THE PROCESS OF MEIOSIS OBJECTIVES 1. What is the product of meiosis? (MCA-II standard) 2. What happens during each step of meiosis? INTRODUCTION 1. Meiosis is a process that reduces the number of chromosomes in new cells to 1/2 the number. 2. The halving of the chromosome number counteracts the division of cells in reproduction. 3. Meiosis produces gametes (sex cells) (germ cells), which are haploid reproductive cells. √ A haploid (1N) cell has only one set of chromosomes. 4. Human gametes are sperm cells and egg cells. 5. Human sperm and egg cells each have 23 chromosomes. 6. The fusion of a human sperm cell and an egg cell results in a zygote that has 46 chromosomes. 7. The resultant zygote is said to be a diploid cell. √ A diploid (2N) cell has two sets of chromosomes. 8. Cells preparing for meiosis first enter interphase, the phase meaning “between cell divisions.” 9. Interphase is subdivided into 3 phases: G1, S, and G2. 10. Replication of DNA occurs during which of the interphase subdivisions? S phase MEIOSIS chromosome chromosome chromosome chromosome cell membrane nuclear envelope centriole centromere aster spindle 1. The nuclear envelope PROPHASE I is starting to disappear . 1. How many chromosomes do you see? 4 2. Each chromosome has 1 or 2 chromatids? 2 3. Each chromosome lines up with its matching chromosome, called a homologue. 4. Because there are two sets of chromosomes present, the cell is said to be diploid (2N). 5. Sometimes chromatids can break off and attach to adjacent chromosomes – a process called crossing - over. METAPHASE I 1. How many chromosomes do you see? 4 2. Each chromosome has 1 or 2 chromatids? 2 3. The chromosomes line up very similar to of mitosis but with one key difference: line up as homologous pairs 4. Spindle fibers have formed in between the centrioles and the chromosomes. 5. The newly formed spindle fibers latch onto ANAPHASE I the centromeres of the chromosomes. 1. How many chromosomes do you see? 4 2. Each chromosome has 1 or 2 chromatids? 2 3. The chromosomes move very similar to mitosis but with one key difference: separating homologous pairs rather than sister chromatids TELOPHASE I 1. What structure is re-forming around the chromosomes? 2. Nuclear membrane How many chromosomes do you see in each re-forming nucleus? 2 3. Each chromosome has 1 or 2 chromatids? 2 4. Because there is now only one set of chromosomes in each nearly-formed cell, the cells are said to PROPHASE II be haploid (1N). 1. At this point meiosis has created 2 cells. 2. Each cell has 2chromosomes. 3. Each chromosome has 1 or 2 chromatids? 2 3. Each chromosome has 2 chromatids because DNA was copied in the S phase of interphase. 4. Note the re-appearance of spindle fibers between the centrioles. METAPHASE II 1. Each cell has 2 chromosomes. 2. Each chromosome has 2 chromatids. 3. Note that the spindle fibers have reached and attached to the centromeres. 4. Note that metaphase II is very similar to metaphase I of meiosis with one difference: ANAPHASE II chromosomes now line up individually rather than as homologous pairs 1. What is pulling the chromosomes apart? Spindle fibers 2. Each cell now has 4 chromosomes. 3. Each chromosome has 1 chromatids. 4. Note that anaphase II is very similar to anaphase I of meiosis with one difference: sister chromatids are pulling apart rather than homologous pairs 1. What structure is re-forming around the chromosomes? TELOPHASE II 2. Nuclear membrane How many chromosomes do you see in each reforming nucleus? 2 3. Each chromosome has 2 chromatids. 4. List one similarity to telophase I of meiosis. Chromosomes have separtated 5. State how telophase II differs from telophase I in two different ways: a. 2 chromosomes in each cell rather than 4 b. 4 cells formed rather than just 2 KEY CONCEPTS 1. Meiosis produces reproductive cells called gametes (sperm and egg). 2. Gametes are specialized cells that have 1/2 the number of chromosomes of the parent cell. 3. Gametes have only one set of chromosomes or are said to be haploid (1N) cells. 4. Gametes are genetically unique because of the prophase I process called crossing over.