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2.6 Cell Division - Coggle Diagram
2.6 Cell Division
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Meiosis
Meiosis is the type of cell division which produces gametes for sexual reproduction. Unlike mitosis, the daughter cells are genetically different from the parent cell and contain just half the number of chromosomes (i.e. they are haploid). When two haploid gametes join during fertilisation, a diploid cell called a zygote is formed. Meiosis involves two rounds of cell division which are referred to as meiosis I and meiosis II.
Meiosis I
Interphase: the DNA replicates so there are now two identical copies of each chromosome (referred to as chromatids).
Prophase I: chromatids condense and arrange themselves into homologous pairs (called bivalents). Crossing over occurs (see below). The nuclear envelope disintegrates and spindle fibres form.
Metaphase I: homologous chromosomes line up along the equator and attach to the spindle fibre by their centromeres.
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Telophase I: chromosomes reach opposite poles of the cell. Nuclear envelope reforms around the chromosomes. Cytokinesis results in the formation of two daughter cells.
Meiosis II
Prophase II: chromosomes condense, nuclear envelope disintegrates and spindle fibres form.
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Telophase II: chromatids reach opposite poles of the cell. Nuclear envelope reforms and cytokinesis takes places. Four genetically unique daughter cells are produced.
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Stem Cells and Potency
Stem cells are unspecialised cells which have the ability to become specialised cells, such as heart cells or neurons. The process by which a stem cell is converted from an unspecialised cell to a specialised cell is called cell differentiation. Stem cells have an unlimited capacity to divide and can produce lots more stem cells by mitosis.
The ability of stem cells to undergo differentiation is referred to as potency - there are different levels of potency:
Totipotent - totipotent cells have the ability to divide into any type of cell (including the extraembryonic cells which make up the placenta and umbilical cord).
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Adult bone marrow contains multipotent adult stem cells which can divide and differentiate to replace old blood cells. They are responsible for forming all the different types of red and white blood cells. In plants, stem cells are present in regions called meristems, found at the tip of the shoot and roots. These have greater potency than adult stem cells, and can divide to form almost any kind of cell.
Levels of Organisation
The cell is the ‘basic building block of life’ and is the smallest functioning part of an organism. A group of cells working together is called a tissue and a collection of tissues all performing a specific function is called an organ. Multiple organs which are connected together are referred to as an organ system.
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Examples of organ systems include the respiratory system, circulatory system, reproductive system and digestive system.
The Stages of Mitosis
Mitosis can be divided into a series of stages depending on what’s going on with the chromosomes in the cell.
Prophase - the chromosomes condense (they become shorter and fatter) and the nuclear envelope disintegrates. The centrioles move to opposite poles of the cell and form spindle fibres.
Metaphase - the chromosomes line up along the middle of the cell. They attach to the spindle fibre by their centromere.
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Telophase & cytokinesis - the two groups of chromsomes decondense (they become long and thin) and a nuclear envelope reforms around them, forming two new nuclei. The cytoplasm divides (cytokinesis) and the plasma membrane pinches off to form two new, genetically-identical cells.
Gametes
Gametes are sex cells (the sperm and egg in humans). Gametes are haploid which means they contain half the number of chromosomes as the rest of the cells which make up our body. This means that when two gametes fuse during sexual reproduction, the fertilised egg (called a zygote) contains the full number of chromosomes i.e. it is diploid. In humans, the diploid number of chromosomes is 46 (23 pairs), which means that gametes contain just 23 chromosomes. During sexual reproduction, the nucleus of the sperm cell fuses with the nucleus of the egg cell - this fusion of nuclei is called fertilisation.
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