3.7
Comparing Mitosis and Meiosis
All multicelled, eukaryotic species grow and repair tissue by mitosis, followed by
cytokinesis. Single-celled eukaryotic species follow the same process to reproduce asexually. Mitosis occurs in nonreproductive cells. In interphase, the cells
have a diploid chromosome number. Before mitotic division occurs, their DNA
is replicated. Reproductive cells also have a diploid chromosome number in
interphase. Before meiosis I, their DNA is replicated. However, there is no DNA
replication between meiosis I and meiosis II. Figures 1 and 2 summarize the similarities and differences between mitosis and meiosis. As you examine Figures 1
and 2, make note of the chromosome number of the cell or cells, whether the
chromosome number is haploid or diploid, and during which stage the chromosome number changes.
The most significant difference between mitosis and meiosis is the end
results. Mitosis results only in clones of the original; all daughter cells are genetically identical to each other and to the parent cell. Meiosis results in four cells
that are different from each other and from the parent. Meiosis, combined with
fertilization, explains the variation in traits that is observed in species that reproduce sexually. The variation occurs through three mechanisms. First, crossing
over during prophase I exchanges genes on the chromosomes. Second, during
metaphase I, the paternal and maternal chromosomes are randomly assorted.
Although homologues always go to opposite poles, a pole could receive all the
maternal chromosomes, all the paternal ones, or some combination. Lastly,
during fertilization, different combinations of chromosomes and genes occur
when two gametes unite.
In later chapters, you will learn about the ways in which meiosis and fertilization contribute to the enormous diversity exhibited by sexually reproducing
organisms. You will learn about how sexual reproduction promotes genetic variability, and how this contributes to the survival of individuals, populations, and
entire species.
Meiosis I
prophase I
The replicated chromosomes condense. Homologous chromosomes
come together in synapsis and
crossing-over occurs.
Chromosomes attach to the spindle.
Figure 1
Stages of meiosis I.
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metaphase I
Homologous chromosomes line
up at the equatorial plate.
anaphase I
Each chromosome separates
from its homologue. They move to
opposite poles of the cell.
telophase I
The nucleus completes its division. The chromosomes are still
composed of sister chromatids.
The cytoplasm divides after
telophase.
3.7
Mitosis
prophase
metaphase
anaphase
telophase
The chromosomes condense,
becoming shorter and thicker. The
centrioles assemble and spindle
fibres attach to the centromeres
of the chromosomes. The nuclear
membrane starts to dissolve.
Chromosomes line up at the
equatorial plate. The nuclear
membrane completely dissolves.
The centromeres divide and the
resulting chromosomes, formerly
chromatids, move to opposite
poles of the cell. An identical
set of chromosomes moves to
each pole.
Chromosomes lengthen again,
the spindle fibres dissolve, and a
nuclear membrane forms around
the chromosomes.
metaphase II
anaphase II
telophase II
Meiosis II
prophase II
The centrioles in the two new
cells move to opposite poles and
new spindle fibres form. The
chromosomes become attached
to the spindle.
Chromosomes line up at the
equatorial plate.
Sister chromatids of each chromosome separate and move to
opposite poles.
The cytoplasm separates, leaving
four haploid daughter cells. The
chromosome number has been
reduced by half. These cells may
become gametes.
Figure 2
Comparsion of the stages in mitosis and meiosis II
Cell Division 109
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
red modelling clay
green modelling clay
plastic knife
sheets of paper
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 numbers as given.
Part 1: Mitosis
1. 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 identical
pieces with a green ball of clay (Figure 3).
4. Draw a line down the length of a sheet of paper. Line up the four chromosomes along the line (Figure 4).
Figure 3
110 Chapter 3
Figure 4
Figure 5
Figure 6
3.7
Figure 7
Figure 8
Figure 9
5. Remove the green balls and move each of the single pieces of clay to opposite
ends of the paper (Figure 5).
6. If the cell is going to divide again, each single chromosome must synthesize
a duplicate during interphase. Make an identical copy
of each piece of clay as before (Figure 6).
Part 2: Meiosis
7. Follow steps 1 to 3 from part 1.
8. Demonstrate crossing-over. Break off a piece of clay from one chromosome
and attach it to the other chromosome (Figure 7). Repeat a few times if
you like.
9. To simulate metaphase I, place the chromosomes on either side of the
equatorial 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 up
along the equatorial plate. Remove the centromere and move chromosomes
to 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 separation?
(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 homologous?
(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?
Cell Division 111