1.5: Mitosis and Meiosis I
In eukaryotic cells, the time and phases from the beginning of one cell division until the beginning of the next cell division is called the cell cycle (Figure 1) . The first phase of the cell cycle is interphase. Interphase is the time during which the cell performs its normal functions and prepares for cell division. Cells spend most of their time in this phase. During interphase, chromosomes are not visible because they are decondensed (present only as a tangled mass of thin threads of DNA with associated proteins, called chromatin). The nuclear membrane is present, and visible, as is the nucleolus .
Figure 1. The cell cycle.
Interphase includes two gap phases, G1 and G2, where the cell increases in size and synthesizes new organelles, enzymes, and other proteins that are needed for cell division. In between the two gap phases, the DNA replicates in preparation for cell division. This stage is called S phase . At the beginning of S phase, chromosomes are single and unreplicated. By the end of S phase, each chromosome has made an exact copy and consists of two sister chromatids . At this point in the cell cycle the sister chromatids are held together tightly at the centromere . While the two sister chromatids are physically joined together they are still considered one replicated chromosome (Figure 2). When the sister chromatids physically separate, later during the cell cycle, they are then considered to be individual chromosomes.
In animal cells, interphase is also when the centrosome (consisting of two centrioles ) is replicated. Spindle fibers form from and radiate outward from the centrosomes to attach to and move chromosomes during cell division.
Figure 2. Chromosomes and sister chromatids.
Interphase is followed by mitosis (in the somatic cells) or meiosis (in reproductive cells), which is when replicated chromosomes and cytoplasm separate, during the process of karyokinesis and cytokinesis respectively.
Chromosomes that are the same length, have the same centromere location and the same gene sequences and positions are called homologous chromosomes. Human somatic cells contain pairs of homologous chromosomes. Each homologous pair consists of one maternal chromosome and one paternal chromosome. Cells that contain two copies of each chromosome are called diploid (2n, where n is the number of different chromosomes in a single set). Human sex cells (eggs and sperm) contain only one copy of each chromosome. Cells with only one copy of each chromosome are haploid (n). When the haploid sperm (n) and egg (n) combine during fertilization this forms a diploid zygote (2n).
MITOSIS
Mitosis is nuclear division that results in two cells containing the same number of chromosomes as the parent cell. Most human cells (skin, muscle, bone, etc.) divide by mitosis. This process is necessary for the normal growth and development of a multicellular eukaryotic organism from a zygote (fertilized egg), as well as growth and the repair and replacement of cells and tissues. At the end of mitosis, two daughter cells are formed that are identical to the original (parent) cell. Mitosis is also a form of asexual reproduction in unicellular eukaryotes.
Mitosis is a complex and highly regulated process. It occurs in the following 4 separate phases: prophase, metaphase, anaphase, and telophase. Telophase is quickly followed by cytokinesis.
Prophase: Cells prepare for division by coiling and condensing their chromatin into chromosomes. By late prophase, individual chromosomes can be seen, each consisting of two sister chromatids joined at a centromere. Spindle fibers begin to form from the centrosomes, which have begun to migrate to opposite “poles” of the cell. The nucleoli and the nuclear membrane degrade. (Figure 3)
Metaphase: Spindle fibers (called kinetochore microtubules or kinetochore spindle fibers) that emanate from the centromeres attach to the kinetochore (a proteinaceous area) on the sister chromatids. The fibers pull and otherwise manipulate the chromosomes to align them on the plane that passes through the center of the cell (metaphase plate) (Figure 4). This “plate” is not an actual structure; it merely signifies the location of replicated chromosomes prior to their impending separation.
Other non-kinetochore spindle fibers or tubules (aka polar microtubules), emanating from the two centrosomes, elongate and eventually overlap with each other near the metaphase plate.
Figure 4. Spindle fibers attaching to kinetochores in metaphase.
Anaphase: The centromeres divide, with the help of separase enzymes, and separate the sister chromatids (Figure 5). This happens simultaneously in all the chromosomes. The kinetochore spindles shorten and pull each chromatid to which they are attached toward the pole (and centrosome) from which they originate. This equally distributes exactly half the chromosomal material to each side of the cell. In late anaphase, the non-kinetochore spindles begin to elongate, lengthening the cell.
Figure 5. Cell in anaphase.
Telophase: The non-kinetochore microtubules continue to elongate, further elongating the cell in preparation for cytokinesis (splitting of the cytoplasm). The chromosomes reach their respective poles. The kinetochores disappear. The two nuclear membranes (one in each half of the cell) begin to form around the chromosomes. Nucleoli reappear and the chromosomes in each soon-to-be new cell begin to decondense back into chromatin. Mitosis is complete at the end of this stage.
Cytokinesis (splitting of the cytoplasm):
In animal cells and all other eukaryotes without a cell wall, cytokinesis is achieved by means of a constricting “belt” of protein fibers that slide past each other near the equator of the cell. As this occurs, the diameter of the belt decreases, pinching the cell to form a cleavage furrow around the cell’s circumference. As constriction proceeds, the furrow deepens until it eventually slices its way into the center of the cell. At this point, the cell is divided into two.
Plant cell walls are far too rigid to be split apart by contracting proteins. Instead, these cells assemble membrane proteins (in vesicles that bud off the Golgi apparatus) in their interior at right angles to the spindle apparatus. This expanding membrane partition, called a cell plate, continues to grow outward until it reaches the interior surface of the plasma membrane and fuses with it. This divides the cell in two.
The phases of Mitosis
Exercise 1: Modeling the Phases of Mitosis
Materials:
- Several sheets of blank paper (continuous printer paper is ideal)
-
Chromosome modeling kits
- Commercially available pop bead kits (e.g Carolina Biological Supply Company, Item #171100)
- Homemade kits may consist of pipe cleaners or yarn or socks, etc. to represent chromosomes
-
The following procedure will be described using a homemade kit consisting of pipe cleaners to represent chromosomes. The pipe cleaner chromosome kit contains:
- 10 each – short red pipe cleaner sticks, short blue pipe cleaner sticks, long pipe cleaner red stick, long blue pipe cleaner sticks (Use as 2 homologous chromosome pairs)
- 5 each – short red plastic lacing cord, short blue plastic lacing cord, long red plastic lacing cord, long blue plastic lacing cord (Use as 2 homologous chromatin pairs)
- 20 white or grey beads (Use as centromeres)
- Several red and blue beads (Use as genes for meiosis crossing-over)
Procedure:
Using models is a great way to represent natural structures and processes that are too small, or too large, or too complex to observe directly. By building chromosomes from the pipe cleaners and manipulating them to model cell division (mitosis and meiosis) you will enhance your understanding of the nature of chromosomes and the cellular structures needed to perform cell division. The pipe cleaner and plastic cord strands are intended to represent two pairs of homologous chromosomes. One pair of homologous chromosomes is longer than the other. Half of each pair is red and represents maternal DNA (genetic material contributed by a female’s egg). The other half of each pair is blue and represents paternal DNA (genetic material contributed by a father’s sperm). Thus, for each pair of homologous chromosomes, one should be red and one should be blue. The thin plastic lacing cord represents chromatin when chromosomes are in an uncoiled, decondensed state. The thicker pipe cleaner chromosomes represent the condensed chromosomes as they prepare for DNA replication and cell division. Alert your instructor if the chromosomes in your bag differ from those below.
This diploid cell with 2 homologous pairs of chromosomes will be modeled as it moves through the following phases of mitosis:
- Interphase (uncondensed DNA) Before Synthesis of DNA (G1)
- Interphase (uncondensed DNA) After Synthesis of DNA (G2)
- Prophase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
- Use the lace cording chromosomes to model the G1 phase of interphase (before synthesis of the DNA). On the paper draw the cell membrane, nucleus, nucleolus, centrioles.
- Use the lace cording chromosomes to model the G2 phase of interphase (after each chromosome was replicated during S phase). Use white beads to represent centromeres. Thread sister chromatids through a white bead to represent the duplicated chromosomes attached at the centromere. Centrioles would move toward opposite poles of the nucleus. Be sure to draw the cell membrane, nucleus, nucleolus, and centrioles on the paper.
- Use the pipe cleaner chromosomes to model the prophase stage of mitosis. The chromosomes are condensed and distributed throughout the nucleus. On your paper be sure to note that the nuclear membrane begins to break down and spindle fibers begin to form and radiate toward the chromosomes. Spindle fibers attach to kinetochore proteins at the centromeres of the chromosomes.
- Use the pipe cleaner chromosomes to model metaphase . Line up the individual chromosomes on the equator (middle) of the cell. Sister chromatids remain attached at the centromere during metaphase.
- Model anaphase by removing the white beads (centromere) from the sister chromatids to separate and move them toward opposite poles of the cell. After separation at the centromere, the chromatids are now called chromosomes.
- Anaphase ends and telophase begins when chromosomes reach opposite poles of the cell. Nuclear division happens in telophase. Nuclear envelopes and nucleoli reappear. Condensed chromosomes begin to decondense and uncoil. The formation of separate nuclear envelopes divide the nuclei and marks the end of telophase.
- Model cytokinesis by drawing the formation of a cleavage furrow to divide the cytoplasm into two and form two separate cells.
How do the daughter cells you formed compare to the original parent cell? _______________________________________________________________________
Observe the phases of Mitosis in Plant Cells
Exercise 2: Observing the Phases of Mitosis in the Onion Root Tip
Materials:
- Prepared slide of the onion root tip
- Compound light microscope
Procedure:
- Examine a slide of a longitudinal section of an onion root tip. Adjust the slide to view the region just above the root cap, where there are likely to be dividing cells.
- Focus on the dividing cells using the 4x scanning objective lens, then switch to the 10x objective and then the 40x objective.
- Survey the slide to find a cell in each phase of mitosis. Draw a cell for each phase below.
Interphase
DNA is uncondensed and in the form of chromatin. Individual chromosomes are not visible. The nuclear membrane is intact. The nucleolus is visible.
Prophase
Chromatin begins to condense into distinguishable chromosomes. These “puffy” structures are seen throughout the nucleus. Nucleoli begin to disappear. In late prophase (often called prometaphase) the nuclear membrane is no longer visible.
Metaphase
The chromosomes line up in the middle of the cell. Spindle fibers attach to kinetochores at the centromere and extend to the poles of the cell.
Anaphase
Centromeres split, separating each former chromatid into two individual chromosomes. The chromosomes move toward opposite poles.
Telophase and Cytokinesis
Chromosomes reach the poles. The nuclear envelopes begin to reform. The formation of a cell plate forms between the two cells to carry out cytokinesis.
Observe the phases of Mitosis in Animal Cells
Exercise 3: Observing the Phases of Mitosis in the Whitefish Blastula
Materials:
- Prepared slide of whitefish blastula
- Compound light microscope
Procedure:
The blastula is an early embryonic stage where many of the cells are dividing at any one time.
- Focus on the dividing cells using the 4x scanning objective lens, then switch to the 10x objective and then the 40x objective.
- Survey the slide to find a cell in each phase of mitosis. Draw a cell for each phase below.
Interphase
The DNA is uncondensed and in the form of chromatin. Individual chromosomes are not visible. The nuclear membrane is intact. The nucleolus is visible.
Prophase
Chromatin begins to condense and chromosomes are distinguishable. These “puffy” structures are seen throughout the nucleus. The nucleoli begin to disappear. In late prophase (often called prometaphase) the nuclear membrane is no longer visible.
Metaphase
The chromosomes line up in the middle of the cell. Spindle fibers attach to kinetochores at the centromere and extend to the poles of the cell.
Anaphase
Centromeres split, separating each former chromatid into two individual chromosomes. The chromosomes move toward opposite poles.
Telophase and Cytokinesis
Chromosomes reach the poles. The nuclear envelopes begin to reform. A cleavage furrow forms between the two cells to carry out cytokinesis.
How Long Does a Cell Spend in Each Phase of the Cell Cycle?
Exercise 4: Determining Time Spent in Different Phases of the Cell Cycle (Optional)
Materials:
- Laptop
- Calculator
Procedure:
- Obtain a laptop.
- Open a web browser and go to the following site: http://www.biology.arizona.edu
- This site will provide an interactive test of your ability to identify the stages of mitosis. It will also allow you to calculate the duration of the stages identified in the laboratory exercise you just completed, but the website will give standard results for the entire class.
- Go to the Cell Biology section, and find the activity “ Online Onion Root Tips ”.
- Keep clicking on “Next” at the bottom of the page until you get to the screen: Determining Time Spent in Different Phases of the Cell Cycle
- Click on “Next” at the bottom of the page.
- Identify each stage shown to you by the program. When a picture of a cell pops up in a stage of mitosis, simply click on the phase in which the cell belongs. If you make a mistake, read the explanation for why you were mistaken before making a new selection.
- When you are finished, use the formula given below and record your results in the table.
- The duration of each stage of mitosis can be determined by using the following formula. Compute the length of time for each stage and place your calculations in the table below.
(Number of cells in a stage ÷ Total number of cells) x 1440 (min in a day) = minutes a cell spends
in each stage in one day
|
Number of cells in each stage |
Interphase ______ |
Prophase ______ |
Metaphase ______ |
Anaphase _______ |
Telophase _______ |
Total 36 |
|
Percent of cells (# of cells / Total) |
100% |
|||||
|
Time (in minutes) spent in Stage – use calculation above |
References
Belwood, Jacqueline; Rogers, Brandy; and Christian, Jason, Foundations of Biology Lab Manual (Georgia Highlands College). “Lab 10: Mitosis & Meiosis,” (2019). Biological Sciences Open Textbooks . 18. CC-BY
https://oer.galileo.usg.edu/biology-textbooks/18
MEIOSIS
Introduction to Meiosis (aka “Reduction Division”)
Meiosis is a special type of cell division in which the daughter cells produced have half the number of chromosomes (n) as their parent cell. This division occurs in the reproductive organs (gonads -- testes of males or ovaries of females) of species that reproduce sexually, and results in the formation of gametes (eggs or sperm) that contain half the number of chromosomes as the original cell. Sexual reproduction involves the joining of gametes (fertilization) to form a zygote, which then has two copies of each chromosome (2n). Meiosis is a critical process, as it increases genetic diversity within a species.
Cells that divide by meiosis prepare for cellular division (during interphase) much like every other cell. Meiosis progresses through the same phases as mitosis (prophase, anaphase, metaphase, telophase, and cytokinesis). However, unlike mitosis, meiosis involves two rounds of cellular division (meiosis I and meiosis II). Meiosis involves only one round of DNA replication where each chromosome replicates to form sister chromatids. Therefore, when meiosis is completed, each daughter cell contains only half the number (n) of chromosomes as the original cell.
The stages of meiosis:
Meiosis I
Prophase l
Metaphase l
Anaphase l
Telophase l
Cytokinesis l
Meiosis II
Prophase ll
Metaphase ll
Anaphase ll
Telophase ll
Cytokinesis ll
Prophase I: During prophase of meiosis I, the chromosomes join in homologous pairs. Homologous chromosomes (aka homologs) are the same length, and carry genetic information (genes) for the same traits, but not necessarily the same versions (alleles) of the gene.
For example, human chromosome #19 contains a gene for eye color. In one person, one allele might code for blue eyes and the other allele codes for green eyes. Since every human inherits two copies of chromosome 19 (one from the mother’s egg and one from the father’s sperm) a person could have 2 blue alleles, 2 green alleles, or one of each.
Paired homologous chromosomes are called tetrads and are said to be in synapsis. During synapsis, equivalent pieces of homologous chromatids are exchanged between the chromosomes. This is called crossing-over and can occur several times along the length of the chromosomes. As occurs in the mitotic division, prophase of meiosis I also involves the degradation of the nuclear membrane and formation of spindle fibers.
Metaphase I: Metaphase of meiosis I occurs when the joined homologous chromosome pairs are moved to the center of the cell by spindle fibers (Figure 6). The fibers arrange the pairs so that homologs are on opposite sides of the metaphase plate (aka equatorial plane).
Figure 6. Homologous pairs line up at the equatorial plate in Metaphase l.
Anaphase I follows, as homologs are pulled apart, toward opposite poles of the cell (Figure 7). [*Note: this is significantly different from the separation of sister chromatids that occurs during mitosis].
Figure 7. In Anaphase l mitotic spindles pull homologs to opposite poles of the cell.
Telophase I marks the end of meiosis I, as new nuclei form and cytokinesis separates the cytoplasm forming two daughter cells.
At the end of meiosis I, the two daughter cells have half the number of chromosomes as did their parent cell. Thus, the cells have been reduced from diploid (2n) to haploid (n) (Figure 8). [n refers to the number of chromosomes in a set that are characteristic for a species. Humans have one set (n) of 23 unique chromosomes (n = 23). A diploid human cell has 2 sets (2n) of 23 unique chromosomes (2n = 46).
Figure 8. Meiosis l results in two haploid cells.
Meiosis II follows meiosis I, which proceeds very much like mitosis.
During Prophase II , chromosomes containing two sister chromatids are lined up on the equator of each daughter cell by the spindle fibers. This is completed by the end of Metaphase II (Figure 9).
Figure 9. Metaphase ll of Meiosis
The centromeres separate and sister chromatids are pulled to each pole of the cell during Anaphase ll (Figure 10) . Cytokinesis II occurs after Telophase II to complete cell division and ultimately the production of four (4) daughter cells (Figure 11). The cells produced (egg or sperm, in humans) are haploid (n rather than 2n) and will either unite (via fertilization) or die. They do not divide further on their own as meiosis is not a cycle.
Figure 10. Anaphase ll of Meiosis
Figure 11. Meiosis results in four haploid cells.
The Phases of Meiosis
Exercise 1: Modeling the Phases of Meiosis
Materials:
- Several sheets of blank paper (continuous printer paper is ideal)
-
Chromosome modeling kits
- Commercially available pop bead kits (e.g Carolina Biological Supply Company, Item #171100)
- Homemade kits may consist of pipe cleaners or yarn or socks, etc. to represent chromosomes
-
The following procedure will be described using a homemade kit consisting of pipe cleaners to represent chromosomes. The pipe cleaner chromosome kit contains:
- 10 each – short red pipe cleaner sticks, short blue pipe cleaner sticks, long pipe cleaner red stick, long blue pipe cleaner sticks (Use as 2 homologous chromosome pairs)
- 5 each – short red plastic lacing cord, short blue plastic lacing cord, long red plastic lacing cord, long blue plastic lacing cord (Use as 2 homologous chromatin pairs)
- 20 white or grey beads (Use as centromeres)
- Several red and blue beads (Use as genes for meiosis crossing-over)
Procedure:
A diploid cell with 2 homologous pairs of chromosomes (as in the previous modeling exercise) will be modeled as it moves through the meiosis. First, you will model meiosis l. Then, you will model meiosis ll as described below.
Model Meiosis l (1 diploid cell → 2 haploid cells)
-
tetrads form, crossing over occurs, homologues separate
- Interphase Before Synthesis of DNA (G1)
- Interphase After Synthesis of DNA (G2)
- Prophase l
- Metaphase l
- Anaphase l
- Telophase l
- Cytokinesis l
Model Meiosis ll (2 haploid cells → 4 haploid cells)
- sister chromatids separate
- Prophase ll
- Metaphase ll
- Anaphase ll
- Telophase ll
- Cytokinesis ll
Modeling Meiosis l
- Use the lace cording chromosomes to model the G1 phase of interphase (before synthesis of the DNA). On the paper draw the cell membrane, nucleus, nucleolus, centrioles.
- Use the lace cording chromosomes to model the G2 phase of interphase (after each chromosome was replicated during S phase). Use white beads to represent centromeres. Thread sister chromatids through a white bead to represent the duplicated chromosomes attached at the centromere. Centrioles would move toward opposite poles of the nucleus. Be sure to draw the cell membrane, nucleus, nucleolus and centrioles on the paper.
- Use the pipe cleaner chromosomes to model prophase l of meiosis. Prophase l of meiosis is notably different than prophase of mitosis. Arrange condensed chromosomes so that homologous chromosomes are paired. Put homologous chromosomes next to each other on your lab bench to simulate this process of synapsis (homologs pairing). Homologous pairs of chromosomes paired in this way are called tetrads .
When tetrads form, the inner non-sister chromatids of the tetrad pair can exchange DNA by a process known as crossing over . Pieces of equivalent segments of non-sister chromatids can be exchanged from one chromatid to the other. Crossing over can occur several times along the length of the chromosomes. Use red and blue beads to represent exchanged segments of chromatids on the inner non-sister chromatids of the tetrad pairs. Place a blue bead on an inner red (maternal) chromatid to represent DNA exchanged from the paternal chromatid. Place a red bead on an inner blue (paternal) chromatid to represent DNA from the maternal chromatid. Make a minimum of 1 crossover for each pair of homologous chromosomes.
- Use the pipe cleaner chromosomes to model metaphase l . Metaphase l of meiosis is also notably different than metaphase of mitosis. Spindle fibers move homologous pairs to line up along the equatorial plane of the cell. Line up the homologous pairs of chromosomes so that homologues are paired together but the maternal and paternal chromosomes are on opposite sides of the equatorial plate along the middle of the cell. Every pair of chromosomes is arranged independent of another. The side of the equatorial plate where each chromosome is arranged is completely random and independent of the side of the equatorial plate on which other chromosomes are located. Therefore, there are several different arrangements that can occur (Figure 12).
Figure 12. Independent Assortment in a cell with 2 homologous pairs.
- In anaphase l of meiosis, homologous chromosomes separate toward opposite poles of the cell. For each homologous pair, move the duplicated maternal chromosome and duplicated paternal chromosome to opposite poles of the cell.
- Anaphase ends and telophase l begins when chromosomes reach opposite poles of the cell. Arrange the chromosomes in groups at opposite ends of the cell. Nuclear division happens in telophase. The formation of separate nuclear envelopes divide the nuclei and mark the end of telophase.
- Model cytokinesis l by drawing the formation of a cleavage furrow to divide the cytoplasm into two and form two separate cells. These two cells will enter meiosis ll. Note that each daughter cell has half the number of chromosomes as the parental cell. Thus, the cells have been reduced from diploid (2n) to haploid (n) . [n refers to the number of pairs of chromosomes that are characteristic for a species . Humans have a “n” of 23, so a diploid human cell has 2(23), or 46 chromosomes].
Meiosis ll
- There is no DNA replication before the second cell division stage of meiosis. The stages of meiosis ll proceed very much like mitosis. The two cells created in meiosis l will enter into prophase ll . The chromosomes in each cell contain two sister chromatids, which are condensed and distributed throughout the nucleus. On your paper be sure to note that the nuclear membrane begins to break down and spindle fibers begin to form and radiate toward the chromosomes. Spindle fibers attach to kinetochore proteins at the centromeres of the chromosomes.
- Use the pipe cleaner chromosomes to model metaphase ll . Line up the individual chromosomes on the equator (middle) of the cell. Sister chromatids remain attached at the centromere during metaphase ll.
- Model anaphase ll by removing the white beads (centromere) from the sister chromatids to separate and move them toward opposite poles of the cell. After separation at the centromere, the chromatids are now called chromosomes.
- Anaphase ll ends and telophase ll begins when chromosomes reach opposite poles of the cell. Nuclear division happens in telophase. Nuclear envelopes and nucleoli reappear. Condensed chromosomes begin to decondense and uncoil. The formation of separate nuclear envelopes divide the nuclei and marks the end of telophase.
- Model cytokinesis ll by drawing the formation of a cleavage furrow to divide the cytoplasm of each cell into two. A total of four cells exist at the end of cytokinesis ll.
How many chromosomes are in the original parental cell? ___________________
How many chromosomes are in each daughter cell? _______________________
Are the chromosomes in daughter cells identical to the chromosomes in the original parental cell? ________________________________________________
Questions for Review
- What is the meaning of diploid? What abbreviation do we use to represent diploid? Name 2 diploid cells in humans.
- What is the meaning of haploid? What abbreviation do we use to represent haploid? Name 2 haploid cells in humans.
- In what stage of the cell cycle does S phase occur? Explain why the DNA must be duplicated during the S phase of the cell cycle, prior to mitosis taking place.
- What specific feature of cytokinesis in animal cells can you use to distinguish this process from cytokinesis in plant cells?
Practical Challenge Questions
- In the circle below, sketch a 2n=6 diploid cell in metaphase of mitosis. Be sure to label the centromere, centrioles, and spindle fibers.
- In the circle below, sketch a 2n=6 haploid cell in metaphase l of meiosis.
- A monogenic gene gives rise to a trait from a single set of alleles. A polygenic gene gives rise to a trait from several sets of alleles. Give an example of a monogenic and polygenic trait.
References
Belwood, Jacqueline; Rogers, Brandy; and Christian, Jason, Foundations of Biology Lab Manual (Georgia Highlands College). “Lab 10: Mitosis & Meiosis,” (2019). Biological Sciences Open Textbooks . 18. CC-BY
https://oer.galileo.usg.edu/biology-textbooks/18