Meiosis A process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes to form gametes, or sex cells Two distinct divisions, called meiosis I and meiosis II
In Humans All human body cells have 46 chromosomes (coiled up DNA). What does that mean? Every cell MUST have the same amount of chromosomes. WHY does every cell have the same amount of chromosomes? All human body cells (somatic cells) that make up your body came from a single cell.
Somatic cells: The 46 chromosomes come in pairs These pairs are called homologous chromosomes
Homologous Chromosomes Homologous Chromosome: Chromosomes that are the SAME (length and traits). Key Point: They are similar, but NOT IDENTICAL
Mitosis Review The daughter cells are IDENTICAL to the parent cell HOW?? They have the same number of chromosomes
Chromosome Number How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? 46 If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? 92 In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have? 23
Diploid Cells A diploid when the cell has a full set of chromosomes ( ½ from mom and ½ from dad) Somatic cells are DIPLOID cells For Humans: DIPLOD CELL = 46 Chromosomes
Haploid Cells A haploid when the cell has half the set ( ½ ) of chromosomes Sex cells are HAPLOID cells For humans a diploid cell has 46 chromosomes. How many chromosomes would a sex cell (haploid) have? 23 chromosomes
In Sexual Reproduction Two haploid sex cells come together to form a diploid cell. These sex cells are also called Gametes Haploid sex cell 23 Chromosomes Female sex cell = Egg Haploid sex cell 23 Chromosomes Male sex cell = Sperm Diploid cell 46 Chromosomes Egg + Sperm = Zygote
Fruit Fly Chromosomes Not all organisms have the same number of chromosomes Fruit Fly Example: Somatic Cell has 8 Chromosomes Four chromosomes come from its male parent; Four come from its female parent These two sets of chromosomes are called: Homologous Chromosomes
Diploid Cells Diploid: Full set of homologous chromosomes Represented by the symbol 2N Fruit Fly: the diploid number is 8. (2N = 8) Humans: 2N = 46
Haploid Cells Haploid: Half set of chromosomes Gametes (sex cells sperm, egg) are haploid N = single set of chromosomes in a sperm or egg cell Fruit Fly: Haploid Number is 4 (N = 4) Humans: N=23
Mitosis: Makes Somatic Cells Diploid 46 Chromosomes Diploid 46 Chromosomes Diploid 46 Chromosomes Creates. 2 identical diploid cells
Meiosis: Makes Sex Cells Diploid 46 Chromosomes Haploid 23 Chromosomes Haploid 23 Chromosomes Haploid 23 Chromosomes Haploid 23 Chromosomes Creates. 4 different haploid cells
Gametes to Zygotes Haploid cells produced by meiosis are gametes: male animals gametes sperm plants pollen contain haploid sperm cell female animals gametes eggs plants - egg cell contain haploid cells Fertilization the fusion of male and female gametes generates new combinations of alleles in a zygote Zygote undergoes cell division by mitosis and eventually forms a new organism
Key differences between types of cells Body Cells Somatic cells : Examples: Number of chromosomes: Sex Cells Gametes: Brain cell, skin cell, etc. Egg and sperm Examples: 46 (diploid) 23 (haploid) Number of chromosomes: RELATIONSHIP BETWEEN THESE TYPES OF CELLS: Somatic cells will always have TWICE the number of chromosomes as gametes
Check For Understanding: 1. Unlike mitosis, meiosis results in a) 4 identical, diploid cells b) 2 genetically different haploid cells c) 2 genetically different diploid cells d) 4 genetically different haploid cells
CHECK FOR UNDERSTANDING 1. If a horse has 12 chromosomes in its gametes, how many chromosomes are in it s skin cells? a) 12 b) 24 c) 48 d) 6 2. If there are 10 chromosomes in the parent cell at the start of meiosis, how many chromosomes will be in the daughter cells at the end of meiosis? a) 10 b) 20 c) 5 d) 40
Check for Understanding Question: What is the relationship between the number of chromosomes in the zygote and the number of chromosomes in a gamete? A ZYGOTE (diploid) HAS TWICE THE NUMBER OF CHROMOSOMES AS THE GAMETES (haploid) (Because two haploids come together to form the diploid zygote)
Mitosis or Meiosis? Makes gametes Meiosis Makes somatic cells Creates haploid cells Creates diploid cells Asexual reproduction Mitosis Meiosis Mitosis Mitosis
Mitosis or Meiosis? Creates cells with the same number of chromosomes as the original Mitosis Involved in sexual reproduction Meiosis Creates cells with ½ the number of chromosomes as the original Meiosis Creates 4 daughter cells Meiosis Creates genetically identical offspring Mitosis
Check for Understanding 1. If human were to have 46 chromosomes in its skin cells, how many chromosomes are in a diploid cell? 46 2. What about a haploid cell? 23 3. In Humans, N=? 23
Check for Understanding In spinach, 2N = 12. What is the haploid number? How many chromosomes are in a spinach leaf? The Red King Crab has 208 chromosomes. What is the diploid number? How many chromosomes are in the sperm cells? 6 208 12 104 The Great White Shark has 82 chromosomes. What is N? 41 How many chromosomes would be in a brain cell? 82
Check for Understanding 1. Name an example of a cell that is a GAMETE. Egg or Sperm 2. An organism has 15 chromosomes in its sperm cell. How many chromosomes will be in that organism s skin cell (somatic cell)? 30 3. Mitosis ends with a Diploid (Diploid or Haploid) cell. 4. Meiosis ends with a Haploid (Diploid or Haploid) cell.
Meiosis Overview Goal: Sexual reproduction (one cell from mom + one cell from dad) 2 Phases: Meiosis I Meiosis II End Result: 4 cells with HALF the chromosomes of the original cell (4 HAPLOID CELLS)
Stages of Meiosis
Interphase 1 Before meiosis I begins, cell undergoes interphase Chromosomes replicate
Meiosis 1 Begins: Prophase I Chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.
Prophase I Homologous Chromosomes pair up and form tetrads, they undergo a process called crossing-over (chromatids of the homologous chromosomes cross over one another) crossed sections of the chromatids are exchanged (new combinations of genes)
Chromosomes Cross-Over During Prophase 1, sister chromatids crossover. That means that the DNA from your mom and the DNA from your dad mix. Each chromosome at the end will have DNA from both parents and is DIFFERENT
Metaphase I Spindle fibers attach to each homologous chromosome Paired homologous chromosomes line up across the center of the cell.
Anaphase I Spindle fibers pull each homologous chromosome pair toward opposite ends of the cell
Telophase I and Cytokinesis Nuclear membranes reform Cytokinesis follows Telophase I, forming two new cells.
Meiosis I Ends Results in two cells (daughter cells) each of which has four chromatids
Meiosis II Begins Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II NO INTERPHASE
Prophase II Chromosomes each consisting of two chromatids become visible
Metaphase II Chromosomes line up in the center of each cell
Anaphase II Paired chromatids separate
Telophase II and Cytokinesis - Each of the four daughter cells produced in meiosis II receives two chromatids, they are HAPLOID
Why is crossing over important? Crossing over leads to genetic diversity (which is the difference in a species). Genetic Diversity is the major advantage of sexual reproduction because it provides genetic variation.
Meiosis Mitosis Summary/ Overview
Meiosis v. Mitosis Meiosis Two divisions Homologous chromosomes line up and move to separate daughter cells. Reduces chromosome number by half Produces 4 genetically different haploid cells. Early step in sexual reproduction. Mitosis One Division Two sets of genetic material separate, each daughter cell receives one complete set of chromosomes Doesn t change chromosome number Makes 2 identical diploid cells Form of asexual reproduction
Chromosomal Mutations Chromosomal mutations are unexpected changes in a particular chromosome This includes the addition of an extra chromosome or the deletion of a chromosome.