Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. BIOLOGY A GUIDE TO THE NATURAL WORLD FOURTH EDITION DAVID KROGH Preparing for Sexual Reproduction: Meiosis

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings An Overview of Meiosis

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. An Overview of Meiosis In human beings, nearly all cells have paired sets of chromosomes, meaning these cells are diploid.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Overview of Meiosis Meiosis is the process by which a single diploid cell divides to produce four haploid cells—cells that contain a single set of chromosomes.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Overview of Meiosis The haploid cells produced through meiosis are called gametes. Female gametes are eggs Male gametes are sperm. They are the reproductive cells of human beings and many other organisms.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis Compared to Mitosis Figure 10.1 Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets of chromosomes. The members of each pair are homologous––the same in size and function. Two pairs of homologous chromosomes are shown within the cells in both the mitosis and meiosis figures. In each homologous pair, one chromosome (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other chromosome (in blue) comes from the father of this person. Prior to the initiation of both mitosis and meiosis, the chromosomes duplicate. In both processes, each chromosome is now composed of two sister chromatids. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, homologous chromosomes—not sister chromatids—line up on opposite sides of the metaphase plate. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate. The cells divide again, yielding four haploid cells. somatic cell duplication gamete precursor Homologous means the same in size and function 2n 1n2n1n 2n homologous pairs MitosisMeiosis In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid or “1n” state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on opposite sides of the plate. The sister chromatids of each chromosome then separate. division

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Overview of Meiosis When the haploid sperm and haploid egg fuse, a diploid fertilized egg (or zygote) is produced, setting into development a new generation of organism.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings The Steps in Meiosis

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Steps in Meiosis In meiosis, there is one round of chromosome duplication followed by two rounds of cell division.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The Steps in Meiosis There are two primary stages to meiosis, meiosis I and meiosis II.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis I In meiosis I, chromosome duplication is followed by a pairing of homologous chromosomes with one another, during which time they exchange reciprocal sections of themselves.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis I Homologous chromosome pairs then line up at the metaphase plate. One member of each pair is on one side of the plate, the other member is on the other side.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis I These homologous pairs are then separated, in the first round of cell division, each of them becoming part of a separate daughter cell.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis II In meiosis II, the chromatids of the duplicated chromosomes are separated into separate daughter cells.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 10.2 (a) Meiosis I (b) Crossing over (c) Independent assortment Homologous chromosomes link as they condense, forming tetrads. DNA has already duplicated Microtubules move homologous chromosomes to metaphase plate. Microtubules separate homologous chromosomes (sister chromatids remain together). Crossing over occurs. Independent assortment occurs. duplicated paternal chromosome Exchange of parts of non-sister chromatids. sister chromatids non-sister chromatids Diploid Prophase IEnd of interphase Metaphase I Anaphase I duplicated maternal chromosome tetrad First important source of genetic variation Two haploid daughter cells result from cytokinesis. (Brief) Sister chromatids line up at new metaphase plate. Sister chromatids separate. Four haploid cells result. Telophase II Metaphase II Metaphase I In the sequence above, homologous chromosomes lined up this way in Metaphase I but they could have lined up this way, yielding a different outcome. Compare these cells to the cells above Haploid Telophase IProphase II Metaphase II Anaphase IITelophase II cytokinesis Meiosis II Second important source of genetic variation Metaphase I Random alignment of maternal/paternal chromosomes at the metaphase plate.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings What is the Significance of Meiosis?

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. What is the Significance of Meiosis? Meiosis generates diversity by ensuring that the gametes it gives rise to will differ genetically from one another.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis Generates Diversity Meiosis is unlike regular cell division, or mitosis, which produces daughter cells that are exact genetic copies of parent cells. Meiosis generates genetic diversity in two ways.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis Generates Diversity First, in prophase I of meiosis, homologous chromosomes pair with each other. In the process called crossing over or recombination, they exchange reciprocal chromosomal segments with one another.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis Generates Diversity Second, in metaphase I of meiosis, there is a random alignment or independent assortment of maternal and paternal chromosomes on either side of the metaphase plate. This chance alignment determines which daughter cell each chromosome will end up in.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis Generates Diversity The genetic diversity brought about by meiosis and sexual reproduction is responsible, to a significant extent, for the great diversity of life- forms seen in the living world today.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis Generates Diversity Evolution is spurred on by differences among offspring, and meiosis and sexual reproduction ensure such differences. By contrast, asexual reproduction, as is seen in bacteria and other organisms, produces organisms that are exact genetic copies, or clones, of the parental organism.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Meiosis and Sex Outcome

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis and Sex Outcome Human females have 23 matched pairs of chromosomes—22 pairs of autosomes and one pair of sex-determining chromosomes, which in females are X chromosomes.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis and Sex Outcome Human males have 22 autosomes, one X chromosome, and one Y chromosome.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. The X and the Y Figure 10.4

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis and Sex Outcome Each egg that a female produces has a single X chromosome in it.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis and Sex Outcome Each sperm that a male produces has either an X or a Y chromosome within it.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis and Sex Outcome If a sperm with a Y chromosome fertilizes an egg, the offspring will be male.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis and Sex Outcome If a sperm with an X chromosome fertilizes the egg, the offspring will be female.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Gamete Formation in Humans

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Sperm Formation The starting cells in human male gamete formation, spermatogonia, are diploid cells that are stem cells in that they give rise not only to a set of specialized cells—the diploid primary spermatocytes—but to more spermatogonia as well.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Sperm Formation Primary spermatocytes go through meiosis, producing haploid secondary spermatocytes. These in turn give rise to the spermatids that develop into mature sperm cells.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Egg Formation In adult females, there may be no counterparts to the male spermatogonia—no cells that function as reproductive stem cells. This is currently a matter of scientific controversy.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Egg Formation Female gamete formation begins with cells called oogonia, most or all of which are produced prior to the birth of the female.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Egg Formation These give rise to primary oocytes—the cells that will go through meiosis I, thus producing haploid oocytes.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Sperm and Egg Formation Figure 10.6 spermatogonium oogonium egg polar bodies (will be degraded) Meiosis I Meiosis II primary oocyte secondary oocyte secondary spermatocytes primary spermatocyte The diploid spermatogonium cell produces a primary spermatocyte. The primary spermatocyte goes through meiosis I, yielding two haploid secondary spermatocytes. The secondary spermatocytes go through meiosis II, yielding four haploid spermatids, which will develop into mature sperm cells. Before the birth of the female, a cell called an oogonium develops into a primary oocyte; this cell enters meiosis I, but remains there until it matures in the female ovary, beginning at puberty. On average, one primary oocyte per month will complete meiosis I. In this process, an unequal meiotic division of cellular material leads to the production of one polar body and one secondary oocyte, which enters into meiosis II. Only secondary oocytes that are fertilized by sperm will complete meiosis II and develop into an egg. The three polar bodies that are produced by meiosis I and II will be degraded. SpermatogenesisOogenesis spermatids polar body

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Egg Formation The vast majority of primary oocytes never complete meiosis, however. It is only the single primary oocyte released each month, in the process of ovulation, that completes meiosis I.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Egg Formation Only those ovulated oocytes that are fertilized by sperm complete meiosis II.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Egg Formation Only one of the cells produced in meiosis will have the potential to develop into a haploid egg. The other cells receive very little cytoplasmic material during the process of cell division and thus are destined to become nonfunctional cells called polar bodies.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Life Cycles: Humans and Other Organisms

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Life Cycles: Humans and Other Organisms Not all reproduction is sexual reproduction, meaning reproduction that works through a fusion of two reproductive cells.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Asexual Reproduction Most types of organisms are capable of asexual reproduction, although such reproduction is rare among more complex organisms and is never carried out by mammals or birds.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Asexual Reproduction Asexual reproduction can take several forms, including binary fission, vegetative reproduction, and regeneration.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Asexual Reproduction Bacteria reproduce through one form of asexual reproduction, called binary fission, in which a given bacterial cell replicates its single chromosome and then divides in two.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Asexual Reproduction Plants can engage in vegetative reproduction. Other organisms, such as worms and sea stars, can carry out regeneration, in which a new, complete organism can be formed from a portion of an existing one.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Regeneration Figure 10.10