Lecture – 3 Cell division : Mitosis & Meiosis by Dr. Praveen Kumar Asst. Prof. GPB MSSSoA, CUTM, Odisha Cell Division All cells are derived from pre-existing cells New cells are produced for growth and to replace damaged or old cells Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals) 2 Original DNA strand DNA Replication DNA must be copied or replicated before cell division Each new cell will then have an identical copy of Two new, identical DNA strands 3 Cell Cycle DNA Copied Cells prepare for Division Cells Mature 4 Daughter Cells Cell Divides into Identical cells Interphase - G1 Stage 1st growth stage after cell division Cells mature by making more cytoplasm & organelles Cell carries on its normal metabolic activities 5 Interphase – S Stage Synthesis stage DNA is copied or replicated Two identical copies of DNA Original DNA 6 Interphase – G2 Stage 2nd Growth Stage Occurs after DNA has been copied All cell structures needed for division are made (e.g. centrioles) Both organelles & proteins are synthesized 7 What’s Happening in Interphase? What the cell looks like Animal Cell What’s occurring 8 Mitosis Division of the nucleus Also called karyokinesis Only occurs in eukaryotes Has four stages Doesn’t occur in some cells such as brain cells 9 Four Mitotic Stages Prophase Metaphase Anaphase Telophase 10 Early Prophase Chromatin in nucleus condenses to form visible chromosomes Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal) Cytoplasm Nuclear Membrane Nucleolus Chromosomes Late Prophase Nuclear membrane & nucleolus are broken down Chromosomes continue condensing & are clearly visible Spindle fibers called kinetochores attach to the centromere of each chromosome Spindle finishes forming 12 between the poles of the cell Metaphase Chromosomes, attached to the kinetochore fibers, move to the center of the cell Chromosomes are now lined up at the Equator of Cell equator Pole of the Cell 13 Anaphase Occurs rapidly Sister chromatids are pulled apart to opposite poles of the cell by kinetochore fibers 14 Telophase Sister chromatids at opposite poles Spindle disassembles Nuclear envelope forms around each set of sister chromatids Nucleolus reappears CYTOKINESIS occurs Chromosomes reappear as chromatin 15 Cytokinesis Means division of the cytoplasm Division of cell into two, identical halves called daughter cells In plant cells, cell plate forms at the equator to divide cell In animal cells, cleavage furrow forms to split cell 16 Daughter Cells of Mitosis Have the same number of chromosomes as each other and as the parent cell from which they were formed Identical to each other, but smaller than parent cell Must grow in size to become mature cells (G1 of Interphase) 17 Meiosis Formation of Gametes (Eggs & Sperm) 18 Facts About Meiosis Preceded by interphase which includes chromosome replication Two meiotic divisions --- Meiosis I and Meiosis II Called Reduction- division Original cell is diploid (2n) Four daughter cells produced that are monoploid (1n) 19 Facts About Meiosis Daughter cells contain half the number of chromosomes as the original cell Produces gametes (eggs & sperm) Occurs in the testes in males (Spermatogenesis) Occurs in the ovaries in females (Oogenesis) 20 Why Do we Need Meiosis? It is the fundamental basis of sexual reproduction Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote 21 Replication of Chromosomes Occurs in Interphase Replication is the process of duplicating a chromosome Occurs prior to division Replicated copies are called sister chromatids Held together at centromere 22 Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Meiosis I Meiosis II Diploid Diploid 23 Haploid Meiosis I: Reduction Division Nucleus Early Prophase I (Chromosome number doubled) Spindle fibers Nuclear envelope Late Prophase I Metaphase I Anaphase I Telophase I (diploid) 24 Prophase I Early prophase Homologs pair. Crossing over occurs. 25 Late prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments. Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Join to form a TETRAD 26 Called Synapsis Crossing-Over Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring 27 Metaphase I Homologous pairs of chromosomes align along the equator of the cell 28 Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres. 29 Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. 30 Meiosis II Gene X Only one homolog of each chromosome is present in the cell. Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. 31 Meiosis II: Reducing Chromosome Number Prophase II Metaphase II 32 Telophase II Anaphase II 4 Identical haploid cells Prophase II Nuclear envelope fragments. Spindle forms. 33 Metaphase II Chromosomes align along equator of cell. 34 Anaphase II Equator Pole Sister chromatids separate and move to opposite poles. 35 Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. 36 Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome 37 Comparison of Divisions Meiosis Mitosis Number of divisions Number of daughter cells Genetically identical? 1 2 Yes 2 4 No Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role 38 Growth and repair Sexual reproduction