108 Chapter 3

3.7 Comparing Mitosis and MeiosisAll multicelled, eukaryotic species grow and repair tissue by mitosis, followed bycytokinesis. Single-celled eukaryotic species follow the same process to repro-duce asexually. Mitosis occurs in nonreproductive cells. In interphase, the cellshave a diploid chromosome number. Before mitotic division occurs, their DNAis replicated. Reproductive cells also have a diploid chromosome number ininterphase. Before meiosis I, their DNA is replicated. However, there is no DNAreplication between meiosis I and meiosis II. Figures 1 and 2 summarize the sim-ilarities and differences between mitosis and meiosis. As you examine Figures 1and 2, make note of the chromosome number of the cell or cells, whether thechromosome number is haploid or diploid, and during which stage the chromo-some number changes.

The most significant difference between mitosis and meiosis is the endresults. Mitosis results only in clones of the original; all daughter cells are genet-ically identical to each other and to the parent cell. Meiosis results in four cellsthat are different from each other and from the parent. Meiosis, combined withfertilization, explains the variation in traits that is observed in species that repro-duce sexually. The variation occurs through three mechanisms. First, crossingover during prophase I exchanges genes on the chromosomes. Second, duringmetaphase I, the paternal and maternal chromosomes are randomly assorted.Although homologues always go to opposite poles, a pole could receive all thematernal chromosomes, all the paternal ones, or some combination. Lastly,during fertilization, different combinations of chromosomes and genes occurwhen two gametes unite.

In later chapters, you will learn about the ways in which meiosis and fertil-ization contribute to the enormous diversity exhibited by sexually reproducingorganisms. You will learn about how sexual reproduction promotes genetic vari-ability, and how this contributes to the survival of individuals, populations, andentire species.

prophase I telophase Ianaphase Imetaphase I

The replicated chromosomes con-dense. Homologous chromosomescome together in synapsis andcrossing-over occurs.Chromosomes attach to the spindle.

Homologous chromosomes lineup at the equatorial plate.

Each chromosome separatesfrom its homologue. They move toopposite poles of the cell.

The nucleus completes its divi-sion. The chromosomes are stillcomposed of sister chromatids.The cytoplasm divides aftertelophase.

Meiosis I

Figure 1

Stages of meiosis I.

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prophase metaphase anaphase telophase

The cytoplasm separates, leavingfour haploid daughter cells. Thechromosome number has beenreduced by half. These cells maybecome gametes.

Sister chromatids of each chro-mosome separate and move toopposite poles.

Chromosomes line up at theequatorial plate.

The centrioles in the two newcells move to opposite poles andnew spindle fibres form. Thechromosomes become attachedto the spindle.

prophase II metaphase II anaphase II telophase II

Meiosis II

The chromosomes condense,becoming shorter and thicker. Thecentrioles assemble and spindlefibres attach to the centromeresof the chromosomes. The nuclearmembrane starts to dissolve.

Chromosomes line up at theequatorial plate. The nuclearmembrane completely dissolves.

The centromeres divide and theresulting chromosomes, formerlychromatids, move to oppositepoles of the cell. An identicalset of chromosomes moves toeach pole.

Chromosomes lengthen again,the spindle fibres dissolve, and anuclear membrane forms aroundthe chromosomes.

Mitosis

Figure 2

Comparsion of the stages in mitosis and meiosis II

110 Chapter 3

Activity 3.7.1

Comparing Mitosis and Meiosis

In this investigation, you will model and compare the events of mitosis and meiosis.

Materials

blue modelling clay plastic knifered modelling clay sheets of papergreen modelling clay pencil

Procedure

For each step, make a coloured sketch of your model with appropriate labels.Include brief descriptions of your steps and make sure to use the same step num-bers as given.

Part 1: Mitosis1. Take some red clay and roll it between your hands to create a piece 10 cm

long and about as thick as your finger. Make another piece about 5 cm long.

2. Repeat step 1 with the blue clay.

3. Make an identical copy of each piece of clay. Then attach the identicalpieces with a green ball of clay (Figure 3).

4. Draw a line down the length of a sheet of paper. Line up the four chromo-somes along the line (Figure 4).

Figure 3 Figure 4 Figure 5 Figure 6

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5. Remove the green balls and move each of the single pieces of clay to oppositeends of the paper (Figure 5).

6. If the cell is going to divide again, each single chromosome must synthesizea duplicate during interphase. Make an identical copyof each piece of clay as before (Figure 6).

Part 2: Meiosis7. Follow steps 1 to 3 from part 1.

8. Demonstrate crossing-over. Break off a piece of clay from one chromosomeand attach it to the other chromosome (Figure 7). Repeat a few times ifyou like.

9. To simulate metaphase I, place the chromosomes on either side of theequatorial plate, represented by a line drawn on a piece of paper (Figure 8).

10. Choose one of the haploid daughter cells and line the chromosomes upalong the equatorial plate. Remove the centromere and move chromosomesto opposite poles (Figure 9).

Analysis

Part 1: Mitosis(a) In step 3, what process did you model? (b) What do the red and blue pieces of clay represent? What do the green balls

of clay represent?(c) In step 4, what is the diploid chromosome number of the cell?(d) What phase of mitosis does the model represent?(e) In step 5, what structure do the single pieces of clay represent after separa-

tion?(f) What phase of mitosis does the model represent?(g) In step 6, how many chromosomes are in each of the daughter cells?(h) Compare the daughter cells with the parent cell.

Part 2: Meiosis(i) In steps 1 to 3, on what basis are chromosomes considered to be homolo-

gous?(j) What is the diploid chromosome number?(k) In step 8, what must happen before the homologous chromosomes can

cross over?(l) In which phase does crossing over occur?(m) What happens during crossing over?

Figure 7 Figure 8 Figure 9