Meiosis

Inheritance of Traits Heredity – the transmission of traits from one generation to the next Variation – offspring differ in appearance from parents and siblings Genetics – the scientific study of heredity and hereditary variation

Inheritance of Traits Parents pass traits to their offspring in the form of “genes” which are segments of DNA. Genes are coded information that enable cells to make enzymes and other proteins that are responsible for producing an organism’s traits.

Inheritance of Traits The cells responsible for passing genetic information from parents to their offspring are “gametes.” Most DNA is stored in chromosomes within the nucleus, (but a tiny amount is found in mitochondria and chloroplasts). A gene’s location within a chromosome is called its “locus.”

Methods of Reproduction Asexual Sexual One parent Genetically identical offspring “Clone” Mitosis Two parents Genetically diverse offspring Meiosis Methods of Reproduction

Chromosomes Human somatic cells (cells other than gametes) have 46 chromosomes – 23 from mom and 23 from dad. Diploid = cell with 2 sets of chromosomes 2n = 2(number of chromosomes in a set) = 2(23) = 46 23rd pair are sex chromosomes XX = female XY = male Gametes are Haploid = n = have one set of chromosomes = 23

Ordered display of condensed chromosome pairs Karyotype Ordered display of condensed chromosome pairs

Meiosis and Fertilization Meiosis = cell division to reduce the number of sets of chromosomes from 2 to 1 in gametes Only occurs in reproductive organs Fertilization = union of gametes in which the nuclei of the two cells fuse Results in a haploid (2n) zygote (fertilized egg cell) which matures by mitosis

Variety of Sexual Life Cycles Alternation of generations Result (regardless of life cycle): genetic variation among offspring

Stages of Meiosis Two cell divisions produce 4 haploid cells Interphase – chromosome replication Result: 1 diploid cell with replicated chromosomes Meiosis 1 – homologous chromosomes separate Result: 2 haploid cells with replicated chromosomes Meiosis 2 – sister chromatids separate Result: 4 haploid cells with unreplicated chromosomes

Stages of Meiosis Prophase 1 Metaphase 1 Anaphase 1 Chromosomes condense Crossing over Metaphase 1 Homologous chromosomes align at metaphase plate Anaphase 1 Sister chromatids remain attached at centromere Homologous chromosomes move toward opposite poles Telophase 1 and Cytokinesis Division into two haploid cells Chromosomes are still double Prophase 2 Spindle apparatus forms Metaphase 2 Chromosomes align single file on metaphase plate Sister chromatids are not identical Anaphase 2 Centromeres separate Sister chromatids come apart and move to opposite poles Telophase 2 and Cytokinesis Nuclei form and chromosomes decondense Each of the 4 daughter cells is genetically distinct Stages of Meiosis

Sources of Genetic Variation Fun Fact: the number of possible combinations of maternal and paternal chromosomes in one person’s gametes is about 8 million!!! 3 Ways… 1. Independent Assortment Each pair of chromosomes is randomly oriented with its maternal or paternal homologue closer to a given pole during Metaphase 1.

Sources of Genetic Variation Fun Fact: In humans, one to three crossover events occur per chromosome pair!! 3 Ways… 2. Crossing Over In Prophase 1, as homologous chromosomes align, the DNA molecules of two non-sister chromatids are broken at the same place and then rejoined to each other’s DNA, producing chromosomes with new combinations of genes At Metaphase 2, sister chromatids can be oriented in two alternative ways which also increases variation

Sources of Genetic Variation Fun Fact: God made you genuinely unique!! 3 Ways… 3. Random Fertilization Which sperm meets which egg? Not counting crossing over, this would be any of about 64 trillion diploid combinations!