SAT II Biology M : DNA Structure and Function

Study concepts, example questions & explanations for SAT II Biology M

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Example Questions

Example Question #64 : Sat Subject Test In Biology

If a structural gene in an organism's genome is comprised of 29% guanine nucleotides, what percentage of the gene is comprised of cytosine nucleotides?

Possible Answers:

29%

19%

42%

It cannot be determined from the given information.

21%

Correct answer:

29%

Explanation:

This question is designed to catch a) students who are not reading the question carefully, and b) students unsure of which nucleotides pair with which.

The correct answer is 29%, because cytosine pairs with guanine in a 1:1 ratio. If you answered 21%, then you likely thought the question was more complex than it was.

Example Question #1 : Dna Structure And Function

There is a certain type of chemical bonding between the paired nucleotides on each strand of DNA which helps maintain the double-helix structure of DNA by attracting each strand to the other. What type of bonding is responsible for this?

Possible Answers:

Peptide

Ionic

Covalent

Phosphodiester

Hydrogen

Correct answer:

Hydrogen

Explanation:

The correct answer is hydrogen bonding, and each nucleotide attracts its pairing mate because they have corresponding number of hydrogen bonds. Adenine is attracted to thymine to create two hydrogen bonds, and cytosine is attracted to guanine to form three hydrogen bonds. While phosphodiester bonds are very important in creating the strand of DNA, they are not the bond that keeps the two strands in the double helix structure.

Example Question #66 : Sat Subject Test In Biology

With respect to DNA, the terms 3' and 5' (pronounced 3-prime and 5-prime, respectively) are used in order to refer to one strand or the other. What do these two terms signify?

Possible Answers:

3' refers to the unbound methyl group and 5' refers to the unbound hydroxyl group at the end of each DNA strand.

3' refers to the unbound hydroxyl group and 5' refers to the unbound carboxyl group at the end of each DNA strand.

3' refers to the unbound sulfhydral group and 5' refers to the unbound phosphate group at the end of each DNA strand.

3' refers to the unbound phosphate group and 5' refers to the unbound hydroxyl group at the end of each DNA strand.

3' refers to the unbound hydroxyl group and 5' refers to the unbound phosphate group at the end of each DNA strand.

Correct answer:

3' refers to the unbound hydroxyl group and 5' refers to the unbound phosphate group at the end of each DNA strand.

Explanation:

When nucleotides bond together and form DNA strands, the first and last nucleotides in the strand have slightly different structures than the rest of the nucleotides between them. On one end of the strand, the nucleotide has an exposed hydroxyl group bound to the third carbon in the carbon ring: this end of the strand is thus called 3'. On the opposite end of the strand, the nucleotide has a phosphate group attached to the 5' carbon in the carbon ring, and is thus called the 5' end. These two groups are exposed because they are used in the bonding of nucleotides to one another to form the strand, but each strand ends with one nucleotide that only is bound on one side: thus, leaving either the hydroxyl or phosphate group exposed (depending on which end you are observing).

These terms are useful because they allow us to discuss the directionality of DNA-related events- if we didn't have terms for directionality the concept would be much more confusing. Example: "DNA polymerase synthesizes the new DNA strand in the 5'-3' direction." Without 3'/5' how would we determine which way the reaction occurs?

Example Question #2 : Dna Structure And Function

If a structural gene in an organism's genome is comprised of 33% adenine nucleotides, what percentage of the gene is comprised of cytosine nucleotides?

Possible Answers:

33%

34%

It cannot be determined from the given information.

17%

22%

Correct answer:

17%

Explanation:

According to Chargaff's rule, DNA nucleotides pair in a 1:1 ratio. Therefore, if we know how much of the particular gene is made up of one nucleotide, we can extrapolate that known variable to find the other three unknown variables.

To do so, you must remember that adenine pairs with thymine, and cytosine pairs with guanine (A-T, C-G), and that since the ratio between each pair is 1:1 then a gene with 33% adenine must also have 33% thymine. Combine these numbers and subtract from 100: the number leftover is the % of total cytosine and guanine in the gene.

100% - 66% = 34%

Finally, since we know that 34% of the DNA is both C and G, and that the ratio between C-G is 1:1, C and G must both be 17%.

Example Question #3 : Dna Structure And Function

A sample of DNA is sequenced and found to contain  guanine. What percentage of thymine does it contain?

Possible Answers:

Correct answer:

Explanation:

In DNA, guanine pairs with cytosine and adenine pairs with thymine. In RNA, which does not have thymine, adenine pairs with uracil. Thus, if a sample contains  guanine, it also contains  cytosine. Together, the two make up  of the  total. The remaining  is divided evenly between the paired adenine and thymine molecules, so the DNA sample contains  percent each of adenine and thymine.  is the correct answer.

Example Question #4 : Dna Structure And Function

Which of the following is NOT found in DNA?

Possible Answers:

Ribose

Phosphate groups

Cytosine

Thymine

Deoxyribose

Correct answer:

Ribose

Explanation:

DNA and RNA are both made of sugar-phosphate backbones. Ribose is the sugar found in RNA; deoxyribose is the sugar found in DNA. DNA also contains the nucleic acid bases adenine, guanine, cytosine, and thymine. Both DNA and RNA contain phosphate groups as part of the backbone.

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