Question 1

DNA is isolated from a virus and found to contain 27% A, 27% T, 23% C and 23% G. The DNA is likely to be:

Accepted Answer

The percentages of A, T, C, and G add up to 100%, indicating that the DNA is double stranded. Additionally, single stranded DNA would not have equal percentages of each base since it would not require base pairing. The presence of RNA contamination or the circular nature of the DNA cannot be determined from the information given.

Question 2

Which one of the following is LEAST likely to be found in a nucleotide?

Accepted Answer

A nucleotide is made up of a five-carbon sugar, a nitrogen-containing base, and one or more phosphate groups. A side chain, also known as a side group, is typically found in amino acids, which are the building blocks of proteins, and not in nucleotides.

Question 3

Which one of the following represents an actual Watson-Crick base pair with the LARGEST number of hydrogen bonds?

Accepted Answer

Cytosine and guanine form a base pair with three hydrogen bonds, while all other base pairs form two hydrogen bonds.

Question 4

In a DNA strand, successive nucleotides are linked by:

Accepted Answer

Nucleotides in a DNA strand are linked by 3'-5' phosphodiester bonds between the sugar (3' carbon) of one nucleotide and the phosphate (5' carbon) of the next nucleotide.

Question 5

Sometimes the sequence of DNA gets mutated and an adenine is paired to a cytosine. Why is this interaction unstable?

Accepted Answer

Adenine usually pairs with thymine through hydrogen bonds, while cytosine pairs with guanine through hydrogen bonds in DNA. Adenine and cytosine have different chemical groups that form hydrogen bonds, and their positions are not compatible with each other, leading to instability in the interaction.

Question 6

In double-stranded DNA, the sugar-phosphate backbones are:

Accepted Answer

In the double helix structure of DNA, the sugar-phosphate backbones are on the outside, and there are two grooves of unequal size known as the major and minor grooves. This arrangement is crucial for the binding of proteins to specific DNA sequences.

Question 7

Which one of the following activities would LEAST likely be found in a topoisomerase?

Accepted Answer

Topoisomerases are enzymes that are involved in the regulation of DNA supercoiling. They can relax or induce twists in double-stranded DNA, cut phosphodiester linkages, and reform them. However, they do not add nucleotides to the end of a DNA strand, so choice C would be the least likely activity found in a topoisomerase.

Question 8

In double-stranded DNA, the amount of A equals that of T and the amount of C equals that of G because:

Accepted Answer

The complementary base pairing between A and T, and C and G ensures that the amount of each base is equal in double-stranded DNA. This is because A always pairs with T and C always pairs with G, forming hydrogen bonds between the two strands. Therefore, if there is a certain amount of A on one strand, there must be an equal amount of T on the complementary strand, and the same goes for C and G.

Question 9

Which type of protein does DNA wrap around so that it can be packaged in a cell?

Accepted Answer

DNA wraps around histone proteins to form chromatin structure, which allows the DNA to be packaged tightly within the cell nucleus.

Question 10

If the first nucleotide after the promoter on the top DNA strand is changed from G to T, what would the resulting nucleotide be on the mRNA?

Accepted Answer

During transcription, RNA polymerase synthesizes mRNA in a 5' to 3' direction, using the DNA template strand. The mRNA is complementary to the DNA template strand. If the DNA sequence changes from G to T on the coding strand (which corresponds to the top strand mentioned), the base on the mRNA that pairs with this changed nucleotide would be A, because RNA uses uracil (U) instead of thymine (T), but since the option for U is not given and the question likely refers to the base that pairs with T during transcription, A (adenine) is the correct answer.

Question 11

In the DNA of certain bacterial cells, 16% of the nucleotides are adenine. What are the percentages of the other nucleotides in the bacterial DNA?

Accepted Answer

According to Chargaff's rules, in DNA, the amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C). Since 16% of the nucleotides are adenine, 16% must also be thymine. This leaves 68% for guanine and cytosine combined, so each must be 34%.

Question 12

Which of the following statements about DNA is/are CORRECT?

Accepted Answer

DNA typically exists as a double helix with about 10 base pairs per turn, and the strands coil to the right. Adenine pairs with thymine, not cytosine, and nucleotides are linked by phosphodiester bonds, not peptide bonds.

Question 13

Which one of the following statements about RNA is INCORRECT?

Accepted Answer

RNA is typically single-stranded, unlike DNA, which is usually double-stranded.

Question 14

A five-carbon sugar connected to a purine or pyrimidine ring is MOST accurately referred to as a(n) _____.

Accepted Answer

The answer of A five-carbon sugar connected to a purine...

Question 15

Why aren't nucleosides incorporated into DNA?

Accepted Answer

Nucleosides are composed of a nitrogenous base and a sugar molecule but lack the phosphate group necessary for the formation of phosphodiester bonds that link nucleotides together to form DNA. Therefore, nucleosides cannot be incorporated into DNA.

Question 16

Occasionally, a double-stranded DNA molecule contains a uracil base (U) instead of T. If a U were present in the template strand of DNA, what base do you think would be incorporated into the RNA transcript at that position?

Accepted Answer

RNA polymerase recognizes and pairs the uracil (U) with adenine (A) for transcription, instead of thymine (T) which pairs with adenine (A) in DNA replication. Therefore, if the template strand contains U instead of T, RNA polymerase will incorporate A in the RNA transcript instead of T.

Question 17

In a nucleotide, the phosphate is attached to the _____ carbon of the sugar, and the base is attached to the _____ carbon of the sugar.

Accepted Answer

In a nucleotide, the phosphate group is attached to the 5' carbon of the sugar (usually ribose or deoxyribose), and the nitrogenous base is attached to the 1' carbon of the sugar.

Question 18

If the first nucleotide after the promoter on the bottom DNA strand in the simulation is changed from C to T, what would the resulting nucleotide be on the mRNA? Test your prediction by making the change and running the simulation.

Accepted Answer

During transcription, DNA is used as a template to make mRNA. The DNA base C (cytosine) pairs with G (guanine) in DNA, but when transcribed to mRNA, the pairing changes so that the DNA's G pairs with mRNA's C. If the DNA base changes from C to T (thymine), then the corresponding mRNA base would change to A (adenine). However, RNA does not use thymine; instead, it uses uracil (U) to pair with adenine. Therefore, if the DNA base changes from C to T, the resulting mRNA base would be U (uracil).

Question 19

Chromatin is composed of:

Accepted Answer

Chromatin is composed of DNA wrapped around histone proteins. RNA may also be present, but it is not a primary component of chromatin. Acidic proteins are not typically part of chromatin composition. Chromatin is not surrounded by a membrane as it is found within the nucleus of a cell.

Question 20

Which one of the following nucleotides hydrogen base pairs with adenine in an RNA molecule?

Accepted Answer

In RNA, uracil base pairs with adenine. Cytosine base pairs with guanine. Thymine is not found in RNA, it is replaced by uracil. Therefore, the correct answer is (C) uracil.

Quiz 3: Nucleic Acids and Transcription