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The Cell Cycle Mitosis phase Prophase Metaphase Anaphase Telophase The Chromosome Hemoglobin gene Hemoglobin gene These are two chromosomes. Chromosomes are only temporary structures that serve the purpose of boxing up the DNA in a package that prevents information getting lost during the process of cell division. What is shown above is called a pair of homologous chromosomes. One member of each pair is inherited from each of your parents. They contain the same genes (beads) in the same locations. What may differ is the form of that gene. The actual trait that is expressed in you is dependent on the information on both chromosomes. Karyotype: A picture of an organism’s chromosomes. This picture is taken right after all the DNA copies itself in preparation for cell division. By studying a karyotype, certain problems can be detected in an organism. Normal Male 46 Down’s Syndrome (Trisomy 21) 47 Normal Female 46 Klinefelter’s Syndrome (Male 47) Interphase Cell membrane Nuclear Membrane DNA During interphase, the cell is performing its day to day activities. Prior to cell division, the cell grows in size, replicates its DNA and makes more copies of all the organelles. The cell spends most of its life in this state. Prophase Centromere Centriole Chromatids (2 identical chromosomes) During prophase, the DNA is now visible as chromosomes. Each chromosome consists of two identical parts called chromatids. The two chromatids are joined together at a point called the centromere. The nuclear membrane dissolves and organelles called centrioles move to opposite sides of the cell. Metaphase Spindle fibers The chromatid pairs position themselves vertically, down the middle of the cell. The centrioles, at east and west ends of the cell are like winches on a tow truck. Cable -like structures called spindle fibers extend from each centriole and connect with the centromere of each chromatid pair. Anaphase Like a cable being wound around a winch, the spindle fibers pulled to opposite sides of the cell, seperateing the pair of chromatids. Telophase New nuclear membranes One pair of homologous chromosomes New nuclear membranes surround each of chromosomes. Notice that this is still one cell but has two copies of DNA. There is one important part of the cell’s life cycle to complete…. Cytokinesis Animal Cell Cleavage furrow Plant Cell Cell Plate During cytokinesis, the cytoplasm and organelles are distributed equally between the two new cells. In animals cells, the membrane constricts down the middle to form two cells. In plant cells, a new cell wall is formed down the middle, separating the one megacell into two smaller ones. Now the chromosomes will unwind and the DNA will appear as a hazy mass as it once did during interphase. Each cell will now carry on its specific functions A Human Being Sperm cell 23 chromosomes haploid = N = 23 + Egg cell 23 chromosomes haploid = N = 23 Stem Cells = Zygote 46 chromosomes diploid = 2N = 46 Mitosis 2N Cell Specialization Muscle Cells 2N Mitosis 2N Cell Specialization Skin Cells 2N Mitosis 2N Cell Specialization 2N Cell Specialization Brain Cells Gonads Cell at this stage are PLURIOPOTENT, meaning they can become anything at all. MEIOSIS (the creation of sex cells) Meiosis is very similar to mitosis. The main difference is that the cell goes through TWO divisions instead of just one. So the result is not two identical diploid cells, but four cells having only one member of each chromosome pair (haploid). Look at the pictures below that point out the differences from mitosis. Prophase I: Chromatid pairs form a TETRAD (a group of four). In mitosis, the pairs were separate from one another. METAPHASE 1: Through a process called “crossing over”, the chromatids become intertwined with each other and actually exchange pieces with one another. ANAPHASE 1: The tetrads line up in the cell during metaphase 1 and are pulled apart during anaphase 1. Note that the sections of the chromosomes are not the same as when they came out of interphase (all red or all yellow). The chromatids are still connected by the centromere. During Telophase / Cytokinesis 1, the cells split, but the cells have two copies of one chromosome pair. Because these are sex cells, they only want one… so…… … they go through a Prophase II, Metaphase II, Anaphase II…. … and finally Cytokinesis II. The result is four cells that have only one member of each homologous pair, or 23 chromosomes. Nondisjunction: the failure of chromosomes to divide during meiosis