Transcript Meiosis
Chromosomes & Meiosis Unit 5: Mendelian Genetics Learning Goals 1. Define homologous chromosomes and explain diploid and haploid. 2. Describe the overall process of meiosis. 3. Explain crossing over including the step in meiosis where it takes place and why it is important. 4. Describe the two types of gamete formation and how fertilization occurs. Chromosome Structure Made of DNA wrapped around histones (proteins) Sections of DNA make up genes Homologous Chromosomes Each body cell has 2 sets of chromosomes Homologous Chromosomes = a pair of chromosomes that have the same genes at the same locations 1 came from mom 1 came from dad Homologous chromosomes From mom From dad Chromosome Number Diploid (2n): cells that have 2 sets of homologous chromosomes (2n) Human body cells are diploid (46 chromosomes or 23 pairs) Haploid (n): cells that have 1 set of chromosomes (n) sex cells (sperm & egg) are haploid (23 chromosomes total) 1) A bug has a haploid number n=5. What is the diploid number (2n)? 2) A crocodile has a diploid number 2n=50. What is the haploid number (n)? Diploid 2n meiosis Haploid gamete n meiosis n Haploid gamete Karyotypes A karyotype shows an individual’s complete set of chromosomes Autosomes = Chromosome #1-22 Sex chromosomes = Chromosome #23 XX = Female XY = Male Karyotypes are studied to determine irregularities in the chromosomes and can show genetic disorders. Human Female Karyotype Human Male Karyotype Down Syndrome Female Trisomy On the karyotype you were given circle and label the following: Autosomes Sex chromosomes 1 set of homologous chromosomes *Determine and label the gender! Meiosis Cell division that results in the formation of gametes: Gametes = Sex Cells (sperm & eggs) Human gametes have 23 chromosomes (22 autosomes + 1 sex chromosome) 23 Egg 23 Steps of Meiosis Interphase DNA is copied before the start of Meiosis I. This makes 2 identical sister chromatids Meiosis 1 Four steps: Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Prophase 1 The homologous pairs of chromosomes line up side by side. (tetrad = 4) Crossing-over Homologous chromosomes can Cross-over and exchange a piece of the chromosome randomly Crossing-over is very important because it creates genetic variation (new combinations of genes), which makes us all different. This is why you do not look exactly like your siblings. Crossing over happens randomly. However, some genes are linked, or close together on the chromosome, meaning they are more likely to be inherited together. Red hair Freckles Chromosome 16 After crossing-over, the homologous chromosomes separate & the cell divides Meiosis II Four steps: Prophase 2 Metaphase 2 Anaphase 2 Telophase 2 Sister chromatids separate & the cells divide. Result is 4 genetically different haploid daughter cells Meiosis Overview Gametes produced during meiosis are needed for sexual reproduction Cells (diploid) divide twice resulting in 4 daughter cells (haploid) Each cell has half the number of chromosomes as the original cell Each new cell is genetically different Mendel’s Law of Segregation Genes segregate (separate) into different cells during meiosis Each gamete only gets one possible gene Principal of Independent Assortment Genes for different traits can segregate (separate) independently during the formation of gametes. The inheritance of one trait is not determined by the inheritance of another trait Ex: The gene for eye color is not connected to the gene for hair color. Independent assortment and crossing-over account for the many genetic variations observed in plants, animals, and other organisms. Assume there was only one cross-over in a man’s sperm and one cross-over in a woman’s egg, they have the possibility of producing 4,951,760,200,000,000,000,000,000,000 genetically different children Spermatogenesis In males meiosis occurs in the testes produces 4 sperm 23 23 23 23 Spermatogenesis Oogenesis In females meiosis occurs in the ovaries produces only 1 egg and 3 polar bodies Polar Bodies Egg 23 Oogenesis Fertilization: the combining of sperm and egg to form a zygote 23 23 Zygote 46 First cell of a new organism Only one sperm and one egg come together The first sperm to reach the egg will be the one to fertilize it. Learning Goals 1. Define homologous chromosomes and explain diploid and haploid. 2. Describe the overall process of meiosis. 3. Explain crossing over including the step in meiosis where it takes place and why it is important. 4. Describe the two types of gamete formation and how fertilization occurs.