Cell division

• Mitosis is the division of the cell’s nucleus into two identical daughter nuclei containing the same number of chromosomes as the mother cell
• The function of mitosis is to create two genetically identical daughter cell with the genome of the mother cell. The process involves replication (=duplication) of DNA (all chromosomes). In order for separation of duplicated DNA to work, the DNA (normally a very long molecule) needs to supercoil. Replication is said to be proofread and checked for errors by the cell’s machinery.
• Remember that mitosis occurs only in eukaryotic cells, while prokaryotic cells divide by binary fission.
• In plant and animal cells, the process of cytokinesis differs. In plant cell, the kinesis results from the transport of vesicles to the cell equator leading to their eventual fusion and formation of the plasma membrane. The vesicles bring cellulose to form the cell wall around the newly formed plasma membrane. In animal cells, the division of cytoplasm is a result of an invagination of the plasma membrane. Actin and myosin are the contractile fibres that create this invagination called cleavage furrow
• Cell life cycle


  1. G1 phase

  2. S phase

  3. G2 phase
• G1 phase


  Phase in which cells spend the majority of their lifespan: period of growth and performance of daily functions
• S phase


  Phase that occurs once the cell has decided to undergo mitosis: period of DNA synthesis (replication)
• G2 phase


  Phase where the cell does its last preparations for mitosis: cell duplicates organelles and prepares enzymes and proteins needed for mitosis
• The name cyclin should help you remember that the concentrations of these proteins go through cycles or vary throughout the cell cycle in response to internal and external signals. An increase or decrease in the concentration of cyclins will influence the progression of the cell cycle.
• Mitosis consists of 4 phases that can be distinguished under the microscope. Due to supercoiling of the DNA, the chromosomes become visible and can be tracked during these phases.
• Prophase: nuclear envelope is fractured, chromosomes are becoming thick, centrioles are located and the poles of the cell.
• Prophase: DNA supercoils, chromosomes condense and become visible. • Nuclear envelope breaks down. • Spindle microtubules start forming at the poles of the cell. • The cell contains double the DNA compared to its G1 phase, the same number of chromosomes. • Begins after S phase has occurred
• Metaphase: chromosomes located at the equator of the cell, each X representing a chromosome, and they’re ordered in one single row. Spindle fibres originate at the poles and attach to centres of X-es each pole, with one chromosome having one spindle from each pole.
• Anaphase: fibres are shortening towards the poles, and dragging one leg of the X towards the pole. Equal number of chromosome legs are moving to each pole. Sister chromatids (legs of each chromosomes, containing identical copies of DNA) are pulled to opposite poles by spindle microtubules. • Now there is an equal number of chromosomes (DNA molecules) at each pole, but overall, the cell now has double the number of chromosomes compared to prophase.
• Telophase: two nuclei beginning to form at each pole, and the chromosomes uncoiling and becoming longer.• Chromosomes begin to uncoil as the nuclear envelope reforms around them. • The cell contains two identical nuclei and awaits the division of cytoplasm and organelles (= Cytokinesis).
• When observing a tissue or a group of cells under a microscope, it is easy to calculate the rate of division of the cells/tissue in question. This is done by the following formula: Mitotic index = number of cells undergoing mitosis divided by total number of cells