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Cell division
The document outlines the processes of cell reproduction, focusing on mitosis and meiosis, highlighting their key phases and differences. Mitosis is described as somatic cell division leading to two genetically identical daughter cells, while meiosis involves two divisions resulting in four haploid cells with genetic variation due to crossing over. Additionally, the document delves into the cell cycle, interphase stages, and specific terminologies related to chromosomes and cell division.
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Cell division
- 2. Table Of Contents: 1.Introduction to types 2. Mitosis 3. Meiosis 4. Difference b/w Mitosis And Meiosis By: Rohit Satyam BT 2nd Yr
- 3. Two Basic ProcessIn Cell Reproduction 1. Cell-Growth: Period of synthesis and duplication of various components of cell. 2. Cell-Division: Adult cell divides into two daughter.
- 4. Fig: 1. CellCycle G0
- 5. Cell Cycle Definition: Allthose changes which occur during cell growth and cell division are collectively called Cell Cycle. Two Parts of Cell Cycle: 1.Interphase 2. M-Phase
- 6.
- 8. Interphase • Also knownas RESTING PHASE(no visible changes occur)/ Metabolically active phase/ preparatory phase/ energy phase. • Three sub phases are: 1. G1/ Gap-I/ Post mitotic/ Pre-synthetic/ First growth phase: • Pooling of Amino Acid & Nucleotides for protein and Nucleic Acid synthesis respectively. • Energy molecules and enzyme synthesis • Carbohydrates, Lipids and Protein Synthesis • Chromosome are Fully Extended
- 9. 2. S/ Syntheticphase: • DNA replicates semiconservately forming two sister chromatids joined at centromere/ primary constriction. •Histone synthesis NOTE: In this phase the DNA amount doubles(4N) But the chromosomes number remain same 3. G2/ Gap-2/ Post Synthetic/ Pre mitotic phase/ second growth phase: Increase in Nuclear Volume •Synthesis of: 1. Spindle Proteins 2. Three types of RNA molecules 3. ATP molecules 4. Mitochondria Duplication 5. Damaged DNA is Repaired
- 10. M-Phase/ Mitotic Phase/D-phase • It involves the separation of SISTER CHROMATIDS & their redistribution into daughter cells. • Orderly distribution of cell organelles • It consists of two stages: 1. Karyokinesis: (Karyon: Nucleus And Kinesis: movement): It involves division of nucleus for which nucleus develops a constriction in centre and becomes dumbell-shaped. Constriction divides the parent nucleus into two daughter nucleus. 2. Cytokinesis: ( kytos= cell): Division of cytoplasm. A constriction in Plasma membrane develops a constriction and deepens centripetally and finally divides into two cells.
- 11. Terminology 1. Homologous Chromosomes:The cell has two sets of each chromosome; one of the pair is derived from the mother and the other from the father. The maternal and paternal chromosomes in a homologous pair have the same genes at the same loci, but possibly different alleles. 2. Kinetochores: On the surface of each centromere, there are two disc like areas where the spindle fibres attaches. They are k/a Kinetochores.
- 16. Pair of homologous chromosomesin diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Homologous chromosomes separate Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes Interphase Meiosis I Meiosis II 2 1
- 17. Mitosis •It is alsocalled Somatic Cell Division or Equational Division. •In this, mature somatic cell divides in such a way that chromosome number is kept constant in daughter cell equal to parent cell, so that daughter cell are quantitatively & qualitatively similar to Parent Cell, so it is called Equational Division.
- 18. •Mitosis produces two geneticallyidentical cells. •Mitosis is referred to in the following stages: prophase, metaphase, anaphase, and telophase. Mitosis
- 19. •In prophase, thecell begins the process of division. •The chromosomes condense. Prophase
- 21. Prophase • Nuclear envelopedisappears. •Centrioles migrate to opposite poles of the cell. •Asters and spindle fibers form.
- 22.
- 23. Metaphase • The chromosomesline up at the equator of the cell (metaphase plate), with the centrioles at opposite ends and the spindle fibers attached to the centromeres. Centriole Centriole Spindle fibers Metaphase plate
- 28. Anaphase • In anaphase,the centromeres divide. • At this point, each chromosome goes from having 2 sister chromatids to being 2 separate chromosomes
- 29. The spindle fiberscontract and the chromosomes are pulledto opposite poles.
- 30. Telophase • In telophasethe nucleus actually divides. • The chromosomes are at the poles of the cell. • The nuclear envelope re- forms around the two sets of chromosomes.
- 31. Cytokinesis • The divisionof the cytoplasm. • In animal cells, a Cleavage Furrow forms and separates Daughter Cells Cleavagefurrowin a dividingfrogcell.
- 32. Cytokinesis • In plant cells,a Cell Plate forms and separates Daughter Cells.
- 33. ANIMAL VS. PLANTMITOSIS • ANIMAL CELL – Centriole and aster present – Daughter cells separated by cleavage furrow • PLANT CELL – No visible centriole or aster – Daughter cells separated by cell plate
- 34. The Stages ofMeiosis • After chromosomes duplicate, two divisions follow – Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes – Meiosis II (equational division) sister chromatids separate • The result is four haploid daughter cells with unreplicated chromosomes
- 35. Meiosis It is calledREDUCTIONAL DIVISION because it involves formation of 4 daughter cells which have half chromosome number to those in their parental cell.
- 37.
- 38. •Meiosis I ispreceded by interphase, when the chromosomes are duplicated to form sister chromatids •The sister chromatids are genetically identical and joined at the centromere •The single centrosome replicates, forming two centrosomes
- 39. Division in meiosisI occurs in four phases –Prophase I –Metaphase I –Anaphase I –Telophase I and cytokinesis
- 40. Prophase I •Prophase Itypically occupies more than 90% of the time required for meiosis •Chromosomes begin to condense •In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
- 41. Sister chromatids of oneduplicated chromosome Key Maternal set of chromosomes (n 3) Paternal set of chromosomes (n 3) Key 2n 6 Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)
- 54. 1. Leptotene/ Leptonema(leptos= Thin tene= Thread) • Also called THIN THREADED STAGE • Volume of nucleas increases. • Formation of Aster • Condensation of nuclear chromatin by dehydration and spiralization . 2. Zygotene/ Zygonema: • Pairing( k/a Synapsis or Syndesis) of homologous chromosomes to form Bivalents. It occurs due to forces of attraction between alleles on Homologous Chromosomes • Pairing of Homologous chromosomes in Zipper Fashion No of Bivalents= ½ of total no of chromosomes in a diploid cell. • Further condensation of chromosomes and moving away of asters. • Under EM, a filamentous ladder-like nucleoproteinous complex called Synaptonemal Complex is observed.
- 56. 3. Pachytene/ Pachynema/Thick Thread stage: • Further condensation of chromosome. Sister chromatids are clearly visible and are joined at centromere and is called Dyad. Each bivalent has 2 dyad and is called a tetrad(i.e. 4 chromatids). • Recombination sometimes takes place wherein exchange of genes or crossing over b/w two non sister chromatids of Homologous Chromosomes occurs at the points called Recombination Nodules. This is enzimetically controlled process and is regulated by recombinase enzyme. • Moving away of Asters.
- 57. Crossing Over • Crossingover produces recombinant chromosomes, which combine DNA inherited from each parent • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
- 58. •In crossing over,homologous portions of two nonsister chromatids trade places •Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome
- 59. 4. Diplotene/ Diplonema: •Nuclear membrane disappears and Nucleoli start disappearing. • Desynapsis- Is separation of Homologous Chromosomes begins due to dissolution of synaptonemal complex. • Chismata are Visible & Terminilization. 5. Diakinesis • Terminilization Completed. But still Homologous chromosomes are attached at the ends. •Formation of Spindle & Complete disappearance of spindles.
- 61. Metaphase I • Inmetaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad • Microtubules from the other pole are attached to the kinetochore of the other chromosome
- 62.
- 64. Anaphase I •In anaphaseI, pairs of homologous chromosomes separate •One chromosome moves toward each pole, guided by the spindle apparatus •Sister chromatids remain attached at the centromere and move as one unit toward the pole
- 65. Telophase I andCytokinesis •In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids •Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
- 66. •In animal cells,a cleavage furrow forms; in plant cells, a cell plate forms. •No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated.
- 67. Prophase I MetaphaseI Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example. Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids.
- 68. Meiosis II Division inmeiosis II also occurs in four phases •Prophase II •Metaphase II •Anaphase II •Telophase II and cytokinesis •Meiosis II is very similar to mitosis
- 69. Prophase II •In prophaseII, a spindle apparatus forms •In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate
- 70. Metaphase II •In metaphaseII, the sister chromatids are arranged at the metaphase plate •Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical •The kinetochores of sister chromatids attach to microtubules extending from opposite poles
- 71. Anaphase II •In anaphaseII, the sister chromatids separate. •The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles.
- 72. Telophase II andCytokinesis •In telophase II, the chromosomes arrive at opposite poles. •Nuclei form, and the chromosomes begin decondensing.
- 73. •Cytokinesis separates thecytoplasm. •At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes. •Each daughter cell is genetically distinct from the others and from the parent cell.
- 75. Prophase II MetaphaseII Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
- 76. MEIOSIS I: Separateshomologous chromosomes Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example. Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids. MEIOSIS I: Separates sister chromatids Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
- 78. Figure 13.11-5 Prophase I ofmeiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes
- 79. • Three eventsare unique to meiosis, and all three occur in meiosis l • Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information • At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes • At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate
- 80. Figure 13.9 Prophase Duplicated chromosome MITOSIS Chromosome duplication Parent cell 2n 6 Metaphase Anaphase Telophase 2n 2n Daughter cells of mitosis MEIOSIS MEIOSIS I MEIOSIS II Prophase I Metaphase I Anaphase I Telophase I Haploid n 3 Chiasma Chromosome duplication Homologous chromosome pair Daughter cells of meiosis I Daughter cells of meiosis II n n n n SUMMARY Property Mitosis Meiosis DNA replication Number of divisions Synapsis of homologous chromosomes Number of daughter cells and genetic composition Role in the animal body Occurs during interphase before mitosis begins One, including prophase, metaphase, anaphase, and telophase Does not occur Two, each diploid (2n) and genetically identical to the parent cell Enables multicellular adult to arise from zygote; produces cells for growth, repair, Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the
Editor's Notes
- #55 The synaptonemal complex is a protein structure that forms between homologous chromosomes (two pairs of sister chromatids) during meiosis and is thought to mediate chromosome pairing, synapsis, and recombination. It is now evident that the synaptonemal complex is not required for genetic recombination[citation needed]. Research has shown that not only does it form after genetic recombination but mutant yeast cells unable to assemble a synaptonemal complex can still engage in the exchange of genetic information.[citation needed] It is currently thought that the SC functions primarily as a scaffold to allow interacting chromatids to complete their crossover activities. The synaptonemal complex is a tripartite structure consisting of two parallel lateral regions and a central element.