Mitosis_LMS.pdf

Mitosis & Meiosis Dr. Mahmood Al Mashhadani Objectives: Following this lecture, the student should be able to: 1. Understand key stages and mechanisms of mitosis and meiosis 2. Distinguish between the two processes and recognize their significance in cell biology. 3. Explore the roles of...

Mitosis & Meiosis Dr. Mahmood Al Mashhadani Objectives: Following this lecture, the student should be able to: 1. Understand key stages and mechanisms of mitosis and meiosis 2. Distinguish between the two processes and recognize their significance in cell biology. 3. Explore the roles of mitosis and meiosis in genetic diversity, reproduction, and cellular development. Mitosis divsion of nucleus NOT cells Loading… Mitosis and Chromosome Complement: Mitosis is a precise process of nuclear division. Ensures each daughter cell receives a diploid complement of chromosomes identical to the parent cell. Cytokinesis: Mitosis is usually accompanied by cytokinesis. Cytokinesis divides the cell into two daughter cells. DNA Segregation during Cell Division Chromosome Duplication: Each chromosome is duplicated before nuclear division begins. Chromosome duplication coincides with the replication of the DNA molecule. Chromosome Separation: Each chromosome divides longitudinally into identical halves. The separated chromosome halves move to opposite directions. Each chromosome half is included in one of the two daughter nuclei. Before Mitosis rmmbr mitosis is part of cell cycle cells divide when signal comes to them u can see chromosomes clearly in metaphase - DO NOT FORGET Interphase and Preparation for Mitosis: In non-mitotic cells, chromosomes are not visible under a light microscope. This stage is called interphase. Loading… During interphase, DNA is replicated in the S phase (synthesis phase). DNA replication leads to chromosome duplication. Interphase Periods: G1 and G2 are periods before and after S where DNA replication does not occur. The cell cycle is described in the sequence: G1 → S → G2 → M. Cell Cycle Players cyclins are certain proteins that regulate cell cycle before the checkpoints these are major regulator of cells cycles they activate cyclin-dependant kinases and these kinases add phospohate groups to carry out certain functions/roles cyclins then degrade after done with their job  Regulation of the Cell Cycle we will have cancer, some DNA missing, some function not carried out. checkpoints are VERY IMPORTANT Regulation of the Cell Cycle: The cell cycle is actively regulated and identical in all eukaryotes. Transitions from G1 into S and G2 into M are checkpoints. G1/S checkpoint: Transition requires sufficient time since the last mitosis or a certain cell size. G2/M checkpoint: DNA replication and repair must be complete for the M phase to begin. oncogenes --> push the cell to next part all cancers have defective P53 thats why they can divide p53 is tumour repressor --> gardian of genome that everything is correct and then allows cell to go to next phase and inhibit it if things are are abnormal The TP53 gene provides instructions for making a protein called tumor protein p53 (or p53). This protein acts as a tumor suppressor, which means that it regulates cell division by keeping cells from growing and dividing (proliferating) too fast or in an uncontrolled way. The spindle assembly checkpoint (SAC)! most important checkpoint is in metaphase which is the spindle assembly checks if chormosme have aligned correctly Cell Cycle Regulators  Mitosis Cell Cycle Duration: Cell cycle duration varies with cell type, usually taking 18 to 24 hours in higher eukaryotes. Mitosis is the shortest period, taking 0.5 to 2 hours. Mitosis Loading… Prophase Chromosomes condense into visible structures within the nucleus. Each chromosome consists of two chromatids held together at the centromere. The nuclear envelope disintegrates, and nucleoli disappear. Metaphase The mitotic spindle forms from microtubules that extend from centrosomes. Centrosomes have duplicated and migrated to opposite poles of the cell. Microtubules attach to chromosomes at the kinetochores. Chromosomes align along the metaphase plate. Mitosis Anaphase Sister chromatids are pulled toward opposite spindle poles, now considered Telophase individual chromosomes. A nuclear envelope forms around each compact group of chromosomes. Anaphase Movement Nucleoli are reformed. Movement is driven by the progressive The spindle apparatus disappears. shortening of spindle fibers attached to Chromosomes undergo decondensation until they are no longer visible as the centromeres. discrete entities. This shortening pulls chromosomes in The two daughter nuclei gradually assume a typical interphase appearance. opposite directions toward the poles. Cytokinesis At the completion of anaphase: Chromosomes lie in two groups near The cytoplasm of the cell divides into two by a deepening furrow around the opposite poles of the spindle. periphery. Each group contains the same number of chromosomes as was present in the original interphase nucleus. Mitosis  Meiosis DNA multiplied once DNA multiplied once and divided twice - haploid is divided once - diploid result Meiosis Meiosis this is called tetrads important for crossing over to occur and thats important for genetic variation Meiosis Vs Mitosis  Meiosis Meiosis I: Interphase: DNA is replicated, resulting in duplicated chromatin. Prophase I: Homologous chromosomes pair up in a process called synapsis. Crossing over occurs, where homologous chromosomes exchange genetic material. Chromosomes condense and become visible under a microscope. Metaphase I: Homologous chromosome pairs align at the metaphase plate. Each pair consists of one chromosome from the mother and one from the father. Anaphase I: Homologous chromosomes are separated and pulled toward opposite poles of the cell. This reduces the chromosome number by half. Telophase I: Chromosomes arrive at the poles, and the cell divides. This results in two haploid cells, each containing one set of chromosomes. Meiosis II: Prophase II: Chromosomes, each still composed of two sister chromatids, condense again. Metaphase II: Chromosomes align individually at the metaphase plate. Anaphase II: Sister chromatids are separated and pulled toward opposite poles of the cell. Telophase II: Chromatids reach the poles, and the cell divides again. This results in four haploid cells, each with a single set of chromosomes. End Result: Outcome: The original diploid cell has been divided into four genetically distinct haploid cells, each with half the chromosome number of the original cell. Crossover Chromosomes also play a role in genetic variation. Humans have 23 pairs of chromosomes, with one set inherited from each parent. The recombination and exchange of genetic material between homologous chromosomes during a specialized form of cell division called meiosis results in the shuffling and mixing of genetic information. This process contributes to the genetic diversity seen within human populations. Mitosis and Meiosis errors!  most mistakes most most that happens in mitosis and meoisis are becuz of SPINDLE FIBERS Sources of mitotic errors Cohesion defects Centrosome amplification 2 poles in this cell Tetraploidy cells are not divided into 2 Sources of mitotic errors Spindle assembly checkpoint (SAC) defects most COMMON cause!!! The spindle assembly checkpoint (SAC) ensures the fidelity of chromosome segregation. misaligned chromsmoes will divide In early stage of mitosis, misaligned chromosomes activate the SAC to inhibit APC/C. This prevents premature onset of chromosome segregation. Only when all chromosomes achieve chromosome bi- orientation, the SAC will be inactivated to allow cells to start The APC/C’s (Anaphase-promoting complex) main function is to trigger the transition from metaphase to anaphase chromosome segregation. Sources of mitotic errors Spindle attachment defects (a) In amphitelic attachment, the sister kinetochores are correctly connected to microtubules from opposite poles, resulting in a bioriented chromosome. no spindle on the other side (b) In a monotelic attachment, only one of the sister chromatids is connected to a spindle pole; the chromosome is mono-oriented. we have spindle fibers but are (c) In a syntelic attachment, both sister on same side kinetochores are attached to a single spindle pole, and the chromosome is mono-oriented. (d) In a merotelic attachment, usually one or, extra fibers rarely, both sister kinetochores are connected to both poles instead of one. Chromosomes are bioriented in merotelic attachments.  Consequences of mitotic errors Mitotic errors lead to DNA damage Mitotic errors can trigger activation of p53  Aneuploidy Is a chromosomal anomaly in which a cell has an extra or missing chromosome  Chromosomal Abnormalities Chromosomal instability (CIN) we have either 47 or 48 in number  Aneuploidy vs Euploidy Euploidy: Refers to a cell or organism with the correct or normal number of chromosomes. 2 pair lost Aneuploidy: Refers to a cell or organism with an abnormal number of chromosomes, which deviates from the normal euploid number one extra There may be a single set (monoploidy), two sets (diploidy), or multiple sets (polyploidy, i.e. triploid, tetraploid, pentaploid, hexaploid, etc.) of chromosomes Chromosomal Abnormalities flipped a piece deleted swapped between chromosomes they cry like cat we will come back to this later so dont worry we will talk about this too we will come back more to this diseases  radiotherapy dmamages DNA as a side effect Isochromosomes Dicentric chromosomes 2 centromere Loading… u have to know the names u will be asked about them in exma Ring chromosomes telomeres cut and then chormosome becomes like ring  Deletions Duplications Deletions occur when a chromosome breaks Duplications occur when part of a chromosome is and some genetic material is lost. abnormally copied (duplicated). Deletions can be large or small, and can occur This type of chromosomal change results in extra copies of anywhere along a chromosome. genetic material from the duplicated segment.  Inversions usally doesnt cause disease becuz nothing is deleted things get balanced out An inversion occurs when a chromosome breaks in two places; the resulting piece of DNA is reversed and re- inserted into the chromosome. Genetic material may or may not be lost as a result of the chromosome breaks. An inversion that includes the chromosome's constriction point (centromere) is called a pericentric inversion. An inversion that occurs in the long (q) arm or short (p) arm and does not involve the centromere is called a paracentric inversion. Translocations A translocation occurs when a piece of one chromosome breaks off and attaches to another chromosome. This type of rearrangement is described as balanced if no genetic material is gained or lost in the cell. If there is a gain or loss of genetic material, the translocation is described as unbalanced.  Isochromosomes An isochromosome is a chromosome with two identical arms. Instead of one q arm and one p arm, an isochromosome has two q arms or two p arms. As a result, these abnormal chromosomes have an extra copy of some genes and are lacking copies of genes on the missing arm. Dicentric chromosomes Unlike normal chromosomes, which have one centromere, a dicentric chromosome contains two centromeres. Dicentric chromosomes result from the abnormal fusion of two chromosome pieces, each of which includes a centromere. These structures are unstable and often involve a loss of some genetic material.  Ring chromosomes Ring chromosomes usually occur when a chromosome breaks in two places, typically at the ends of the p and q arms, and then the arms fuse together to form a circular structure. The ring may or may not include the centromere, depending on where on the chromosome the breaks occur. In many cases, genetic material near the ends of the chromosome is lost. Chromosomal Abnormalities you have to know these terms becuz we have to answer in exam using these terms