Meiosis and Sexual Reproduction

Chapter 7

Meiosis Section 1

• Meiosis – a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes or spores
• Meiosis involves only one replication of DNA, but two divisions of the nucleus, Meiosis I and Meiosis II

• Prophase I – chromosomes condense -homologous chromosomes pair up
• Crossing over –when portions of a chromatid on one homologous chromosomes are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome

• Crossing over is an efficient way to produce genetic recombination the formation of new combinations of genes
• As a result of crossing-over, the two chromatids of a chromosome no longer contain identical genetic material
• Also provides a source of genetic variation
• Has an enormous impact on how rapidly organisms change

• Metaphase I - pairs of homologous chromosomes are moved by the spindle to the equator of the cell – pairs remain together
• Anaphase I - homologous chromosomes separate. The chromatids do not separate at their centromeres – each chromosome is still composed of two chromatids – genetic material has recombined

• Telophase I – individual chromosomes gather at each of the poles - cytoplasm divides (cytokinesis) forming two new cells
• Chromosomes do not replicate between meiosis I and meiosis II

• Prophase II – a new spindle forms around the chromosomes
• Metaphase II – chromosomes line up along the equator and are attached at their centromeres to spindle fibers

• Anaphase II – centromeres divide –chromatids move to opposite poles
• Telophase II – nuclear envelope forms around each set of chromosomes – result of meiosis is four haploid cells

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Meiosis and Genetic Variation

• Three mechanisms make key contribution to genetic variation: independent assortment, crossing over, and random fertilization
• Independent assortment random distribution of homologous chromosomes during meiosis

• Fertilization of an egg by a sperm is random, the number of possible outcomes is squared (223 x 223 = 64 trillion)
• The number of genetic combinations that can occur among gametes is practically unlimited

Video on Meiosis

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio19.swf::Stages%20of%20Meiosis

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Importance of Genetic Variation

• Meiosis and the joining of gametes are essential to change
• The pace of change appears to increase as the level of genetic variation increases
• Natural selection does not always favor genetic change
• Modern organisms are little changed from their ancestors, slowing the pace of change

Meiosis and Gamete Formation

• Spermatogenesis – the process by which sperm are produced in male animals, which occurs in the testes
• Large cell (2n) undergoes meiosis I, forming 2 cells (1n) that undergo meiosis II, forming 4 haploid cells (1n) that change to form and develop a tail to become sperm

• Oogenesis – the process by which gametes are produced in female animals – occurs in the ovaries
• During cytokinesis following meiosis I, the cytoplasm divides unequally
• Cell receiving almost all the cytoplasm will become the ovum – the smaller cells are called polar bodies and will not survive
• Ovum has a rich storehouse of nutrients to nourish the young organism if fertilization occurs

Classwork/Homework

• Chapter review p. 156 1-3, 5a, 6-8, 11,16, 18

Sexual Reproduction�Section 2

Sexual and Asexual Reproduction

• Reproduction, the process of producing offspring, can be asexual or sexual
• Asexual reproduction – a single parent passes copies of all of the genes to each of the offspring, there is no fusion of haploid cells – results in uniform progeny
• Clone – an organism that is genetically identical to its parent – an individual produced by asexual reproduction

• Prokaryotes reproduce by a type of asexual reproduction called binary fission
• Asexual reproduction is advantageous in:

a) stable environments b) conditions in which physical damage is frequent and severe

c) harsh environments where sexual reproduction often fails

Types of Asexual Reproduction

Fission – the separation of a parent into two or more individuals of about equal size

Fragmentation – a type of reproduction in which the body breaks into several pieces which can develop into adults when missing parts are regrown

• Budding – new individuals split off from existing ones
• Bud may break off & become independent or remain attached to the parent

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Genetic Diversity

• Asexual reproduction is the simplest and most primitive method
• Allows organisms to produce many offspring in a short period of time, without using energy to produce gametes or find a mate
• Sexual reproduction provides a powerful means of quickly making different combinations of genes among individuals

• Hypothesis of sexual evolution is based on that many enzymes that repair DNA are involved in meiosis
• Sexual reproduction would have to work at fantastic precision – just not for one species but for all species
• Only diploid cells can repair certain kinds of chromosome damage
• Many modern protests are haploid most of the time, & reproduce asexually

Sexual Life Cycles in Eukaryotes

• Life cycle – the entire span in the life of an organism from one generation to the next
• Eukaryotes that undergo sexual reproduction can have one of three types of sexual life cycles:

a) haploid

b) diploid

c) alternation of generations

Haploid Life Cycle

• Is the simplest of sexual life cycles
• Haploid cells occupy the major portion of life cycle
• Zygote is the only diploid cell, & undergoes meiosis immediately after it is formed
• Give rise to gamete by mitosis not meiosis
• During meiosis of the zygote, chromosome damage is repaired

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Diploid Life Cycle

• Outstanding characteristic is the adult individuals are diploid, each individual inheriting chromosomes from two parents
• Fertilization – the gametes (sperm & egg) join to produce a diploid zygote which divides by mitosis
• Gametes are the only haploid cells in diploid cycle

Cloning by Parthenogenesis

• Read 153 Exploring Further
• How Does Parthenogenesis occur?
• What are some organisms that reproduce using this method?
• Are mammals thought to be able to reproduce this way? Explain.

Alternation of Generation

• Plants, algae, and some protists have a life cycle that regularly alternates between a haploid phase and a diploid phase
• Sporophyte – the diploid phase in the life cycle that produces spores
• Spore – a haploid reproductive cell produced by meiosis that is capable of developing into an adult without fusing with another cell

• Gametophyte – the haploid phase that produces gametes by mitosis
• Gametes fuse and give rise to the diploid phase
• Sporophyte and gametophyte generations take turns
• All three involve an alternation of haploid & diploid phases
• Only differ in which phases become multicellular

Classwork/Homework

• Chapter review p. 156 & 157

questions 4, 5b-c, 9, 10, 19, and

Standardized Test Prep (STP)