Quick Lab
OPEN-ENDED INQUIRY
Modeling DNA Replication
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Cut out small squares of white and yellow paper to represent phosphate and sugar molecules. Then, cut out small strips of blue, green, red, and orange paper to represent the four nitrogenous bases. Build a set of five nucleotides using your paper strips and tape. Look back at Figure 12–5 if you need help.
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Using your nucleotides, tape together a single strand of DNA. Exchange strands with a partner.
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Model DNA replication by creating a strand that is complementary to your partner's original strand.
Use Models Taping together the nucleotides models the action of what enzyme?
Evaluate In what ways does this lab accurately represent DNA replication? How could you improve the lab to better show the steps of replication?
Analyze and Conclude
Telomeres DNA at the tips of chromosomes are known as telomeres (Figure 12–9). This DNA is particularly difficult to replicate. Cells use a special enzyme, called telomerase, to solve this problem by adding short, repeated DNA sequences to the telomeres. In rapidly dividing cells, such as stem cells and embryonic cells, telomerase helps to prevent genes from being damaged or lost during replication. Telomerase is often switched off in adult cells. In cancer cells, however, telomerase may be activated, enabling these cells to grow and proliferate rapidly.
FIGURE 12–9 Telomeres The telomeres are the white (stained) part of the blue human chromosomes.
Replication in Living Cells
How does DNA replication differ in prokaryotic cells and eukaryotic cells?
DNA replication occurs during the S phase of the cell cycle. As we saw in Chapter 10, replication is carefully regulated, along with the other critical events of the cycle so that it is completed before a cell enters mitosis or meiosis. But where, exactly, is DNA found inside a living cell?
The cells of most prokaryotes have a single, circular DNA molecule in the cytoplasm, containing nearly all the cell's genetic information. Eukaryotic cells, on the other hand, can have up to 1000 times more DNA. Nearly all of the DNA of eukaryotic cells is found in the nucleus, packaged into chromosomes. Eukaryotic chromosomes consist of DNA, tightly packed together with proteins to form a substance called chromatin. Together, the DNA and histone molecules form beadlike structures called nucleosomes, as described in Chapter 10. Histones, you may recall, are proteins around which chromatin is tightly coiled.
Table of Contents
- Formulas and Equations
- Applying Formulas and Equations
- Mean, Median, and Mode
- Estimation
- Using Measurements in Calculations
- Effects of Measurement Errors
- Accuracy
- Precision
- Comparing Accuracy and Precision
- Significant Figures
- Calculating With Significant Figures
- Scientific Notation
- Calculating With Scientific Notation
- Dimensional Analysis
- Applying Dimensional Analysis