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Download Unit 1.1.3a - Cell Division, Cell Diversity and Cellular Organisation
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SICM Tuition Biology AS Cell Division, Cell Diversity and Cellular Organisation As we have seen with the cell ultrastructure, cells contain material, which is vital for life: they contain genetic material, which is passed on when they replicate. Replication can occur in two ways: mitosis or meiosis. We will look at these in a bit more detail later, but first, we want to look at the cell cycle as a whole. In fact, the cell division itself is only a minor part of the cycle of a cell. Mitosis and the cell cycle Mitosis is part of the cell cycle. the existing cell divides into two new cells (daughter cells) does not result in a change in chromosome number daughter cells are identical to each other and the parent cell G1 - “Gap 1” - S - “Synthesis” - DNA replication so that each chromatid manufactures an identical partner – chromosome G2 - “Gap 2” - chromosomes begin to condense (get ready to appear) M - “Mitosis” - nuclear division C - cytokinesis - G2 cell grows and develops M S C G1 the division of the cytoplasm and separation of daughter cells distinct from nuclear division. It involves the constriction that divides the cell into two It is usually well underway by the end of the Telophase and does not involve the formation of the cell plate As you can see from the illustration above, mitosis only takes up a small percentage of the cell cycle. The remaining parts are described in some detail, but basically, the genetic information is copied (S phase) and is checked at numerous stages. This makes sure that there is no incorrect or incomplete material copied. If there is, then the cell undergoes apoptosis – cell death. So what is mitosis?! SICM Tuition Biology AS Mitosis This is the name given to the form of cell division which occurs in normal body cells. The result of this division is: - 2 genetically identical daughter cells - the chromosome number is maintained This type of cell division is used for growth, repair and replacement. Once the process of cell division has started, it is a continuous process and 4 distinct stages have been identified by humans: P M A T Interphase - this is often termed the “resting phase” (but it is not really – lots does occur) - no chromosomes are visible - DNA replication occurs in this phase. Prophase - chromosomes appear; longitudinally split (i.e. double stranded) - each strand = a chromatid (i.e. a molecule of DNA) - a chromosome is made of two chromatids - the 2 chromatids making up a chromosome are genetically identical centromere DNA chromatid - the centriole: o a small body just outside the nucleus o this divides and each half migrates to the opposite side of the nucleus o as the centrioles move apart, they give rise to spindle fibres o the spindle fibres become attached to the centromere of the chromosomes when the chromosomes are lined up at the equator of the cell SICM Tuition Biology AS Metaphase - the double membrane of the nucleus disappears - the chromosomes become aligned along the equator of the cell - the chromosomes are attached to the spindle fibres by their centromere centriole centromere spindle fibre Anaphase - a short, rapid phase - the centromere splits in half and each half repels each other - at the same time, the spindle fibres shorten Telophase - this covers the period in which the two groups of chromosomes arrive at the poles and become surrounded by a nuclear membrane - the spindle fibres disappear and the cell cleavage appears - the cytoplasm divides into two by means of formation of a cell membrane (or lamella in plants) between the two sets of chromosomes - the telophase may be either a transitory period – short enough only for DNA replication to occur before the cells divide again – or long : permanent interphase may follow. Separation of the chromatids due to… (a) (b) the centromeres split and each half repels each other half the spindle fibres shorten SICM Tuition Biology AS Role of spindle fibres (a) attach the chromosomes / chromatids by the centromere (b) shorten to separate chromatids (i.e. movement of chromosomes) Energy for process from ATP (i.e. respiration) Is interphase a “resting phase”? No… (a) DNA replication occurs (i.e. each chromatid manufactures a genetically identical partner – see DNA stages later on) (b) a build up of ATP for the process of DNA replication and cell division Outcome (a) (b) (c) 2 daughter cells same chromosome number as each other daughter / parent cell genetically identical to each other and parent cell (i.e. no variation) Consequence of mitosis (a) (b) from fertilised egg: - repeated cell divisions give rise to a population of cells which later become tissues and organs - all cells have the same genetic complement each individual of a species has a characteristic number of chromosomes e.g. humans: 46 (i.e. 23 pair of homologous chromosomes) Homologous chromosomes: In a “diploid” organism (such as most animals) there are two copies of the entire genome. The homologous chromosomes are the two similar chromosomes – one originating with the father and one originating with the mother - that may be paired with each other. The X and Y chromosomes are homologous but not identical (c) Genetic information carried on chromosomes varies (d) Some cells lose their genetic material once they are developed (e.g. red blood cells, xylem cells) (e) mitosis is used for growth, repair and replacement SICM Tuition Biology AS In the same way as cells go through their cell cycle, yeast does the same. When dividing, it forms “buds” and releases them as is shown in the diagram below: Budding Newly budded yeast cell OK..so that’s mitosis. What about meiosis? What’s the difference?!!? OK…well, the letters are definitely different!! But what else?! Mitosis - each daughter cell produced is genetically identical to each other AND parent cell the chromosome number remains the same this occurs in animals for growth and repair this occurs for plants in the tip of the stem and the tip of the root mitosis only has one division mitosis produces 2 cells Meiosis - the chromosome number is halved each cell produced is genetically different (due to crossing over) this takes place in gamete formation in humans: testis and ovary this takes place in plants in the anther and the ovary meiosis divides twice successively meiosis produces 4 cells So that’s normal cell division. But how do all our cells form? We don’t have one of each cell when the egg is fertilised by the sperm. So mitosis, which replicates the cell cannot form specialised cells….can it?! Stem cells Stem cells are unspecialised cells, which can differentiate and renew to form other types of cells. “ A “generic” cell that can make exact copies of itself indefinitely. In addition, a stem cell has the ability to produce specialised cells for various tissues in the body, such as heart muscle, brain tissue, and liver tissue. ”
