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Transcript
Meiosis Section 10 - 1 • Learning Targets: • I can… – Describe the basic process of meiosis – Compare and contrast mitosis and meiosis Chromosomes • Hair color, eye color, height, etc. are all characteristics that are passed on from a parent to child – Each of these characteristics are called traits – Instructions for each trait are located on chromosomes found in the nucleus of cells Chromosomes – DNA on chromosomes are arranged in segments that control the production of proteins • The DNA segments are called genes – Each chromosome consists of hundreds of genes • Each gene plays an important role in determining the characteristics and functions of the cell Homologous Chromosomes • Human body cells have 46 chromosomes – Each parent contributes 23 chromosomes, resulting in 23 pairs of chromosomes – Each pair (one chromosome from each parent) are called homologous chromosomes • Each homologous chromosome will be the same size/shape and the centromere will be in the same position Homologous Chromosomes • Each homologous chromosome carries genes that control the same inherited traits – Ex: the gene for earlobe type will be located at the same position on both homologous chromosomes • Though both genes will code for earlobe type, they will not necessarily code for the same earlobe type Homologous Chromosomes Haploid Cells • In order to maintain the same chromosome number from generation to generation, an organism produces gametes – Gametes: sex cells that have half the number of chromosomes • Each human gamete contains 23 chromosomes – Gametes are haploid cells • Haploid: cells with a single (n) set of chromosomes Diploid Cells • When one haploid gamete combines with another haploid gamete fertilization occurs – The cell created will have 2n chromosomes (n from mother and n from father) • This is called a diploid cell • In humans diploid cells have 46 chromosomes or 23 homologous chromosomes Meiosis • Gametes are formed during a process called Meiosis – Meiosis is a type of cell division that reduces the number of chromosomes – Occurs in the reproductive structures of organisms that reproduce sexually – Meiosis involves 2 consecutive cell divisions called meiosis I and meiosis II Meiosis I • Steps of Meiosis I 1. Interphase: Undergo interphase like mitosis – Cells carry out various metabolic processes and replicate DNA Meiosis I 2. Prophase I – Replicated chromosomes become visible • Consist of 2 sister chromatids – – Homologous chromosomes begin to form pairs as they condense in a process called synapsis Homologous chromosomes are held tightly together Meiosis I • Prophase I cont… – Crossing Over: process during which chromosomal segments are exchanged between a pair of homologous chromosomes – Centrioles move to the cell’s opposite poles, spindle fibers form and bind to the sister chromatids at the centromere Meiosis I • Prophase I Crossing Over Meiosis I 3. Metaphase I • Pairs of homologous chromosomes line up at the equator • Spindle fibers attach to each homologous chromosome Meiosis I 4. Anaphase I • Homologous chromosomes separate • Each member of the pair is guided by spindle fibers and moves toward opposite poles • Unlike mitosis, when homologous chromosomes split in anaphase, each still has 2 sister chromatids Meiosis I • Anaphase I Meiosis I 5. Telophase I • Homologous chromosomes reach the cell’s opposite poles • Each pole only contains one member of the original pair of homologous chromosomes • Note that each chromosome may not be identical because of crossover that occurred during synapsis • Cytokinesis also happens at this point Meiosis II • Steps of Meiosis II 1. Prophase II – Spindle apparatus forms in each new cell from Meiosis I – Chromosomes condense – Spindle fibers attach to each chromosome Meiosis II 2. Metaphase II • Centromeres of chromosomes line up randomly at the equator of each cell 3. Anaphase II • Sister chromatids are pulled apart at the centromere by the spindle fibers • Sister chromatids move toward opposite poles Meiosis II 4. Telophase II • Sister chromatids reach opposite poles • Nuclear membrane and nuclei reform • At the end of Meiosis II, cytokinesis occurs • Results in 4 haploid cells Meiosis Importance of Meiosis • Meiosis provides variation among offspring – This occurs when homologous chromosomes line up at the equator during metaphase I randomly • Since crossing over may have occurred, the chromosomes are not identical, therefore each of the 4 haploid cells may not be identical Asexual v. Sexual Reproduction • Asexual Reproduction – organisms inherit all of its chromosomes from a single parent – Therefore, the new individual is genetically identical to its parent • Some organisms use both asexual and sexual reproduction – Considered less advance because it has been shown that beneficial genes multiply faster with sexual reproduction