Ch 13 Meiosis and Sexual Life Cycles

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Explain what causes the traits of parents (such as hair color) to show up in their offspring

Parents pass genes to their offspring; the genes program cells to make specific enzymes and other proteins, whose cumulative action produces an individual's inherited traits

What is the original source of variation among the different alleles of a gene?

Mutations in a gene lead to the different versions (alleles) of that gene

The diploid number for fruit flies is 8, and the diploid number for grasshoppers is 46. If no crossing over took place, would the genetic variation among off-spring from a given pair of parents be greater in fruit flies or grasshoppers? Explain

Without crossing over, independent assortment of chromosomes during meiosis I theoretically can generate 2^n possible haploid gametes, and random fertilization can produce 2^n x 2^n possible diploid zygotes. Because the haploid number (n) of grasshoppers is 23 and that of fruit flies is 4, two grasshoppers would be expected to produce a greater variety of zygotes than would two fruit flies.

Under what circumstances would crossing over during meiosis NOT contribute to genetic variation among daughter cells?

If the segments of the maternal and paternal chromatids that undergo crossing over are genetically indetical and thus have the same two alleles for every gene, then the recombinant chromosomes will be genetically equivalent to the parental chromosomes. Crossing over contributes to genetic variation only when it involves the rearrangement of different alleles

A human cell containing 22 autosomes and a Y chromosome is
a. a sperm
b. an egg
c. a zygote
d. a somatic cell of a male
e. a somatic cell of a female


Which life cycle stage is found in plants but not animals?
a. gamete
b. zygote
c. multicellular diploid
d. multicellular haploid
e. unicellular diploid


Homologous chromosomes move toward opposite poles of a dividing cell during
a. mitosis
b. meiosis I
c. meiosis II
d. fertilization
e. binary fission


Meiosis II is similar to mitosis in that
a. sister chromatids separate during anaphase
b. DNA replicates before the division
c. the daughter cells are diploid
d. homologous chromosomes synapse
e. the chromosome number is reduced


If the DNA content of a diploid cell in the G1 phase of the cell cycle is x, then the DNA content of the same cell at metaphase of meiosis I would be
a. .25 x
b. .5 x
c. x
d. 2x
e. 4x


If we continued to follow the cell lineage from the previous question, then the DNA of a single cell at metaphase of meiosis II would be
a. .25 x
b. .5 x
c. x
d. 2x
e. 4x


How many different combinations of maternal and paternal chromosomes can be packaged in gametes made by an organism with a diploid number of 8 (2n=8)
a. 2
b. 4
c. 8
d. 16
e. 32


Many species can reproduce either asexually or sexually. What might be the evolutionary significance of the switch from asexual to sexual reproduction that occurs in some organisms when the environment becomes unfavorable?

In asexual reproduction you produce a genetic clone of the organism. Thus there is no genetic variability. All the genes are the same.
Sexual reproduction and crossing over in Prophase I of meiosis will allow for genetic variability to be produced. This is important evolutionary since this genetic variability will confer selective advantages/disadvantages on different individuals and will drive evolution (survival of the fittest) to adverse environmental changes.

The diagram above (p. 261) represents a meiotic cell in a certain individual. The individual from whom this cell was takekn has inherited different alleles for each gene (freckles and black hair from one parent and no freckles and blond hair from the other). Predict allele combinations in the gametes resulting from this meiotic event (it will help you if you draw out the rest of meiosis, labelling alleles by name). List other possible combinations of these alleles in this individual's gametes.


If a horticulturist breeding gardenias succeeds in having a single plant with a particularly desirable
set of traits, which of the following would be her most probable and efficient route to establishing a
line of such plants?

A) Backtrack through her previous experiments to obtain another plant with the same traits.
B) Breed this plant with another plant with much weaker traits.
C) Add nitrogen to the soil of the offspring of this plant so the desired traits continue.
D) Force the plant to self-pollinate to obtain an identical one.
E) Clone the plant asexually to produce an identical one


The human genome is minimally contained in which of the following?
A) the entire human population
B) every human cell
C) the entire DNA of a single human
D) each human gene
E) each human chromosome


Which of the following defines a genome?
A) a karyotype
B) the complete set of an organism's genes
C) the complete set of an organism's polypeptides
D)representation of a complete set of a cell's polypeptides
E) the complete set of a species' polypeptides


Which of these statements is false?
A) In humans, the 23rd pair, the sex chromosomes, determines whether the person is female
(XX) or male (XY).
B) In humans, each of the 22 maternal autosomes has a homologous paternal chromosome.
C) Sexual life cycles differ with respect to the relative timing of meiosis and fertilization.
D) At sexual maturity, ovaries and testes produce diploid gametes by meiosis.
E) Single, haploid (n) sets of chromosomes in ovum and sperm unite during fertilization,
forming a diploid (2n), single-celled zygote.


Which of the following is an example of alternation of generations?
A) A grandparent and grandchild each have dark hair, but the parent has blond hair.
B) A diploid cell divides by mitosis to produce two diploid daughter cells, which then fuse to
produce a tetraploid cell.
C) A haploid mushroom produces gametes by mitosis, and the gametes undergo fertilization,
which is immediately followed by meiosis.
D) A diploid animal produces gametes by meiosis, and the gametes undergo fertilization to
produce a diploid zygote.
E) A diploid plant (sporophyte) produces, by meiosis, a spore that gives rise to a multicellular,
haploid pollen grain (gametophyte).


A given organism has 46 chromosomes in its karyotype. We can therefore conclude which of the

A) It must be a primate.
B) It must be human.
C) It must be sexually reproducing.
D) Its gametes must have 23 chromosomes.
E) It must be an animal.


A triploid cell contains three sets of chromosomes. If a cell of a usually diploid species with 42
chromosomes per cell is triploid, this cell would be expected to have which of the following?

A) 21 chromosome pairs and 21 unique chromosomes
B) 63 chromosomes in 31 1/2 pairs
C) 63 chromosomes in 21 sets of 3
D) 63 chromosomes, each with three chromatids


If a cell has completed the first meiotic division and is just beginning meiosis II, which of the
following is an appropriate description of its contents?

A) It has half the amount of DNA as the cell that began meiosis.
B) It has half the chromosomes but twice the DNA of the originating cell.
C) It is identical in content to another cell from the same meiosis.
D) It has the same number of chromosomes but each of them has different alleles than another
cell from the same meiosis.
E) It has one-fourth the DNA and one-half the chromosomes as the originating cell.


Which of the following might result in a human zygote with 45 chromosomes?
A) failure of the egg nucleus to be fertilized by the sperm
B) an error in the alignment of chromosomes on the metaphase plate
C) an error in either egg or sperm meiotic anaphase
D) fertilization of a 23 chromosome human egg by a 22 chromosome sperm of a closely related
E) lack of chiasmata in prophase I


The somatic cells of a privet shrub each contain 46 chromosomes. To be as different as they are
from human cells, which have the same number of chromosomes, which of the following must be

A) Privet shrubs must be metabolically more like animals than like other shrubs.
B) Genes of privet chromosomes are significantly different than those in humans.
C) Genes on a particular privet chromosome, such as the X, must be on a different human
chromosome, such as number 18.
D) Privet cells cannot reproduce sexually.
E) Privet sex cells have chromosomes that can synapse with human chromosomes in the


How do cells at the completion of meiosis compare with cells that have replicated their DNA and
are just about to begin meiosis?
A) They have the same number of chromosomes and half the amount of DNA.
B) They have half the number of chromosomes and half the amount of DNA.
C) They have half the amount of cytoplasm and twice the amount of DNA.
D) They have twice the amount of cytoplasm and half the amount of DNA.
E) They have half the number of chromosomes and one-fourth the amount of DNA.


Which of the following occurs in meiosis but not in mitosis?
A) production of daughter cells
B) condensation of chromatin
C) synapsis of chromosomes
D) chromosome replication
E) alignment of chromosomes at the equator


Experiments with cohesins have found that
A) a protein that cleaves cohesins would cause cellular death.
B) cohesins are protected from destruction throughout meiosis I and II.
C) cohesins are cleaved from chromosomes at the centromere before anaphase I.
D) cohesins are protected from cleavage at the centromere during meiosis I.
E) a protein cleaves cohesins before metaphase I.


Which diagram represents anaphase I of meiosis? (see figure)
A) I B) II C) IV D) V E) VI


Which diagram(s) represent anaphase II of meiosis? (see figure)
A) II only
B) III only
C) IV only
D) V only
E) either II or V


17) A certain female's number 12 chromosomes both have the blue gene and number 19 chromosomes
both have the long gene. As cells in her ovaries undergo meiosis, her resulting eggs (ova) may
have which of the following?

A) either one blue or one orange gene in addition to either one long or one short gene
B) one chromosome 12 with one blue gene and one chromosome 19 with one long gene
C) either two number 12 chromosomes with blue genes or two with orange genes
D) either two number 19 chromosomes with long genes or two with short genes


If a female of this species has one chromosome 12 with a blue gene and another chromosome 12
with an orange gene, and has both number 19 chromosomes with short genes, she will produce
which of the following egg types?
A) three-fourths blue long and one-fourth orange short gene eggs
B) only orange short gene eggs
C) one-half blue short and one-half orange short gene eggs
D) only blue short gene eggs
E) three-fourths blue short and one-fourth orange short gene eggs


How is natural selection related to sexual reproduction as opposed to asexual reproduction?
A) Sexual reproduction utilizes far less energy than asexual reproduction.
B) Sexual reproduction results in many new gene combinations, some of which will lead to
differential reproduction.
C) Sexual reproduction results in the most appropriate and healthiest balance of two sexes in the
D) Sexual reproduction allows the greatest number of offspring to be produced.
E) Sexual reproduction results in the greatest number of new mutations.


If the DNA content of a diploid cell in the G1 phase of the cell cycle is x, then the DNA content of
the same cell at metaphase of meiosis I would be
A) x. B) 0.25x. C) 2x. D) 4x. E) 0.5x


If the DNA content of a diploid cell in the G1 phase of the cell cycle is x, then the DNA content of a
single cell at metaphase of meiosis II would be

A) 0.25x. B) 0.5x. C) 2x. D) 4x. E) x.


How many different combinations of maternal and paternal chromosomes can be packaged in
gametes made by an organism with a diploid number of 8 (2n= 8)?
A) 2 B) 4 C) 16 D) 32 E) 8



-Reproductive cells
-contain 23 chromosomes


-discrete units of hereditary information consisting
of a specific nucleotide sequence in DNA (or RNA in some


A specific location along the length of a chromosome is referred to this


When referring to several locations along a chromosome or several chromosomes

Asexual reproduction

A type of reproduction involving only one parent
that produces genetically identical offspring by
budding or by the division of a single cell or the
entire organism into two or more parts.
• Involves mitosis
• Variations between parent and offspring and among
offspring are due to mutations(change in DNA
• Offspring are clones(genetically identical

Sexual reproduction

A type of reproduction in which two parents give
rise to offspring that have unique combinations of
genes inherited from the gametes of the two
• Offspring are genetically differentfrom their parents
and from their siblings.
• Involves meiosis (and mitosis)

Compare somatic and germ line cells

somatic cells= all cells of body except reproductive cells
germ cells= reproductive cells


• Display of metaphase chromosomes, arranged by size
-prepared from isolated somatic cells, treated with a drug to stimulate mitosis
-size of chromosome, position of centromere, pattern of stained bands are determined by a karyotype


-chromosomes 1-22

Sex chromosomes

X and Y chromosomes.
Determine the sex of an individual

Haploid vs diploid

Haploid: contain 1 set (1n) of the genome, 22 autosomes + sex
chromosome = 23 chromosomes

Diploid: contain 2 sets (2n) of the genome, 2
(22 autosomes + sex chromosome)


-Union of gametes, culminating in fusion of their nuclei


-Resulting fertilized egg is dipliod because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines


-The two stage cell division in eukaryotic cells that results in daughter cells with half the chromosome number of the original cell

Draw and label the three main sexual life cycles

(page 252 fig 13.6) animals, plants, and fungi


-multicellular haploid stage in plants and some species of algae


-multicellular diploid stage in plants and some species of algae

What stage of meiosis separates homologous chromosomes?

Meiosis I

Which stage of meosis separates sister chromatids?

Meiosis II

Meiosis I

-includes prophase 1, metaphase 1, anaphase 1, telophase 1 and cytokinesis

Meiosis II

-includes prophase II, metaphase II, anaphase II, telophase II and cytokenisis

Prophase I

-homologs loosely pair along their lengths
-paired homologs become physically connected to eachother along their lengths by a zipper-like protein structure, the synaptonemal complex; state is called synapsis
-crossing over begins during pairing and synaptonemal complex formation
-synapsis has ended and chromosomes in each pair have moved apart slightly
-each homologous pair has one or more x-shaped regions called chiasmata
-centrosome movement, spindle formation, and nuclear envelope breakdown occurs as in mitosis

Metaphase I

Pairs of homologous chromosomes are now arranged at metaphase plate with one chromosome in each pair facing each pole
-both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the other homolog are attached to microtubules from opposite pole

Anaphase I

-breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allows homologs to separate
-homologs move towards opposite poles
-sister chromatid cohesion persists at centromere causing chromatids to move as a unit toward same pole

Telophase and Cytokenisis I

-two haploid cells form; each chromosome still consists of two sister chromatids
-a cleavage furrow forms in animal cells
-no chromosome duplication occurs

Prophase II

-spindle apparatus forms
-in late prophase II, chromosomes still composed of two chromatids associated at centromere, move toward the metaphase II plate

Metaphase II

-chromosomes are positioned at metaphase plate as in mitosis
-because of crossing over in meiosis I, the two sister chromatids of each chromosome are NOT genetically identical
-the kinetochores of sister chromatids are attached to microtubules extending from opposite poles

Anaphase II

-breakdown of protines holding the sister chromatids together at the centromere allows chromatids to separate.
-chromatids move toward opposite poles as individual chromosomes

Telophase II and cytokinesis

-Nuclei form, chromosomes begin decondensing and cytokenisis occurs
-meiotic division of one parent cell produces four daughter cells, each with a haploid set of unduplicated chromosomes
-four daughter cells are genetically distinct from one another and from parent cell

Compare mitosis and meiosis

- DNA replication occurs during interphase before mitosis begins,
-one cell division,
-synapsis of homologous chromosomes does not occur,
-two (diploid) daughter cells produced which are genetically identical to parent cell,
-enables multicellular adult to arise from zygote

-DNA replication occurs during interphase before meiosis I begins, two cell divisions,
-synapsis of homologous chromosomes occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion,
-produces 4 (haploid) daughter cells, containing half as many chromosomes as parent cell,
-genetically different,
-produces gametes; reduces number of chromosome sets by half and introduces genetic variability

synaptonemal complex

-Occurs during prophase I
-zipper-like protein structure...paired homologs become physically connected to each other along their lengths
-state is called synapsis

Crossing over

-A genetic rearrangement between non-sister chromatids involving the exchange of corresponding segments of DNA molecules, begins during pairing and synaptonemal complex formation, and is completed while homologs are in synapsis

When can crossing over result in new combinations of genetic information?

-crossing over produces recombinant chromosomes: individual chromosomes that carry genes (DNA) derived from two different parents
-in a single crossover event, the DNA of 2 non-sister chromatids is broken by specific proteins at precisely corresponding points, and the two segments beyond the crossover point are joined to the other chromatid
-a paternal chromatid is joined to a piece of a maternal chromatid
-produces chromosomes with new combinations of maternal and paternal alleles


-crossing over between nonsister chromatids of a homologous pair of chromosomes can be visualized with the light microscope and is called a chiasma
-many crossing overs, called a chiasmata

At what stage of meiosis does the reduction in chromosome
number occur?

Telophase I of Meiosis I

Describe the movement of the chromosomes during meiosis

Pairs of homologous chromosomes line up at the metaphase plate, with one chromosome in each pair facing each pole
Homologs move toward opposite poles, guided by spindle apparatus
-in meosis II the chromosomes still composed of two chromatids associated at the centromere, move toward metaphase II plate

Independent assortment

The first meiotic division results in the independent
assortment of maternal and paternal chromosomes
to the daughter cells.
• A particular replicated chromosome has a 50:50
(1/2) chance of assorting to a particular daughter
-two choices for each replicated chromosome
-two separate events
-The number of different combinations is given by the formula
-With 23 homologous chromosome pairs (X and Y do
pair during meiosis), there are 223 or about 8 million
different combinations possible
• Thus, each gamete (sperm or ovum) represents only
1 of the 8 million genetically distinct gametes that
an individual can produce!

Random fertilization

-adds to genetic variation arising from meiosis

-fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion diploid combinations

Why does random fertilization increase the amount of diversity produced
by independent

-each male and female gamete represents one of 8.4 million possible chromosome combinations due to independent assortment..fusion of gametes increases this amount of diversity

What is the significance of genetic diversity?

-genetic variation is the raw material for evolution by natural selection
-mutations are the original source of this variation
-natural selection results in the accumulation of genetic variations favored by the environment
-as environment changes, population may survive if in each generation, at least some of the members and cope effectively with new conditions

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