself for cell division.  M phase is the phase where actual cell division occurs.  Period of cell cycle varies from organism to organism.  A human cell divides approximately every 24 hours.  A yeast cell divides every 90 minutes. © 2022, Aakash BYJU'S. All rights reserved Interphase M Phase Interphase  Interphase is the most active phase of the cell cycle. G2  During this phase, cell growth and DNA replication takes place. M Phase  It lasts for more than 95% of the duration of the cell cycle.  It is also called as the resting phase as there is no apparent activity related to cell division.  Interphase is further divided into 3 stages: o G1 phase o S phase o G2 phase © 2022, Aakash BYJU'S. All rights reserved CELL CYCLE S G1 G1 Phase  G1 phase or Gap 1 phase is the longest phase of interphase. G1 phase  It is present between mitosis and initiation of DNA replication.  In this phase, the cell grows in size.  Also, active synthesis of RNA and proteins takes place in this phase. G2  The cell organelles duplicate during this phase. M Phase CELL CYCLE S G1 © 2022, Aakash BYJU'S. All rights reserved S Phase  The genetic material, in most of the organisms, is present as DNA in the nucleus.  The DNA contains all the instructions required to build and run a cell down to the very minute detail.  In S phase the DNA molecules are duplicated. This occurs through the process of DNA replication. Parental molecule Daughter DNA molecules  DNA replication is the process of copying a DNA molecule to produce two identical DNA molecules. G 2 M Phase CELL CYCLE S G1 © 2022, Aakash BYJU'S. All rights reserved S Phase  Even though the DNA content gets doubled during the S phase, the chromosome number remains the same since the 2 copies of the DNA strand are still attached to each other.  In animal cells, centrioles also duplicate in this phase.  The centrioles help in distributing the duplicated genetic material equally. G2 M Phase CELL CYCLE Before S phase (G1 Phase) © 2022, Aakash BYJU'S. All rights reserved S After S Phase G1 G2 Phase  G2 phase or gap2 phase is present in between S phase and M phase.  Mitochondria, chloroplast and Golgi bodies duplicate in this phase.  DNA synthesis stops at this phase.  There is production of proteins required for the actual dividing phase.  By the end of the G2 phase, the cell is now ready for cell division.  In animal cells, the mitotic division takes place only in the diploid somatic cells. G2  In plant cells, mitotic division is seen both in diploid and haploid cells. M Phase CELL CYCLE S G1 © 2022, Aakash BYJU'S. All rights reserved G0 Phase  G0 phase is also called as quiescent stage. G2  Cells that do not divide, exit from cell cycle in G1 phase and enter into inactive G0 phase. M Phase  Some cells enter G0 phase permanently and never divide again. o Examples: Heart cells, nerve cells.  Cells that temporarily enter G0 phase can enter G1 phase and undergo division. o Example: Cambial cells that undergo division and help in secondary growth in plants. © 2022, Aakash BYJU'S. All rights reserved CELL CYCLE S G1 G0 Phase Cell Cycle Checkpoints: Interphase G1/S checkpoint M  It is the main checkpoint for a cell to progress or halt cell cycle.  It checks for nutrients, growth factors, DNA damage  If conditions are not favourable, the cell exits G1 phase and enters G0 phase. G2 S DNA synthesis G1 G0 G2/M checkpoint  Cell checks for DNA damage and ensures that DNA replication is done without errors.  G2 checkpoint, before M phase, ensures that cell division proceeds and healthy daughter cells are formed.  If errors have occurred during DNA replication, then cell pauses allowing the cell to undergo repair.  If errors are not rectified, then cell undergoes programmed cell death, where the cell’s lysosomes release their hydrolytic enzymes to destroy itself. © 2022, Aakash BYJU'S. All rights reserved Mitotic Phase  The mitosis phase of the cell division phase includes two steps: Mitosis phase Karyokinesis The replicated chromosomes separate and two nuclei are formed. Following karyokinesis, the cytoplasm divides and this results in the formation of two daughter cells. (Karyon- Nucleus, Kinesis Movement) © 2022, Aakash BYJU'S. All rights reserved Cytokinesis Following the karyokinesis, the cytoplasm divides and this results in the formation of two daughter cells. (Cytos- Cell/ Hollow, Kinesis - Movement) Mitosis Phase  Mitosis was first observed by Strasburger in plant cell and by Walter Flemming in animal cell.  The term mitosis was given by Walter Flemming.  A type of cell division that produces: ○ Two similar daughter cells ○ Having the same number of chromosomes as parent cell. Mitosis Karyokinesis Prophase © 2022, Aakash BYJU'S. All rights reserved Metaphase Cytokinesis Anaphase Telophase Stages of Karyokinesis: Prophase  Prophase is the first phase of karyokinesis.  It is the longest phase in terms of the time taken for completion.  The chromatin fibres start condensing during the early prophase and form a condensed mass.  Since, it resembles a condensed ball of wool, early prophase is also known as the spireme stage (tangle or coil of filament).  By the late prophase, they further condense to form the chromosomes. © 2022, Aakash BYJU'S. All rights reserved Stages of Karyokinesis: Prophase  During prophase the nuclear membrane degenerates and the nucleolus disappears.  If nuclear membrane disappears during the mitosis, it is called eumitosis and if the nuclear membrane remains intact it is called premitosis.  Disintegration of endoplasmic reticulum and Golgi apparatus also takes place.  The centrosomes with replicated centrioles start moving towards the opposite poles.  Each centrosome radiates microtubules known as asters. Aster rays help the centrioles to hold their place in the cytoplasm.  In animal cells, mitosis is called amphiastral. In plant cells, it is called anastral.  The centrioles form spindle fibres. © 2022, Aakash BYJU'S. All rights reserved Centrioles Nuclear membrane Stages of Karyokinesis: Metaphase  The complete degradation of the nuclear membrane marks the start of metaphase.  The chromosomes come to lie at the equatorial plate (equidistant from the two poles). This process is known as congression.  Congression occurs with the assembly of the mitotic spindle that mediates the microtubule-chromosome interactions required for the movement of chromosomes.  The centromere is surrounded by a small disc shaped structure called kinetochore. The kinetochore form the site of attachment of microtubules.  Chromosomes are observed to be the thickest and the shortest at this stage. © 2022, Aakash BYJU'S. All rights reserved Stages of Karyokinesis: Anaphase  The centromere splits.  The sister chromatids separate into two identical and independent chromosomes.  Each chromatid now has its own centromere.  The spindle fibres pull the chromatids along with the centromere towards their respective poles.  The chromatids move to opposite poles.  Half of them reach one pole and the other half reach the other.  During migration, the centromere of chromosomes face towards the poles. The chromatids or arms of chromosomes trail behind. © 2022, Aakash BYJU'S. All rights reserved Stages of Karyokinesis: Telophase  Chromosomes cluster at opposite poles.  They start decondensing into chromatin fibres and their individuality is lost as discrete elements.  The nucleolus, ER, and Golgi apparatus reappear.  The nuclear envelope develops around the chromatin at each pole, forming two daughter nuclei. © 2022, Aakash BYJU'S. All rights reserved Formation of two daughter nuclei during telophase Cytokinesis Cell furrow formation  In animal cells, cytokinesis is achieved by the formation of a furrow.  Furrow appears in the plasma membrane and deepens towards the centre in a centripetal fashion.  Furrows from both the sides join at the centre, dividing the cytoplasm into two.  The formation of cell furrow is aided by microfilaments and microtubules. © 2022, Aakash BYJU'S. All rights reserved Cytokinesis Cell plate formation  In plant cells, wall formation starts at the centre of the cell and grows outwards.  The formation of the new cell wall begins with the formation of a cell plate.  Fragments from the Golgi complex (phragmoplast), which are known as vesicles, fuse together to form cell plates.  The cell plate is laid in a centrifugal manner.  The cell plate represents the middle lamella between the walls of two adjacent cells.  Mitochondria and plastids get distributed between the two daughter cells. © 2022, Aakash BYJU'S. All rights reserved Significance of Mitosis  Growth: Mitosis causes growth and development in multicellular organisms.  Plants can grow from a tiny zygote to huge organisms due to mitosis.  Helps in maintenance of proper surface area to volume ratio of a cell  Repair: The old and worn-out cells are replaced by new cells.  Reproduction: Unicellular organisms reproduce (multiply) through mitosis.  In unicellular organisms, replication of cells is synonymous with growth.  Regeneration: Mitosis causes cell growth that causes the revival of the lost body parts in animals such as starfish, planaria, the tail of a lizard, etc. © 2022, Aakash BYJU'S. All rights reserved Regulation of Cell Cycle Metaphase checkpoint  Checks for chromosome spindle attachment. Regulation of cell cycle  Cyclins are proteins that bind to and activate the cyclin-dependent kinases (CDK’s).  Cyclin-CDK complexes control the progression of a cell from one phase to the next phase of the cell cycle.  A stage-specific cyclin binds to a CDK and takes the cell through a checkpoint.  To move to the next phase, the previous cyclin is degraded and a new cyclin specific for the next stage binds to CDK, and the cell progresses into the next phase. © 2022, Aakash BYJU'S. All rights reserved M G2 G1 S DNA synthesis G0 Regulation of Cell Cycle Start of M phase G2/M-phase cyclin Degradation of G2/M- phase cyclin CDK M Degradation of - phase cyclin S S S-phase progression S-phase cyclin G1 /S-phase cyclin G1 G2 S-phase CDK CDK CDK G1 /S-phase CDK Start of S phase Degradation of G1 /Sphase cyclin © 2022, Aakash BYJU'S. All rights reserved Meiosis  The termmeiosis was coined by Farmer and Moore in 1905.  Meiosis is the kind of cell division where the chromosome number is reduced to half in the daughter cells.  It involves two sequential cycles of nuclear division but only one cycle of DNA replication.  It is reductional division that occurs in diploid germ cells. The single cells divide twice to produce four cells. Each daughter cell contains half the amount of genetic information. Diploid mother cell (2n) 46 MEIOSIS Haploid daughter cells (n) © 2022, Aakash BYJU'S. All rights reserved 23 23 23 23 Meiosis  Germ cells undergo meiosis to give rise to haploid gametes.  Gamete formation happens by meiosis instead of mitosis. 46  It ensures that gametes are haploid. o Parent cells in humans have two pairs of chromosomes, i.e., 23 + 23 chromosomes. They are diploid. o After meiotic cell division, the four daughter cells formed have only half the number of chromosomes, i.e., 23 chromosomes. o Such cells with only half the set of chromosomes are known as haploid cells. o Haploid cells are denoted by 2n/2 = n  After fertilisation, the diploid phase is restored. © 2022, Aakash BYJU'S. All rights reserved 23 46 Diploid gamete mother cell 46 Diploid zygote 23 Haploid gametes Steps of Meiosis  Meiosis involves two sequential cycles: Meiosis © 2022, Aakash BYJU'S. All rights reserved Meiosis I Meiosis II Prophase I Prophase II Metaphase I Metaphase II Anaphase I Anaphase II Telophase I Telophase II Meiosis I  Meiosis -I is initiated after the parental chromosomes have replicated to produce identical sister chromatids in the S phase.  It is longer and more complex when compared to the prophase of mitosis.  Unlike mitosis, meiotic prophase I has five substages based on the chromosomal changes in the nucleus. Prophase I Leptotene © 2022, Aakash BYJU'S. All rights reserved Zygotene Pachytene Diplotene Diakinesis Prophase-I Leptotene (Bouquet stage)  Leptotene is the ‘thin thread’ stage.  Chromatin fibres start condensing.  Chromosomes become gradually visible under light microscope. Zygotene  Zygotene is the paired thread stage. Bivalent  The two chromosomes which are similar in form, size and structure are called homologous chromosomes.  Homologous pairs come together to form a synaptonemal complex.  The homologous chromosomes come to lie side by side in pairs and this pairing is known as synapsis.  The pair of synapsed chromosomes is known as the bivalent or tetrad. © 2022, Aakash BYJU'S. All rights reserved Tetrad Homologous chromosomes Prophase-I Pachytene  Pachytene is the ‘thick thread’ stage, as the synapse chromosomes appear thick. Chiasma  Bivalent chromosomes are clearly visible. They appear as tetrads.  By the end of pachytene, the recombination between the homologous chromosomes is complete and the two chromatids are linked at the site of crossing over.  Recombination nodules appear on the non-sister chromatids of homologous chromosomes. Recombination nodule  The exchange of genetic material between the non-sister chromatids of homologous chromosomes takes place, which is also known as crossing over.  Recombination is catalysed by enzyme recombinase. © 2022, Aakash BYJU'S. All rights reserved Chromatids Prophase-I Diplotene  Diplotene is the ‘twin thread’ stage.  In this stage, dissolution of the synaptonemal complex occurs.  The homologous chromosomes start separating i.e. desynapsis occurs.  Recombined homologous chromosomes separate from each other at all sites except at the site of crossover.  X-shaped chiasmata is observed.  In oocytes of some vertebrates, diplotene lasts for month or years. It is called dictyotene stage.  Terminalisation of chiasmata starts in this stage. © 2022, Aakash BYJU'S. All rights reserved X-shaped chiasmata Prophase-I Diakinesis  This is the last stage of prophase I.  The chromosomes are fully condensed.  Termination of chiasmata is observed.  Spindle apparatus assembles.  The nuclear membrane breaks down.  Nucleolus also disappears. © 2022, Aakash BYJU'S. All rights reserved Metaphase- I  As the early metaphase-I starts, the microtubules arise from the opposite spindle poles of the spindle apparatus.  Microtubules are made up of tubulin protein.  They provide both mechanical support and cell movement.  As it enters late metaphase-I, bivalent chromosomes align on the equatorial plate. Early metaphase I  Microtubules from opposite poles attach to the homologous chromosomes.  These microtubules attach to the kinetochores of the pair of homologous chromosomes. © 2022, Aakash BYJU'S. All rights reserved Late metaphase I Anaphase- I  In this phase, the homologous chromosomes are separated.  The spindle microtubules pull the homologous chromosomes towards the opposite poles, respectively.  The sister chromatids of the homologous chromosomes are associated with each other at the centromere.  Reduction in the number of chromosomes occurs during anaphase- I © 2022, Aakash BYJU'S. All rights reserved Anaphase I Telophase- I  Telophase- I is the final step of meiosis -I.  The chromosomes reach the poles.  The spindle fibres completely disappear.  At this stage, the nuclear membrane and the nucleolus reappear after the homologous chromosomes have separated.  It produces two daughter nuclei each containing half the number of chromosomes but double the amount of nuclear DNA. © 2022, Aakash BYJU'S. All rights reserved Telophase I Cytokinesis  Telophase I is followed by cytokinesis.  Cytokinesis is the process where the cytoplasm is divided equally into daughter cells.  The daughter cells formed at the end of meiosis have bivalent chromosomes, and this chromosome is also known as a dyad (one pair of chromosomes from the tetrad). Interkinesis  It is a short-lived stage between meiosis I and meiosis II.  During this phase, the chromosomes are elongated but do not form chromatin fibres.  This stage has no DNA replication.  The RNA and protein required during meiosis -II are synthesized during this phase. © 2022, Aakash BYJU'S. All rights reserved Meiosis- II Meiosis © 2022, Aakash BYJU'S. All rights reserved Meiosis I Meiosis II Prophase I Prophase II Metaphase I Metaphase II Anaphase I Anaphase II Telophase I Telophase II Prophase II  This phase is initiated after cytokinesis I and is simpler than prophase I of meiosis I.  In early prophase II, the nuclear membrane starts to disintegrate.  Chromatin fibres begin to condense to form chromosomes.  As the cell enters late prophase II, the nuclear membrane disintegrates, and chromosomes become compact.  The centrioles also move towards the opposite ends. © 2022, Aakash BYJU'S. All rights reserved Early prophase II Late prophase II Metaphase II, Anaphase II Metaphase -II  Condensed chromosomes align at the equatorial plate.  The microtubules of the spindle apparatus get attached to the sister chromatids at kinetochore. Metaphase II Anaphase-II  The microtubules of the spindle pull the sister chromatids to the opposite poles.  The centromere of the sister chromatids splits. Anaphase II © 2022, Aakash BYJU'S. All rights reserved Telophase II  It marks the end of meiosis II.  The nuclear membrane and the nucleolus reappear.  The chromosomes decondense into chromatin.  The spindle fibers degenerate. Telophase II © 2022, Aakash BYJU'S. All rights reserved Cytokinesis  Telophase II is followed by cytokinesis.  The cytoplasm is divided into daughter cells.  The end of cytokinesis is marked by the tetrad of the haploid cells.  The two cells give rise to four cells or a tetrad of cells. 4 haploid cells Cytokinesis © 2022, Aakash BYJU'S. All rights reserved Significance of Meiosis  Meiosis produces gametes for sexual reproduction.  It conserves the specific chromosome number of each species in the category of sexually reproducing organisms.  It increases the genetic variability from one generation to the next.  Genetic variations contribute to evolution. © 2022, Aakash BYJU'S. All rights reserved Meiosis- II vs Mitosis Similarities:  Both are equatorial divisions.  The sister chromatids are separated during the anaphase to become the chromosomes of the daughter cells.  Microtubules attach from the opposite directions to the centromere of each sister chromatid pair.  Chromosomes decondense during telophase. Differences: Mitosis Meiosis -II Mitosis occurs in diploid somatic cells. Meiosis -II always occurs in haploid germ cells. Mitosis is always followed by DNA replication. Meiosis-II is not followed by DNA replication. After mitosis, the daughter cells are exactly similar to one another and the parent cell. The daughter cells formed are neither similar to each other nor similar to the parent cell. © 2022, Aakash BYJU'S. All rights reserved Summary Cell cycle Cell division Interphase G1 phase: Cell grow in size Protein production Nutrients synthesis S phase: DNA replication Centriole duplication G2 phase: Cell growth Protein production © 2022, Aakash BYJU'S. All rights reserved Checkpoints in cell division M phase Karyokinesis: Chromosomes separate Two nuclei are formed Cytokinesis: Cytoplasm divides Two daughter cells formed G1 /S checkpoint: i) Check for nutrients ii) Growth factors iii) DNA damage G2/M checkpoint: i) Check for cell size ii) DNA replication Metaphase checkpoint: i) Checks for chromosome spindle attachment. Summary Karyokinesis Mitosis Karyon = Nucleus; Kinesis = Movement It is the division of the nucleus. Prophase - Condensation of chromatin fibres - Nuclear membrane degenerates Metaphase - Chromosomes are attached to spindle fibres. - Chromosomes are arranged in the equatorial plane. Anaphase - Centromere splits and chromatids separate. - Chromatids move to opposite poles. Telophase - Chromosome reach the poles - Disappearance of spindle fibres - Decondensation of chromosomes © 2022, Aakash BYJU'S. All rights reserved Cytokinesis Cytos = Cell, Kinesis = Movement It is the division of the cytoplasm. Cell furrow formation Observed in animals Cell plate formation Observed in plants Summary Meiosis Meiosis I DNA replication in Interphase Centromere intact Dyad of cells Reduction division © 2022, Aakash BYJU'S. All rights reserved Meiosis II No DNA replication Centromere splits Tetrad of cells Equational division