Chapter 10 Meiosis Textbook pages 270 - 276
Reproduction • Reproduction - process of producing offspring. • Some offspring are produced by two parents. • Some offspring are produced by just one parent. • Whether an organism is identical or similar to its parent is determined by the way the organism reproduces.
Asexual Reproduction • Production of identical offspring: • Mitosis • Binary Fission • Budding Today were going to talk about sexual reproduction at the cellular level.
Sexual Reproduction • Practiced by most eukaryotic organisms • In sexual reproduction, two parents give genetic material to produce offspring that are geneticallydifferent from their parents. • Each parent produces gametes (reproductive cell); sperm and eggs. • A gamete from one parent fuses with a gamete from the other parent to form a zygote. • This process is called fertilization.
Germ Cells & Somatic Cells • Germ cells (gametes) are cells that are specialized for sexual reproduction (sperm & egg). • Somatic cells are all other body cells. They DO NOT participate in sexual reproduction.
CHROMOSOMES: All cells contain chromosomes. There are two types of chromosomes found in the nucleus of cells: autosomes sex chromosomes Your body cells contain 23 pairs of homologous chromosomes: • 23 chromosomes from your mom, and • 23 chromosomes from your dad
NEW Term! • Homologous chromosomes are chromosomes that make up a pair; one from each parent • Are the same length • Have the same genes
Your body cells contain 23 pairs of homologous chromosomes: For each homologous pair, one chromosome comes from mom and one comes from dad.
LOOK AT THE KARYOTYPE BELOW: Chromosome pairs 1-22 are autosomes. The 23rd pair are the sex chromosomes: X and Y which determinegender: XX = XY = female male
Haploid vs. Diploid • Cells that are diploid (somatic cells) have two sets of chromosomes (2n) • Cells that are haploid (gametes) have one set of chromosomes (n)
Body Cells - somaticcells - diploid (2n) Diploid (2n) cells – have two copies of every chromosome - half the chromosomes come from each parent Gametes - sex cells (sperm and egg) - have DNA that can be passed to offspring - haploid (n) Haploid (n) cells – have one copy of each chromosome - gametes have 22 autosomes and 1 sex chromosome
Key Terms • Diploid refers to complete chromosome sets in a cell (or 2N) • N= the number of chromosomes in a gamete (sex cell) • N chromosomes from the female parent + N chromosomes from the male parent= 2N • For humans, 2N = 46
Key Terms • Haploid refers to single chromosomes in a cell (or N) • One-half of the 2N set (the maternal N or the paternal N) • Gametes (i.e. sperm, egg sex cells) are haploid cells • For humans, N = 23
Bottom Line • diploid means there are the full number of chromosomes in cells • haploid means that there are half the number of chromosomes in cells
Mitosis and meiosis are types of nucleardivision that make different types of cells. Meiosis - occurs in sex cells - Makes 4 haploid cells from diploid cells - produces gametes Mitosis - occurs in somatic cells - makes more diploid cells Make a Venn diagram to compare and Contrast Mitosis and Meiosis!
Meiosis reduces the chromosome number and creates genetic diversity. homologouschromosomes sister chromatids sister chromatids Cells go through two rounds of division in meiosis: • Meiosis I and Meiosis II - each round has four phases • Meiosis I – pairs of homologous chromosomes separate • Meiosis II – sister chromatids divide
Meiosis Interphase Occurs…. • Replication of DNA • Chromatin condenses Sounds familiar right???
Meiosis I • Occurs after DNA has been replicated in interphase • Divides homologous chromosomes in four phases: • prophase I metaphase I anaphase I telophase I • After cytokinesis, two cells are produced with one homologous • chromosome in each
PROPHASE I • Homologous chromosomes pair up • Crossing over occurs • Nuclear envelope breaks down • Spindles form
What is Crossing Over? • Chromosomal segments (genetic information) are exchanged between a pair of homologous chromosomes Crossing Over Creates Genetic diversity
Which set of chromatids illustrates the result of a single crossover of the homologous chromosomes?
METAPHASE I • Spindle fibers attach to the centromeres • Homologous Chromosomes Line up at the equator • chromosome pairs line up randomly in cell
ANAPHASE I • Homologous chromosomes separate and move to the opposite poles of the cell
TELOPHASE I • The cell divides • The result is two. 2 Haploid daughter cells with exchanged genetic information • The daughter cells are not genetically identical **During Telophase I Cytokinesis occurs at the same time!!**
Anaphase I Interphase I Prophase I Metaphase I . Review Meiosis I Telophase I
Meiosis II • Meiosis II divides sister chromatids in four phases: • prophase II metaphase II anaphase II telophase II • After cytokinesis, four new cells are produced, each with half • the chromosome number (haploid)
Nucleus breaks up • Spindles form and attach to centromere
Sister chromatids separate • Chromosomes move to opposite poles of cell
Cells divide • 4 haploid daughter cells result • Each cell is genetically different
Prophase II Metaphase II Anaphase II Telophase II . Review Meiosis II
Haploid cells develop into mature gametes: Gametogenesis - the production of gametes - differs between females and males • Sperm become streamlined and mobile • with the addition of flagella • Sperm primarily contribute DNA to an • embryo • Eggs contribute DNA, cytoplasm and • organelles to an embryo • During meiosis, the egg gets most of • the contents • The other cells form polarbodies
Sources of Variations and Differences 1. Crossing over • occurs during prophase I • homologous chromosomes swap pieces • this chromosome swapping creates variations 2. Random assortment of chromosomes • chromosome pairs line up randomly in cell • when the pairs separate, this creates cells with • genetic variations 3. Fertilization • which sperm gets to the egg first?? • it is random and can create variations in • offspring
So what are we making during Mitosis??? • What are we making during Meiosis??
Videos that show side by side view • http://highered.mcgraw-hill.com • http://www.pbs.org/wgbh/nova/miracle/divi_flash.html
WHY MEIOSIS? • MITOSIS – RESULTS IN GENETICALLY IDENTICAL OFFSPRING – INCLUDING THE # CHROMOSOMES • WHAT WOULD HAPPEN IF THE EGG AND SPERM HAD THE SAME # OF CHROMOSOMES AS THE BODY CELLS? EGG = 46 CHROMOSOMES SPERM = 46 CHROM. ZYGOTE = 46 + 46 = 92 CHROMOSOMES = NOT HUMAN