cell division cycle final

• The tiny, highly motile sperm cell contributes, essentially, only its genetic material.
• The cell cycle assumes a period of 24 hours, but some cells complete the cycle in less than a hour and others take many days.
• The relative lengths of the four phases of the cycle vary, with G1 exhibiting the most variation.
• The continuity of life is based on the reproduction of cells, or cell division.
• In some cases, the division of one cell to form two reproduces an entire organism, as when a unicellular organism, such as Amoeba, divides to form duplicate offspring.
• Cell division also enables multicellular organism, such as a human, to grow and develop from a single cell, the fertilized egg.
• Even after the organism is full grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear or accidents.
• For example, dividing cells in your bone marrow continuously supply new blood cells.
• A cell that has undergone growth, and is ready to divide, is referred to as a mother cell.
• The division of a mother cell results in two approximately equal daughter cells.
• Cell division involves the distribution of identical genetic material – DNA – to the two daughter cells.
• A cell’s total hereditary endowment of DNA is referred to as its genome.
• The genome consists of one or more very long DNA molecules.
• Arranged along the length of each DNA molecule are hundreds of thousands of genes, the hereditary units that specify an organism’s traits.
• Mitosis (division of the nucleus) is just one part of a cell division cycle, or cell cycle.
• In fact the mitotic (M) phase, when the division of the nucleus (mitosis) and the cytoplasm (cytokinesis) actually take place, is the shortest part of the cell cycle.
• Successive mitotic divisions alternate with a much longer interphase, which usually accounts for about 90% of the time that elapses during each cell cycle.
• Interphase consists of three periods of growth, referred to as the G1 phase (first gap), the S phase, and G2 phase (second gap).
• During all three subphases of interphase, the cell grows by synthesizing proteins and producing cytoplasmic organelles.
• Chromosomes are duplicated only during the S phase (S stands for synthesis, as in DNA synthesis).
• Interphase is followed by mitosis proper, a complex process that is divided into four stages namely; prophase, metaphase, anaphase and telophase.
• The entire process of mitosis occurs in continuum, and not a series of discrete occurrences.
• Mitosis is the kind of division that takes place in the somatic or body cells.
• Mitosis is involved during growth and in the repair and replacement of worn out tissues.
• Mitosis is also the type of division involved in the asexual reproduction of organisms.
• In contrast, meiosis is a variation of cell division that produces the gametes (sperm and eggs, in animals; microspores and megaspores in plants).
• Meiosis occurs only in the gonads (ovary and testis) of animals and in the sporangia of plants and yield daughter cells that have half as many chromosomes as the parent cell.
• During the reduction division of meiosis, the chromosomal pairs are divided so that each gamete contains one of each type of chromosome (It is haploid).
• The second involves the longitudinal separation of chromatids in each of these two haploid nuclei, resulting in production of four haploid cells.
• Each chromosome has two identical threads irregularly coiled about each other, each thread being called a chromatid.
• Just as in mitotic prophase, each chromosome contains two chromatids co
• Within the nucleus are one or more small, round, dense bodies called nucleoli and extremely long and thin chromosomes which appear as granular network of darkly-staining materials called chromatin.
• The two chromatids of each chromosome are known as sister chromatids since they are formed as a result of DNA duplication.
• At the end, two daughter cells, quantitatively and qualitatively the same as the original mother cell are formed.
• Since a zygotic nucleus contains twice as many chromosomes as the gametic nuclei at fertilization, and since chromosome number remains constant from generation to generation, there must be a process which reduces the diploid chromosome complement back to the haploid number.
• Meiosis is a special kind of cell division consisting of two nuclear divisions which follow each other in rapid sequence.
• The two chromatids of each chromosome separate at the centromere, one moves to one pole and the other toward the other pole, while the arms of the chromatids are dragged passively.
• The non-dividing cell has a distinct nucleus enclosed by a definite membrane.
• In meiosis I, the cells gradually increase in size as they proceed through prophase I.
• This stage, which occurs between two successive mitosis, is very active biologically; such that DNA duplication takes place during this stage.