Process of meiosis CSCOPE Unit 08 Lesson 02

Asexual vs. sexual reproduction Asexual Reproduction Sexual Reproduction Uses only mitosis Produces clones—genetically identical offspring Fast and effective reproduction to spread organism No diversity Combination of two haploid sex cells Fertilization: combines genetic information from two separate cells with ½ of the original genetic information of parent organism Gametes are from separate parents. (female = egg, male = sperm) Zygote (new individual) has two sets of chromosomes, one from each parent Process changes diploid cells to haploid gametes—leading to an increase in diversity of offspring

Asexual vs. sexual reproduction What is the advantage of an organism that reproduces sexually rather than asexually? An organism that reproduces sexually will have more genetic diversity than one that reproduces asexually.

Meiosis i Four Phases: Genetic diversity results from meiosis I. Prophase I Metaphase I Anaphase I (and) Telophase I Genetic diversity results from meiosis I. Graphic (and subsequent graphics adapted from): by Boumphreyfr (Own work) [CC-BY-SA-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Prophase I Homologous (same) chromosomes pair (synapsis) Homologous pairs called bivalents—two chromosome with four chromatids Nuclear membrane dissolves Chromosomes attach to spindle fibers and begin to move

Metaphase I Homologous chromosomes (bivalents) line up at the equator to form tetrads. Random orientation results in diversity. 50/50 chance of getting either parent’s homologue for each chromosome Tetrad

Anaphase I Homologous chromosomes separate. Chromosomes with TWO chromatids move to opposite poles. Daughter cells will have 23 chromosomes (haploid) but with two chromatids.

Telophase I Nuclear membrane begins to reform Cell may quickly move to meoisis II

Meiosis II Similar to mitosis BUT no synthesis phase (no DNA replication) Chromatids are NOT identical because of crossing over (recombination). Separates chromatids and produces two daughter cells with 23 chromosomes that have one chromatid each

Important Concepts!!! Crossing Over: Exchange of genetic information between homologous chromosomes that results in recombinant chromosomes and genetic diversity This is a central idea for the next two units on genetics and evolution. It is important that students have a firm grasp of the meaning of the term.

Important Concepts!!! Independent Assortment: Chromosomes randomly line up during metaphase I, and there are various ways that the homologous chromosomes may line up—again resulting in genetic diversity http://www.mhhe.com/biosci/ap/ap_prep/bioD6.html This is a central idea for the next two units on genetics and evolution. It is important that students have a firm grasp of the meaning of the term. http://www.mhhe.com/biosci/ap/ap_prep/bioD5.html