a guide to the natural world david krogh © 2011 pearson education, inc. chapter 10 lecture outline...

A Guide to the Natural World

David Krogh© 2011 Pearson Education, Inc.

Chapter 10 • Lecture OutlinePreparing for Sexual Reproduction: Meiosis

Biology

Fifth Edition

© 2011 Pearson Education, Inc.

10.1 An Overview of Meiosis


An Overview of Meiosis

• In human beings, nearly all cells have paired sets of chromosomes, meaning these cells are diploid.


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.


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.


Homologousmeans the

same in sizeand function

Mitosis Meiosis

somaticcell

gameteprecursor

duplication2n 2n

2n 2n

2n 2n

2n 2ndivision

2n 2n n n

divisionn n

n n n n

homologouspairs

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.

duplication

2. Prior to the initiation of both mitosis and meiosis, the chromosomes

duplicate. In both processes, each chromosome is now composed of

two sister chromatids.

3. 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.

4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate.

division5. 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 “n” state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on opposite sides of the plate.

division6. The sister chromatids of each chromosome then separate.

7. The cells divide again, yielding four haploid cells.

Figure 10.1


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.


10.2 The Steps in Meiosis


The Steps in Meiosis

• In meiosis, there is one round of chromosome duplication followed by two rounds of cell division.


The Steps in Meiosis

• There are two primary stages to meiosis, meiosis I and meiosis II.


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.


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.


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.


Meiosis

Suggested Media Enhancement: MeiosisTo access this animation go to folder C_Animations_and_Video_Files

and open the BioFlix folder.


Meiosis II

• In meiosis II, the chromatids of the duplicated chromosomes are separated into separate daughter cells.


The Steps of MeiosisPrior tomeiosis

Meiosis IDiploi

d metaphaseplate

End ofinterphaseDNA has alreadyduplicated

Homologous chromosomes link as they condense, forming tetrads.

Prophase I

Crossing overoccurs

(see Figure 10.3)

Microtubulesmove homologouschromosomesto metaphase plate.

Metaphase I

Anaphase IMicrotubules

separate homologouschromosomes(sister chromatidsremain together).Independent

assortment occurs

(see Figure 10.4)

Figure 10.2


Telophase ITwo haploiddaughter cellsresult fromcytokinesis.

Prophase II(Brief)

Metaphase IISister chromatidsline up at newmetaphase plate.

Anaphase IISister chromatidsseparate.

Telophase IIFour haploidcells result.

cytokinesis

cytokinesis

Haploid Meiosis IIThe Steps of Meiosis

Figure 10.2


10.3 The Significance of Meiosis


The Significance of Meiosis

• Meiosis generates diversity by ensuring that the gametes it gives rise to will differ genetically from one another.


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.



• 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.



• 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.



• 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.



• 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.


10.4 Meiosis and Sex Outcome


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.



• Human males have 22 autosomes, one X chromosome, and one Y chromosome.


The X and the Y

Figure 10.6



• Each egg that a female produces has a single X chromosome in it.



• Each sperm that a male produces has either an X or a Y chromosome within it.



• If a sperm with a Y chromosome fertilizes an egg, the offspring will be male.



• If a sperm with an X chromosome fertilizes the egg, the offspring will be female.


10.5 Gamete Formation in Humans


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.


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.


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.


Egg Formation

• Female gamete formation begins with cells called oogonia, most or all of which are produced prior to the birth of the female.


Egg Formation

• These give rise to primary oocytes—the cells that will go through meiosis I, thus producing haploid oocytes.


Spermatogenesis

spermatogonium

1. The diploid spermatogonium cell produces a primary spermatocyte.

primaryspermatocyte

2. The primary spermatocyte goes through meiosis I, yielding two haploid secondary spermatocytes.

secondaryspermatocytes

3. The secondary spermatocytes go through meiosis II, yielding four haploid spermatids, which will develop into mature sperm cells.

spermatids

Meiosis I

Meiosis II

Oogenesis

oogonium

primaryoocyte

polarbody

secondaryoocyte

polar bodies(will be degraded) egg

1. 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.

2. 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.

3. 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.

Sperm and Egg Formation in Humans

Figure 10.8


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.


Egg Formation

• Only those ovulated oocytes that are fertilized by sperm complete meiosis II.


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.


10.6 Life Cycles: Humans and Other Organisms


Life Cycles: Humans and Other Organisms

• Not all reproduction is sexual reproduction, meaning reproduction that works through a fusion of two reproductive cells.


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.



• Asexual reproduction can take several forms, including binary fission, vegetative reproduction, and regeneration.



• 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.



• 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.


Regeneration

Figure 10.11

a guide to the natural world david krogh © 2011 pearson education, inc. chapter 10 lecture outline...

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