Reading Comprehension

Source: Knewton

For the past twenty years, forensic scientists involved in criminal investigations have been making use of the DNA fingerprinting process to identify perpetrators of illicit acts. The fingerprinting process takes advantage of the fact that each individual is composed of a unique and particular genetic make-up. Even monozygotic twins (commonly known as identical twins) are not two equal manifestations of the same genetic copy; each twin embodies certain characteristics borne from distinct genetic mutations that occurred in that twin, separately, during development. Restriction fragment length polymorphism (RFLP) analysis is a technique used to compare samples of genetic material made up of billions of nucleotides, which are the primary component molecules of DNA. Restriction enzymes are the key to RFLP analysis: these enzymes recognize specific palindromic nucleotide sequences along a strand of DNA and cut, or "splice," the DNA wherever they encounter the sequence in question. By comparing DNA fragment lengths of a nucleotide polymer after splicing, laboratory analysts expose variation in the nucleotide sequences of distinct samples of DNA. In RFLP analysis, DNA samples-such as those that have been treated with restriction enzymes-undergo a process called gel electrophoresis, which employs an electrical field to pull the DNA through a cross linked polymer gel. The negatively charged end of any DNA strand or fragment is then attracted to the positively charged cathode of the apparatus, where geneticists can observe and sort the genetic material by length and mass.

Although RFLP analysis was the first profiling technique to gain widespread application, most DNA scientists currently prefer to analyze short tandem repeat (STR) regions of DNA. STR sequences are segments of DNA located on non-coding regions where nucleotide patterns are repeated over and over again next to each other. STR sequences are never expressed genetically; in fact, DNA fingerprinting is the only purposeful application of STR sequences that has yet been uncovered. The discovery of variable number of tandem repeats among distinct individuals led to the development of STR analysis, which measures the exact number of repeating units and compares it to the number of repeating units in another DNA sample. One way this method differs from RFLP analysis is that STR analysis uses probes to attach markers to certain STR regions on the strand of DNA. Then, a polymerase chain reaction (PCR) copying process, which can reliably turn a small number of DNA molecules into billions within several hours, is employed to discover the lengths of the short tandem repeats.

The STR method has been used to identify 13 different standardized loci-locations on a chromosome-that are recognizable to all DNA scientists and are used throughout the United States to identify genetic differences. Public and private forensic laboratories use these markers to generate unique DNA profiles. The high variability in the nature of the STR regions under analysis for forensic testing results in a high level of discrimination between one DNA profile and another; the likelihood that any two individuals will have the same 13-loci DNA profile can be as low as one in one billion.

Q1
The passage suggests that which of the following can be inferred about non-coding regions of a DNA strand?

A: They can only be analyzed using STR analysis.
B: They do not provide information relevant to DNA fingerprinting.
C: They cannot be analyzed using RFLP analysis techniques.
D: They consist primarily of repeating STR nucleotide patterns.
E: They carry segments with no known genetic function. *

Q2
The author mentions the probes involved in STR analysis in order to make which of the following points?

A: The STR analysis method analyzes non-coding regions that are never expressed genetically.
B: Probes are used in the STR analysis method to identify palindromic nucleotide DNA sequences.
C: STR analysis does not use molecules that recognize and cut palindromic nucleotide DNA sequences. *
D: The length of tandem repeats will vary between samples of DNA analyzed using STR analysis.
E: The probes in STR analysis perform the same function on DNA as restriction enzymes do in RFLP analysis.

Q3
The author of the passage would most likely agree with which of the following statements?

A: DNA fingerprinting is the most accurate method available by which to use genetic markers to identify perpetrators in criminal investigations.
B: The most effective strategy for DNA analysts would be to employ both RFLP and STR analysis techniques in DNA profiles, as each has value independent from the other.
C: The low likelihood that subjects have identical nucleotide sequences is the primary factor underlying the success of DNA fingerprinting. *
D: RFLP analysis should no longer be used to analyze DNA samples involved in criminal investigations.
E: The discovery of new standardized loci would result in a decrease in the level of discrimination between the DNA profiles of distinct individuals.

Q4
According to the passage, what would be the most likely result if gel electrophoresis were to be performed on a strand of DNA that had not yet been treated with restriction enzymes?

A: Fragments of DNA would be separated by mass, but not by length.
B: The DNA strand would be cut at locations other than at palindromic nucleotide sequences.
C: DNA fragments would move unrestricted through the cross linked polymer gel toward a positive cathode.
D: Only one piece of genetic material would be pulled through the cross linked polymer gel. *
E: Fragments of DNA would be separated according to length with the aid of an electrical field.


Can someone please explain about these problems? I did wrong in all four.