3 - Nuclear Division.pdf

Nuclear Division WEEK 3 Learning Objectives Describe the structure of chromosome Identify and discuss the stages in the eukaryotic cell cycle Describe the cytokinesis in animal and plant cells Explain the significance of mitosis.  Sister chromatids...

Nuclear Division WEEK 3 Learning Objectives Describe the structure of chromosome Identify and discuss the stages in the eukaryotic cell cycle Describe the cytokinesis in animal and plant cells Explain the significance of mitosis.  Sister chromatids duplicated chromosomes Centromere attachment site of sister chromatids Telomere repeated nucleotide sequences at the two ends of chromosome; essential for stability Kinetochore Spindle microtubule mediate spindle–microtubule pull the chromosomes apart to attachment; control the opposite poles of cell, chromosomal movement during allowing two daughter cells to mitosis and meiosis be identical A normal human karyotype contains 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes, generally arranged in order from largest to smallest. Somatic Gametes cells Diploid (2n) cells are cells with pairs of homologous chromosomes, containing two complete sets of chromosomes, one from each parent Haploid (n) cells are cells that contain only one of the homologues, having a single set of unpaired chromosome. Cell Cycle the series of events that take place in a cell, resulting in cell growth, replication of DNA, and division of cytoplasm and organelles to produce two daughter cells. Prokaryotic Cell Cycle Binary Fission Performs asexual reproduction at optimal conditions. Replication of DNA does not occur in phases. Growth - Long process Each DNA copy moves to separate location on plasma membrane. No spindle apparatus Cell Division - Rapid process and nuclear membrane Cell elongates; are involved. Forms a septum at the center; Plasma membrane grows inwards and pinches off; Produces two new identical daughter cells. Eukaryotic Cell Cycle Mitosis and Meiosis Cells may or may not divide, depends on various factors: Type of cells, nutrients, signals etc… Bone marrow, skin – divide once per day Nerve, muscle cells – stop dividing once matured Interphase A period of growth before a dividing cell enters mitosis. Longest stage in cell cycle (90% or more) Cell cycle phase Cellular events Duration First gap phase (G1) Growth and increase in cell size Metabolic cellular activities Produce ribosomes and RNA 10 hours Cytoplasmic materials and organelles increase Synthesis phase (S) DNA synthesis Replicates DNA 8 hours Joining of sister chromatids Second gap phase (G2) Intensive cellular synthesis / post-DNA synthesis 5 hours Produces new proteins Continues to increase in size Mitosis Division of nucleus and cytoplasm to produce two daughter cells with chromosomes identical to the parent cells. Allows body to repair by replacing the dead cells In animal cells, centrosome contains centrioles and asters that can organize spindle microtubules and chromosomes during division. In plant cells, centrosomes are absent. They depend on enzymes to to form and organize the spindle fibers. Vesicles filled with cell wall components form a cell plate to connect the fibers  In plant cell In animal cell Mitosis - Prophase Nucleoli disappear (late) Mitotic spindle forms in cytoplasm Nuclear envelope disintegrates (late) Centrioles move to opposite end Condensation of chromosomes Mitosis - Metaphase Spindle fully develops and extends to Chromosomes align at metaphase centre of the cell plate an imaginary plane perpendicular to the spindle fibres Mitosis - Anaphase Sister chromatid separates, forming Daughter chromosomes move to daughter chromosomes opposite poles of cell Mitosis – Telophase & Cytokinesis Nucleoli reappears Nuclear envelopes are formed Chromatin fibres uncoil Cleavage furrow forms to Splitting of cytoplasm and Produces two daughter pull membrane inwards equal allocation of organelles cells (diploid) Cytokinesis in Plant Cell Furrowing is impossible with cell wall on surface. Vesicles comprising cell wall materials from Golgi are formed. Vesicles enlarge gradually and fuse to form cell plate. New cell wall forms between membranes of cell plate.  Mitosis in onion root tip and kidney cells https://www.youtube.com/watch?v=5-ur7bWqlDQ https://www.youtube.com/watch?v=N97cgUqV0Cg Meiosis Division of nucleus to produce four daughter cells not identical to each other or the parent. Each daughter cell (n, haploid) contains half the chromosome number of the parent nucleus (2n, diploid). To produce sex cells and maintain a species or generation. Cells divide twice.  Meiosis I – Prophase I Nucleolus disappears and centrosomes migrate away from each other Chromosome condenses (Leptotene) Homologous chromosomes attract each other and undergo synapsis, forming bivalents (Zygotene) Crossing over occurs at chiasma, sister chromatids are no longer Synapsis: Homologous chromosomes come together and pair identical (Pachytene) Bivalents: Two sets of paired chromosomes lay side-by-side Chiasma becomes more visible after synapsis ends (Diplotene) Crossing over: Exchange of segments between non-sister chromatids, resulting in genetic recombination Nuclear membrane disintegrates (Diakinesis) and spindle fibres are Chiasma: Regions where non-sister chromatids are attached formed. 5 stages of Prophase I Meiosis I – Metaphase I Spindle fibers are fully developed. Bivalents arrange themselves across Each points to one of the poles only equatorial plate Meiosis I – Anaphase I Chromatids are pulled towards Centromeres do not split opposite poles Meiosis I – Telophase I & Cytokinesis Chromosome number reduces Nuclear envelope reforms Animal cell goes into interphase II, but no DNA replication in S phase Meiosis II – Prophase II The nucleoli and nuclear envelope New set of spindle fibers are formed. disperse Meiosis II – Metaphase II Chromosomes aligned at metaphase plate Meiosis II – Anaphase II Centromeres are split Spindle fibers pull chromatids to opposite poles Meiosis II – Telophase II & Cytokinesis II Nuclear envelopes reforms Chromatin fibers uncoil Cleavage furrow forms in animal cell Cell wall formation in plant cell Produces four daughter cells (haploid)  Mitosis produces two diploid daughter cells; Meiosis produces four haploid daughter cells. DNA is replicated once only during interphase (before mitosis and meiosis I. No replication before meiosis II Crossing over occurs in prophase I of meiosis only; No crossing over occurs in mitosis and meiosis II. In meiosis metaphase I, paired homologous chromosomes align at Summary metaphase plate. In mitosis, individual chromosomes align. In meiosis anaphase I, centromeres remain intact with homologous chromosomes. In mitosis, sister chromatids separate. Regulation of Cell Cycle Internal Chemical Factors Transition signaled by cyclins and cyclin dependent kinases (CDKs) Regulation of Cell Cycle Internal Signals at Checkpoint Kinetochores not attached to microtubules will send signals to inactivate anaphase-promoting complex (delay the anaphase stage) Regulation of Cell Cycle External Signals at Checkpoint Platelet-derived growth factor (PDGF) released by damaged body cells stimulates fibroblast growth to heal injury External Physical Factors Anchorage dependence Cells must adhere to substrate Density-dependent inhibition Crowding inhibits cell division Cancer cells typically divide about 20-50 times more than healthy cell before they stop do not respond to body’s control mechanisms divide excessively, invade other tissues, and can kill the organism if left unchecked may create their own growth factors and abnormal signaling system form tumour when successfully evade immune system End of slides