Learning Assignment for Ch. 12- Meiosis
The previous chapter focused on the type of nuclear and cell division that
gives rise to most of the cells in your body (the “somatic” cells, which are
all genetically identical and have the same number of chromosomes). Sperm
and eggs, however, have one-half the number of chromosomes as somatic cells
and each gamete is genetically different from each other. In fact, of all
the billions of sperm produced throughout the life of a man, it is unlikely
that any two have the same sets of alleles! This chapter focuses on meiosis
and how this type of nuclear (cell) division produces genetic variation in
populations of sexually reproducing organisms.
LEARNING GOALS:
1. Understand the events that occur during meiosis I and II (especially the
condition of the chromosomes during each phase of meiosis).
2. Understand how meiosis results in genetic variation in populations.
3. Understand why certain fruits are seedless.
GENERAL QUESTIONS:
1. P. 250 and Table 12.2 (p. 253)- The cell below is diploid (2 sets of
chromosomes; each set consist of 3 chromosomes). Is it in G1 or G2?
Using the chromosome shapes provided, draw the cell as it would look if
it were haploid in G1. Draw the cell as it would look if it were haploid
in G2. Are each set of sister chromatids genetically identical or
genetically different? Draw the cell as it would look if it were
triploid in G1 and also triploid in G2. Although we’ll focus on haploid
and diploid cells this semester, some cells (e.g., certain types of plant
cells) are triploid (3 sets of chromosomes), tetraploid (four sets of
chromosomes), etc.!
G1 or G2?
Haploid, G2
Haploid, G1
Triploid,
G1
Triploid,
G2
2. Fig. 12.4, and pp. 253-256- Meiosis occurs in two phases (meiosis I and
meiosis II). In meiosis I, the homologous chromosomes are separated
(this phase also introduces genetic variation!). In meiosis II, the
sister chromatids are separated (the sequence of events is essentially
the same as those in mitosis). Using the diploid cell below (notice it
is in G2…each chromosome has already replicated), sketch the cell as it
would appear in the following phases: prophase I, metaphase I, anaphase
I, telophase I (and cytokinesis). Take one of the cells that was formed
after telophase I and sketch it as it would appear during prophase II,
metaphase II, anaphase II, telophase II (and cytokinesis). Don’t worry
too much about centrosomes, etc., but focus primarily on the position of
the chromosomes.
Notice that crossing over occurs during prophase I—what is “crossing
over” and what is the significance of this process (see also Fig.
12.6)?
Consider the two cells formed after telophase I—are they haploid or
diploid? Are they genetically identical or different?
How many cells are ultimately formed as a result of a single cell
undergoing meiosis? Are they haploid or diploid? Are they
genetically identical or genetically different?
3. Fig. 12.8- Hopefully you saw that crossing over is one mechanism by which
genetic variation is produced in gametes. However, even if crossing over
didn’t occur, gametes would still have A LOT of genetic variation among
them! How? The position of the homologous chromosomes during metaphase
I is totally random and will determine the combination of chromosomes
that ends up in each sperm or egg cell formed! The cell below shows one
possible arrangement of chromosomes during metaphase I. Notice that two
sets of homologous chromosomes are present (one maternal and one
paternal).
What are the three genes labeled in the cell?
What six alleles are present?
How many chromosomes are present?
How many pairs of homologous chromosomes would be present if this were
a human skin cell, liver cell, or a cell in an ovary or testicle that
was getting ready to pass through meiosis to produce eggs or sperm?
Indicate the combinations of alleles that would be found in the gametes
after this cell passed through meiosis. What other possible arrangements of
chromosomes are there during metaphase I? What allele combinations would
result after those cells passed through meiosis?
blue
eyes
red hair
free earlobes
blonde hair
attached earlobes
brown
eyes
4. P. 259- A simple formula can be used to calculate the number of possible
combinations of alleles in cells formed by meiosis: 2n where “n” = the
haploid number of chromosomes. The cell above is diploid with six
chromosomes. Therefore, the haploid number of chromosomes = three. How
many possible combinations of alleles are possible in cells formed as a
result of the above cell passing through meiosis?
How many possible combinations of alleles are possible in human sperm and
eggs (again, ignore the genetic variation caused by crossing over at this
time)?
Considering your answer to the previous question, what is the chance
(likelihood) that a particular sperm fertilized a particular egg some 18,
20, 25 or so years ago? Now you can see how truly unique you are!!!
5. P. 266- We used a diploid cell to illustrate the normal sequence of
events of meiosis. Now consider a triploid cell (lets say with a total
of 21 chromosomes- 3 sets of 7). How many chromosomes would be in each
of the cells formed if the triploid cell underwent meiosis? Why are
bananas seedless?