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SAT Writing and Language Post-Test

SAT Writing and Language Post-Test

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?