biology form 4 chapter 5

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  1 Divided into 5.1 MITOSIS Cell division Process by which a cell, called the parent cell, divides into two cells, called daughter cells. Mitosis  A process of nuclear division which produces two genetically identical daughter nuclei. The significance of mitosis 1. Replaces dead cells. E.g skin cells 2. Repaired and replaced damaged cells. 3. Basis of asexual reproduction in unicellular organism.  –  binary fission 4. Increases the number of cells in all living organisms, thus, allowing growth and development in multicellular organism. 5. Results in the formation of two daughter nuclei which are genetically identical to each other and to parent nucleus. The nuclei contain the same number of chromosomes, same genetic material as parent nucleus. This is to ensure that the daughter cells perform the same function as the parent cell.  Nuclear division cytoplasmic division (cytokinesis) Cell division (2 stages) Mitosis Meiosis Chromosome : A thread-like structure which composed of a long and linear DNA molecule that carries genes in a linear sequence which determines the individual characteristics of an organism.  2 Chromosomes and chromosomal number 1. Two type of cells in sexually reproducing organism a. Somatic cell -  Comprise all the cells in an organism, except for the reproductive cell -  Formed through mitosis b. Reproductive cells  –  formed through meiosis 2. Chromosomes are found in nucleus. 3. The number of chromosomes present in the cells of each species of an individual organism is constant.  a. All individuals of the same species have the same chromosomal number but the cells of individuals of a different species have a different chromosomal number. b. For example, -  Onions have 16 chromosomes. -  Fruit fly, Drosophila melanogaster  , has 8 chromosomes. -  Human has 46 chromosomes. 4. The gametes contain only half the number of chromosomes (single set). The chromosomal number is said to be haploid and is designated as n. 5. In human, one set of chromosomes consists of 23 chromosomes. 6. All somatic cell have two sets of chromosomes. Our somatic cells have 46 chromosomes arranged in 23 pairs while each gamete only has 23 chromosomes. 7. The presence of two sets of chromosomes in the nucleus of a cell is known as the diploid number of chromosomes (2n). Male chromosome set Female chromosome set  3 8. Each pair of the chromosomes is referred as homologous chromosomes . They have same structural features. Mitosis Definition: ã  Process of nuclear division ã  Formation of two daughter cells ã  Two daughter cells are genetically identical to the parent cell ã  Maintains the chromosomal number of species ã  Ensures genetic material is passed on to the offspring Cell cycle: The period that extends from the time a new cell is produced until the time the cell completes a cell division. 2 major phases: a. Interphase ã  G1 (Growth phase 1) ã  S phase (DNA synthesis) ã  G2 (Growth phase 2) ã  b. Mitotic cell division (M Phase)   ã  Mitosis i. Prophase ii. Metaphase iii. Anaphase iv. Telophase ã  Cytokinesis Interphase: -  Interphase accounts for about 90% of the cell cycle. -  Interphase is also the stage at which the cell grow larger and prepare for cell division  4 Interphase Description G 1  (gap or growth phase) ã  Begins to acquire and synthesis the materials required for cell division. ã  Synthesis new organelles and proteins, cells grow larger ã  Metabolic rate of the cell is high. ã  Crucial phase: the cell will decide whether o not to divide and complete the cycle to form new cells ã  Chromosomes at this stage are known as chromatin. S phase ã  DNA undergoes replication. ã  A duplicated chromosome consists of two identical sister chromatids = 2 identical DNA double helices G 2   ã  Cell continues grow and remains metabolically active. ã  Enzymes and proteins are synthesized for cell division ã  Cell accumulates energy and completes its final preparations for division.
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