But where did these chromosomes and their genes come

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But Where Did These Chromosomes and Their Genes COME From? They came from your mom and dad. Remember, we said that all human cells (except for sperm and ova) have 46 chromosomes, and the chromosomes were found as two sets of 23 chromosomes each. One set of 23 chromosomes came from your mom in an ovum, and one set of 23 chromosomes came from your dad in a sperm cell. So that means that sperm and ova (sex cells, or gametes ) have only 23 chromosomes, half the number of normal (non-sex, or somatic ) cells. Cells that have two complete sets of chromosomes are described as being diploid , and cells that have only one set of chromosomes are described as being haploid . So sperm and ova are haploid cells. How do sperm and ova come to have only 23 chromosomes? They undergo a special type of cell division called meiosis . By the Numbers Diploid number refers to the number of chromosomes a cell has when it’s in a diploid state. For a human cell, the diploid number is 46. Haploid number means the number of chromosomes a cell has when it’s in a haploid state. Naturally, the haploid number is always one-half the diploid number. For a human being, the haploid number is 23. THE FORMATION OF GAMETES: MEIOSIS The gametes—the sperm and ova—are the only human cells that are haploid. Each has 23 chromosomes. When a sperm and an ovum get together—that is, when the sperm fertilizes the ovum—the chromosomes from the sperm join with the chromosomes in the
ovum. The newly formed cell—the zygote —is diploid. The diploid zygote then undergoes mitosis to begin the new human’s development. We’ll look at the specifics of how a sperm is formed and how an ovum is formed in just a little while. But first let’s go over the basics of meiosis. During Meiosis 1. The cell undergoes DNA replication during interphase, just as it would if it were about to go through ordinary mitosis. All of the chromosomes replicate, and we’re left with a cell that still has 46 chromosomes, each made up of two chromatids joined by a centromere. 2. The replicated chromosomes are split up in the course of two sets of divisions: prophase I, metaphase I, anaphase I, telophase I, and prophase II, metaphase II, anaphase II, and telophase II. 3. The differences between mitosis and meiosis are all found during the first set of divisions: prophase I, metaphase I, anaphase I, and telophase I. Meiosis I Meiosis I consists of four phases: prophase I, metaphase I, anaphase I, and telophase I. Remember that the chromosomes have already replicated and are found as two chromatids held together at the centromere. The biggest difference between these four phases and the four phases of mitosis is that at the very beginning, the homologous chromosomes pair up in a process called synapsis . This changes everything. Prophase I Synapsis occurs during prophase I . All the chromosomes have to find their homologous partner and pair up. Chromosome 1-A has to find chromosome 1-B, chromosome 2-A has to find chromosome 2-B, and so on. It takes a while, and prophase I is the longest phase of meiosis.

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