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Chapter 10.1, Meiosis. Chromosomes and Genes. Recall from chapter 9, during mitosis, the DNA (Chromatin) condenses into x-shaped structures called chromosomes.
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Chromosomes and Genes • Recall from chapter 9, during mitosis, the DNA (Chromatin) condenses into x-shaped structures called chromosomes. • The chromosomes are arranged into segments called genes. Each gene gives instructions to the cell to create proteins that produce characteristics called traits. • Traits, such as hair color, eye color and height, are inherited through genes on chromosomes.
Chromosome Numbers • Each organism has a specific number of chromosomes in each cell. • Humans cells have 46 chromosomes; 23 from your mother, and 23 from your father. • The 46 chromosomes inherited from your parents are called homologous, because each chromosome has a matching chromosome that is the same length, has the same centromere position and they have the same genes that code for particular traits.
Haploid vs. Diploid Cells • All the cells of our body (except sex cells) are called Diploid cells. • Diploid cells have all the chromosomes inherited from your mother and father, and they are represented by the symbol 2n. • The sex cells of your body are called gametes. • Gametes have only half the number of chromosomes as the rest of your body's cells. Gametes are called Haploid cells. Haploid cells are represented by the symbol n.
Where do haploid and diploid cells come from? • During sexual reproduction, two gametes (sperm and egg cells) combine by fertilization. • When two haploid cells combine, a diploid cell is formed to make a zygote. • A zygote later develops into an embryo. • When an organisms become sexually mature, they begin to develop gametes. • The process that forms gametes is called meiosis.
What is Meiosis? • Meiosis is a type of cell division that produces gametes. • Meiosis is different from mitosis because the number of chromosomes are reduced. • Another name for meiosis is reduction division. • Meiosis occurs in all organisms that reproduce sexually. • Meiosis involves two cell divisions called meiosis I and meiosis II.
Meiosis I • Very similar to mitosis with a few differences, so only the differences will be discussed. • In Prophase I, homologous chromosomes (similar pairs from mother and father) pair up and crossing over occurs. • Crossing over is when segments of homologous chromosomes are exchanged. • This is one way that genetic variety and diversity can occur by sexual reproduction.
Meiosis I • In Metaphase I, the homologous chromosomespair up in random order andattach to spindle fibers at the equator. • During Anaphase I, the homologous chromosomes separaterather than the sister chromatids, so each chromosome still consists of two sister chromatids after anaphase I. • The sister chromatids might not be identical in Telophase I because of crossing over.
Meiosis II • After Meiosis I, the two cells are now reduced from 2n (Diploid) to n (Haploid). • At this point a second division occurs that is very similar to mitosis, so we will only go over the major differences. • During Metaphase II, a haploid number of chromosomes line up at the equator. • During Anaphase II, sister chromatids are pulled apart (rather than homologous pairs).
What is the end result? • At the conclusion of Meiosis I and II, the result is 4 separate haploid (n) daughter cells that are each unique and different from each other. • Remember from mitosis, the end product is two identical diploid (2n) daughter cells. • Variety is produced when homologous chromosomes cross over (during Prophase I), and when homologous chromosomes randomly line up at the equator (during Metaphase I). This is produces Independent Assortment.
