Why would you expect equal numbers of guanine and cytosine in a molecule of DNA?

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Solution

The correct option is C 32%According to Chargaff’s rule in a DNA molecule A = T and G = C and A + G + C + T = 100%. Here cytosine (C) is 18% hence G = 18% ∴ A + T = (100 – 36)% ∴ A + T = 64% ∴ A = 32%

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Which of the following choices displays complementary DNA strands?

Correct answer:

3'-ATTGC-5’

5’-TAACG-3’

Explanation:

When determining complementary strands, there are a few important points to consider. First, remember that uracil is only found in RNA. We can eliminate any answer choices that contain uracil. Second, remember that DNA is anti-parallel. This means that the 3' end of the template strand must match up with the 5' end of the complementary strand. Flip any answers that have both 3' ends together. Finally, make sure that the bases align correctly. Adenine and thymine should always pair and cytosine and guanine should always pair.

Template strand: 3'-ATTGC-5’

Complementary strand is antiparallel: 5'-_ _ _ _ _-3'

Complementary strand must have right base pairs: 5'-TAACG-3'

An alternate way of writing this answer would be 3'-GCAAT-5'. This answer would still be correct! It would simply need to be flipped, as in the second step outlined above.

Which of the following could be a segment of DNA?

Possible Answers:

5'-AGATAG-3'

3'-GATCTC-5'

5'-CTAGAG-3'

3'-GATCTC-5'

5'-CTCGTG-3'

3'-GATCTC-5'

5'-CTCGTG-3'

3'-AATGTA-5'

5'-GTCGAT-3'

3'-GATCTC-5'

Correct answer:

5'-CTAGAG-3'

3'-GATCTC-5'

Explanation:

DNA is a double stranded molecule that is antiparallel, meaning the 5' end of one strand matches up with the 3' end of the other strand. Nitrogenous bases hold the strands together via hydrogen bonding. These bases are adenine (A), thymine (T), guanine (G), and cytosine (C). Adenine (A) can only pair with thymine (T), and guanine (G) can only pair with cytosine (C).

The correct answer presents two strands that are antiparallel and show proper binding patterns.

5'-CTAGAG-3'

3'-GATCTC-5'

All other answers have an error in base pairing or ordering.

Which of the following describes the components of a nucleotide? 

Possible Answers:

A phosphate group, a hexose sugar, and a nitrogenous base 

A phosphate group, a pentose sugar, and an amino acid 

ATP, a pentose sugar, and a nitrogenous base 

ATP, a hexose sugar, and an amino acid 

A phosphate group, a pentose sugar, and a nitrogenous base

Correct answer:

A phosphate group, a pentose sugar, and a nitrogenous base

Explanation:

Nucleic acids are one of the main biological macromolecules, and include DNA and RNA. A nucleotide is a DNA monomer, while a ribonucleotide is an RNA monomer. Numerous nucleotides are bonded together by phosphodiester bonds to form a single molecule of DNA. The pattern of nucleotides is used to store and transmit hereditary information. A nucleotide is composed of a phosphate group, a nitrogenous base, and a pentose sugar (deoxyribose).

Which of the following DNA primers has the lowest melting point?

Possible Answers:

GCCACAGGTGGA

GCGGATGTGTCC

CATGCGTAGATC

TGACAAACTTGT

AGGTGGACACCG

Correct answer:

TGACAAACTTGT

Explanation:

Cytosine and guanine form three hydrogen bonds with each other, while adenine and tyrosine only form two hydrogen bonds. This means that strands of DNA with a higher percentage of cytosine and guanine will have higher melting points.

Since we are looking for the sequence with the lowest melting point, we want the lowest percentage of cytosine and guanine, and the highest percentage of adenine and thymine.

A section of double-stranded DNA is composed of 35% adenine bases. What is the percentage of cytosine bases in the section of DNA?

Possible Answers:

More information is needed in order to answer the question

Correct answer:

Explanation:

Since we know that 35% of the bases in the section of DNA are adenine, we can conclude that 35% of the bases are thymine. This is because adenine will always pair with thymine, so there will be just as many thymine bases as adenine bases. Together, adenine and thymine compose 70% of the segment.

This means that 30% of the section is composed of guanine-cytosine pairs.

Since these two bases will be equal in quantity, 15% of the DNA section will be cytosine bases.

In a sample of DNA, 30% of the bases are found to be guanine. What percentage of the bases in the sample are thymine?

Correct answer:

Explanation:

Guanine will pair with cytosine. From this knowledge, we can assume that there will be an equal number of guanine and cytosine residues in the sample. Each guanine must have a cytosine counterpart.

The total composition of the DNA sample must be accounted for by the sum of all the bases.

Use the known values for guanine and cytosine to find the sum of adenine and thymine.

Like cytosine and guanine, adenine and thymine must be present in equal amounts in order to form proper base pairs. We can reasonably assume that half of the remaining DNA will consist of each residue.

An analysis of a DNA sample indicates it contains 35% adenine. What percent of the sample would be cytosine? 

Correct answer:

Explanation:

There are four nitrogenous bases found in DNA: adenine, thymine, cytosine, and guanine. Adenine always binds with thymine, and cytosine always binds with guanine.

Since certain bases always appear in pairs, they will have equal percentages of the DNA composition. The percentage of adenine will equal the percentage of thymine, and the percentage of cytosine will equal the percentage of guanine. Together, these percentages will add to 100%.

We know that the sample is 35% adenine, which tells us that it is also 35% thymine.

We know that cytosine and guanine pair together and will be present in equal amounts, so we can divide this final total by 2 to find our answer.

The sample is 35% adenine, 35% thymine, 15% guanine, and 15% cytosine.

A strand of DNA was measured to contain 22% adenine. What is the DNA's composition of the other bases?

Possible Answers:

22% Adenine, 22% Cytosine, 28% Guanine, 28% Thymine 

22% Adenine, 22% Guanine, 28% Cytosine, 28% Uracil

22% Adenine, 22% Uracil, 28% Cytosine, 28% Guanine

22% Adenine, 22% Guanine, 28% Cytosine, 28% Thymine

22% Adenine, 22% Thymine, 28% Cytosine, 28% Guanine

Correct answer:

22% Adenine, 22% Thymine, 28% Cytosine, 28% Guanine

Explanation:

We can use Chargaff's rule to find the remaining compositional percentages. Adenine always pairs with thymine, so their percentages will be equal. Cytosine always pairs with guanine, so their percentages will also be equal. The sum of all four percentages must equal 100%.

We know that the sample is 22% adenine; this tells us it is also 22% thymine.

Since cytosine and guanine are present in equal amounts, we can simply divide their sum by 2.

The final composition is 22% adenine, 22% thymine, 28% cytosine, and 28% guanine.

Uracil is only found in RNA. 

A strand of RNA is composed of 20% adenine and 30% guanine.

What is the strand's composition of the other bases?

Possible Answers:

Cannot determine from the given information

20% Adenine, 30% Guanine, 30% Uracil, 20% Cytosine

20% Adenine, 30% Guanine, 20% Uracil, 30% Cytosine

20% Adenine, 30% Guanine, 20% Thymine, 30% Cytosine

20% Adenine, 30% Guanine, 30% Thymine, 20% Cytosine

Correct answer:

Cannot determine from the given information

Explanation:

Chargaff's rule only applies to DNA. RNA is single-stranded, and thus, no base pairing occurs.

Think of a strand of DNA. Each base pairs with a specific partner, allowing us to determine their percentages: adenine and thymine are always equal, and cytosine and guanine are always equal. In RNA, with this pairing absent, there is no correlation between the base percentages. A strand could be 20% adenine, 30% guanine, 5% cytosine, and 45% uracil; we simply cannot draw any conclusions.

What are the monomers of DNA?

Possible Answers:

Phosphate-sugar backbone

Nucleotides

Deoxyribose sugars

Nitrogen atoms

Correct answer:

Nucleotides

Explanation:

A polymer is a macromolecule that is made up of subunits that are repeated or very similar. These subunits are called monomers. DNA is a polymer made up of monomer units called nucleotides. Nucleotides are made up of a phosphate group, a five-carbon sugar (deoxyribose, in the case of DNA), and a variable nitrogenous base. There are four different nucleotides that make up the polymer of DNA: thymine, cytosine, adenine, and guanine. These four nucleotides belong to two different classes based on structure. Adenine and guanine are purines that have two carbon-nitrogen rings. Thymine and cytosine are pyrimidines that have only one carbon-nitrogen ring.

All AP Biology Resources

Why would you expect equal numbers of guanine and cytosine in a molecule of DNA but not necessary in a molecule of RNA?

Are there always going to be an equal number of guanine and cytosine in a DNA molecule?

Is there always an equal number of guanine and cytosine Why?

Are the amounts of cytosine and guanine in a nucleotide of DNA the same?