Chapter 6 Glossary, Summary, and Practice Questions

KEY TERMS

anaphase the stage of mitosis during which sister chromatids are separated from each other

binary fission the process of prokaryotic cell division

cell cycle the ordered sequence of events that a cell passes through between one cell division and the next

cell cycle checkpoints mechanisms that monitor the preparedness of a eukaryotic cell to advance through the various cell cycle stages

cell plate a structure formed during plant-cell cytokinesis by Golgi vesicles fusing at the metaphase plate; will ultimately lead to formation of a cell wall to separate the two daughter cells

centriole a paired rod-like structure constructed of microtubules at the center of each animal cell centrosome

cleavage furrow a constriction formed by the actin ring during animal-cell cytokinesis that leads to cytoplasmic division

cytokinesis the division of the cytoplasm following mitosis to form two daughter cells

diploid describes a cell, nucleus, or organism containing two sets of chromosomes (2n)

FtsZ a tubulin-like protein component of the prokaryotic cytoskeleton that is important in prokaryotic cytokinesis (name origin: Filamenting temperature-sensitive mutant Z)

G₀ phase a cell-cycle phase distinct from the G₁ phase of interphase; a cell in G₀ is not preparing to divide

G₁ phase (also, first gap) a cell-cycle phase; first phase of interphase centered on cell growth during mitosis

G₂ phase (also, second gap) a cell-cycle phase; third phase of interphase where the cell undergoes the final preparations for mitosis

gamete a haploid reproductive cell or sex cell (sperm or egg)

gene the physical and functional unit of heredity; a sequence of DNA that codes for a specific peptide or RNA molecule

genome the entire genetic complement (DNA) of an organism

haploid describes a cell, nucleus, or organism containing one set of chromosomes (n)

homologous chromosomes chromosomes of the same length with genes in the same location; diploid organisms have pairs of homologous chromosomes, and the members of each pair come from different parents

interphase the period of the cell cycle leading up to mitosis; includes G₁, S, and G₂ phases; the interim between two consecutive cell divisions

kinetochore a protein structure in the centromere of each sister chromatid that attracts and binds spindle microtubules during prometaphase

locus the position of a gene on a chromosome

metaphase the stage of mitosis during which chromosomes are lined up at the metaphase plate

metaphase plate the equatorial plane midway between two poles of a cell where the chromosomes align during metaphase

mitosis the period of the cell cycle at which the duplicated chromosomes are separated into identical nuclei; includes prophase, prometaphase, metaphase, anaphase, and telophase

mitotic phase the period of the cell cycle when duplicated chromosomes are distributed into two nuclei and the cytoplasmic contents are divided; includes mitosis and cytokinesis

mitotic spindle the microtubule apparatus that orchestrates the movement of chromosomes during mitosis

oncogene a mutated version of a proto-oncogene, which allows for uncontrolled progression of the cell cycle, or uncontrolled cell reproduction

origin the region of the prokaryotic chromosome at which replication begins

prometaphase the stage of mitosis during which mitotic spindle fibers attach to kinetochores

prophase the stage of mitosis during which chromosomes condense and the mitotic spindle begins to form

proto-oncogene a normal gene that controls cell division by regulating the cell cycle that becomes an oncogene if it is mutated

quiescent describes a cell that is performing normal cell functions and has not initiated preparations for cell division

S phase the second, or synthesis phase, of interphase during which DNA replication occurs

septum a wall formed between bacterial daughter cells as a precursor to cell separation

telophase the stage of mitosis during which chromosomes arrive at opposite poles, decondense, and are surrounded by new nuclear envelopes

tumor suppressor gene a gene that codes for regulator proteins that prevent the cell from undergoing uncontrolled division

CHAPTER SUMMARY

The Genome

Prokaryotes have a single loop chromosome, whereas eukaryotes have multiple, linear chromosomes surrounded by a nuclear membrane. Human somatic cells have 46 chromosomes consisting of two sets of 22 homologous chromosomes and a pair of nonhomologous sex chromosomes. This is the 2n, or diploid, state. Human gametes have 23 chromosomes or one complete set of chromosomes. This is the n, or haploid, state. Genes are segments of DNA that code for a specific protein or RNA molecule. An organism’s traits are determined in large part by the genes inherited from each parent, but also by the environment that they experience. Genes are expressed as characteristics of the organism and each characteristic may have different variants called traits that are caused by differences in the DNA sequence for a gene.

The Cell Cycle

The cell cycle is an orderly sequence of events. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages. In eukaryotes, the cell cycle consists of a long preparatory period, called interphase. Interphase is divided into G₁, S, and G₂ phases. Mitosis consists of five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis is usually accompanied by cytokinesis, during which the cytoplasmic components of the daughter cells are separated either by an actin ring (animal cells) or by cell plate formation (plant cells).

Each step of the cell cycle is monitored by internal controls called checkpoints. There are three major checkpoints in the cell cycle: one near the end of G₁, a second at the G₂–M transition, and the third during metaphase.

Cancer and the Cell Cycle

Cancer is the result of unchecked cell division caused by a breakdown of the mechanisms regulating the cell cycle. The loss of control begins with a change in the DNA sequence of a gene that codes for one of the regulatory molecules. Faulty instructions lead to a protein that does not function as it should. Any disruption of the monitoring system can allow other mistakes to be passed on to the daughter cells. Each successive cell division will give rise to daughter cells with even more accumulated damage. Eventually, all checkpoints become nonfunctional, and rapidly reproducing cells crowd out normal cells, resulting in tumorous growth.

Prokaryotic Cell Division

In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and each copy is allocated into a daughter cell. The cytoplasmic contents are also divided evenly to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome and no nucleus. Therefore, mitosis is not necessary in bacterial cell division. Bacterial cytokinesis is directed by a ring composed of a protein called

FtsZ. Ingrowth of membrane and cell-wall material from the periphery of the cells results in a septum that eventually forms the separate cell walls of the daughter cells.

ART CONNECTION QUESTIONS

• Figure 6.4 Which of the following is the correct order of events in mitosis?
• Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus re-forms and the cell divides. The sister chromatids separate.
• The kinetochore becomes attached to the mitotic spindle. The sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus re-forms and the cell divides.

REVIEW QUESTIONS

• A diploid cell has the number of chromosomes as a haploid cell.
• one-fourth
• one-half
• twice
• four times
• An organism’s traits are determined by the specific combination of inherited .
• cells
• genes
• proteins
• chromatids
• Chromosomes are duplicated during what portion of the cell cycle?
• G₁ phase
• S phase
• prophase
• prometaphase
• Separation of the sister chromatids is a characteristic of which stage of mitosis?
• prometaphase
• metaphase
• anaphase
• telophase
• The individual chromosomes become visible with a light microscope during which stage of mitosis?
• prophase
• prometaphase
• metaphase
• anaphase
• What is necessary for a cell to pass the G₂ checkpoint?
• 
  The kinetochore becomes attached to metaphase plate. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus re-forms and the cell divides.
• The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. The kinetochore breaks apart and the sister chromatids separate. The nucleus re-forms and the cell divides.

• cell has reached a sufficient size
• an adequate stockpile of nucleotides
• accurate and complete DNA replication
• proper attachment of mitotic spindle fibers to kinetochores
• are changes to the nucleotides in a segment of DNA that codes for a protein.
• Proto-oncogenes
• Tumor suppressor genes
• Gene mutations
• Negative regulators
• A gene that codes for a positive cell cycle regulator is called a(n) .
• kinase inhibitor
• tumor suppressor gene
• proto-oncogene
• oncogene
• Which eukaryotic cell-cycle event is missing in binary fission?
• cell growth
• DNA duplication
• mitosis
• cytokinesis
• FtsZ proteins direct the formation of a that will eventually form the new cell walls of the daughter cells.
• contractile ring
• cell plate
• cytoskeleton
• septum

CRITICAL THINKING QUESTIONS

• Compare and contrast a human somatic cell to a human gamete.

• Describe the similarities and differences between the cytokinesis mechanisms found in animal cells versus those in plant cells.
• Outline the steps that lead to a cell becoming cancerous.
• 
  Explain the difference between a proto-oncogene and a tumor suppressor gene.
• Name the common components of eukaryotic cell division and binary fission.