Chapter 11 Summary
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Core Concepts Summary
11.1 During cell division, a single parental cell divides into two daughter cells.
Prokaryotic cells divide by binary fission, in which a cell replicates its DNA, segregates its DNA, and divides into two cells. page 220
Eukaryotic cells divide by mitosis (nuclear division) and cytokinesis (cytoplasmic division). Together, mitosis and cytokinesis are known as mitotic cell division. page 222
M phase (mitosis and cytokinesis) alternates with interphase, which consists of G1, S (synthesis), and G2 phases. These four stages together constitute the cell cycle. page 222
Cells that do not need to divide exit the cell cycle and are in G0. page 222
11.2 Mitotic cell division is the basis of asexual reproduction in unicellular eukaryotes and the process by which cells divide in multicellular eukaryotes.
DNA in a eukaryotic cell is packaged as linear chromosomes. page 222
Humans have 46 chromosomes: 22 pairs of homologous chromosomes and 1 pair of sex chromosomes. Each parent contributes one complete set of 23 chromosomes at fertilization. page 222
During S phase, chromosomes replicate, resulting in the formation of sister chromatids held together at the centromere. page 223
Mitosis involves five steps following DNA replication: (1) prophase—
Mitosis is followed by cytokinesis, in which one cell divides into two. In animals, a contractile ring of actin pinches the cell in two. In plants, a new cell wall, called the cell plate, is synthesized between the daughter cells. page 225
11.3 Meiotic cell division is essential for sexual reproduction, the production of offspring that combine genetic material from two parents.
Sexual reproduction involves meiosis and fertilization, both of which are important in increasing genetic diversity. page 226
Meiotic cell division is a form of cell division that reduces the number of chromosomes by half to produce haploid gametes or spores that have one copy of each chromosome. page 226
Fertilization involves the fusion of haploid gametes to produce a diploid cell. page 226
Meiosis consists of two successive cell divisions: The first is reductional (the chromosome number is halved), and the second is equational (the chromosome number stays the same). Each division consists of prophase, prometaphase, metaphase, anaphase, and telophase. page 226
In meiosis I, homologous chromosomes pair and exchange genetic material at chiasmata, or regions of crossing over. In contrast to mitosis, centromeres do not split and sister chromatids do not separate. page 227
Genetic diversity is generated by crossing over and random alignment and subsequent segregation of maternal and paternal homologs on the metaphase plate in meiosis I. page 227
Meiosis II is similar to mitosis, in which chromosomes align on the metaphase plate, centromeres split, and sister chromatids separate from each other. page 228
The similarity of meiosis II and mitosis suggests that meiosis evolved from mitosis. page 231
The division of the cytoplasm differs between the sexes: Male meiotic cell division produces four functional sperm cells, whereas female meiotic cell division produces a single functional egg cell and three polar bodies. page 231
11.4 The cell cycle is regulated so that cell division occurs only at appropriate times and places.
Cells have regulatory mechanisms for promoting and preventing cell division. page 233
Levels of proteins called cyclins increase and decrease during the cell cycle. page 233
Cyclins form complexes with cyclin-
These complexes are activated by signals that promote cell division. page 234
Different cyclin–
The cell also has checkpoints that halt progression through the cell cycle if something is not right. page 235
The p53 protein is an example of a checkpoint protein; it prevents cell division in the presence of DNA damage. page 235
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11.5 Cancer is uncontrolled cell division that results from mutations in genes that control cell division.
Cancer results when mechanisms that promote cell division are inappropriately activated or the normal checks on cell division are lost. page 236
Cancers can be caused by certain viruses carrying oncogenes that promote uncontrolled cell division. page 238
Viral oncogenes have cellular counterparts called proto-
Oncogenes and proto-
Tumor suppressors encode proteins, such as p53, that block cell division and inhibit cancers. page 238
Cancers usually result from several mutations in proto-
Self-Assessment
Compare and contrast the ways in which prokaryotic cells and eukaryotic cells divide.
Self-Assessment 1 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 envelope then dissolves and each pair of chromosomes is 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. Binary fission is like mitosis in many ways although it differs in a few key aspects: (1) The size and shape of the genetic material; bacteria have a single circular chromosome, whereas eukaryotic cells generally have multiple linear chromosomes. (2) The DNA of prokaryotes is attached to the plasma membrane, allowing separation of DNA into the daughter cells by cell growth. Eukaryotic cells have to first dissolve the nuclear membrane and then reform it to separate DNA.
Describe three situations in which mitotic cell division occurs.
Self-Assessment 2 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.
Name the five steps of mitosis, and draw the structure and position of the chromosomes at each step.
Self-Assessment 3 Answer
The five steps in mitosis are: (1) prophase (chromosomes condense, centrosomes radiate spindle 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 Fig. 11.5 for an illustration of the changes in the chromosomes at each step. 
Describe how chromosomes behave in meiosis. State when chromosomes are duplicated (forming sister chromatids) and when they are not duplicated.
Self-Assessment 4 Answer
Meiosis basically consists of two consecutive cell division cycles called meiosis I and meiosis II. Prior to meiosis, chromosomes are duplicated, forming sister chromatids. In meiosis I, homologous chromosomes pair, cross over, and separate into two different daughter cells. During meiosis II, sister chromatids separate into two more daughter cells. The result of this process is four haploid daughter cells, each having unique genetic information.
Compare and contrast mitotic cell division and meiotic cell division in terms of number of products, number of cell divisions, and the processes unique to each.
Self-Assessment 5 Answer
Mitosis is preceded by one round of DNA synthesis and produces two genetically identical daughter cells, each with 46 chromosomes, in one round of division. Meiosis is also preceded by one round of DNA synthesis, but consists of two rounds of cell division to produce four genetically different daughter cells, with 23 chromosomes each. See Fig. 11.12.
Name two ways in which meiotic cell division creates genetic diversity, and explain how each occurs.
Self-Assessment 6 Answer
Genetic diversity is created in meiotic cell division by the crossing over of chromosomes and random alignment of bivalents in metaphase I. In crossing over, homologous chromosomes of maternal origin and paternal origin exchange 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.
Explain how cytokinesis differs between animal and plant cells.
Self-Assessment 7 Answer
In animal cells, cytokinesis involves a contractile ring made of actin. This contractile ring forms against the inner face of the cell membrane at the equator of the cell. When the ring contracts, the cytoplasm is pinched in half, forming two new daughter cells. In plant cells, cytokinesis involves the growth of a new cell wall called a cell plate in the middle of the dividing cell. The new plate fuses with the original cell wall at the perimeter of the cell and two new daughter cells are formed.
Describe the roles of cyclins and cyclin-
dependent kinases in the cell cycle. Self-Assessment 8 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. Give three examples of checkpoints that the cell monitors before proceeding through the cell cycle.
Self-Assessment 9 Answer
Three examples of checkpoints during the cell cycle are (1) the spindle assembly checkpoint, which is active before anaphase and ensures that all chromosomes are attached to the spindles; (2) the DNA damage checkpoint, which is active in the G1 phase and checks for DNA damage; and (3) the DNA replication checkpoint, which is active in G2 and makes sure all the DNA is replicated.
Describe what an oncogene, a proto-
oncogene, and a tumor suppressor gene do. Self-Assessment 10 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.