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Meiosis and Life Cycle Patterns

Most organisms produce offspring by a process of sexual reproduction, in which a gamete from

one parent joins a gamete from the other parent to form a zygote (or fertilized egg). This

process results in offspring that have a combination of parental chromosomes and provides a

source of genetic variation for species. However, the chromosome number for a species isconstant, so at some time prior to sexual reproduction, chromosome number must be reduced

or the species chromosome number would double with each generation. Meiosis is the

process that reduces the chromosome number by half.

The process of meiosis accomplishes this reduction by first pairing, and then separating,

homologous or matching pairs of chromosomes. Cells that contain pairs of homologouschromosomes are called diploid. When a cell has chromosome pairs, we refer to the diploid

number of chromosomes. For humans, the diploid number of chromosomes is 46, comprised of

23 homologous pairs of chromosomes. Homologous chromosomes also form the basis of the

variation we see in the genetic traits of living organisms. Meiosis, therefore, has a secondfunction for living organisms: maintaining genetic variation.

Meiotic products have half as many chromosomes as their parent cells, and are haploid. Thenucleus of a haploid cell will have one set of chromosomes and no homologous pairs. A human

gamete will have 23 chromosomes and no pairs.

Meiosis involves two divisions. The first division, Meiosis I, involves pairing the homologous

chromosomes and then separating the homologues into separate cells (which reduces the

chromosome number by half). Because any cell division is preceded by DNA duplication, the

second division of meiosis (Meiosis II) involves the separation of the individual duplicatedchromosomes.

In this laboratory you will observe slides of meiosis in the lily flower. You will use pop-beadchromosomes to simulate the process of meiosis, including simulating crossing over and

recombination, and then compare, with diagrams, the process of oogenesis, which produces

eggs, with spermatogenesis, which produces sperm in animals. You will also look at meiosis in

the context of an organism's life cycle, comparing and contrasting the timing of meiosis inplants, animals, and haploid organisms.

Exercise I Meiosis in the Lily antherMeiosis in plants occurs in structures called sporangia. The sporangia of flowering plants are

located in the flower. Flowers have both male and female sporangia. The male sporangium is

called the anther and the female sporangium is an ovule. For comparison, the sporangia ofconifers are located in their cones. The brown fuzzy spots on the undersides of fern leaves are

fern sporangia.

Your instructor may show a series of slides showing meiosis in the anther (male sporangium)and ovule (female sporangium) of the lily, Lilium sp. before you try to find meiotic stages with

the microscope slides. Since the lily is a plant, the meiotic products will not directly form

gametes, but will develop by mitosis into structures (gametophytes) that will eventually containsperm (the pollen grain) and egg (the embryo sac).

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Materials Needed

Slide projector and slides of meiosis in lily anthers and ovulesPrepared slides of:

Lilium anther, prophase I

Lilium anther, first division (heterotypic division)

Lilium anther, second division (homeotypic division)Lilium anther, pollen tetrad

ProcedureObserve the prepared slides and micrographs of the lily anther to locate the stages meiosis.

Your instructor will provide you with an orientation to the structure of the anther so that you will

know where to locate the meiotic figures. As you observe the different stages of meiosis be surethat you can recognize the events of both meiosis I and of meiosis II. The cells that do meiosis

(the sporogenous tissue) in the anther are spherical. Each anther chamber has several cells,

and each lily flower has six anthers. You may or may not find meiotic figures in all the anther

sections on your slide. Although it would be ideal to have a slide with just one stage of meiosis,the sporogenous tissue of the anther does meiosis on its own time frame, not for the

convenience of biology students trying to see the stages of meiosis in real cells.

Meiosis I

Recall that prior to meiosis, in premeiotic interphase, DNA duplication occurs so the all of the

chromosomes in the cells about to do meiosis are duplicated.

There are two slides for meiosis I: Lilium anther, prophase I and Lilium anther, first division

(heterotypic division)

Prophase I

In prophase I, homologous chromosomes pair up in a process called synapsis. This uses

proteins along the chromosomes to join the homologues together. The homologues literally joinalong their length. At several points, called chiasmata, the four chromatids of the paired

homologous chromosomes intertwine. Chromatids of the homologous chromosomes can break

at one or more chiasmata and exchange equivalent bits of DNA with their homologue. This

exchange is called crossing over and is mediated by enzymes. If the exchanged pieces of thehomologues were different forms of a gene, then recombination occurs. The sister chromatids

now have some genetic variation; they are no longer precisely identical.

Early Prophase I Synapsed Homologous Pairs

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Metaphase I

Homologous pairs of chromosomes, still synapsed, are moved to the equator by the spindlecomplex. The alignment is random; some "maternal" chromosomes will orient facing one pole

along the equator; others face the opposite pole. Spindle microtubules just attach to

homologous chromosomes as they find them from the respective poles of the cell.

Anaphase I

The homologous chromosomes are separated from each other and pulled toward opposite

poles, officially reducing the chromosome number. Duplicated chromosomes are not affectedduring Anaphase I. The sister chromatids are still tightly bound to each other by their

centromeres. No homologous chromosome pairs are present at the end of Anaphase I. Each

cluster of chromosomes at the respective poles of the cell will have one of each type ofhomologous chromosome. The pairing and separation of homologues that you have just

observed is the key to reducing chromosome number while maintaining all of the genetic

information.

Telophase I and Interkinesis

Each nucleus that will form around the set of chromosomes at each pole will have half the

number of chromosomes as the pre-meiotic cell, each with one set of the homologouschromosomes

Cytokinesis typically will form two cells. (Each chromosome is still duplicated; this occurred inpre-meiotic interphase.) The cells are just preparing for the second division.

Metaphase I Anaphase I Telophase I

Meiosis II

Meiosis II is just like a mitosis; duplicated chromosomes are distributed equally into new cells,

each with the same number of chromosomes as the original cell The differences are that youare starting with two cells, and forming a total of four new cells, and the two cells contain no

homologous chromosomes.

Prophase II

New spindle apparatus is formed in each of the two cells from telophase I

The still-duplicated chromosomes stretch out and then recondense.

Spindle microtubules from each pole attach to the kinetochores of each of the sister chromatids.

Metaphase IIThe duplicated chromosomes are aligned along the equator by the spindle complex in each ofthe two cells.

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Anaphase II

Centromeres of sister chromatids are separated from each other and the now singlechromosomes are pulled to the poles of the two cells.

Telophase II and Cytokinesis

Each new nucleus formed has half the number of the original chromosomes and each nucleushas one of each type of homologous chromosome. Cytokinesis results in a total of four new

cells, the haploid products.

Prophase II Metaphase II Anaphase II Telophase II

Exercise II Meiosis Simulation - Optional

This exercise will give you the opportunity to review your understanding of meiosis by

diagramming the process below. A clear understanding of the behavior of chromosomes, and inparticular, homologous chromosomes and how they differ from sister chromatids, is essential to

understanding the process of meiosis and its reduction of chromosome number.

Recall that when the homologous chromosomes are synapsed during prophase I of meiosis,

crossing-over, or recombination occurs. During crossing-over homologous chromosomes

exchange small pieces of non-identical chromatids. The point where crossing-over occurs iscalled a chiasma, and several chiasmata occur in a single pair of homologous chromosomes.

Recombination has genetic importance. All of the genes located on a single chromosome are

inherited as one unit, so that the genes located on one chromosome are said to be "linked"together for purposes of inheritance. Crossing-over permits the separation of linked genes,

which results in a greater diversity of gene combinations in the cells formed by meiosis.

Because of crossing-over, no two meiotic products are exactly the same.

Diagram the process of meiosis on the following page showing crossing-over by exchanging

"bits" of your homologous chromosomes during prophase I.

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Nucleus in Prophase I

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Independent Assortment of Homologous Chromosomes during Meiosis

A source of genetic variation in meiosis is the alignment of homologous chromosomes at theequator during metaphase I. Some maternal chromosomes will align towards one pole and

some towards the other pole. Each pair of chromosomes assorts independently of any other

pair, relative to maternal/paternal origin during each meiotic event. This is known as

Independent Assortment. Diagram the possible ways that two pairs of homologouschromosomes can align at metaphase I and the resulting haploid products in telophase II below.

Premeiotic Interphase in G2

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Exercise III: Meiosis and the Life Cycles of Organisms

Just as each cell has a cell cycle, organisms have a life cycle. For most, the life cycle includessexual reproduction. Meiosis is something that takes place at just one point in any sexually

reproducing organism's life cycle. The timing of meiosis and whether the organism spends its

assimilative life composed of diploid or haploid cells (or part of its life composed of haploid cells,

and part with diploid) determines the life cycle pattern. Life cycles can be diploid, haploid or analternation of generations.

Diploid Life CycleIn animals, meiosis generally produces just haploid sex cells, or gametes, which at fertilization

start the next generation. The only haploid cells of the animal are egg or sperm, and the

respective maturation processes are called oogenesis and spermatogenesis. When gametesfuse, the zygote grows by mitosis producing the adult stage. All cells will be diploid and all cells

are produced by mitosis. The animal life cycle is a diploid life cycle.

Haploid Life CycleIn many protists, and some fungi, haploid gametes fuse to form a zygote, but the zygote

immediately does meiosis forming single haploid cells. In protists, which remain single cell

organisms, the nucleus is then haploid. At some time, a single cell will become a gamete andfuse with another to make a zygote, or haploid cells may do mitosis to make more individuals

asexually.

Haploid life cycles can be more complex. Fungi and some algae may make multicellular haploid

organisms from the single-celled meiotic product by mitosis. At some time, special areas of the

haploid body will become gamete-making structures (often called gametangia), and haploid

gametes are then formed by mitosis.

Alternation of Generations

Most plants have both a multicellular haploid stage and a multicellular diploid stage in their lifehistories. This is called the alternation of generations. For most plants, either the diploid stage

or the haploid generation predominates so most humans just haven't had the opportunity to

become acquainted with these different forms of a plant's life.

In plants, the structure in which meiosis occurs is called a sporangium. The multicellular diploid

plants, or parts of plants that produce sporangia are called sporophytes (spore-making plant).

Meiosis does not directly produce gametes, but produces haploid cells, called spores that inturn, grow, by mitosis, into multicellular haploid structures called gametophytes (gamete-

making plant). Gametophytes eventually produce structures that make gametes by mitosis.

Earlier you observed the process of meiosis in the male sporangium of the lily flower and sawthe male spores that will develop into the male lily gametophytes.

No matter how the life cycle varies, each gamete has a unique combination of chromosomes,

each zygote will be unique, and genetic variation is both maintained and obtained withinspecies.

Compare three typical life cycle patterns shown and look at the timing of meiosis in the life cycleof each. Then compare the life cycle of the fern illustrated with the human life cycle to help you

get a better idea of the alternation of generations life cycle typical of plants.

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Diploid Life Cycle Haploid Life Cycle Alternation of Generations

Human Life Cycle Fern Life Cycle

Fern Sporangia Fern Gametophyte