Meiosis Study Notes PDF

Meiosis Meiosis During sexual reproduction: oReproductive cells called gametes unite to form a new individual. § Gametes are called sperm and eggs in both animals and plants. o The fusion of gametes is called fertilization. Meiosis is nuclear division that leads to halving of chromosome number. oGam...

Meiosis Meiosis During sexual reproduction: oReproductive cells called gametes unite to form a new individual. § Gametes are called sperm and eggs in both animals and plants. o The fusion of gametes is called fertilization. Meiosis is nuclear division that leads to halving of chromosome number. oGametes contain half the number of chromosomes as a somatic cell. § At fertilization, the full chromosome number is restored. Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Chromosomes Carry Genes A gene is a section of DNA that influences one or more hereditary traits. o Different versions of a specific gene are called alleles. Chromosomes of the same type are called homologous chromosomes, or homologs. oA pair of homologs is called a homologous pair. oHomologous pairs contain the same genes in the same position along the chromosome. § However, the two homologs are not identical. § Homologs may contain different alleles. Chromosomes Every organism has a characteristic number of chromosomes. Sex chromosomes determine the sex of the individual. oIn many mammals: § Females have two X chromosomes. § Males have an X and a Y chromosome. Autosomes are non-sex chromosomes. Ploidy A karyotype identifies the number and types of chromosomes present in a species. Many organisms, including humans, are diploid. o They have a pair of homologs of each chromosome. o They have two alleles of each gene. What is the diploid number for a human cell? How many autosomes would you find in one of your skin cells? How many sex chromosomes would you find in one of your skin cells? Ploidy Other organisms are haploid. They have only one of each type of chromosome. They have just one allele of each gene. Ploidy Each species has a haploid number (n). Indicates the number of distinct types of chromosomes present. Sex chromosomes count as a single type. A cell’s ploidy (n, 2n, 3n, etc.) indicates the number of complete chromosome sets it contains. Diploid (2n) cells have: A paternal chromosome that came from the father. A maternal chromosome that came from the mother. Organisms or cells with three or more versions of each type of chromosome are called polyploid (3n, 4n, etc.). Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Overview of Meiosis Before meiosis begins, each chromosome in the diploid (2n) parent cell is replicated. When replication is complete, each chromosome has two identical sister chromatids. They remain attached along most of their length. What protein holds the sister chromatids together? Cohesins Overview of Meiosis Meiosis consists of two cell divisions: 1. Meiosis I: the two homologs of each chromosome pair separate into two daughter cells. Each daughter cell has one set of chromosomes. 2. Meiosis II: the sister chromatids of each chromosome separate into two daughter cells. Each of the two daughter cells from meiosis I divides, resulting in four haploid cells. Video Meiosis https://www.youtube.com/watch?v=D1_-mQS_FZ0&t=2s Stages of Meiosis Spindle Apparatus List the parts of the spindle apparatus. MTOC Astral microtubule Kinetochore microtubule Polar microtubule What is the function of each of these structures? Stages of Meiosis Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Prophase I The homolog pairs come together in a pairing process called synapsis. Synapsis (Bivalent formation) Sister chromatids Sister chromatids oThe structure that results from synapsis is called a bivalent, consisting of two homologs. oThe chromatids of the homologs are called non-sister chromatids. Non-sister chromatids Synapsis and Crossing Over 1. As chromosomes condense, sister chromatids stay joined along their entire length by cohesins. Synapsis and Crossing Over 2. Homologs pair by synapsis and are held together by proteins called the synaptonemal complex. Synapsis and Crossing Over 3. Breaks are made in the DNA and cross over occurs between corresponding segments of non-sister chromatids. Synapsis and Crossing Over 4. The synaptonemal complex disassembles and homologs are held together only at chiasmata. Synapsis and Crossing Over (Prophase I) Meiosis I Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Meiosis II There is no DNA synthesis between the two meiotic divisions. In meiosis II, the process resembles that of mitosis. Comparing Mitosis and Meiosis Comparing Mitosis and Meiosis Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Draw a generalized sexual life cycle. Animal Life Cycle In animals, the products of meiosis are gametes. Each gamete is haploid, containing a single set of chromosomes. During fertilization, gametes fuse to form a single cell called a zygote. This process restores the original chromosome number. Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Genetic Variation Genetic Variation - Asexual reproduction Asexual reproduction produces clones that are genetically identical to one another and to the parent. Genetic Variation Sexual reproduction produces offspring with unique chromosome complements. oOnly sexual reproduction results in a shuffling of the alleles of the parents into the offspring. o Meiosis results in four cells with a chromosome composition different from each other and from the parent cells. § Independent shuffling of maternal and paternal chromosomes. § Crossing over during meiosis I. o Fertilization also introduces variation as haploid sets of chromosomes combine to make a unique offspring. Independent Assortment Homologous pairs of chromosomes orient randomly at metaphase I of meiosis. oIn independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs. oThe number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number. For humans (n = 23), there are 8.4 million (223) possible combinations of chromosomes. Independent Assortment Video https://www.youtube.com/watch?v=5I8BnmdzoS8 Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent. In crossing over, homologous portions of two non-sister chromatids trade places. Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome. Crossing Over Video https://www.youtube.com/watch?v=U1k0v4MLuMk&t=2s Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg). o The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations. o Crossing over adds even more variation. Lecture Outline 1. Homologous Chromosomes and Ploidy 2. Meiosis a) Meiosis I b) Meiosis II 3. Life Cycles 4. Genetic Variation 5. Errors in Meiosis Errors in Meiosis Errors in meiosis are common. oOver a third of conceptions are spontaneously terminated because of problems in meiosis. One infant in every 781 live births in the Canada have Down syndrome. o Caused by an extra copy of chromosome 21. o Called trisomy 21. Nondisjunction A nondisjunction occurs if both homologs or both sister chromatids move to the same pole of the parent cell. Nondisjunction Nondisjunction Leads to trisomy Nondisjunction results in gametes that: Contain an extra chromosome (n + 1). Lack one chromosome (n − 1). Fertilization of an n + 1 gamete leads to trisomy. Fertilization of an n − 1 gamete leads to monosomy. Cells with too many or too few of a chromosome are called aneuploid. Leads to monosomy Nondisjunction Video https://www.youtube.com/watch?v=SbrVw1jrZxE&t=1s