Chapter 13 - Meiosis and Sexual Life Cycles

Question 1

Question 2

a. If 2n = 14, how many chromosomes will be present in the somatic cells of an animal? _______

How many chromosomes will be found in gametes? _______

b. If n = 14, how many chromosomes will be found in diploid somatic cells? _______

How many sets of homologous chromosomes will be found in gametes? _______

c. If 2n = 28, how many chromatids will be found in a cell in which DNA synthesis has occurred prior to cell division? ______

What is the difference between sister and nonsister chromatids?

Answer 2

a. 14 ; 7
b. 28 ; 1
c. 56. Sister chromatids are produced when a chromosome replicates. They are joined at the centromere and attached along their lengths. Nonsister chromatids are found on different chromosomes of a homologous pair (homologs).

Question 3

Complete this diagram of sexual life cycles by filling in the type of cell division, the type of cells, and the ploidy (n or 2n).

Answer 3

a. meiosis
b. fertilization
c. zygote

Question 4

Complete this diagram of sexual life cycles by filling in the type of cell division, the type of cells, and the ploidy (n or 2n).

Answer 4

d. mitosis
e. gametes
f. fertilization
g. zygote
h. meiosis
i. spores

Question 5

Complete this diagram of sexual life cycles by filling in the type of cell division, the type of cells, and the ploidy (n or 2n).

Answer 5

j. gametes
k. fertilization
l. 2n
m. zygote
n. meiosis

Question 6

The following diagrams represent some of the stages of meiosis (not in the right order). Label these stages.

Now place these stages in the proper sequence.

_____ _____ _____ _____ _____

Answer 6

a. metaphase II
b. prophase I
c. anaphase I
d. interphase
e. metaphase I
f. anaphase II

Proper sequence: d, b, e, c, a, f.

Question 7

a. How many different assortments of maternal and paternal chromosomes are possible in a human gamete?
b. In a human zygote, how many diploid combinations of parental chromosomes are possible?
c. Why does the number in part b still underestimate the possible genetic variations on a zygote?

Answer 7

a. 2²³, approximately 8.4 million
b. 70 trillion (2²³ x 2²³)
c. This number of combinations does not take into account the additional variation of recombinant chromosomes produced by crossing over.

Question 8

Label the following diagram to review the terms that describe replicated chromosomes in a diploid cell.

Answer 8

a. sister chromatids
b. centromere
c. pair of homologous chromosomes
d. nonsister chromatids

Question 9

Describe the key events of these stages of meiosis.

a. Interphase
b. Prophase I
c. Metaphase I
d. Anaphase I
e. Metaphase II
f. Anaphase II

Answer 9

a. Chromosome replication, sister chromatids attached at centromere and by sister chromatid cohesion
b. Synapsis of homologous pairs (synaptonemal complex), crossing over evidental at chiasmata
c. Homologous pairs line up independently at metaphase plate.
d. Homologous pairs of chromosomes separate and homologs move toward opposite poles, sister chromatids remain attached at centromere.
e. Haploid set of chromosomes, each consisting of two sister chromatids, aligns at metaphase plate; sister chromatids not identical due to crossing over.
f. Sister chromatids separate and move to opposite poles as chromosomes.

Question 10

Create a concept map to help you organize your understanding of the similarities and differences between mitosis and meiosis.

Answer 10

Question 11

What is a karyotype?

a. a genotype of an individual
b. a pictorial display of an individual’s chromosomes
c. a blood type determination of an individual
d. a unique combination of chromosomes found in a gamete
e. a species-specific diploid number of chromosomes

Answer 11

b. a pictorial display of an individual’s chromosomes

Question 12

A synaptonemal complex would be found during

a. prophase of mitosis.
b. fertilization of syngamy of gametes.
c. metaphase II of meiosis.
d. prophase of meiosis I.
e. anaphase I of meiosis.

Answer 12

d. prophase of meiosis I.

Question 13

What are autosomes?

a. sex chromosomes
b. chromosomes that occur singly
c. chromosomal abnormalities that result in genetic defects
d. chromosomes found in mitochondria and chloroplasts
e. none of the above

Answer 13

e. none of the above

Question 14

Which of the following statements is not true?

a. The restoration of the diploid chromosomes number after halving in meiosis is due to fertilization.
b. Sister chromatid cohesion at areas where crossing over has occurred holds homologous chromosomes together until anaphase I.
c. Recombinant chromosomes are produced when both maternal and paternal chromosomes independently assort into the same gamete.
d. In mitosis, separation of sister chromatids in anaphase results in two identical cells.
e. Separation of sister chromatids in anaphase II does not usually result in two identical cells.

Answer 14

c. Recombinant chromosomes are produced when both maternal and paternal chromosomes independently assort into the same gamete.

Question 15

During the first meiotic division (meiosis I),

a. homologous chromosomes separate.
b. the chromosomes number is reduced in half.
c. crossing over between nonsister chromatids occurs.
d. paternal and maternal chromosomes assort randomly.
e. all of the above occur.

Answer 15

e. all of the above occur.

Question 16

A cell with a diploid number of 6 could produce gametes with how many different combinations of maternal and paternal chromosomes?

a. 6
b. 8
c. 12
d. 64
e. 128

Answer 16

b. 8

Question 17

The DNA content of a diploid cell is measured in the G₁ phase. After meiosis I, the DNA content of one of the two cells produced would be

a. equal to that of the G₁ cell.
b. twice that of the G₁ cell.
c. one-half that of the G₁ cell.
d. one-fourth that of the G₁ cell.
e. impossible to estimate due to independent assortment of homologous chromosomes.

Answer 17

a. equal to that of the G₁ cell.

Question 18

In most fungi and some protists,

a. the zygote is the only haploid stage.
b. gametes are formed by meiosis.
c. the multicellular organism is haploid.
d. the gametophyte generation produces gametes by mitosis.
e. reproduction is exclusively asexual.

Answer 18

c. the multicellular organism is haploid.

Question 19

In the alternation of generations found in plants,

a. the sporophyte generations produces spores by mitosis.
b. the gametophyte generation produces gametes by mitosis.
c. the zygote will develop into a sporophyte generation by meiosis.
d. spores develop into the haploid sporophyte generation.
e. the gametophyte generation produces spores by meiosis.

Answer 19

b. the gametophyte generation produces gametes by mitosis.

Question 20

Which of the following is least likely to be a source of genetic variation in sexually reproducing organisms?

a. crossing over
b. replication of DNA during S phase before meiosis I.
c. independent assortment of chromosomes
d. random fertilization of gametes
e. mutation

Answer 20

b. replication of DNA during S phase before meiosis I.

Question 21

Meiosis II is similar to mitosis because

a. sister chromatids separate.
b. homologous chromosomes separate.
c. DNA replication precedes the division.
d. they both take the same amount of time.
e. haploid cells are almost always produced.

Answer 21

a. sister chromatids separate.

Question 22

Pairs of homologous chromosomes

a. have identical DNA sequences in their genes.
b. have genes for the same characters at the same loci.
c. are found in gametes.
d. separate in meiosis II.
e. have all of the above characteristics.

Answer 22

b. have genes for the same characters at the same loci.

Question 23

Asexual reproduction of a diploid organism would

a. be impossible.
b. involve meiosis.
c. produce identical offspring.
d. show variation among sibling offspring.
e. involve spores produced by meiosis.

Answer 23

c. produce identical offspring.

Question 24

In a sexually reproducing species with a diploid number of 8, how many different diploid combinations of chromosomes would be possible in the offspring?

a. 8
b. 16
c. 64
d. 256
e. 512

Answer 24

d. 256

Question 25

The calculation of offspring in question 14 includes only variation resulting from

a. crossing over.
b. random fertilization.
c. independent assortment of chromosomes.
d. a, b, and c.
e. only b and c.

Answer 25

e. only b and c.

Question 26

How many chromatids are present in metaphase II in a cell undergoing meiosis from an organism in which 2n = 24?

a. 12
b. 24
c. 36
d. 48
e. 96

Answer 26

b. 24

Question 27

Which of the following would not be considered a haploid cell?

a. daughter cell after meiosis II
b. gamete
c. daughter cell after mitosis in gametophyte generation of a plant
d. cell in prophase I
e. cell in prophase II

Answer 27

d. cell in prophase I

Question 28

Which of the following is not true of homologous chromosomes?

a. They behave independently in mitosis.
b. They synapse during the S phase of meiosis.
c. They travel together to the metaphase plate in meiosis I.
d. They are held together during synapsis by a synaptonemal complex.
e. Crossing over between nonsister chromatids of homologous chromosomes is indicated by the presence of chiasmata.

Answer 28

b. They synapse during the S phase of meiosis.

Question 29

Which of the following describes why or how recombinant chromosomes add to genetic variability?

a. They are formed as a result of random fertilization when two sets of chromosomes combine in a zygote.
b. They are the result of mutations that change alleles.
c. They randomly orient during metaphase II and the nonequivalent sister chromatids separate in anaphase II.
d. Genetic material from two parents is combined on the same chromosome.
e. Both c and d are true.

Answer 29

e. Both c and d are true.

Question 30

Genetics is the scientific study of the transmission of traits from parents to offspring (heredity) and the variation between and with generations.

Offspring aquire genes from parent sby inheriting chromosomes.

Question 31

A cell in G₂ before meiosis compared with one of the four cells produced by that meiotic division has

a. twice as much DNA and twice as many chromosomes.
b. four times as much DNA and twice as many chromoromes.
c. four times as much DNA and four times as many chromosomes.
d. half as much DNA but the same number of chromosomes.
e. half as much DNA and half as many chromosomes.l

Answer 31

b. four times as much DNA and twice as many chromoromes.

Question 32

The inheritance of traits from parents to offspring involves the transmission of discrete units of information coded in segments of DNA known as genes. Most genes contain instructions for synthesizing enzymes and other proteins that then guide the development of inherited traits.

Precise copies of an organism’s genes are packaged into gametes (sperm and eggs). Upon fertilization, genes from both parents are passed on to offspring. The DNA of a eukaryotic cell is packaged along with various proteins into a species-specific number of chromosomes. The entire complement of DNA is called the genome. A gene’s locus is its location on a chromosome.

Question 33

In asexual reproduction, a single parent passes copies of all its genes to its offspring. A clone is a group of genetically identical offspring. In sexual reproduction, an individual receives a unique combination of genes inherited from two parents.

Question 34

In somatic cells, there are two chromosomes of each type, known as homologous chromosomes or homologs. A gene controlling a particular character is found at the same locus on each chromosome of a homologous pair.

A karyotype is an ordered display of an individual’s condensed chromosomes. Isolate somatic cells are stimulated to undergo mitosis, arrested in metaphase, and stained. A computer uses a digital photograph to arrange chromosomes into homologous pairs by size and shape.

Sex chromosomes detemine the sex of a person: Females have two homologous X chromosomes; males have nonhomologous X and Y chromosomes. Chromosomes ohter than the sex chromosomes are called autosomes.

Somatic cells contain a set of chromosomes from each parent. These are diploid cells, each with a diploid number of chromosomes, abbreviated 2n. Gametes, egg and sperm, are haploid cells and contain a single set of chromosomes. The haploid number (n) of chromosomes for humans is 23.

Question 35

Fertilization, or fusion of sperm and ovum (egg), produces a zygote containing both paternal and maternal set of chromosomes. The diploid zygote then divides by mitosis to produce the somatic cells of the body, all of which contain the diploid number (2n) of chromosomes.

Meiosis is a special type of cell division that halves the chromosome number and provides a haploid set of chromosomes to each gamete. Gametes are produced by meiosis from specialized germ cells in the gonads. An alternation between diploid and haploid numbers of chromosomes, involving the processes of fertilization and meiosis, is characteristic of sexually reproducing organisms.

Question 36

In most animals, meiosis occurs in the formation of gametes, which are the only haploid cells in the life cycle.

Plants and some species of algae have a type of life cycle called alternation of generations that includes both diploid and haploid multicellular stages. The multicellular diploid sporophyte produces haploid spores by meiosis. These spores undergo mitosis and develop into a multicellular haploid organism, the gametophyte, which produces gametes by mitosis. Gametes fuse to form a diploid zygote, which develops into the next sporophyte generation.

In most fungi and some protists, the only diploid stage is the zygote. Meiosis occurs after the gametes fuse, producing haploid cells that divide by mitosis to create a unicellular or multicellular haploid organism. Gametes are produced by mitosis in these organisms.

Question 37

Meisosis reduces the number of chromosome sets from diploid to haploid.

In meiosis, chromosome replication is followed by two consecutive cell divisions: meiosis I and meiosis II, producing four haploid daughter cells, each with one set of chromosomes.

Question 38

In interphase, each chromosome replicates, producing two genetically identical sister chromatids that remain attached at the centromere and along their length (called sister chromatid cohesion). During prophase I, homologous chromosomes synapse. Crossing over may occur between nonsister chromatids, forming chiasmata.

in metaphase I, homologous pairs line up on the metaphase plate with their kinetochores attached to spindle fibers from opposite poles. Each pair separates in anaphase I, with one homolog moving toward each pole. In telophase I, a haploid set of chromosomes, each composed of two sister chromatids, reaches each pole. Cytokinesis usually occurs during telophase I. There is no replication of genetic material prior to the second division of meiosis.

Meiosis II looks liek a regular mitotic division, in which chromosomes line up individually on the metaphase plate, and sister chromatids separate and move apart in anaphase II. These sister chromatids, however, are not genetically identical due to crossing over in prophase I. At the end of telophase II, there are four haploid daughter cells.

Question 39

Mitosis produces daughter cells that are genetically identical to the parent cell. Meiosis produces haploid cells that differ genetically from their parent cell and from each other.

The three unique events that produce this result occur during meiosis I: In prophase I, when homologous chromosomes are held together along thier lengths by the synaptonemal complex (synapsis), genetic material is rearranged by crossing over between nonsister chromatids. Crossovers are visible later in this stage as X-shaped regions called chiasmata where sister chromatid cohesion holds the original sister chromatids together. In metaphase I, chromosomes line up in pairs, not as individuals, on the metaphase plate. During anaphase I, the homologous pairs separate and one homolog (with sister chromatids still attached at the centromere) goes to each pole.

Sister chromatids are held together by protein complexes called cohesins. Enzymes cleave the cohesins along the arms of sister chromatids so that homologs can separate in anaphase I. A protein named shugoshin protects the cohesins at the centromere from cleavage until anaphase II, when the sister chromatids separate.

Meiosis I is called a reductional division because it reduces the chromosome sets from two (diploid) to one (haploid). The sister chromatids of homolog do not separate until meiosis II, sometimes called the equational division.

Question 40

Mutations that result in different alleles are the original source of genetic variation. The three mechanisms that generate genetic variation in sexual reproduction are independent assortment of chromosomes, crossing over, and random fertilization.

Each homologous pair lines up independently at the metaphase plate — the orientation of the maternal and paternal chromosomes is random. The number of possible combinations of maternal and paternal chromosomes in gametes is 2ⁿ, where n is the haploid number.

In prophase I, homologous segments of nonsister chromatids are exchanged by crossing over, froming recombinant chromosomes with new genetic cominations of maternal and paternal genes on the same chromosome. The no-longer-equivalent sister chromatids assort independently during meiosis II.

The random nature of fertilization adds to the genetic variability established in meiosis.

Question 41

In Darwin’s theory of evolution by natural selection, genetic variations present in a population result in adaptation as the individuals with the variations best suited to an environment produce the most offspring. The process of sexual reproduction and mutation are the sources of this variation.