Name Class Date. 4. How many chromosomes does a human cell have before dividing? a. unlimited b. 23 c. 46 d. 12

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1 Skills Worksheet Directed Reading B Section: How DNA Works 1. How much DNA does a human cell contain? a. 30,000 m b. less than 1 m c. about 2 m d. more than 10 m UNRAVELING DNA 2. What is DNA often bundled into? a. proteins b. chromosomes c. bases d. traits 3. What is a string of nucleotides called? a. traits b. loops c. a cell d. a gene 4. How many chromosomes does a human cell have before dividing? a. unlimited b. 23 c. 46 d. 12 Holt Science and Technology 11 Genes and DNA

2 GENES AND PROTEINS 5. What do three bases form? a. DNA b. gene c. chromatin d. amino acid 6. What does each gene have instructions for making? a. a protein b. a cell c. an amino acid d. DNA Proteins and Traits 7. What do proteins act as? a. organisms b. bases c. codes d. chemical triggers and messengers Help from RNA 8. What other type of molecule helps make proteins? a. RAD b. RNA c. DNR d. DTR Holt Science and Technology 12 Genes and DNA

3 The Making of a Protein Read the words in the box. Read the sentences. Fill in each blank with the word or phrase that best completes the sentence. ribosome transfer RNA messenger RNA 9. A mirrorlike copy of a DNA segment is called 10. The RNA copy goes through a protein assembly line called a 11. The message is translated by and a protein is made. CHANGES IN GENES Mutations Circle the letter of the best answer for the question. 12. Changes in the bases of DNA are called what? a. extras b. roller coasters c. accidents d. mutations Holt Science and Technology 13 Genes and DNA

4 Read the words in the box. Read the sentences. Fill in each blank with the word or phrase that best completes the sentence. deletion substitution insertion 13. When a base is left out, the change is called a(n) 14. When an extra base is added, the change is called a(n) 15. When the wrong base is used, the change is called a(n) Do Mutations Matter? 16. Which of these does not result from mutation? a. an improvement b. a harmful change c. cell repair d. no change How Do Mutations Happen? 17. What is one way mutations happen? a. because of ribosomes b. because DNA wears out c. because of random errors d. because of sugars Holt Science and Technology 14 Genes and DNA

5 18. What are agents that might damage DNA called? a. ribosomes b. mutagens c. generations d. regulations AN EXAMPLE OF SUBSTITUTION 19. What can a mutation produce? a. a bad sunburn b. transfer RNA c. the wrong protein d. cigarette smoke 20. What is caused by of a substitution mutation? a. blue eyes b. sickle cell disease c. height d. red hair USES OF GENETIC KNOWLEDGE Genetic Engineering 21. What is it called when scientists change an organism s genes? a. DNA fingerprinting b. genetic engineering c. genetics d. transfer RNA Holt Science and Technology 15 Genes and DNA

6 22. What happened when scientists added a firefly gene to a plant? a. The firefly glowed. b. The plant died. c. The plant glowed. d. The plant vanished. Genetic Identification 23. How are the unique patterns in DNA identified? a. DNA fingerprinting b. genetic engineering c. genetics d. transfer RNA 24. What is an organism that has an exact copy of another s DNA called? a. a twin b. a brother or sister c. a clone d. a child Holt Science and Technology 16 Genes and DNA