BIOLOGY 11 IB 2.5: CELL DIVISION

ASSESSMENT STATEMENTS 2.5.1Outline the stages in the cell cycle, including interphase (G 1, S, G 2 ), mitosis, and cytokinesis 2.5.2State that tumors (cancers) are the result of uncontrolled cell division and that these can occur in any organ or tissue 2.5.3State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication, and an increase in the number of mitochondria and/or chloroplasts 2.5.4Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase, and telophase) 2.5.5Explain how mitosis produces two genetically identical nuclei 2.5.6State that growth, embryonic development, tissue repair, and asexual reproduction involve mitosis

THE CELL CYCLE The cell cycle consists of three main phases: 1.Interphase (G 1, S, G 2 )  interphase involves the cells increase in size and doubling of cytoplasmic components and chromosomes 2.Mitosis  Produces two cells (daughter cells) with the same genetic material 3.Cytokinesis  Physical division of the cells Note: We study the cell cycle in discrete stages but they occur in a continuum.

RATE OF CELL DIVISION Different types of cells divide at different rates. Some types of cells like bone marrow reproduce rapidly (1 hour) Other extreme certain cells like nerve and muscle cells never reproduce once they mature (stay in interphase) Other cells divide only when needed for repair (liver cells) Importance in any cell to maintain balance = homeostasis If one kind of cell begins to reproduce too rapidly, the normal organization will be disrupted. This is what causes cancer (multiple cells uncontrollably dividing to form a tumour)

REGULATION OF CELL CYCLE Scientists are still trying to figure it out One known is cell-to-cell contact Once cells enter DNA replication (S phase) they are committed to complete the cell cycle meaning division Scientists are studying this point in interphase to determine what triggers the cell to undergo DNA replication Their focus has been on  chemical controls like proteins  Genes required for cell division

WHY RESEARCH THE CELL CYCLE? Mitosis is the process behind growth of organisms, development of embryos, tissue repair, and asexual reproduction. This knowledge may be used to replace cells damaged by injury or disease Help live longer lives Control the spread of cancers

INTERPHASE Longest and most variable part of the cell cycle In G 1 phase the major event is growth of the cell. At the beginning of G 1 the cell is the smallest it will ever be. In S phase the main activity is replication of the DNA of the cell, the chromosomes. This phase is sometimes referred to as the synthesis phase. G 2 is a second growth phase, where the cell grows and makes preparations for mitosis. During this phase, organelles may increase in number, DNA begins to condense from chromatin to chromosomes, and microtubules may begin to form.

DNA PACKAGING During G2 the chromatin (elongated DNA and histones) begins to condense. This is accomplished via a process called supercoiling. First the DNA wraps around histones to produce nucleosomes. The nucleosomes are further wrapped into a solenoid. Solenoids group together in looped domains. Looped domains coil together to produce the chromosome.

DNA NAMING Before replication, in the S phase, chromosomes are composed of one molecule of DNA. After replication, the chromosome includes two molecules of DNA. These two identical molecules are held together by the centromere, and each molecule is referred to as a chromatid. Together they are called sister chromatids. During mitosis the sister chromatids separate, then they do each is called a chromosome and his its own centromere.

MITOSIS Mitosis is made up of 4 phases:  Prophase - condensation of chromosomes, microtubule organization  Metaphase - microtubule attachment and chromosomal organization  Anaphase - separation of chromosomes  Telophase - chromosomal decondensation and cell separation Using page 41 of the textbook draw a diagram of each stage in your notes, and provide a short summary of what is going on.

CYTOKINESIS Happens after nuclear division In animal cells  inward pinching of fluid plasma membrane to form cleavage furrows In plant cells  due to the relatively firm cell wall, a cell plate is formed. The cell plate occurs midway between the two poles of the cell and moves outward toward the side of the cell from a central region.