Chapter 3

Lesson 2

Quick Summary

• Chromosomes contain instructions that control traits of organisms.
• Each organism has a certain number and kind of chromosomes in each of its cells.
• The DNA of a chromosome forms genes.

Understand

Why are chromosomes important?

They contain instructions that guide the activities of cells.

Infer

If you looked at a cell and were able to see its chromosomes, what would you know about the cell? Explain.

The cell would be ready to divide. The chromosomes in most cells are not visible at other times because the DNA and protein are spread throughout the nucleus.

Deduce

Suppose Organism A has 110 chromosomes and Organism B has 108 chromosomes. What do you know about the two organisms? Explain.

They are different species. Each species has a particular number of chromosomes.

Quick Summary

• Pairs of the same four base materials make up all DNA strands.
• The instructions that DNA gives a cell depend on the order of the base pairs.
• During mitosis, two complete strands of DNA are formed from one DNA strand. The two new strands are identical to the original.

Summarize

When a copy of a DNA strand is being made, where do the extra bases come from?

Extra bases float in the nucleus of a cell.

Assess

Tell what would happen if a copy of a strand of DNA had a mismatched base pair arrangement. Why?

A mutation would occur because the bases match in the pattern TA or AT, and GC or CG.

Compare

How are the bases on a rung of DNA like puzzle pieces?

They fit together only in a certain way.

Vocabulary

Mutate - to change, undergo mutation

Mutation - a sudden departure from the parent type in one or more heritable characteristics, caused by a change in a gene or a chromosome.

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Checkpoint p. 59

1. What is a gene?

A section of DNA within a chromosome. A series of base pairs or rungs (DNA).

2. Use the Internet to find out how Watson and Crick used Franklin’s X rays to figure out the structure of DNA.

Watson and Crick realized that strands of DNA looked like twisted ladders, that they learned how genes and chromosomes copy themselves in forming new cells, and patterns formed by base pairs.

Checkpoint p. 61

1. Describe the structure of DNA.

DNA is made up of pairs of bases. Two base pairs fit together to form the “rungs” of the DNA “ladder.”

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2. Why is it important that base A can pair only with base T and base G can pair only with base C?

The way the bases are paired determines the instructions they give to the cell. If any base could pair with any other base, the new strands of DNA would not be identical.

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3. How does DNA make an exact copy of itself?

DNA “unzips” and the base pairs come apart. Free-floating bases within the cell’s nucleus pair with the separated bases on the DNA strand. The process continues until two complete double strands of DNA are formed.