5 Steps to a 5: AP Biology 2017 (2016)
Review the Knowledge You Need to Score High
IN THIS CHAPTER
Summary: This chapter teaches you what you need to know about cell division in prokaryotes (binary fission), the cell cycle, and cell division in eukaryotes (mitosis and meiosis). In addition, it discusses the life cycles of various organisms.
There are four main stages in the cell cycle—G1 , S, G2 , and M.
The stages of mitosis are: prophase, metaphase, anaphase, telophase, and cytokinesis.
Crossing over occurs during prophase I of meiosis.
Examples of cell division control mechanisms: growth factors, checkpoints, density-dependent inhibition, and cyclins/protein kinases.
Sources of cell variation: crossover, 2 n possible gametes, and random pairing of gametes.
Cell division, the process by which cells produce more of their kind, can occur in several ways. In this chapter, we discuss cell division in prokaryotes (binary fission), the cell cycle, and cell division in eukaryotes (mitosis and meiosis). After comparing mitosis and meiosis, we will touch on the life cycles of various organisms.
Cell Division in Prokaryotes
Prokaryotes are simple single-celled organisms without a nucleus. Their genetic material is arranged in a single circular chromosome of DNA, which is anchored to the cell membrane. As in eukaryotes, the genetic material of prokaryotes is duplicated before division. However, instead of entering into a complex cycle for cell division, prokaryotes simply elongate until they are double their original size. At this point, the cell pinches in and separates into two identical daughter cells in a process known as binary fission (Figure 9.1 ).
Figure 9.1 Binary fission.
The Cell Cycle
Eukaryotic cell reproduction is a bit more complicated. The cell cycle functions as the daily planner of growth and development for the eukaryotic cell. It tells the cell when and in what order it is going to do things, and consists of all the necessary steps required for the reproduction of a cell. It begins after the creation of the cell and concludes with the formation of two daughter cells through cell division. It then begins again for the two daughter cells that have just been formed. There are four main stages to the cell cycle, and they occur in the following sequence: phases G1 , S , G2 , and M (Figure 9.2 ). Phases G1 and G2 are growth stages; S is the part of the cell cycle during which the DNA is duplicated; and the M phase stands for mitosis, the cell division phase.
Figure 9.2 Pie chart showing the four main stages of the cell cycle.
BIG IDEA 3.A.2
In eukaryotes, information is passed to the next generation via the cell cycle .
Stages of the Cell Cycle
G1 phase . During the first growth phase of the cell cycle, the cell prepares itself for the synthesis stage of the cycle, making sure that it has all the necessary raw materials for DNA synthesis.
S phase . The DNA is copied so that each daughter cell has a complete set of chromosomes at the conclusion of the cell cycle.
G2 phase . During the second growth phase of the cycle, the cell prepares itself for mitosis (for producing body cells) and/or meiosis (for producing gametes), making sure that it has the raw materials necessary for the physical separation and formation of daughter cells.
M phase . Mitosis is the stage during which the cell separates into two new cells.
The first three stages of the cycle (G1 , S, and G2 ) make up the portion of the cell cycle known as interphase. A cell spends approximately 90 percent of its cycle in this phase. The other 10 percent is spent in the final stage, mitosis.
The amount of time that a cell requires to complete a cycle varies by cell type. Some cells complete a full cycle in hours, while others can take days to finish. The rapidity with which cells replicate also varies. Skin cells are continually zipping along through the cell cycle, whereas nerve cells do not replicate—once they are damaged, they are lost for good. This is one reason why the death of nerve cells is such a problem—these cells cannot be repaired or regenerated through mitotic replication.
During mitosis, the fourth stage of the cell cycle, the cell actually takes the second copy of DNA made during the S phase and divides it equally between two cells. Single-cell eukaryotes undergo mitosis for the purpose of asexual reproduction. More complex multicellular eukaryotes use mitosis for other processes as well, such as growth and repair.
Mitosis consists of four major stages: prophase, metaphase, anaphase, and telophase. These stages are immediately followed by cytokinesis —the physical separation of the newly formed daughter cells. During interphase, chromosomes are invisible. The chromatin —the raw material that gives rise to the chromosomes—is long and thin during this phase. When the chromatin condenses to the point where the chromosome becomes visible through a microscope, the cell is said to have begun mitosis. The AP Biology exam is not going to ask you detailed questions about the different stages of mitosis; just have a general understanding of what happens during each step.
Prophase . Nucleus and nucleolus disappear; chromosomes appear as two identical, connected sister chromatids; mitotic spindle (made of microtubules) begins to form; centrioles move to opposite poles of the cell (plant cells do not have centrioles).
Metaphase . For metaphase, think middle. The sister chromatids line up along the middle of the cell, ready to split apart.
Anaphase . For anaphase, think apart. The split sister chromatids move via the microtubules to the opposing poles of the cell—the chromosomes are pulled to opposite poles by the spindle apparatus. After anaphase, each pole of the cell has a complete set of chromosomes.
Telophase . The nuclei for the newly split cells form; the nucleoli reappear, and the chromatin uncoils.
Cytokinesis . Newly formed daughter cells split apart. Animal cells are split by the formation of a cleavage furrow, plant cells by the formation of a cell plate.
Figure 9.3 is a pictorial representation of the stages of mitosis.
Figure 9.3 The stages of mitosis.
Here are the definitions for words you may need to know:
Cell plate: plant cell structure, constructed in the Golgi apparatus, composed of vesicles that fuse together along the middle of the cell, completing the separation process.
Cleavage furrow: groove formed (in animal cells) between the two daughter cells that pinches together to complete the separation of the two cells after mitosis.
Cytokinesis: the actual splitting of the newly formed daughter cells that completes each trip around the cell cycle—some consider it part of mitosis; others regard it as the step immediately following mitosis.
Mitotic spindle: apparatus constructed from microtubules that assists the cell in the physical separation of the chromosomes during mitosis.
Control of Cell Division
Control of the cell cycle is important to normal cell growth. There are various ways in which the cell controls the process of cell division:
1. Checkpoints . There are checkpoints throughout the cell cycle where the cell verifies that there are enough nutrients and raw materials to progress to the next stage of the cycle. The G1 checkpoint, for example, makes sure that the cell has enough raw materials to progress to and successfully complete the S phase.
2. Density-dependent inhibition . When a certain density of cells is reached, growth of the cells will slow or stop because there are not enough raw materials for the growth and survival of more cells. Cells that are halted by this inhibition enter a quiescent phase of the cell cycle known as G0 . Cancer cells can lose this inhibition and grow out of control.
3. Growth factors . Some cells will not divide if certain factors are absent. Growth factors, as their name indicates, assist in the growth of structures.
4. Cyclins and protein kinases . Cyclin is a protein that accumulates during G1 , S, and G2 of the cell cycle. A protein kinase is a protein that controls other proteins through the addition of phosphate groups. Cyclin-dependent kinase (CDK) is present at all times throughout the cell cycle and binds with cyclin to form a complex known as MPF (maturation or mitosis promoting factor). Early in the cell cycle, because the cyclin concentration is low, the concentration of MPF is also low. As the concentration of cyclin reaches a certain threshold level, enough MPF is formed to push the cell into mitosis. As mitosis proceeds, the level of cyclin declines, decreasing the amount of MPF present and pulling the cell out of mitosis.
Sam (12th grader): “Control mechanisms are an important theme for this test. Be able to write about them.”
Haploid Versus Diploid Organisms
One thing that is often a major source of confusion for some of my students is the distinction between being haploid and being diploid. Let’s start with a definition of the terms:
A haploid (n ) organism is one that has only one copy of each type of chromosome. In humans, this refers to a cell that has one copy of each type of homologous chromosome.
A diploid (2n ) organism is one that has two copies of each type of chromosome. In humans, this refers to the pairs of homologous chromosomes.
During the discussion of meiosis below, the terms haploid and diploid will be used often. Whenever we say “2n ,” or diploid, we are referring to an organism that contains two full sets of chromosomes. The letter n is used to represent the number of sets of chromosomes. So if an organism is said to have 4n chromosomes, this means that it has four complete sets of chromosomes. Humans are diploid, and consist of 2n chromosomes at all times except as gametes, when they are n . Humans have 23 different chromosomes; there are two full sets of these 23 chromosomes, one from each parent, for a total of 46 chromosomes. Human sex cells have 23 chromosomes each.
Now that we have armed you with the knowledge of the distinction between haploid and diploid, it is time to dive into the topic of meiosis, which occurs during the process of sexual reproduction. A cell destined to undergo meiosis goes through the cell cycle, synthesizing a second copy of DNA just like mitotic cells. But after G2 , the cell instead enters meiosis, which consists of two cell divisions, not one. The second cell division exists because the gametes to be formed from meiosis must be haploid. This is because they are going to join with another haploid gamete at conception to produce the diploid zygote. Meiosis is like a two-part made-for-TV miniseries. It has two acts: meiosis I and meiosis II. Each of these two acts is divided into four steps, reminiscent of mitosis: prophase, metaphase, anaphase, and telophase.
Homologous chromosomes resemble one another in shape, size, function, and the genetic information they contain. In humans, the 46 chromosomes are divided into 23 homologous pairs. One member of each pair comes from an individual’s mother, and the other member comes from the father. Meiosis I is the separation of the homologous pairs into two separate cells. Meiosis II is the separation of the duplicated sister chromatids into chromosomes. As a result, a single meiotic cycle produces four cells from a single cell. The cells produced during meiosis in the human life cycle are called gametes.
Again, the AP Biology exam is not going to test your mastery of the minute details of the meiotic process. However, a general understanding of the various steps is important:
Prophase I . Each chromosome pairs with its homolog. Crossover (synapsis) occurs in this phase. The nuclear envelope breaks apart, and spindle apparatus begins to form.
Metaphase I . Chromosomes align along the metaphase plate matched with their homologous partner. This stage ends with the separation of the homologous pairs.
Anaphase I . Separated homologous pairs move to opposite poles of the cell.
Telophase I . Nuclear membrane reforms; the process of cytoplasmic division begins.
Cytokinesis . After the daughter cells split, the two newly formed cells are haploid (n ).
As discussed earlier, meiosis consists of a single synthesis period during which the DNA is replicated, followed by two acts of cell division. With the completion of the first cell division, meiosis I, the cells are haploid because they no longer consist of two full sets of chromosomes. Each cell has one of the duplicated chromatid pairs from each homologous pair. The cell then enters meiosis II.
Prophase II . The nuclear envelope breaks apart, and spindle apparatus begins to form.
Metaphase II . Sister chromatids line up along the equator of the cell.
Anaphase II . Sister chromatids split apart and are called chromosomes as they are pulled to the poles.
Telophase II . The nuclei and the nucleoli for the newly split cells return.
Cytokinesis . Newly formed daughter cells physically divide.
Figure 9.4 is a pictorial representation of the stages of meiosis I and II.
Figure 9.4 The stages of meiosis.
In humans, the process of gamete formation is different in women and men. In men, spermatogenesis leads to the production of four haploid sperm during each meiotic cycle. In women, the process is called oogenesis. It is a trickier process than spermatogenesis, and each complete meiotic cycle leads to the production of a single ovum, or egg. After meiosis I in females, one cell receives half the genetic information and the majority of the cytoplasm of the parent cell. The other cell, the polar body, simply receives half of the genetic information and is cast away. During meiosis II, the remaining cell divides a second time, and forms a polar body that is cast away, and a single haploid ovum that contains half the genetic information and nearly all the cytoplasm of the original parent cell. The excess cytoplasm is required for proper growth of the embryo after fertilization. Thus, the process of oogenesis produces two polar bodies and a single haploid ovum.
To review, why is it important to produce haploid gametes during meiosis? During fertilization, a sperm (n ) will meet up with an egg (n ), to produce a diploid zygote (2n ). If either the sperm or the egg were diploid, then the offspring produced during sexual reproduction would contain more chromosomes than the parent organism. Meiosis circumvents this problem by producing gametes that are haploid and consist of one copy of each type of chromosome. During fertilization between two gametes, each copy will match up with another copy of each type of chromosome to form the diploid zygote.
Before moving on, there are a few important distinctions between meiosis and mitosis that should be emphasized.
In meiosis during prophase I, the homologous pairs join together. This matching of chromosomes into homologous pairs does not occur in mitosis. In mitosis, the 46 chromosomes simply align along the metaphase plate alone.
An event of major importance that occurs during meiosis that does not occur during mitosis is known as crossover (also known as crossing over ) (Figure 9.5 ). When the homologous pairs match up during prophase I of meiosis, complementary pieces from the two homologous chromosomes wrap around each other and are exchanged between the chromosomes. Imagine that chromosome A is the homologous partner for chromosome B. When they pair up during prophase I, a piece of chromosome A containing a certain stretch of genes can be exchanged for the piece of chromosome B containing the same genetic information. This is one of the mechanisms that allows offspring to differ from their parents. Remember that crossing over occurs between the homologous chromosome pairs, not the sister chromatids.
Figure 9.5 Crossover.
The AP Biology exam characteristically will ask a question or two about the various types of life cycles for plants, animals, and fungi. A life cycle is the sequence of events that make up the reproductive cycle of an organism. Let’s take a quick look at the three main life cycles.
The most complicated life cycle of the three is that of plants, also called the alternation of generations (Figure 9.6 ). It is referred to by this term because during the life cycle, plants sometimes exist as a diploid organism and at other times as a haploid organism. It alternates between the two forms. Similar to the other life cycles, two haploid gametes combine to form a diploid zygote, which divides mitotically to produce the diploid multicellular stage: the sporophyte. The sporophyte undergoes meiosis to produce a haploid spore. Mitotic division leads to the production of haploid multicellular organisms called gametophytes. The gametophyte undergoes mitosis to produce haploid gametes, which combine to form diploid zygotes . . . and around and around they go.
Figure 9.6 Plant life: alternation of generations.
The human life cycle (Figure 9.7 ) is pretty straightforward. The only haploid cells present in this life cycle are the gametes formed during meiosis. The two haploid gametes combine during fertilization to produce a diploid zygote. Mitotic division then leads to formation of the diploid multicellular organism. Meiotic division later produces haploid gametes, which continue the cycle.
Figure 9.7 Human life cycle.
The life cycle of fungi (Figure 9.8 ) is different from that of humans. Fungi are haploid organisms, with the zygote being the only diploid form. Like humans, the gametes for fungi are haploid (n ), and fertilization yields a diploid zygote. But in this life cycle, instead of dividing by mitosis, the zygote divides by meiosis to form a haploid organism. Another difference in this life cycle is that the gametes are formed by mitosis, not meiosis—the organism is already haploid, before forming the gametes.
Figure 9.8 Fungus life cycle.
Here is some trivia about life cycles that might come in handy on the exam. The only diploid stage for a fungus is the zygote. The only haploid stage for a human is the gamete. Of the plant life cycles, the moss (bryophyte) is an exception in that its prominent generation is the gametophyte. For ferns, conifers (cone-producing plants), and angiosperms (flowering plants), the prominent generation is the sporophyte. The dominant sporophyte generation is considered more advanced evolutionarily than a dominant gametophyte generation. These different plant types will show up again later in Chapter 14 .
Sources of Cell Variation
NYC teacher: “Knowing the sources of variation is important.”
What makes us different from our parents? Why do some people look amazingly like their parents while others do not? The process of cell division provides ample opportunity for variation. Remember that during meiosis, homologous chromosome pairs align together along the metaphase plate. This alignment is a completely random process, and there is a 50 percent chance that the chromosome in the pair from the individual’s mother will go to one side, and a 50 percent chance that the chromosome in the pair from the individual’s father will go to that side. This is true for all the homologous pairs in an organism. This means that 2 n possible gametes can form from any given set of n chromosomes. For example, in a 3-chromosome organism, there are 23 = 8 possible gametes. In humans, there are 23 homologous pairs. This comes out to 223 (8,388,608) different ways the gametes can separate during gametogenesis.
BIG IDEA 3.C.2
Crossing over and random assortment during meiosis increase variation .
Another source of variation during sexual reproduction is the random determination of which sperm meets up with which ovum. In humans, the sperm represents one of 223 possibilities from the male gamete factory; the ovum, one of 223 possibilities from the female gamete factory. All these factors combine to explain why siblings may look nothing like each other.
A third major source of variation during gamete formation is the crossover (or crossing over ) that occurs during prophase I of meiosis. It is very important for you to remember that this process happens only during that stage of cell division. It does not occur in mitosis.
1 . Which of the following plant types has the gametophyte as its prominent generation?
2 . During which phase of the cell cycle does crossing over occur?
A. Metaphase of mitosis
B. Metaphase I of meiosis
C. Prophase I of meiosis
D. Prophase of mitosis
E. Anaphase I of meiosis
For questions 3–6, please use the following answer choices:
3 . During this phase, the split sister chromatids, now considered to be chromosomes, are moved to the opposite poles of the cell.
4 . During this phase the nucleus deteriorates, and the mitotic spindle begins to form.
5 . During this phase, the two daughter cells are actually split apart.
6 . During this phase, the sister chromatids line up along the equator of the cell, preparing to split.
7 . Which of the following organisms is diploid (2n ) only as a zygote and is haploid for every other part of its life cycle?
8 . Which of the following statements is true about a human meiotic cell after it has completed meiosis I?
A. It is diploid (2n ).
B. It is haploid (n ).
C. It has divided into four daughter cells.
D. It proceeds directly to meiosis II without an intervening intermission.
9 . Which of the following is not true about cyclin-dependent kinase (CDK)?
A. It is present only during the M phase of the cell cycle.
B. When enough of it is combined with cyclin, the MPF (mitosis promoting factor) formed initiates mitosis.
C. It is a protein that controls other proteins using phosphate groups.
D. It is present at all times during the cell cycle.
10 . Which of the following statements about meiosis and/or mitosis is incorrect?
A. Mitosis results in two diploid daughter cells.
B. Meiosis in humans occurs only in gonad cells.
C. Homologous chromosomes line up along the metaphase plate during mitosis.
D. Crossover occurs during prophase I of meiosis.
E. Meiosis consists of one replication phase followed by two division phases.
Answers and Explanations
1 . B —Bryophytes, or mosses, are the plant type that has the gametophyte (haploid) as its dominant generation. The others in this question have the sporophyte (diploid) as their dominant generation.
2 . C —Crossover occurs in humans only in prophase I. Prophase I is a major source of variation in the production of offspring.
3 . C
4 . A
5 . E
6 . B
7 . C —The life cycle for fungi is different from that of humans. Fungi exist as haploid organisms, and the only time they exist in diploid form is as a zygote. Like humans, the gametes for fungi are haploid (n ) and combine to form a diploid zygote. Unlike in humans, the fungus zygote divides by meiosis to form a haploid organism.
8 . B —Human cells start with 46 chromosomes arranged in 23 pairs of homologous chromosomes. At this time they are 2n because they have two copies of each chromosome. After the S phase of the cell cycle, the DNA has been doubled in preparation for cell division. The first stage of meiosis pulls apart the homologous pairs of chromosomes. This means that after meiosis I, the cells are n , or haploid—they no longer consist of two full sets of chromosomes.
9 . A —CDK is present at all times during the cell cycle. It combines with a protein called cyclin, which accumulates during interphase of the cell cycle, to form MPF. When enough MPF is formed, the cell is pushed to begin mitosis. As mitosis continues, cyclin is degraded, and when the concentration of MPF drops below a level sufficient to maintain mitotic division, mitosis grinds to a halt until the threshold is reached again next time around the cycle.
10 . C —Answer choices A, B, D, and E are all correct. C is incorrect because homologous pairs of chromosomes pair together only during meiosis. During mitosis, the sister chromatid pairs align along the metaphase plate, separate from the homologous counterpart.
You should be familiar with the following terms:
Binary fission: prokaryotic cell division; double the DNA, double the size, then split apart.
Cell cycle: G1 → S → G2 → M → growth1 → synthesis → growth2 → mitosis → etc.
Interphase: G1 + S + G2 = 90 percent of the cell cycle.
Cytokinesis: physical separation of newly formed daughter cells of cell division.
Cell division control mechanisms:
1. Growth factors: factors that when present, promote growth, and when absent, impede growth.
2. Checkpoints: a cell stops growing to make sure it has the nutrients and raw materials to proceed.
3. Density-dependent inhibition: cell stops growing when certain density is reached—runs out of food!!!
4. Cyclins and protein kinases: cyclin combines with CDK to form a structure known as MPF that pushes cell into mitosis when enough is present.
Haploid (n ): one copy of each chromosome.
Diploid (2n ): two copies of each chromosome.
Homologous chromosomes: chromosomes that are similar in shape, size, and function.
Spermatogenesis: the process of male gamete formation (four sperm from one cell).
Oogenesis: the process of female gamete formation (one ovum from each cell).
Life cycles: Sequence of events that make up the reproductive cycle of an organism.
• Human: zygote (2n ) → multicellular organism (2n ) → gametes (n ) → zygote (2n )
• Fungi: zygote (2n ) → multicellular organism (n ) → gametes (n ) → zygote (2n )
• Plants: zygote (2n ) → sporophyte (2n ) → spores (n ) → gametophyte (n ) → gametes (n ) → zygote (2n )
Sources of variation : crossover, 2 n possible gametes that can be formed, random pairing of gametes.
1 . All of the following act as cell division control mechanisms EXCEPT
(A) growth factors.
(B) cyclins and protein kinases.
(C) density-dependent inhibition.
(D) alternation of generations.
2 . During which stage of the cell cycle does the plant prepare itself for mitosis and or meiosis by making sure that it has the raw materials necessary for the physical separation and formation of daughter cells?
(A) G1 phase
(B) S phase
(C) G2 phase
(D) M phase
3 . During which stage of mitosis are the chromosomes pulled to opposite poles of the cell by the spindle apparatus?
4 . The sporophyte is the prominent generation in the alternation of generations life cycle of each of the following EXCEPT
Answers and Explanations
1 . D —Alternation of generations is a plant life cycle, so named because during the cycle, the plant sometimes exists as a diploid organism and at other times as a haploid organism. This cycle is not involved in cell division control.
2 . C —If you know the order of the cell cycle, this one is easy. The order is G1 → S → G2 → M. So clearly the second growth phase prepares for the meiosis or mitosis phase (M).
3 . C —For anaphase, think “apart.” The split sister chromatids move via the microtubules to the opposing poles of the cell. After anaphase, each pole of the cell has a complete set of chromosomes.
4 . A —Bryophytes (mosses) are the exception to the general plant life cycle rule in that they are the only plants to have the gametophyte as their prominent generation.