Chapter 2: Reproduction and Chromosome Transmission
Student Learning Objectives
Upon completion of this chapter you should be able to:
1.
2.
3.
4.
5.
6.
7.
Know the general features of chromosomes.
Understand the process of binary fission in bacteria.
Know the stages of mitosis and recognize diagrams associated with this process.
Understand the process of cytokinesis and how it differs in animals and plants.
Know the end result of mitosis in terms of number of cells and their chromosome content.
Know the stages of meiosis and the cellular processes that are involved with each stage.
Understand the process of gamete formation in both plants and animals.
2.1 General Features of Chromosomes
Overview
This chapter begins with a description of eukaryotic and prokaryotic cells. While the
principles of genetics are basically the same for all organisms, there are some differences in the
structure of the eukaryotic and prokaryotic genomes, as well as variations in the processes of gene
structure and expression. Therefore, it is important at this point in the course that you establish an
understanding of the differences between these two types of cells. The section also introduces the
topic of karyotype, which is a mechanism of visualizing the chromosomal complement of a given
cell. This plays an important role in the identification of chromosomal abnormalities, a topic that
will be discussed in Chapter 8. For now, you should concentrate on understanding how
karyotypes are prepared (Figure 2.2).
The last portion of this first section introduces an important concept – the diploid nature
of chromosomes in many eukaryotic organisms. It is crucial that you begin to distinguish between
homologs (similar chromosomes) and the concept of sister chromatids (next section). Consider
the chromosome to be a filing cabinet, where the individual files within the cabinet represent
genes. As we will see in the following sections, we get one set of cabinets (homologs) from our
mother, and a second set from our father. Since our parents are of the same species, it makes
sense that the organization of the cabinets be the same, meaning that the physical location (locus)
of each file (gene) is the same in each cabinet. But our parents are not clones of each other, and so
there may be minor variations in the content of the files. Figure 2.3 illustrates this concept.
Outline of Key Terms
Somatic cells
Germ cells
Gametes
Karyotype
Diploid
Homologs
Locus (Loci)
Alleles
Homozygous
Heterozygous
Reproduction
Chromosomes
Chromatin
Prokaryotes
Nucleoid
Eukaryotes
Organelles
Nucleus
Cytogenetics
Cytogeneticist
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Focal Points



The differences between prokaryotic and eukaryotic cells (pages 19-21)
The procedure for making a human karyotype (Figure 2.2)
A comparison of homologous chromosomes (Figure 2.3)
Exercises and Problems
For each of the following, match the information provided with its appropriate term. Answers
may be used more than once, or not at all.
_____ 1. The site in the eukaryotic cell where the majority of the genetic information is located.
_____ 2. A photographic representation of the chromosomes in a cell.
_____ 3. A cell that does not have its genetic information contained in a nucleus.
_____ 4. The cells of humans belong to this general class of cells.
_____ 5. The structures of living cells that carry the genetic information.
_____ 6. Bacteria belong to this general class of cells.
_____ 7. Another term for gametes.
_____ 8. Membrane-bound structures that have specific functions in eukaryotic cells.
a. organelles
b. nucleus
c. somatic cell
d. germ cells
e. eukaryotic cells
f. prokaryotic cells
g. karyotype
h. nucleoid
i. chromosomes
For questions 9 to 14, match the following terms with their correct definition.
_____ 9.
Locus
_____ 10. Homologs
_____ 11. Gene
_____ 12. Diploid
_____ 13. Homozygous
_____ 14. Heterozygous
a. The physical location of a gene on a chromosome.
b. An individual having two chromosomes with identical alleles for a trait.
c. A unit of genetic information, codes for a single trait.
d. Term used to identify organisms that have a pair of each chromosomes.
e. Chromosomes that are members of a pair.
f. An individual having two chromosomes with different alleles for a trait.
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For questions 15 to 18, indicate whether the statement is true (T) or false (F). If false, change the
statement to make it true.
_____15. Homologous chromosomes are identical in the genetic information that they contain.
_____16. Alleles for a trait occupy the same loci on homologous chromosomes.
_____17. Eukaryotic organelles, other than the nucleus, that contain DNA include the lysosome
and Golgi body.
_____18. Cytogenetics is the field of genetics that involves the microscopic examination of
chromosomes.
2.2 Cell Division
Overview
This section of the chapter examines the process of cellular division in both prokaryotes
and eukaryotes. This process allows for asexual reproduction in both types of organisms, and
enables mullticellularity in most eukaryotic species.
Most prokaryotes divide by binary fission (Figure 2.4). The process is relatively simple
compared to eukaryotic cell division. Indeed, eukaryotic cells progress through an elaborate cell
cyle in order to divide (Figure 2.5). However, before jumping directly into mitosis, be sure that
you understand the basic purpose of the S phase, during which the sister chromatids (Figure 2.6b)
are formed. Chapter 11 will examine the process of DNA replication in greater detail. Students
frequently have a difficult time with the stages of mitosis, specifically with regards to
chromosome number, if they do not comprehend the purpose of the S phase.
Outline of Key Terms
Chromatids
Sister chromatids
Centromere
Kinetochore
Mitosis
Asexual reproduction
Binary fission
Cell cycle
Interphase
Restriction point
Focal Points



Binary fission (Figure 2.4)
The eukaryotic cell cycle (Figure 2.5)
Chromosomes following DNA replication (Figure 2.6)
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Exercises and Problems
In the diagram below, label each of the phases of the cell cycle.
__________ 1.
__________ 2.
__________ 3.
__________ 4.
__________ 5.
__________ 6.
__________ 7.
__________ 8.
__________ 9.
__________ 10.
__________ 11.
__________ 12.
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For questions 13 to 16, complete the sentence with the most appropriate term(s):
13. Most bacteria divide by a process called ________.
14. The G1, S, and G2 phases are collectively known as _______.
15. The ______ is a group of proteins that are bound to the centromere.
16. A cell in the ____ phase is either temporarily out of the cell cycle, or will never divide again.
2.3 Mitosis and Cytokinesis
Overview
After addressing the topic of bacterial cell division, we turn our attention to cell division
in eukaryotic species. These processes include mitosis and meiosis. This section begins with a
discussion of the mitotic spindle apparatus, which organizes and sorts eukaryotic chromosomes.
Note how the mitotic spindle of animal cells is composed of three types of microtubules (aster,
kinetochore, and polar), each of which serves a particular function (Figure 2.7).
Mitosis is nuclear division and involves five stages. The purpose of each of these stages
is outlined in the following table.
Stage of Mitosis
General Characteristics
Prophase
Chromosomes condense.
Nuclear membrane begins to disassociate.
Mitotic spindle forms.
Centrosomes (if present) begin to move to opposite poles of
the cell.
Nuclear envelope is completely disassociated.
Spindle fibers interact with the kinetochore.
Chromosomes align along a central line (metaphase plate) of
the cell.
Sister chromatids separate and begin to move towards
opposite poles of the cell.
Each chromatid is now considered to be a chromosome.
Chromosomes arrive at the poles of the cell and decondense.
Nuclear envelope reforms around the two sets of
chromosomes.
Prometaphase
Metaphase
Anaphase
Telophase
It is sometimes easier to understand mitosis if you mark one of the chromosomes and follow it,
and its homolog, through the stages. The animations provided with the text assist in the
visualization of the process, and also help you to understand that mitosis is a fluid series of steps.
Cytokinesis is the last stage of the division process. Whereas mitosis was nuclear
division, cytokinesis is cytoplasmic division. Note that attempts are not made to provide each cell
with the same number of each organelle. Organelles will be replicated as needed as soon as the
new cell enters into the G1 phase of the cell cycle. You should also examine the differences
between cytokinesis in animal and plant cells.
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Outline of Key Terms
Mitosis
Prophase
Condensed chromosomes
Prometaphase
Metaphase
Metaphase plate
Anaphase
Telophase
Cytokinesis
Cleavage furrow
Cell plate
Mitotic spindle (apparatus)
Microtubule-organizing centers
(MTOCs)
Centrosomes
Centrioles
Spindle pole
Interphase
Decondensed chromosomes
Focal Points



The structure of the mitotic spindle in a typical animal cell (Figure 2.7)
The process of mitosis (Figure 2.8, pages 27-29)
The mechanisms of cytokinesis (Figure 2.9)
Exercises and Problems
For questions 1 to 9, match each statement with the term that best describes the stage of the cell
cycle or mitosis that it is associated with. Terms may be used more than once.
_____ 1.
The chromosomes arrange themselves along a line in the center of the cell.
_____ 2.
The sister chromatids are formed.
_____ 3.
The sister chromatids separate, forming new chromosomes.
_____ 4.
The nuclear envelope reforms around the chromosomes.
_____ 5.
The spindle fibers attach to the kinetochore.
_____ 6.
The centrosomes begin to move to opposite poles of the cell.
_____ 7.
The chromosomes condense.
_____ 8.
Gap phases of the cell cycle.
_____ 9.
The nuclear envelope begins to disassociate.
a. prophase
b. S phase
c. prometaphase
d. telophase
e. anaphase
f. metaphase
g. G1 and G2 phases
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For questions 10 to 15, match the diagram with the correct label.
_____ 10.
a. telophase
b. prometaphase
c. interphase
d. anaphase
e. metaphase
f. prophase
_____ 11.
_____ 12.
_____ 13.
_____ 14.
_____ 15.
10
13
11
14
12
15
16. Using the number for each label, place these stages in their correct order.
For questions 17 and 20, complete the sentences with the most appropriate term(s)
17.
18.
19.
20.
In animal cells, cytokinesis is marked by the formation of a ________.
In plant cells, cytokinesis is marked by the formation of a ________.
There are _____ chromatids present in a human somatic cell in metaphase of mitosis.
In a human somatic cell, _____ chromosomes move to each pole in anaphase of mitosis.
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2.4 Meiosis
Overview
For most organisms, the majority of the cell division in the organism following the
formation of the zygote is by the process of mitosis. The daughter cells of mitotic division have
the same chromosome number as the parent cell, and with some minor exceptions, contain
identical genetic information. However, organisms also have a need to produce cells with a
reduced number of chromosomes, as is the case with gamete formation in animals. This requires a
modified form of cell division called meiosis.
Meiosis accomplishes two things: 1) a reduction in chromosome number, and 2) genetic
variation. The reduction in chromosome number is the result of two consecutive cell divisions,
without an intervening S phase (DNA replication). Genetic variation is achieved via two different
processes that occur during meiosis I: crossing-over between non-sister chromatids of
homologous chromosomes in prophase I (Figure 2.10), and random alignment of homologous
chromosomes in metaphase I (Figure 2.12). Another distinctive feature of metaphase I is the
attachment of kinetochore microtubules to the sister chromatids. Note how the two pairs of sister
chromatids are attached to opposite poles (See Figure 2.13). (Remember that in mitosis, and
meiosis II for that matter, a pair of sister chromatids is attached to only one pole).
Typically students understand mitosis, but not meiosis. However, if you recognize that
meiosis follows the same fundamental principles as mitosis, with the exception of a few changes
to provide for variation and reduction of chromosome number, then the process becomes easier to
understand. The best means of studying meiosis is to follow Figure 2.12 closely. As was the case
with mitosis, it is often helpful to mark one of the chromatids and then follow it through the
various stages of meiosis.
The section ends with a discussion of mitosis vs. meiosis. Be sure to study Table 2.1 as it
summarizes the events of each of the substages, as well as the outcome.
Outline of Key Terms
Synapsis
Bivalents
Synaptonemal complex
Crossing over
Chiasma (chiasmata)
Tetrads
Meiosis
Haploid
Meiosis I
Meiosis II
Prophase I
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
Focal Points



The process of crossing over in prophase I (Figures 2.10 and 2.11)
The stages of meiosis in an animal cell (Figure 2.12)
A comparison of mitosis, meiosis I, and meiosis II (Table 2.1)
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Exercises and Problems
For questions 1 to 5, match the stage of prophase I to its correct description.
_____ 1. Diakinesis
_____ 2. Zygotene
_____ 3. Diplotene
_____ 4. Leptotene
_____ 5. Pachytene
a. Homologous chromosomes recognize each other by synapsis.
b. The chromosomes begin to condense, and are now visible using a microscope.
c. The stage in which crossing over occurs.
d. The synaptonemal complex disappears.
e. The final stage of prophase I.
For questions 6 to 16, use the following key:
a. Statement applies to mitosis only
b. Statement applies to meosis only
c. Statement applies to both mitosis and meiosis
d. Statement applies to neither mitosis nor meiosis
_____ 6.
Produces identical daughter cells.
_____ 7.
Involves a single division.
_____ 8.
Involves two divisions.
_____ 9.
Occurs in prokaryotic cells.
_____ 10. Occurs in eukaryotic cells.
_____ 11. Occurs in skin cells.
_____ 12. Is preceded by DNA replication.
_____ 13. Includes crossing-over between homologous chromosomes.
_____ 14. Halves the chromosome number.
_____ 15. Keeps the chromosome number intact.
_____ 16. Involves formation of the synaptonemal complex.
17. A cell has 20 chromosomes at the end of meiosis. How many chromatids are found in a cell
at the prophase I stage?
For questions 18 to 20, consider an organism with a diploid chromosome number of 2n = 14.
18. How many random chromosome alignments are possible at metaphase I of meiosis?
19. How many bivalents are present in a cell in prophase I of meiosis?
20. How many chromatids are present in a cell in metaphase II of meiosis?
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2.5 Sexual Reproduction
Overview
Earlier in the chapter we examined the process of asexual reproduction. This section
discusses sexual reproduction, which is a common mechanism employed by eukaryotes to
produce offspring. During sexual reproduction, different types of haploid gametes are produced,
which then fuse with each other via the process of fertilization to begin the life of the new
organism.
It is important to note here that meiosis in animals occurs in conjunction with
gametogenesis. In spermatogenesis, a diploid spermatocyte undergoes meiosis to produce four
haploid spermatids, which then differentiate into mature sperm (Figure 2.14A). In oogenesis, a
diploid oocyte undergoes meiosis to produce one large haploid egg and up to three small haploid
polar bodies (Figure 2.14B). However, in plants gametes are not formed directly from meiosis.
Rather, meiosis produces the gametophyte stage, which in turn produces gametes by mitosis
(Refer to Figure 2.15).
Outline of Key Terms
Gametogenesis
Oogenesis
Spermatogenesis
Plant generations
Sporophyte
Gametophyte
Pollen grain
Embryo sac
Endosperm
Sexual reproduction
Fertilization
Haploid gametes
Isogamous organisms
Heterogamous organisms
Sperm cells
Egg cell (ovum)
Focal Points


Gametogenesis in animals (Figure 2.14)
The formation of gametes in angiosperms, or flowering plants (Figure 2.15)
Exercises and Problems
For questions 1 to 6, complete the sentence with the most appropriate term(s):
1. The process by which gametes are produced is called ________.
2. Some simple eukaryotic species are ________, producing morphologically-similar gametes.
3. Most eukaryotic species are _______, producing two morphologically-different gamete types.
4. In mammals, the female gamete is fertilized at the ________ oocyte stage.
5. In mammals, the immediate precursor of a sperm cell is called the _________.
6. In plants, gametes are produced by the gametophyte via ________ cellular division.
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For questions 7 to 11, use the following key:
a. Statement applies to spermatogenesis only
b. Statement applies to oogenesis only
c. Statement applies to both spermatogenesis and oogenesis
d. Statement applies to neither spermatogenesis nor oogenesis
_____ 7.
Produces haploid cells.
_____ 8.
Produces four functional gametes.
_____ 9.
Produces a single functional gamete.
_____ 10. Occurs in plant species.
_____ 11. Starts from a diploid cell.
For questions 12 to 16, match the term on the left with its correct description on the right.
_____ 12. Embryo sac
a. The female gametophyte.
_____ 13. Gametophyte
b. The haploid generation of the plant.
_____ 14. Endosperm
c. A 3n structure that serves as food-storage for embryo.
_____ 15. Sporophyte
d. The diploid generation of the plant.
_____ 16. Pollen grain
e. The male gametophyte
Chapter Quiz
1. Which of the following is characteristic of meiosis but NOT mitosis?
a. Chromosomes are duplicated prior to division.
b. Sister chromatids are attached to each other at the centromere.
c. Nuclear membrane and nucleolus disappear in prophase.
d. Homologous chromosomes pair.
e. Four identical daughter cells are produced.
2. Which of the following best describes meiosis II as it occurs in humans? (The term n refers to
the number of chromosome sets)
a. 4n → 2n
b. 2n → 1n
c. 2n → 2n
d. 1n → 2n
e. 1n → 1n
3. An organism has 20 pairs of chromosomes at the start of mitosis, how many chromosome pairs
will each cell have at the end of mitosis?
a. 5
b. 10
c. 20
d. 40
e. none of the above
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4. An animal has 40 chromosomes in its somatic cells. A cell in metaphase II of meiosis is
expected to have a total of ______ chromosomes and ______ chromatids.
a. 40
;
20
b. 20
;
20
c. 20
;
40
d. 20
;
10
e. 10
;
20
5. During which of the following stages of the cell cycle is the DNA of the organism replicated?
a. prophase
b. metaphase
c. S phase
d. G2 phase
e. G1 phase
6. Bacteria reproduce by which of the following?
a. mitosis
b. meiosis
c. binary fission
d. all of the above
7. Homologous chromosomes may differ in which of the following?
a. overall length
b. allele composition
c. genetic loci
d. centromere position
e. none of the above
8. During which of the following stages of meiosis does synapsis and crossing over occur?
a. prophase I
b. telophase II
c. prophase II
d. metaphase
e. cytokinesis
9. Which of the following represents the correct sequence of stages during interphase?
a. G1 → G2 → G0 → S
b. G0 → G1 → G2 → S
c. G0 → G1 → S → G2
d. G1 → G0 → S → G2
e. G1 → S → G2 → G0
10. The diploid stage in plants is termed the
a. microphyte.
b. megaphyte
c. gametophyte.
d. sporophyte.
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Answer Key for Study Guide Questions
This answer key provides the answers to the exercises and chapter quiz for this chapter. Answers
in parentheses ( ) represent possible alternate answers to a problem, while answers marked with
an asterisk (*) indicate that the response to the question may vary.
2.1
1. b
2. g
3. f
4. e
5. i
6. f
7. d
8. a
9. a
10. e
11. c
12. d
13. b
14. f
15. F, similar but not
necessarily identical
16. T
17. F, mitochondria and
chloroplasts
18. T
2.2
1. Interphase
2. S phase
3. G2 phase
4. Prophase
5. Prometaphase
6. Metaphase
7. Mitosis
8. M
9. Anaphase
10. Telophase
11. Cytokinesis
12. G1 phase
13. binary fission
14. interphase
15. kinetochore
16. G0
2.3
1. f
2. b
3. e
4. d
5. c
6. a
7. a
8. g
9. a
10. e
11. f
12. b
13. d
14. a
15. c
16. 15-11-12-10-13-14
17. cleavage furrow
18. cell plate
19. 92
20. 46
2.4
1. e
2. a
3. d
4. b
5. c
6. a
7. a
8. b
9. d
10. c
11. a
12. c
13. b
14. b
15. a
16. b
17. 80
18. 27 (128)
19. 7
20. 28
2.5
1. gametogenesis
2. isogamous
3. heterogamous
4. secondary
5. spermatid
6. mitotic
7. c
8. a
9. b
10. d
11. c
12. a
13. b
14. c
15. d
16. e
Quiz
1. d
2. e
3. c
4. c
5. c
6. c
7. b
8. a
9. d
10. d
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