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PCC BIO 100IN - Mitosis and Meiosis Reading

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Mitosis and Meiosis Lab Bio 100 Instructor Deborah Bird page 1 Mitosis and Meiosis Reading What is the difference between a non-dividing cell and a dividing cell? The chromatin inside the nucleus of a non-dividing cell is being used by the cell for the manufacture of proteins. We will go into this aspect of cells in the next unit, but let’s learn a few things about the normal cell function before we go into cell division. A Non-dividing Cell Makes Proteins Proteins are so important to organisms that genes literally control all the phases of life from metabolism, size, color, physiology, development, reproduction etc. In genetics the site where a gene resides is called a locus (loci is plural).The heredity material DNA found in the chromatin of an non-dividing cells is organized into units called genes. Each gene is literally a code in the form of codons that tells the cell how to make a protein. Each codon will tell the cell which amino acid to place next in the protein. Go to the web site below to see the relationship between DNA, genes, Messenger RNA and the making of proteins . The web site actually shows a gene being read and a Messenger RNA forming. The messenger RNA will go out into the cell to the site of a ribosome and begin protein production. Go to How Life Works and click play. (http://www.eowilson.org/index.php?option=com_content&task=view&id=28&Itemid=46) See the reading on protein synthises on the homepage for more information. We will cover this topic in the next unit. A dividing cell is not making proteins When a cell is going to go through mitosis or meiosis the chromosomes appear on the scene. Each chromosome is a condensed form of DNA that forms wherever a cell is going to replicate itself or make a sex cell. The DNA is not in the form of chromosomes when the cell is just doing its normal function as you saw in the film. The chromosomes appear when a cell is getting ready to divide. The chromosomes keep the DNA material intact while the cell is dividing or making sex cells. When a cell forms chromosomes the nucleus of the cell breaks down and the chromosomes become visible with a simple light microscope. You can see that the chromosomes pairs look alike in the cell. They not only look alike, they carry the same gene for the same trait at the same locus. These pairs of chromosomes are called the homologous chromosomes. You inherited one half of the pair from your mother and one-half from your father. For humans we have 46 chromosomes so we have 23 pairs of homologous chromosomes that carry the same genes. Organisms whose chromosomes come in pairs are called diploid. The diploid condition is represented by 2n.. So 2n means they come in pairs Winding up the DNA into chromosomes also serves another function – it allows the genes to interact with its partner, but only during meiosis. When the sex cells are made cells start out diploid and become haploid, containing one half of their chromosome complement.Mitosis and Meiosis Lab Bio 100 Instructor Deborah Bird page 2 Diagram of a duplicated and condensed metaphase eukaryotic chromosome. (1) Chromatid – one of the two identical parts of the chromosome after S phase. (2) Centromere – the point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm. Chromosomes vary widely between different organisms. The chromosomes do not line up together as pairs in mitosis.Mitosis and Meiosis Lab Bio 100 Instructor Deborah Bird page 3 Mitosis Mitosis is the process of copying the DNA exactly and then splitting the DNA evenly between two cells. The whole idea is to start and end with exactly the same DNA sequence you started with and the exact same number of chromosomes. We will covering this in class> Meiosis Meiosis, on the other hand, is the process that splits the homologous pairs apart. You could think of it as a forced divorce. If you’re going to make sex cells you have to accomplish two tasks by the end of meiosis; separate the homologous chromosomes and you are given the chance to exchange genetic material before you separate from your partner forever. The homologous chromosomes pair up to say “goodbye” locus by locus and this is called synapsis. During synapsis genetic material can be exchanged at that locus. At the end of meiosis you have haploid cells – the chromosome number has been halved by separating the homologous pairs and possibly new genetic combinations have occurred at the loci. The new cells are haploid. Haploid is represented by n. For humans the diploid condition is 46 chromosomes and the haploid condition is 23 chromosomes. These pairs will go through synapses at the beginning of the meiosis process. Homologous Pairs of Chromosomes of


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