Mastering Concepts
9.1
1. How do asexual and sexual reproduction differ?
Asexual reproduction requires only one parent and produces offspring that are identical,
except for mutations. Sexual reproduction requires two parents and produces genetically
variable offspring.
2. How can asexually reproducing organisms acquire new genetic information?
Mutations can create new gene variants, and some asexually reproducing organisms can
acquire new genetic information by exchanging DNA. For example, conjugation can
increase genetic diversity in bacteria and Paramecium.
3. Why does sexual reproduction persist even though it requires more energy than asexual
reproduction?
Sexual reproduction produces the variation needed for a population to survive a changing
environment. Mutations do generate variation among asexual organisms, but not quickly
enough or in great enough degrees to be successful in a rapidly changing environment.
9.2
1. What are autosomes and sex chromosomes?
Autosomes are chromosomes that are the same regardless of sex, whereas the sex
chromosomes determine whether an individual is male or female.
2. Draw a hypothetical karyotype for a cell with a diploid number of 8.
The drawing should include four homologous pairs of chromosomes. The members of
each pair should not be attached to each other, but they should look alike. Each of the
four pairs should have a unique combination of size/centromere location/banding pattern.
9.3
1. What is the difference between somatic and germ cells?
Germ cells can undergo meiosis and produce haploid gametes; somatic cells are body
cells that divide mitotically.
2. How do haploid and diploid nuclei differ?
A diploid cell (2n) has two full sets of chromosomes, with one set coming from each
parent. A haploid cell (n) has only one set of chromosomes.
3. What are the roles of meiosis, gamete formation, and fertilization in a sexual life
cycle?
In meiosis, a diploid cell divides twice to produce four haploid nuclei, reducing the
chromosome number by half. Meiosis also creates new allele combinations in the
haploid nuclei. Gamete formation packages the haploid chromosomes into reproductive
cells (sperm and egg cells). Fertilization merges haploid gametes from two parents,
producing a new diploid cell with half its chromosomes coming from each parent.
4. What is a zygote?
A zygote is a fertilized egg cell; it is the first cell of a new diploid organism.
9.4
1. What happens during interphase?
In interphase the cell grows and synthesizes all molecules necessary for cell division.
DNA replicates, transforming each chromosome into a pair of connected sister
chromatids. Chromatin begins to condense, and spindle proteins are produced.
2. How do the events of meiosis I and meiosis II produce four haploid cells from one
diploid germ cell?
In the diploid germ cell, meiosis I divides the homologous pairs of chromosomes into two
separate haploid cells. In each of these cells, meiosis II then divides the sister chromatids
of each chromosome. The overall result is four non-identical haploid daughter cells.
9.5
1. How does crossing over shuffle alleles?
Crossing over shuffles alleles when part of one chromosome switches places with part of
its homologous chromosome. The result is recombinant chromosomes with some alleles
arising from each parent
2. Explain how to arrive at the estimate that one human couple can produce over 70
trillion unique offspring.
During metaphase I, 23 homologous pairs of chromosomes (consisting of four
chromatids) align at the equator of the cell. Each homologous pair can align in either of
two ways. For 23 homologous pairs, there are 223 (8,388,608) possible alignments. Then,
during fertilization, any one of the woman’s 8,388,608 possible eggs can be fertilized by
any one of a man’s 8,388,608 possible sperm. The square of 8,388,608 therefore gives
the number of possible offspring combinations -- about 70.3 trillion.
3. How are identical twins different from fraternal twins?
Identical (monozygotic) twins arise from a single zygote and are therefore genetically
identical to each other. Fraternal (dizygotic) twins arise from two separate fertilized eggs
and are therefore as similar (or dissimilar) as any two non-identical siblings.
9.6
1. In what ways are mitosis and meiosis similar?
Mitosis and meiosis are similar in that both divide the chromosomes of a eukaryotic cell.
Overall, the processes have a similar progression of stages, and similar mechanisms move
chromosomes.
2. In what ways are mitosis and meiosis different?
Mitosis occurs in somatic cells at any life cycle stage, whereas meiosis is restricted to
germ cells at particular times in the life cycle. Mitosis involves only one cell division;
meiosis involves two. Mitosis yields two daughter cells; meiosis yields four. Cytokinesis
occurs once for every DNA replication event of mitosis; cytokinesis occurs twice for
every DNA replication event of meiosis. Crossing over occurs only in meiosis;
homologous chromosomes do not align with one another in mitosis. Daughter cells
produced by mitosis are identical; daughter cells produced by meiosis are genetically
variable. Daughter cells from mitosis are used for growth and repair and in asexual
reproduction; daughter cells from meiosis are used in sexual reproduction.
9.7
1. Draw a diagram to show how nondisjunction of all chromosomes during meiosis I in
one parent could lead to polyploid offspring. (Use 2n=6 for the starting cells; assume the
others parent’s gamete contributes the normal number of chromosomes.)
The diagram should show all three homologous pairs of chromosomes failing to separate
during anaphase I of meiosis. The products will be one cell containing all six
chromosomes (a total of 12 chromatids) and one cell with no chromosomes. When the
cell containing all of the chromosomes undergoes meiosis II, it will generate gametes
containing six chromosomes. If this abnormal gamete were fertilized by a normal haploid
gamete containing three chromosomes, the zygote would be triploid: it would have nine
chromosomes instead of the six in a normal diploid cell.
2. How can deletions, duplications, inversions, and translocations cause illness?
Deletions, duplications, inversions, and translocations can delete, duplicate, or damage
genes, which may affect the normal production of proteins. Chromosome duplication
may be the least harmful, because the “spare” genes can mutate while the others continue
their normal functions. Chromosome deletions may mean that an individual cannot make
some proteins. Inversion produces symptoms because chromosomes may not align
properly during meiosis, causing fertility problems, miscarriage, or birth defects. In
translocation, chromosomes may be missing parts, or genes can be broken. Affected
individuals may lack proteins, produce harmful proteins, or have fertility problems.
9.8
1. What are the stages of sperm development in humans?
Mitosis in a diploid spermatogonium produces diploid primary spermatocytes; meiosis I
in a diploid primary spermatocyte produces haploid secondary spermatocytes; meiosis II
in secondary spermatocytes produces haploid spermatids; spermatids mature into haploid
sperm cells.
2. What are the stages of development of an egg cell in humans?
Mitosis in a diploid oogonium produces diploid primary oocytes; meiosis I in a diploid
primary oocyte produces a secondary oocyte and a much smaller polar body (both
haploid); after fertilization, meiosis II in the secondary oocyte produces a large haploid
ovum and another small polar body.
3. How does gamete production in plants differ from that in animals?
In plants, gamete production occurs in multicellular haploid gametophytes, which
produce haploid gametes by mitosis. The gametes unite in fertilization, and the zygote
develops into a multicellular diploid sporophyte plant. Meiosis in cells of the sporophyte
plant produces haploid spores that germinate into the haploid gametophyte generation.
9.9
1. Why are the offspring of a male and a hermaphrodite more variable than the offspring
of a self-reproducing hermaphrodite?
A male and a hermaphrodite are two different individuals, and they are not genetically
identical to each other. Their offspring therefore inherit half of their alleles from each
parent; because meiosis scrambles alleles, the offspring are not identical to one another.
In contrast, a self-reproducing hermaphrodite produces gametes, but when those gametes
unite at fertilization, the offspring are identical to the hermaphrodite.
2. Under what conditions does evolution select for sexual reproduction in C. elegans?
The presence of genetically variable pathogens selects for sexual reproduction in C.
elegans.
Write It Out
1. Explain why evolution often selects traits that promote genetic diversity.
Organisms that reproduce asexually are successful as long as the environment stays the
same. But most habitats on Earth are continuously changing; they may become hotter,
colder, wetter, drier, sunnier, shadier, more acidic, more basic, etc. In that case,
organisms that reproduce sexually are likely to have the greatest reproductive success
because their offspring are genetically variable. At least some of their offspring are likely
to survive as the environment changes.
2. Describe a situation in which asexual reproduction might be more likely than sexual
reproduction.
Asexual reproduction is most likely in habitats that undergo little change, such as the
bottom of the ocean.
3. Most cells in a sexually reproducing organism have two sets of chromosomes. Explain
this observation and describe its significance to meiosis.
Sexually reproducing organisms have two parents, each of which contributes one set of
chromosomes. But when a diploid organism reproduces sexually, it cannot simply
combine one of its diploid cells with a diploid cell from a mate. If it did, the chromosome
number would double in the next generation. Instead, the organism must produce haploid
gametes that combine with the haploid gametes from another organism. Meiosis produces
these gametes.
4. Sketch the relationship between mitosis, meiosis, and fertilization in a sexual life
cycle.
[Answer will be visual and based on Figure 9.5.]
5. What is the difference between haploid and diploid cells? Are your skin cells haploid
or diploid? What about germ cells? Gametes?
Haploid cells have one set of chromosomes, whereas diploid cells have two sets. Skin
cells and germ cells are diploid, but gametes are haploid.
6. Some male veterans of the Gulf War in Iraq claim that their children have birth defects
that were caused by toxic substances contaminating the war zone. What types of cells
would a toxin have to have affected in these men to cause birth defects years later?
Explain your answer.
The spermatogonia were probably affected since they eventually give rise to sperm cells,
which will fertilize an egg and result in the offspring. Since somatic cells are not involved
in reproduction, mutations to body cells are not passed to offspring.
7. How does crossing over produce variation among gametes?
In crossing over, homologous chromosomes align and swap segments with each other.
After crossing over is complete, the chromosomes have new allele combinations. Since
crossing over occurs at random, the gametes produced in one round of meiosis are not
identical to those produced in a subsequent round of meiosis.
8. Draw all possible metaphase I chromosomal arrangements for a cell with a diploid
number of 8. How many unique gametes are possible for this species? Is this number an
underestimate or an overestimate? Why?
Drawings should show 16 possible unique gametes from 8 possible unique metaphase I
configurations. This number is an underestimate of the possible variation because it does
not account for crossing over.
9. Is it possible for a boy–girl pair of twins to be genetically identical? Why or why not?
No, a boy-girl pair of twins must be genetically different (because sex is determined by
genes on the sex chromosomes). The boy-girl combination results from two separate
sperm fertilizing two separate egg cells.
10. List some examples of chromosomal abnormalities, and explain how each relates to
an error in meiosis.
Extra or missing sex chromosomes and extra autosomes result from failure of
chromosomes to separate correctly in meiosis. Chromosomal deletions, duplications,
inversions, and translocations result from failures in homologous chromosomes to align
properly during prophase I or from mistakes during crossing over.
11. Could nondisjunction occur during mitosis? Compare and contrast the likely
consequence of nondisjunction in mitosis vs. meiosis.
Yes, nondisjunction could occur during mitosis, producing a cell with too many or too
few chromosomes. Typically, however, the consequences of nondisjunction in mitosis are
much less significant than when nondisjunction happens in meiosis. The abnormal cell
produced in mitosis will likely die, but others may be available to take its place. In
contrast, nondisjunction in meiosis produces gametes with incorrect chromosome
numbers. After fertilization, every cell of the resulting offspring will have the incorrect
chromosome number. The offspring may not survive, or it may have serious
abnormalities.
12. How does spermatogenesis differ from oogenesis, and how are the processes similar?
Spermatogenesis results in four equally-sized, very small spermatids that mature into
sperm cells. Oogenesis results in one very large egg and three polar bodies. The
processes are similar in that they involve an initial mitotic division and then two meiotic
divisions to produce haploid gametes.
13. Provide examples to support or refute this statement: The products of meiosis are
always haploid cells, whereas the products of mitotic division are always diploid cells.
The first part of the statement is true: unless nondisjunction occurs, diploid cells in
animals and plants undergo meiosis to produce haploid cells. The second part, however,
is true only for some organisms (including humans and most other animals). Plants have
a multicellular gametophyte generation in which haploid cells undergo mitotic division to
produce additional haploid cells; many fungi also produce haploid cells by mitotic
division.
Pull It Together
1. Fit the following terms into this concept map: chromatid, centromere, nondisjunction,
fertilization, and mitosis.
“Fertilization” connects with the phrase “produces a” to “Zygote.” “Mitosis” connects
with the phrase “divides chromosomes during replication of” to “Diploid cells” and
“Haploid cells.” “Chromosomes” connects with the phrase “when replicated, consist of
two” to “Chromatids,” which connects with the phrase “attach to each other at the” to
“Centromere.” “Nondisjunction” connects with the phrase “is the failed separation of” to
“Chromatids.”
2. Create a separate concept map that includes these terms: crossing over, synapsis,
gamete, autosome, and homologous pair. You may add other terms to the map as well.
Answers will vary, but one possibility is to connect “Homologous pair” with the phrase
“aligns during” to “Synapsis,” which could then connect with the word “precedes” to
“Crossing over.” The term “Crossing over” could connect with the phrase “occurs in the
cells that give rise to” to the term “Gametes.” And the term “Gametes” could connect
with the phrase “contain one copy of each” to the term “Autosome.”
3. What two processes in meiosis I generate genetic variation among gametes?
During prophase I, variation is created by crossing over between the members of each
pair of homologous chromosomes. The second source of variation is the random
alignment of the paired chromosomes during metaphase I.