SAT Writing and Language Practice Test: DNA Analysis in a Day

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DNA Analysis in a Day

Jane Saunders, a forensic DNA specialist, arrives at work and finds a request waiting for her: She needs to determine if the DNA of a fingernail with a few skin cells on it 1 match any records in the criminal database.

"Human DNA is a long, double-stranded 2 molecule; each strand consists of a complementary set of nucleotides," she explains. "DNA has four nucleotides: 3 adenine (A), thymine (T), guanine (G) and cytosine (C). On each strand is a sequence of nucleotides that 'match,' or pair up with the nucleotides on the other, or complementary, strand. 4 As a result, when there is an adenine on one strand, there is a thymine on the complementary strand, and where there is guanine on one strand, there is cytosine on the complementary strand."

She begins by 5 moving the DNA from the rest of the sample, transferring it to a 6 reaction tube. She adds a solution of primers, DNA polymerase, and nucleotides. Her goal is to separate the two strands of the DNA molecules and then make complementary copies of each strand.

After mixing the primers, DNA polymerase, and nucleotides with the evidence DNA, Saunders closes the reaction tube and puts it in a thermocycler. It is programmed to raise the temperature to 94°C to separate the double strands into single strands, and then lower the temperature to 59°C to attach the primers to the single strands. Finally, it raises the temperature to 72°C for the DNA polymerase to build the complementary strands. The thermocycler holds each temperature for one minute and repeats the cycle of three temperatures for at least 30 cycles. At the end of each cycle, the number of DNA segments containing the sequence marked by the primers doubles. If the original sample contains only 100 DNA strands, the 7 absolute final solution will have billions of segments. 8

[1] After a short lunch break, Saunders needs to separate and identify the copied DNA segments. [2] She had used primers that bind to 13 specific sites in human DNA called short tandem repeats, or STRs. [3] The 13 STRs are segments of four nucleotides that repeat, such as GATAGATAGATA. [4] "Now here's where the real magic happens!" Saunders says excitedly. [5] "Most DNA is identical for all humans. [6] But STRs vary greatly. [7] The chances of any two humans—other than identical twins—having the same set of 13 STRs is less than one in one trillion." 9

Saunders knows that the detectives will be 10 prepared to hear her findings, so she sits down at her desk to compare her results with the criminal database in the hopes of finding a match. 11 Is it possible that too much time is spent identifying DNA in cases that are relatively easy to solve?


  • B. matches
  • C. has matched
  • D. will be matching


  • B. molecule, each strand consists
  • C. molecule each strand consists
  • D. molecule but each strand consists


  • B. adenine (A), thymine (T), guanine (G), and cytosine (C)
  • C. adenine (A), thymine (T) guanine (G) and cytosine (C)
  • D. adenine (A) thymine (T), guanine (G) and cytosine (C)


  • B. Specifically,
  • C. However,
  • D. Similarly,


  • B. reviewing
  • C. changing
  • D. detaching

6. Which choice most effectively combines the sentences at the underlined portion?

  • A. reaction tube since she adds
  • B. reaction tube, however, she adds
  • C. reaction tube, and adding
  • D. reaction tube, she adds


  • B. absolutely the final solution
  • C. the final solution
  • D. the most final solution

8. Which sentence could be added to the beginning of the paragraph to most effectively establish the central idea?

  • A. The process of testing the DNA includes several steps and many changes in temperature.
  • B. The object of testing the DNA is to recreate many strands of the DNA in question.
  • C. Saunders uses a variety of machines in order to analyze the DNA.
  • D. Saunders would be unable to identify the DNA without the thermocycler.

9. Where should sentence 1 be placed to make the paragraph feel cohesive?

  • A. Where it is now
  • B. After sentence 2
  • C. After sentence 3
  • D. After sentence 4


  • B. eager
  • C. impatient
  • D. conditioned

11. Which choice most effectively establishes a concluding sentence for the paragraph and the passage?

  • B. It takes a good deal of work and expense to identify DNA in the world of modern forensics.
  • C. She takes pride in the fact that her scientific expertise plays such a key role in bringing criminals to justice.
  • D. She marvels at how far science has come in DNA analysis.