CHAPTER SUMMARY

11.1 DURING CELL DIVISION, A SINGLE PARENTAL CELL DIVIDES INTO TWO DAUGHTER CELLS.

11.2 MITOTIC CELL DIVISION IS THE BASIS OF ASEXUAL REPRODUCTION IN UNICELLULAR EUKARYOTES AND THE PROCESS BY WHICH CELLS DIVIDE IN MULTICELLULAR EUKARYOTES.

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11.3 MEIOTIC CELL DIVISION IS ESSENTIAL FOR SEXUAL REPRODUCTION, THE PRODUCTION OF OFFSPRING THAT COMBINES GENETIC MATERIAL FROM TWO PARENTS.

11.4 THE CELL CYCLE IS REGULATED SO THAT CELL DIVISION OCCURS ONLY AT APPROPRIATE TIMES.

11.5 CANCER IS UNCONTROLLED CELL DIVISION THAT USUALLY RESULTS FROM MUTATIONS IN GENES THAT CONTROL CELL DIVISION.

Self-Assessment Question 1

Compare and contrast the ways in which prokaryotic cells and eukaryotic cells divide.

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Model Answer:

Prokaryotic cells reproduce through a process called binary fission. During this process, the cell replicates its DNA, increases in size, and divides into two daughter cells, each having one copy of the parental DNA. Eukaryotic cells go through a similar process, albeit more complex, called mitotic cell division. In this process, cells first replicate their chromosomes in the nucleus. The nuclear envelop then dissolves and each pair of chromosomes are divided by connecting to the mitotic spindle. Once the two full sets of chromosomes are separated, a nuclear envelope forms around each one. The cell then goes through a process called cytokinesis, where it is split into two new daughter cells.

Self-Assessment Question 2

Describe three situations in which mitotic cell division occurs.

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Model Answer:

Three situations in which mitotic cell division occurs are in the development of multicellular organisms, maintenance and repair of organs and tissues, and asexual reproduction of unicellular eukaryotes.

Self-Assessment Question 3

Name the five steps of mitosis, and draw the changes in the structure and position of the chromosomes at each step.

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Model Answer:

The five steps in mitosis are: (1) Prophase (chromosomes condense, centrosomes radiate microtubules and migrate to opposite poles), (2) Prometaphase (microtubules of the mitotic spindle attach to chromosomes), (3) Metaphase (chromosomes align in center of cell), (4) Anaphase (sister chromatids separate and travel to opposite poles), and (5) Telophase (nuclear envelope re-forms and chromosomes decondense).

See Figure 11.5 for an illustration of the changes in the chromosomes at each step.

Self-Assessment Question 4

Describe how chromosomes behave in meiosis. Be able to state when chromosomes are duplicated (forming sister chromatids) and when they are not duplicated.

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Model Answer:

Meiosis basically goes through two cell-division cycles called meiosis I and meiosis II. In meiosis I, chromosomes are duplicated in Prophase I forming sister chromatids. In Anaphase I, homologous chromosomes separate into two different daughter cells. During meiosis II, the chromosomes do not duplicate and instead the sister chromatids are separated into two more daughter cells. The end result of this process is four daughter cells, each having unique genetic information.

Self-Assessment Question 5

Compare and contrast mitotic cell division and meiotic cell division in terms of number of products, number of cell divisions, and processes unique to each.

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Model Answer:

Mitosis goes through one round of DNA synthesis to produce two genetically identical daughter cells, each with 46 chromosomes. Meiosis also goes through one round of DNA synthesis but goes through two rounds of cell division to produce four genetically different daughter cells, with 23 chromosomes each. See Figure 11.12.

Self-Assessment Question 6

Name two ways in which meiotic cell division creates genetic diversity, and explain how each occurs.

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Model Answer:

Genetic diversity is created in meiotic cell division by the crossing over of chromosomes and random alignment of bivalents in metaphase I. Crossing over occurs when homologous chromosomes of maternal origin and paternal origin undergo an exchange of DNA segments, thus creating genetic diversity. Random alignment of bivalents leads to a chromosome set that is a random mix of maternal and paternal homologs.

Self-Assessment Question 7

Explain how cytokinesis differs between animal and plant cells.

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Model Answer:

In animal cells, cytokinesis involves a contractile ring made of actin. In plant cells, it involves the growth of a new cell wall called a cell plate.

Self-Assessment Question 8

Describe the roles of cyclins and cyclin–dependent kinases in the cell cycle.

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Model Answer:

In the cell cycle, cyclins bind to and activate cyclin-dependent kinases (CDKs). Once activated, the CDKs then phosphorylate target proteins involved in promoting cell division. Once activation has occurred, the CDK dissociates from the cyclin and the cyclin degrades.

Self-Assessment Question 9

Give three examples of checkpoints that the cell monitors before proceeding through the cell cycle.

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Model Answer:

Three examples of checkpoints during the cell cycle are (1) Spindle assembly checkpoint, which happens before anaphase and ensures that all chromosomes are attached to the spindles; (2) DNA damage checkpoint, which happens in the G1 phase and checks for DNA damage; and (3) DNA replication checkpoint, which happens in G2 and makes sure all the DNA is replicated.

Self-Assessment Question 10

Describe the differences among a proto-oncogene, an oncogene, and a tumor suppressor gene.

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Model Answer:

An oncogene is a gene that causes cancer. A proto-oncogene is a gene that has the potential to cause cancer when mutated. A tumor suppressor gene blocks specific steps in the development of cancer.