Chapter 7 Glossary, Summary, and Practice Questions

KEY TERMS

alternation of generations a life-cycle type in which the diploid and haploid stages alternate

aneuploid an individual with an error in chromosome number; includes deletions and duplications of chromosome segments

autosome any of the non-sex chromosomes

chiasmata (singular = chiasma) the structure that forms at the crossover points after genetic material is exchanged

chromosome inversion the detachment, 180° rotation, and reinsertion of a chromosome arm

crossing over (also, recombination) the exchange of genetic material between homologous chromosomes resulting in chromosomes that incorporate genes from both parents of the organism forming reproductive cells

diploid-dominant a life-cycle type in which the multicellular diploid stage is prevalent euploid an individual with the appropriate number of chromosomes for their species fertilization the union of two haploid cells typically from two individual organisms gametophyte a multicellular haploid life-cycle stage that produces gametes

germ cell a specialized cell that produces gametes, such as eggs or sperm

haploid-dominant a life-cycle type in which the multicellular haploid stage is prevalent

interkinesis a period of rest that may occur between meiosis I and meiosis II; there is no replication of DNA during interkinesis

karyogram the photographic image of a karyotype

karyotype the number and appearance of an individuals chromosomes, including the size, banding patterns, and centromere position

life cycle the sequence of events in the development of an organism and the production of cells that produce offspring

meiosis a nuclear division process that results in four haploid cells

meiosis I the first round of meiotic cell division; referred to as reduction division because the resulting cells are haploid

meiosis II the second round of meiotic cell division following meiosis I; sister chromatids are separated from each other, and the result is four unique haploid cells

monosomy an otherwise diploid genotype in which one chromosome is missing

nondisjunction the failure of synapsed homologs to completely separate and migrate to separate poles during the first cell division of meiosis

polyploid an individual with an incorrect number of chromosome sets

recombinant describing something composed of genetic material from two sources, such as a chromosome with both maternal and paternal segments of DNA

reduction division a nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus; meiosis I is a reduction division

somatic cell all the cells of a multicellular organism except the gamete-forming cells

sporophyte a multicellular diploid life-cycle stage that produces spores

synapsis the formation of a close association between homologous chromosomes during prophase I

tetrad two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I

translocation the process by which one segment of a chromosome dissociates and reattaches to a different, nonhomologous chromosome

trisomy an otherwise diploid genotype in which one entire chromosome is duplicated

X inactivation the condensation of X chromosomes into Barr bodies during embryonic development in females to compensate for the double genetic dose

CHAPTER SUMMARY

Sexual Reproduction

Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces genetically unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are three main categories of life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and alternation of generations, demonstrated by plants and some algae.

Meiosis

Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. The process that results in haploid cells is called meiosis. Meiosis is a series of events that arrange and separate chromosomes into daughter cells. During the interphase of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four haploid daughter cells, each with half the number of chromosomes as the parent cell. During meiosis, variation in the daughter nuclei is introduced because of crossover in prophase I and random alignment at metaphase I. The cells that are produced by meiosis are genetically unique.

Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell. Meiotic divisions are two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set rather than the two sets the parent cell had. The main differences between the processes occur in the first division of meiosis. The homologous chromosomes separate into different nuclei during meiosis I causing a reduction of ploidy level. The second division of meiosis is much more similar to a mitotic division.

Errors in Meiosis

The number, size, shape, and banding pattern of chromosomes make them easily identifiable in a karyogram and allow for the assessment of many chromosomal abnormalities. Disorders in chromosome number, or aneuploidies, are typically lethal to the embryo, although a few trisomic genotypes are viable. Because of X inactivation, aberrations in sex chromosomes typically have milder effects on an individual. Aneuploidies also include instances in which segments of a chromosome are duplicated or deleted. Chromosome structures also may be rearranged, for example by inversion or translocation. Both of these aberrations can result in negative effects on development, or death. Because they force chromosomes to assume contorted pairings during meiosis I, inversions and translocations are often associated with reduced fertility because of the likelihood of nondisjunction.

ART CONNECTION QUESTIONS

• Figure 7.2 If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

REVIEW QUESTIONS

• What is a likely evolutionary advantage of sexual reproduction over asexual reproduction?
• 
  sexual reproduction involves fewer steps

• less chance of using up the resources in a given environment
• sexual reproduction results in greater variation in the offspring
• sexual reproduction is more cost-effective
• Which type of life cycle has both a haploid and diploid multicellular stage?
• an asexual life cycle
• diploid-dominant
• haploid-dominant
• alternation of generations
• Which event leads to a diploid cell in a life cycle?

• meiosis
• fertilization
• alternation of generations
• mutation
• Meiosis produces daughter cells.
• two haploid
• two diploid
• four haploid
• four diploid
• At which stage of meiosis are sister chromatids separated from each other?
• prophase I
• prophase II
• anaphase I
• anaphase II
• The part of meiosis that is similar to mitosis is

.

CRITICAL THINKING QUESTIONS

• Explain the advantage that populations of sexually reproducing organisms have over asexually reproducing organisms?
• Describe the two events that are common to all sexually reproducing organisms and how they fit into the different life cycles of those organisms.
• Explain how the random alignment of homologous chromosomes during metaphase I contributes to variation in gametes produced by meiosis.
• 
  meiosis I
• anaphase I
• meiosis II
• interkinesis
• If a muscle cell of a typical organism has 32 chromosomes, how many chromosomes will be in a gamete of that same organism?
• 8
• 16
• 32
• 64
• The genotype XXY corresponds to:
• Klinefelter syndrome
• Turner syndrome
• Triplo-X
• Jacob syndrome
• Abnormalities in the number of X chromosomes tend to be milder than the same abnormalities in autosomes because of .
• deletions
• nonhomologous recombination
• synapsis
• X inactivation
• Aneuploidies are deleterious for the individual because of what phenomenon?
• nondisjunction
• gene dosage
• meiotic errors
• X inactivation

• In what ways is meiosis II similar to and different from mitosis of a diploid cell?
• Individuals with trisomy 21 are more likely to survive to adulthood than individuals with trisomy 18. Based on what you know about aneuploidies from this module, what can you hypothesize about chromosomes 21 and 18?