Chapter 7 Meiosis and Sexual Reproduction Section 1: Meiosis Section 2: Sexual Reproduction

Section 1 Meiosis Objectives: Summarize the events that occur during meiosis. Relate crossing-over, independent assortment, and random fertilization to genetic variation. Compare spermatogenesis and oogenesis.

Section 1 Meiosis Formation of Haploid Cells Meiosis and Chromosome Number Meiosis reduces the number of chromosomes by half to form reproductive cells. (46 to 23) When the reproductive cells unite in fertilization, the normal diploid number is restored.(23+23=46)

Stages of Meiosis  Meiosis I –4 stages  Prophase I  MetaphaseI  Anaphase I  TelophaseI /cytokinesis Meiosis II -4 stages Prophase II Metaphase II Anaphase II Telophase II /cytokinesis 2 Stage process

Meiosis I

Meiosis II

Meiosis I PROPHASE I Chromosomes become visible Nuclear envelope disappears homologous chromosomes separate. Crossing-over during prophase I results in the exchange of genetic material between homologous chromosomes.

Meiosis I Metaphase I Pairs of homologous chromosomes Move to equator of cell Spindle fibers attach to chromosomes

Meiosis I  Anaphase I –Homologous chromosomes separate –Pulled to poles by spindle fibers –Chromatids do not separate at their centromeres –Each chromosomes has 2 chromatids, however the genetic material was recombined during crossing over.

Meiosis I  Telophase I –Chromosomes gather at the poles –Each cell has 1 chromosome from each pair homologous chromosome –Cytokinesis occurs –Chromosomes do not replicate again between Meiosis I & II

Meiosis I  In conclusion: –1 diploid cell divides into 2 diploid cells –The resulting 2 cells have a recombination of genetic material from the parent cell through the crossing over of chromosomes.

Meiosis II During Meiosis II, the two chromatids of each chromosome separate. As a result of meiosis II, 4 haploid cells are produced from the original diploid cell.

Meiosis II  Prophase II –New spindle forms around chromosomes

Meiosis II  Metaphase II –Chromosomes line up on equator –Attached to spindle fibers at centromeres

Meiosis II  Ananphase II –Centromeres divide –Chromasomes move to opposite poles

Meiosis II  Telophase II/cytokinesis –Nuclear envelope forms around each set of chromosomes –Spindle breaks down –Cytoplasm divides –Results in 4 haploid cells

Section 7.1 Independent Assortment the random distribution of homologous chromosomes during meiosis contributes to genetic variation in sexually reproducing organisms ( 8 million) gametes can be produced with different gene combinations from 1 original cell Meiosis and Genetic Variation

  Crossing Over and Random Fertilization – –Both crossing-over and the random fertilization of gametes contribute to genetic variation in sexually reproducing organisms. – –2 23 x 2 23 = 64 trillion combinations

Crossing over

Importance of Genetic Variation – –Genetic variation is essential for evolution to occur. –More genetic material available in population, the more the population of individuals can vary

Section 1 In sexually reproducing eukaryotic organisms Meiosis in Males gametes form through the process of spermatogenesis. Meiosis in Females gametes form through the process of oogenesis Meiosis and Gamete Formation

Meiosis in Males  Spermatogenisis –Occurs in the testes –1 germ cell gives rise to 4 sperm cells

Meiosis in females  Oogenisis –Occurs in the ovaries –1 germ cell gives rise to 1 ovum

Section 2 Sexual Reproduction Objectives: Differentiate between asexual and sexual reproduction. Identify three types of asexual reproduction. Evaluate the relative genetic and evolutionary advantages and disadvantages of asexual and sexual reproduction. Differentiate between the three major sexual life cycles found in eukaryotes.

Section 2 Sexual Reproduction Sexual and Asexual Reproduction Asexual Reproduction Asexual reproduction is the formation of offspring from one parent. The offspring are genetically identical to the parent. Sexual Reproduction Sexual reproduction is the formation of offspring through the union of gametes from two parents. The offspring are genetically different from their parents.

Types of Asexual Reproduction There are many types of asexual reproduction such as All types lead to clones of the parent. – –fission – –fragmentation – –Budding – –Sporolation

Types of Asexual Reproduction  Fission: Splitting into two –Bacteria, paramecium  Fragmentation: breaking off of pieces from parent grow into new organism. –Some worms, plants and cyanobacteria

 Budding: parent grows bud that may or may not break off and grow into new offspring. –Hydra, yeast, some plants Sporolation: produce spores during meiosis that are released and grow into new offspring without having to pair with another cell. –Fungi, non- flowering plants

Section 2 Advantages - Disadvantages Genetic Diversity Whereas sexual reproduction increases variation in the population by making possible genetic recombination, asexual reproduction leads to a lack of genetic diversity among offspring. This lack of diversity is a disadvantage in a changing environment.

Evolution of Sexual Reproduction Sexual reproduction may have begun as a mechanism to repair damaged DNA. By recombining genes, enzymes can use genteic material to repair errors or “ bad genes” in a population.