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

MeiosisFormation of Haploid Cells•Meiosis and Chromosome Number Meiosis reduces the number of chromosomes by half to form reproductive cells. When the reproductive cells unite in fertilization, the normal diploid number is restored.•Meiosis I During Meiosis I, homologous chromosomes separate. Crossing-over during prophase I results in the exchange of genetic material between homologous chromosomes.•Meiosis II During Meiosis II, the two chromatids of each chromosome separate. As a result of meiosis, four haploid cells are produced from one diploid cell.

Section 1

MeiosisMeiosis and Genetic Variation•Independent Assortment Independent assortment, the random distribution of homologous chromosomes during meiosis, contributes to genetic variation in sexually reproducing organisms.•Crossing Over and Random Fertilization Both crossing-over and the random fertilization of gametes contribute to genetic variation in sexually reproducing organisms.•Importance of Genetic Variation Genetic variation is essential for evolution to occur.

Section 1

MeiosisMeiosis and Gamete Formation•Meiosis in Males In sexually reproducing eukaryotic organisms, gametes form through the process of spermatogenesis in males.•Meiosis in Females In sexually reproducing eukaryotic organisms, gametes form through the process of oogenesis in females.

Section 2

Sexual ReproductionObjectives:•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 ReproductionMeiosis and Gamete Formation•Meiosis in Males In sexually reproducing eukaryotic organisms, gametes form through the process of spermatogenesis in males.•Meiosis in Females In sexually reproducing eukaryotic organisms, gametes form through the process of oogenesis in females.

Section 2

Sexual ReproductionSexual 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 fission, fragmentation, and budding. All types lead to clones of the parent.

Section 2

Sexual ReproductionSexual and Asexual Reproduction continued

•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.

Section 2

Sexual ReproductionSexual Life Cycles in Eukaryotes•Haploid Life Cycle In the haploid life cycle, the simplest of all life cycles, the haploid cell occupies the major portion of the life cycle.•Diploid Life Cycle In the diploid life cycle, the adults are diploid and the diploid individual occupies the major portion of the life cycle.•Alternation of Generations Some organisms have a life cycle that alternates between diploid and haploid phases.