Reading - Unmasking the Main Idea for CBEST Exam Study Guide
Main idea questions can be put in three categories. The first asks for a simple sentence or title that includes the main topic of the passage. The second asks questions about the author's thoughts or intentions in writing the passage. Then there are those that ask for a paraphrase of all the main ideas in the passage.
Main Idea Questions
Main idea questions take a variety of forms:
- What is the main idea of the passage?
- The best title for this passage would be …
- What is the theme of the passage?
- The central thought of the passage is …
How to Find Main Idea Answers in the Passage
To answer main idea questions, you sometimes do not have to read the whole passage. The main idea is often stated at the beginning or end of the passage. Sometimes you can glean the main idea by looking for the topic sentences of each paragraph of the passage.
Sample Passage and Question
This passage continues the passage on DNA that you began in the previous section.
The goal is to discover the sequence of bases in the DNA. If this is a mitochondrial DNA fragment, the sequence will match the person's mother and maternal relatives. The DNA is divided down the center like a zipper. Heat is used to cause the division. Only one half of the DNA (one side of the zipper) is used. The sequence of bases will be discovered by re-creating the other half of the DNA.
The next goal is to use the half of the DNA that was saved to reconstruct the other half. This process will show the sequence of bases. Bases A and T always bind to each other, and bases C and G always bind to each other. The idea is to put one-half of a DNA strand in a test tube with some free bases and an enzyme that causes the free bases to attach to the half strand, rezipping the zipper. Modified bases are also added so that the location of that base on the "zipper" can be marked. In this way, the sequence of bases can be discovered. Each test tube contains thousands of copies of the saved half of the DNA strand, and a radioactive primer that will attach at the start location of every strand so that all operations start at the same place on every strand. Also included are DNA polymerase, which is an enzyme that acts as a "glue" to attach the free bases to the half DNA strand, and all four bases, which are free and unattached. There is also a modified base; each test tube has a different base that has been modified to act as a marker during reconstruction.
At the end of the process, thousands of reconstructed strands will be in each test tube. Some of these strands will be complete, but some of them will have been terminated by a modified base so they will be shorter. Each strand will have the same sequence of bases, but the strands will terminate at different positions where A is found. The reason that some of the strands do not terminate at base A is that a normal instead of a modified base A has attached at some of the base A locations. There is a test tube for each base. Therefore, there are reconstructed strands that terminate with C in the test tube with modified C bases, strands that terminate with G in the test tube with modified G bases, and strands that terminate with T in the test tube with modified T bases. The four test tubes are used in order to tell the difference between bases, since the strands and bases all look alike. However, the same test can be done in one test tube if fluorescent dyes are used to tag the modified bases.
- Which of the following best describes the main topic of the passage?
- DNA can be linked to clues in a criminal investigation.
- Learning about the genetic code is important.
- The role of modified bases is part of the pieces of reading DNA.
- Dyes are used to tell the difference between bases.
- Reading base A strands is done at the end of the process.
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