Cell Division = The process by which cells produce offspring cells.
Prokaryotic Cells • Binary fission = Division of prokaryotic cells into two offspring cells. • Steps: 1. One chromosome, a circular piece of DNA along with its proteins that is attached to the cell membrane, makes a copy of itself (replication). • 2. The cell continues to grow until it is about twice its original size. • 3. The cell wall forms between the two chromosomes and the cell splits into two new cells.
Eukaryotic Cells • 1. Somatic cells of the body divide by the process called the Cell Cycle. It consists of Interphase, Mitosisand Cytokinesis.
Interphase – Cell spends most of its lifetime in interphase G1 phase – the cell grows to mature size. The cell has divided in the previous step and is appx ½ of its mature size S phase – A copy of DNA in the cell is made – this is for the new cell G2 phase – cell prepares for cell division by making more organelles – it is going to need enough for two new cells G0 – some cells go from G1 to G0. These cells do not divide so they do not need a copy of DNA ex. nerve cells
M phase or Mitosis = Nuclear division process that divides the nucleus in half • Results in 2 new cells with genetic material that is identical to that of the original cell. • The 2 offspring cells are about equal in size. • Each receives a copy of the cells chromosomes. • Each receives half of the cells cytoplasm and organelles.
4 phases: • 1. Prophase- DNA shortens and coils into rod-shaped chromosomes. • Nucleolus and nuclear membrane breakdown and disappear. • Micotubules start to extend from centriole. These microtubles are now called Spindle Fibers
2. Metaphase • Spindle fibers move chromosome to center of the dividing cell.
3. Anaphase • The chromatids of each chromosome separate at the centromere and slowly move, centromere first, toward the opposite poles of the cell. (Now the chromatids are considered individual chromosomes. )
4. Telophase • After chromosomes reach opposite poles, they uncoil. • The spindle fibers disassemble. • The nuclear envelope forms around each set of chromosomes and a nucleolus forms within each.
Cytokinesis = The cytoplasm divides. • In animals, a cleavage furrow forms in the middle of the two poles, pinching the cell into 2 new cells by way of microfilaments. • In plants, a cell plate is formed by which vesicles from the Golgi Apparatus fuse at the midline of the cell.
2. Reproductive cells divide by the process of Meiosis. • Meiosis = Cell division that reduces the number of chromosomes by half in new cells. • Starts after G1, S and G2. • Includes two divisions. • Only occurs in reproductive organs of animals. • Results in 4 new cells, each with half of the original number of chromosomes.
Meiosis I : steps • 1. Prophase 1 – DNA shortens and coils into rod-shaped chromosomes. • Nucleolus and nuclear membrane disappear. • Spindle fibers form. • * Homologous chromosomes pair up in a process called synapsis.
Synapsis = The pairing of homologous chromosomes during meiosis. • Tetrad = A pair of homologous chromosomes lined up so that the genes on one chromosome are near the corresponding genes on the other homologue. • Crossing over = The exchange of pieces of chromosomes that occurs during synapsis. • Crossing over allows genetic material to be exchanged between maternal and paternal chromosomes and results in genetic recombination. • Genetic recombination = Process of mixing up genetic information which produces new combinations of it.
2. Metaphase I • Tetrads randomly line up along the midline. (ie. Some maternal chromosomes go to one pole and some paternal chromosomes go to the same pole.) • Independent assortment = Random alignment and separation of homologous pairs of chromosomes.
Example of Independent Assortment
3. Anaphase I • Each homologous chromosome, made of 2 sister chromatids attached by a centromere, move to opposite poles of the cell.
4. Telophase I • Chromosomes reach the ends of the cell and cytokinesis begins. • Meiosis I results in two haploid cells.
Meiosis II – • This starts with the products of Meiosis I; no copying of DNA occurs. • Sometimes it begins after nuclear membrane forms and sometimes it begins without the nuclear membrane reforming.
Meiosis II : steps • 1. Prophase II – Spindle fibers form and move chromosomes toward midline.
2. Metaphase II • Chromosomes move to midline.
3. Anaphase II • Chromatids separate and move toward opposite poles.
4. TelophaseII • Nuclear membrane reforms around chromosomes of 4 new cells.
Cytokinesis II • Occurs next and 4 new cells are formed, each with half of the original number of chromosomes.
Meiosis
2 Forms of Meiosis in humans • 1. Spermatogenesis= Production of sperm cells that occurs in human male testes. • Spermatids mature into Spermatozoa = mature sperm cells.
2. Oogenesis • = Production of mature egg cells that occurs in human female ovaries. • Only one mature egg cell = ova is formed along with three polar bodies due to the unequal division of the cytoplasm.
Meiosis and Mitosis Compared
