Chapter 9: The Cell Cycle
Telophase
Daughter nuclei are forming. Meanwhile, cytokinesis has started: the cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.
During what phase in the cell cycle would you find the most DNA per cell?
G2
Metaphase
-The centrosomes are now at opposite poles of the cell. -The chromosomes convene at the metaphase plate, a plane that is equidistant between the spindle's two poles. The chromosomes' centromeres lie at the metaphase plate. -For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.
Cytokinesis
-The division of the cytoplasm is usually well under way by late telophase, so the two daughter cells appear shortly after the end of mitosis. -In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two.
The complex of DNA and protein that makes up a eukaryotic chromosome is properly called __________.
Chromatin Explanation: The DNA-protein complex called chromatin is organized into a long, thin fiber. A centromere is the narrow region of the chromosome where the two duplicated chromosomes, sister chromatids, attach to one another. A chromoplast is a pigment-storing organelle and the centrosome is the organelle from which microtubules originate.
Prometaphase
Discrete chromosomes are now visible; each consists of two aligned, identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment.
During interphase, the genetic material of a typical eukaryotic cell is ___________________.
Dispersed in the nucleus as long strands of chromatin.
What human cell would you be unlikely to see divide?
Nerve cell
Which of the following phases of mitosis is essentially the opposite of prometaphase in terms of the nuclear envelope?
Telophase Explanation: In telophase, fragments of the nuclear envelope begin to reassemble, the mitotic spindle breaks down, the chromatin uncoils, and cytokinesis begins.
Anaphase
The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore microtubules shorten.
Prophase
The chromosomes are condensing and the the nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form.
"Cytokinesis" refers to _______________.
The division of the cytoplasm.
Metaphase
The spindle is now complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.
A biochemist measured the amount of DNA in cells growing in the laboratory and found that the quantity of DNA in the cells doubled __________.
between the G1 and G2 phases Explanation: The period between prophase and anaphase of mitosis is characterized by fragmentation of the nuclear envelope, condensation of chromosomes, and their alignment on the metaphase plate. The mitotic (M) phase, separates the duplicated chromosomes from each other. During the period between anaphase and telophase of mitosis, sister chromatids are separated to the opposite poles of the cell and reorganized into new nuclei.
Chromatids are __________.
identical copies of each other if they are part of the same chromosome.
You would know a dividing cell was a plant cell rather than an animal cell if you saw that __________.
it had formed a cell plate. Explanation:
DNA replication occurs in _______________.
the S phase of interphase in both somatic and reproductive cells Explanation: Chromosomes are duplicated only during the S phase ("S" stands for synthesis of DNA) of interphase of the cell cycle. During all three subphases, a cell that will eventually divide grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum. Interphase occurs prior to meiosis and mitosis.
During binary fission in a bacterium __________.
the origins of replication move apart. Explanation: Prokaryotes can undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form two cells. The term binary fission, meaning "division in half," refers to this process. The process in eukaryotes involves mitosis, whereas that in prokaryotes does not. In bacteria, most genes are carried on a single bacterial chromosome that consists of a circular DNA molecule and associated proteins. The process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin of replication, producing two origins. As the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the cell. While the chromosome is replicating, the cell elongates. When replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward, dividing the parent cell into two daughter cells. In this way, each cell inherits a complete genome. Bacteria lack a spindle apparatus and centrioles. In addition, at the origin of replication, the circular DNA molecule is replicated, forming two, not four, identical copies of the bacterial chromosome.
Which of the following represents a mismatch or incorrect description?
metaphase: the nuclear envelope disappears Explanation: In prometaphase, the nuclear envelope fragments and the kinetochores attach to microtubules. During metaphase, the mitotic spindle aligns the chromosomes at the metaphase plate, a region along the equator of the cell.
One event occurring during prophase is __________.
The beginning of the formation of the spindle apparatus.
G2 of Interphase
-A nuclear envelope encloses the nucleus. -The nuclei contains one or more nucleoli (singular, nucleolus) -Two centrosomes have formed by duplication of a single centrosome. Centrosomes are regions in animal cells that organize the microtubules of the spindle. Each centrosome contains two centrioles. -Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed.
Anaphase
-Anaphase is the shortest stage of mitosis, often lasting only a few minutes. -Anaphase begins when the cohesin proteins are cleaved. This allows the two sister chromatids of each pair to part suddenly. Each chromatid thus becomes a full-fledged chromosome. -The two liberated daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first. -The cell elongates as the nonkinetochore microtubules lengthen. -By the end of anaphase, the two ends of the cell have equivalent--and complete--collections of chromosomes.
Prophase
-The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. -The nucleoli disappear. -Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins (sister chromatid cohesion) -The mitotic spindle (named for its shape) begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters. The centrosomes move away from each other, propelled partly by the lengthening microtubules between them.
Prometaphase
-The nuclear envelope fragments. -The microtubules extending from each centrosome can now invade the nuclear area. -The chromosomes have become even more condensed. -Each of the two chromatids of each chromosome now has a kinetochore, a specialized protein structure at the centromere. -Some of the microtubules attach to the kinetochores, becoming kinetochore microtubules, which jerk the chromosomes back and forth. -Nonkinetochore microtubules interact with those from the opposite pole of the spindle.
Telophase
-Two daughter nuclei form in the cell. Nuclear envelopes arise from the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system. -Nucleoli reappear -The chromosomes become less condensed. -Any remaining spindle microtubules are depolymerized. -Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.
In telophase of mitosis, the mitotic spindle breaks down and the chromatin uncoils. This is essentially the opposite of what happens in __________.
Prophase Explanation: During prophase, chromatin condenses into distinct chromosomes and the mitotic spindle begins to form. This structure is responsible for maneuvering the chromosomes around the cell during mitosis.
Which of the following does not occur during mitosis?
Replication of chromosomes Explanation: Mitosis includes five phases during which a variety of events occur. The S phase of interphase is not part of mitosis. During prophase, chromatin condenses into distinct chromosomes and the mitotic spindle begins to form. This structure is responsible for maneuvering the chromosomes around the cell during mitosis. In prometaphase, one event that occurs is that the nuclear envelope fragments. During metaphase, the mitotic spindle aligns the chromosomes at the metaphase plate, a region along the equator of the cell. In anaphase, the mitotic spindle separates the sister chromatids and moves them to opposite ends of the cell. In telophase, the nuclear envelope begins to reform, and cytokinesis begins.
The person credited with first recognizing (in the 1860s) that living cells cannot arise spontaneously, but arise only from previously existing cells, is __________.
Rudolf Virchow
Assume that you are dealing with a species in which the number of chromosomes in each somatic cell is 14. How many sister chromatids are present in the early telophase of mitosis?
0 Explanation: During interphase of the cell cycle, all of a eukaryotic cell's chromosomes are duplicated. Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome. Each sister chromatid has a centromere, a region containing specific DNA sequences where the chromatid is attached most closely to its sister chromatid. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus. For example, the nuclei of human somatic cells (all body cells except the reproductive cells) each contain 46 chromosomes, made up of two sets of 23, one set inherited from each parent. Reproductive cells, or gametes—sperm and eggs—have half as many chromosomes as somatic cells, or one set of 23 chromosomes in humans. In this cell, mitosis starts with 14 chromosomes, each of which consists of two chromatids, because the cell has been through interphase. By telophase, the chromatids have separated and the individual units of DNA (now called chromosomes) are arriving at the poles. There are no chromatids at this point.
A cell biologist carefully measured the quantity of DNA in grasshopper cells growing in cell culture. Cells examined during the G2 phase of the cell cycle contained 200 units of DNA. What would be the amount of DNA at G1 of the cell cycle in one of the grasshopper daughter cells?
100 Units Explanation: The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells. Mitosis is one part of the cell cycle. The mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. Mitotic cell division alternates with a much longer stage called interphase, which often accounts for about 90% of the cycle. During interphase, a cell that is about to divide grows and copies its chromosomes in preparation for cell division. Interphase can be divided into subphases: the G1 phase ("first gap"), the S phase ("synthesis"), and the G2 phase ("second gap"). During all three subphases, a cell that will eventually divide grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum. However, chromosomes are duplicated only during the S phase, which occurs between the two G phases. G2 follows the S phase of interphase. Consequently, DNA has already been replicated. The 200 units of DNA in the sister chromatids of cells in G2 will be divided equally during the M phase, resulting in daughter cells each containing 100 units of DNA, which is the same amount present in the cell during the G1 phase prior to DNA replication. Fifty units would be the quantity of DNA following meiosis. Cell division and DNA replication are very exact. Consequently, the amount of DNA in a cell would never be indicated by a range.
How many maternal chromosomes are present in a somatic human cell not engaged in cell division?
23 chromosomes
If a cell contains 60 chromatids at the start of mitosis, how many chromosomes will be found in each daughter cell at the completion of the cell cycle?
30 chromosomes Explanation: During the cell cycle, all of the chromosomes are duplicated. A cell that enters the cell cycle with 60 chromosomes at the beginning of mitosis would have 30 chromosomes at the end mitosis. Mitosis distributes the chromosomes evenly to the two daughter cells. After mitosis, all eukaryotic cells contain the original number of chromosomes as the parent cell.
If a somatic human cell is just about to divide, it has __________ chromatids.
92 chromatids Explanation: Human somatic cells have 92 chromatids just prior to cell division due to the replication of the 46 chromosomes that occurred during the S phase. The chromatids separate during anaphase of mitosis, resulting in cells that contain 46 chromosomes.
Down syndrome is characterized by cells having three copies of chromosome 21. As a cell in an individual with Down syndrome prepares to enter mitosis, how many chromatids would be present?
94 chromatids Explanation: Down syndrome is characterized by cells having three copies of chromosome 21. As a cell in an individual with Down syndrome prepares to enter mitosis, 94 chromatids would be present. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus. For example, the nuclei of human somatic cells (all body cells except the reproductive cells) each contain 46 chromosomes, made up of two sets of 23, one set inherited from each parent. Reproductive cells, or gametes—sperm and eggs—have half as many chromosomes as somatic cells, or one set of 23 chromosomes in humans. Human somatic cells have 92 chromatids just prior to cell division due to the replication of the 46 chromosomes that occurred during the S phase. Cells from an individual with Down syndrome would have 23 pairs of chromosomes, plus an extra chromosome for a total of 47. At S phase those chromosomes replicate to produce 94 sister chromatids.
Which of the following is involved in the binary fission of bacteria?
Distribution of a copy of the single parental chromosome to each daughter cell Explanation: Prokaryotes can undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form two cells. The term binary fission, meaning "division in half," refers to this process. The process in eukaryotes involves mitosis, whereas that in prokaryotes does not. In bacteria, most genes are carried on a single bacterial chromosome that consists of a circular DNA molecule and associated proteins. The process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin of replication, producing two origins. As the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the cell. While the chromosome is replicating, the cell elongates. When replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward, dividing the parent cell into two daughter cells. In this way, each cell inherits a complete genome. Prophase, prometaphase, metaphase, anaphase, and telophase are all phases of mitosis, which does not occur in bacteria. Because prokaryotes lack a nucleus, disintegration of the nuclear membrane does not occur. Cell plates form in plant cells, not bacteria. Bacteria do not develop a spindle apparatus.
Which of the following processes does not occur in dividing bacteria?
Mitosis Explanation: Prokaryotes can undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form two cells. The term binary fission, meaning "division in half," refers to this process. The process in eukaryotes involves mitosis, whereas that in prokaryotes does not. In bacteria, most genes are carried on a single bacterial chromosome that consists of a circular DNA molecule and associated proteins. The process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin of replication, producing two origins. As the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the cell. While the chromosome is replicating, the cell elongates. When replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward, dividing the parent cell into two daughter cells. In this way, each cell inherits a complete genome. Mitosis does not occur in dividing bacteria. Mitosis might have had its origins in simpler bacterial mechanisms of cell reproduction. Prokaryotes (bacteria) reproduce by a type of cell division called binary fission.