Mitosis and meiosis.pptx

Mitosis and Meiosis Prof. Dr. Deniz Kıraç Interphase • Interphase is the first stage of the cell cycle • During interphase, the cell grows, makes a copy of its DNA and prepares to divide into two cells. • The nuclear membrane is still intact to protect the DNA molecules from undergoing mutation....

Mitosis and Meiosis Prof. Dr. Deniz Kıraç Interphase • Interphase is the first stage of the cell cycle • During interphase, the cell grows, makes a copy of its DNA and prepares to divide into two cells. • The nuclear membrane is still intact to protect the DNA molecules from undergoing mutation. • G1, S and G2 stages takes place in interphase. 2 1st Part of Interphase During the first part of interphase, the cell grows to full size & produces all the structures it needs. 2nd Part of Interphase In the next part of interphase, the cell makes an exact copy of the DNA in its nucleus in a process called replication. At the end of DNA replication, the cell contains two identical sets of DNA. 3 Mitosis • Mitosis is the stage during which the cell’s nucleus divides into two new nuclei. • During mitosis, one copy of the DNA is distributed into each of the two daughter cells. • Mitosis is divided into four parts, or phases:  Prophase  Metaphase  Anaphase  Telophase 4 Prophase • The threadlike chromatin in the cell’s nucleus condenses to form double-rod structures called chromosomes. • Each identical rod in a chromosome is called a chromatid. • The two chromatids are held together by a structure called a centromere. 5 Prophase • Centrioles (or poles) appear and begin to move to opposite end of the cell. • Spindle fibers form between the poles. • The DNA molecules progressively shorten and condense by coiling, to form chromosomes. • The nuclear membrane and nucleolus are no longer visible. • The spindle apparatus has migrate to opposite poles of the cell. Sister chromatids Centrioles Spindle fibers 6 Prophase Animal Cell Plant Cell Spindle fibers Spindle fibers 7 Metaphase • Chromosomes arrange themselves at equator of spindle, become attached to spindle fibres at centromeres Centrioles Spindle fibers 8 Metaphase Animal Cell Plant Cell 9 Anaphase • The spindle fibres shorten and the centromere splits, separated sister chromatids are pulled along behind the centromeres. Centrioles Spindle fibers 10 Anaphase Animal Cell Plant Cell 11 Telophase Nuclei • The chromosomes reach the poles of their respective spindles. They appear as chromatin (threads rather than rods). • Nuclear membrane reform before the chromosomes uncoil. • The spindle fibres disintegrate. • Two new nuclei form. • Mitosis ends. Nuclei Chromatin 12 Telophase Animal Cell Plant Cell 13 14 15 Cell at Interphase Telophase Anaphase nucleus cytoplasm Prophase Metaphase 16 Cytokinesis • The completion of mitosis is accompanied by cytokinesis, giving rise to two daughter cells. • Cytokinesis usually initiates shortly after the onset of anaphase and is triggered by the inactivation of MPF, thereby coordinating nuclear and cytoplasmic division of the cell. • Cytokinesis of animal cells is mediated by a contractile ring of actin and myosin filaments that forms beneath the plasma membrane. • Cleavage proceeds as contraction of the actin-myosin filaments pulls the plasma membrane inward, eventually pinching the cell in half. • The bridge between the two daughter cells is then broken, and the plasma membane is released. 17 Cytokinesis in animal and plant cells 100 µm Cleavage furrow Contractile ring of Microfilaments (actin and myosin) (a) Cleavage of an animal cell (SEM) Vesicles forming cell plate Wall of parent cell 1 µm Cell plate New cell wall Daughter cells Daughter cells (b) Cell plate formation in a plant cell (TEM) 18 • Plant cells divide by forming new cell walls and plasma membranes inside the cell. • In early telophase, vesicles carrying cell wall precursors from the golgi apparatus associate with spindle microtubules and accumulate at the former site of the metaphase plate. • These vesicles then fuse to form a large, membrane-enclosed, disclike structure, and their polysachharide contents assemble to form the matrix of a new cell wall (called a cell plate). • The cell plate expands outward, perpendicular to the spindle, until it reaches the plasma membrane. • The membrane surrounding the cell plate then fuses with the parental plasma membrane, dividing the cell in two. 19 Cytokinesis in animal and plant cells 100 µm Cleavage furrow Contractile ring of Microfilaments (actin and myosin) (a) Cleavage of an animal cell (SEM) Vesicles forming cell plate Wall of parent cell 1 µm Cell plate New cell wall Daughter cells Daughter cells (b) Cell plate formation in a plant cell (TEM) 20 • In cytokinesis, cell membrane moves inward to create two daughter cells – each with its own nucleus with identical chromosomes. • During cytokinesis, the cytoplasm divides, distributing organelles into each of the two new cells. 21 • Each daughter cell has the same number of chromosomes as the original parent cell. • At the end of cytokinesis, each cell enters interphase & the cycle begins again. • The length of each stage & cell cycle varies, depending on the type of cell. 22 Meiosis • One division of the chromosomes followed by two divisions of nucleus and cell • A diploid (2n) parent cell give rise to 4 haploid (n) daughter cells • Forms gametes (sperms and ova) or spores in some plants 1st meiotic division: similar to mitosis except for a highly modified prophase 2nd meiotic division: a typically mitotic division 23 Meiosis I Prophase I Prophase I comprised of five stages. • Leptotene: chromosomes begin to condense. • Zygotene: chromosomes become closely paired. • Pachytene: crossing over occurs. • Diplotene: homologous chromosomes begin to separate but remain attached by the chiasmata. • Diakinesis: chromosomes condense and separate until terminal chiasmata only connect the two chromosomes. 24 • The close association of homologous chromosomes (synapsis) begins during the zygotene stage. • During this stage, a zipperlike protein structure, called the synaptonemal complex, forms along the length of the paired chromosomes. Synaptonemal complex • This complex keeps the homologous chromosomes closely associated and aligned with one another through the pachytene stage. 25 • Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over (chiasmata). • The synaptonemal complex disappears at the diplotene stage and the homologous chromosomes separate along their length. • At this stage, each chromosome pair (called bivalent) consists of four chromatids with clearly evident chiasmata. • Diakinesis represents the transition to metaphase, during which chromosomes become fully condensed. 26 27 28 In general, at prophase I, chromosomes become visible, shorten and fatten 29 Homologous chromosomes pair together (synapsis) to form a bivalent 30 • sister chromatids join at chiasmata where crossing over can occur • nucleolus disappear, • nuclear membrane breaks down • centrioles migrate to poles to form a spindle 31 32 33 34 Metaphase I • Bivalent chromosomes attach to the spindle and align at the metaphase plate. • The bivalents are randomly oriented with respect to the poles such that chromosomes (maternal or paternal or both) are evenly sorted. • Only the chiasmata hold the paired homologues together. 35 Anaphase I Spindle fibres attaching to the centromeres contract and pull sister chromosome of homologous chromosomes to opposite poles. 36 Telophase I • Chromosomes reach opposite poles; • Nuclear envelopes form; • Nucleolus reform; • Spindle fibres disappear; • Chromatids uncoil; • Cell divides into two; • Nucleus may enter into interphase but no DNA replication or • Cell enters prophase II directly 37 38 Meiosis II • The second part of meiosis (meiosis II) is very similar to mitotic division except that DNA synthesis does not occur between the two stages. 39 Prophase II • Nucleolus disappears; • Nuclear envelope breaks down; • Centrioles divide and move to opposite poles; • Spindle develops at right angle to spindle of 1st meiotic division 40 Metaphase II Chromosomes arrange themselves on equator of spindle with spindle fibres attached to centromeres of each chromosome 41 Anaphase II centromeres divide and pulled by spindle fibres to opposite poles, carrying the chromatids with them 42 Telophase II Upon reaching poles, • chromatids unwind • nuclear envelope • nucleolus reform • Spindle disappears, • 2 cells divide to give 4 cells (a tetrad) 43 44 45 Comparision of mitosis and meiosis 46 The Significance of Cell Division Significance of Mitosis Mitosis produce daughter cells which are exact copies of the parental cell for: • Growth • Repair • Asexual reproduction 47 Significance of Meiosis There are 3 ways of variation which is necessary for offspring to be different from their parents • Production and fusion of haploid gametes • Random distribution of chromosomes during metaphase I and consequent independent assortment • Crossing over between homologous chromosomes 48 a) Production and fusion of haploid gametes a sperm (haploid) chromosomes fertilization zygote (diploid) an ovum (haploid) 49 b) Random distribution of chromosomes during metaphase I and consequent independent assortment 50 c) Crossing over between homologous chromosomes 51 Gamete Formation Sperm and ovum are produced by two main processes 1) meiosis and 2) specialized cell differentiation. Gametogenesis differs greatly between spermatogenesis and oogenesis. Spermatogenesis converts the spermatocyte into four spermatids. • During oogenesis, asymmetric cell division produces one large cell and three small ones that degenerate into three polar bodies. 52 Meiosis is arrested at the diplotene stage, during which oocytes grow to a large size. Oocytes then resume meiosis in response to hormonal stimulation and complete the first meiotic division, with asymetric cytokinesis giving rise to a small polar body. Most vertebrate oocytes are then arrested again a Metaphase II 53 54 Meiosis and Fertilization • Binding of a sperm to its receptor signals an increase in Ca 2+ levels in the egg cytoplasm. • This induces surface alterations that prevent additional sperm from entering the egg; which ensures a normal diploid embryo. Fertilization and completion of meiosis