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.
A diploid cell has ________ the number of chromosomes as a haploid cell.
D. four times
An organism’s traits are determined by the specific combination of inherited ________.
Compare and contrast a human somatic cell to a human gamete.
Human somatic cells have 46 chromosomes, including 22 homologous pairs and one pair of nonhomologous sex chromosomes. This is the 2n, or diploid, condition. Human gametes have 23 chromosomes, one each of 23 unique chromosomes. This is the n, or haploid, condition.
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.
Figure 6.2.2 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.
- 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.
D. 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 reforms and the cell divides.
Chromosomes are duplicated during what portion of the cell cycle?
A. G1 phase
B. S phase
Separation of the sister chromatids is a characteristic of which stage of mitosis?
The individual chromosomes become visible with a light microscope during which stage of mitosis?
What is necessary for a cell to pass the G2 checkpoint?
A. cell has reached a sufficient size
B. an adequate stockpile of nucleotides
C. accurate and complete DNA replication
D. proper attachment of mitotic spindle fibers to kinetochores
Describe the similarities and differences between the cytokinesis mechanisms found in animal cells versus those in plant cells.
There are very few similarities between animal cell and plant cell cytokinesis. In animal cells, a ring of actin fibers is formed around the periphery of the cell at the former metaphase plate. The actin ring contracts inward, pulling the plasma membrane toward the center of the cell until the cell is pinched in two. In plant cells, a new cell wall must be formed between the daughter cells. Because of the rigid cell walls of the parent cell, contraction of the middle of the cell is not possible. Instead, a cell plate is formed in the center of the cell at the former metaphase plate. The cell plate is formed from Golgi vesicles that contain enzymes, proteins, and glucose. The vesicles fuse and the enzymes build a new cell wall from the proteins and glucose. The cell plate grows toward, and eventually fuses with, the cell wall of the parent cell.
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 damage.
________ are changes to the nucleotides in a segment of DNA that codes for a protein.
B. Tumor suppressor genes
C. Gene mutations
D. Negative regulators
A gene that codes for a positive cell cycle regulator is called a(n) ________.
A. kinase inhibitor
B. tumor suppressor gene
Outline the steps that lead to a cell becoming cancerous.
If one of the genes that produce regulator proteins becomes mutated, it produces a malformed, possibly non-functional, cell-cycle regulator. This increases the chance that more mutations will be left unrepaired in the cell. Each subsequent generation of cells sustains more damage. The cell cycle can speed up as a result of loss of functional checkpoint proteins. The cells can lose the ability to self-destruct.
Explain the difference between a proto-oncogene and a tumor suppressor gene.
A proto-oncogene is the segment of DNA that codes for one of the positive cell-cycle regulators. If that gene becomes mutated to a form that is overactive, it is considered an oncogene. A tumor suppressor gene is a segment of DNA that codes for one of the negative cell-cycle regulators. If that gene becomes mutated to a form that is underactive, the cell cycle will run unchecked.
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.
Which eukaryotic cell-cycle event is missing in binary fission?
A. cell growth
B. DNA duplication
FtsZ proteins direct the formation of a ________ that will eventually form the new cell walls of the daughter cells.
A. contractile ring
B. cell plate
Name the common components of eukaryotic cell division and binary fission.
The common components of eukaryotic cell division and binary fission are DNA duplication, segregation of duplicated chromosomes, and the division of the cytoplasmic contents.