Eukaryotic Cells and Their DNA Prokaryotic cells have a single chromosome, but eukaryotic cells have many chromosomes. Eukaryotes have more DNA than prokaryotes do. Different kinds of eukaryotes have different numbers of chromosomes. More-complex eukaryotes do not always have more chromosomes than simpler eukaryotes do. For example, fruit flies have 8 chromosomes, potatoes have 48, and humans have 46. Figure 2 shows the 46 chromosomes of a human body cell lined up in pairs. These pairs are made up of similar chromosomes known as homologous chromosomes (hoh MAHL uh guhs KROH muh SOHMZ). Although chromosomes vary in size, the homologous chromosomes in each pair will be very similar. How many chromosomes are in the cells of the human body?
7.2.e
Making More Eukaryotic Cells The eukaryotic cell cycle has three stages. In the first stage, called interphase, the cell grows and copies its organelles and chromosomes. After each chromosome is duplicated, the two copies are called chromatids. Chromatids are held together at a region called the centromere. The joined chromatids condense into an X shape, as shown in Figure 3. After this step, the cell enters the second stage of the cell cycle. In the second stage, the chromatids separate. The process by which chromosomes separate is called mitosis. During mitosis, each new cell receives a copy of each chromosome. Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. In the third stage, cytokinesis, the cell splits into two cells called daughter cells. Cytokinesis is the division of cytoplasm and all of the materials in the cytoplasm. The new daughter cells are exactly the same as each other and as the original cell. For example, each daughter cell receives exactly the same number of chromosomes.
Figure 3 This duplicated chromosome consists of two chromatids. The chromatids are joined at the centromere.
Chromatids Centromere
Figure 2 Human body cells have 46 chromosomes, or 23 pairs of chromosomes.
Mitosis Adventure How does a cell change during a cell cycle? Describe cell division from inside the cell. Go to go.hrw.com, and type in the keyword HY7ACTW.
7.1.b Students know the characteristics that distinguish plant cells from animal cells, including chloroplasts and cell walls. 7.1.e Students know cells divide to increase their numbers through a process of mitosis, which results in two daughter cells with identical sets of chromosomes. 7.2.e Students know DNA (deoxyribonucleic acid) is the genetic material of living organisms and is located in the chromosomes of each cell.
Section 2
The Cell Cycle
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Figure 4
The Cell Cycle
Interphase Before mitosis begins, chromosomes are copied. Each chromosome becomes two chromatids.
Mitosis Phase 1 (Prophase) Mitosis begins. Chromosomes condense from long strands into rodlike structures.
Mitosis Phase 2 (Metaphase) The nuclear membrane is dissolved. Paired chromatids align at the cell’s equator.
Mitosis and the Cell Cycle Figure 4 shows the cell cycle and the phases of mitosis in an animal cell. Mitosis has four phases, as shown and described above. This diagram shows only four chromosomes to make it easy to see what’s happening inside the cell.
Cytokinesis Cell plate
Figure 5 When a plant cell divides, a cell plate forms and the cell splits into two cells.
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Chapter 5
In animal cells and other eukaryotes that do not have cell walls, division of the cytoplasm begins at the cell membrane. The cell membrane begins to pinch inward to form a groove. Eventually, the cell is pinched in half, and two daughter cells form. Cytokinesis is shown at the last step of Figure 4. Eukaryotic cells that have a cell wall—such as the cells of plants, algae, and fungi—go through cytokinesis differently. In this kind of cell, a cell plate forms in the middle of the cell. The cell plate contains the materials for the new cell membranes and for the new cell walls that will separate the new cells. After the cell splits in two, a new cell wall forms where the cell plate was. The cell plate and a late stage of cytokinesis in a plant cell are shown in Figure 5. What is the difference between cytokinesis in an animal cell and cytokinesis in a plant cell? 7.1.b
The Cell in Action
Mitosis Phase 3 (Anaphase) The paired chromatids separate and move to opposite sides of the cell.
Mitosis Phase 4 (Telophase) A nuclear membrane forms around each set of chromosomes, and the chromosomes decondense. Mitosis is complete.
Cytokinesis In a cell that lacks a cell wall, the cell pinches in two. In a cell that has a cell wall, a cell plate forms and separates the cells into two new cells.
Quick Lab The Mitosis Flipbook The process of mitosis involves four phases, but the transition from one phase to the next involves many intermediate stages. In this activity, you will illustrate and assemble a flipbook. The flipbook will show the phases and intermediate stages of mitosis. 1. Punch two holes near the upper edge of 20 index cards. 2. Take four cards from your stack. List a different phase of mitosis on each card. 3. Review the images above. Draw a diagram of each phase of mitosis on the correct card. 4. Use colored pencils to make each chromosome a different color. On each card where the cell part is present, label one example of the chromosome, centromere, and nuclear membrane. 5. On the blank cards, draw the intermediate stages between each phase. Show the gradual transitions of mitosis by making small changes on each card.
6. Put your cards in order. Place a brad in each hole to assemble your flipbook. 7. In what phase of mitosis does the nuclear membrane dissolve? 8. This flipbook shows the actions of chromosomes, centromeres, and the nuclear membrane during mitosis. Is it acceptable to leave out what is happening to the rest of the cell’s organelles? Explain your answer. 9. Why is it important for eukaryotic cells to undergo mitosis?
7.1.e 7.7.d
30 min
Section 2
The Cell Cycle
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