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

DNA secondary structure, the double helix, is held together by hydrogen bonds between base pairs. Specifically, adenine bases pair with thymine bases and guanine bases pair with cytosine bases. Heating a DNA sample disrupts these hydrogen bonds, thus “unwinding” the double helix and denaturing the DNA. Because three hydrogen bonds form between guanine/cytosine base pairs and two hydrogen bonds form between adenine/thymine base pairs, more energy is required to denature the former. DNA with a greater number of guanine/cytosine base pairs denatures at a higher temperature than adenine/thymine base pairs. In fact, there is a linear relationship between the amount of guanine and cytosine in a given DNA molecule, known as the GC content, and the temperature at which the double helix will denature, called the melting point (denoted as tm).

Imagine you have a number of different DNA samples each measuring 250 base pairs in length. Suppose that the samples are dissolved in a buffer containing 1 M NaCl and that they differ only in their GC content. If you were to calculate the melting point for the different DNA molecules, and plot it against the molecule’s GC content, you would get a line, as shown below:

In general, the linear equation used to calculate the melting point of a DNA molecule (in °C) is,

where Na+ is the molar concentration (moles/L) of sodium ions and the length of DNA is measured in base pairs (bp). In the following exercises, assume that [Na+] = 100 mM which implies the melting temperature of a DNA molecule is given by,

Use this equation to answer the following questions:

Calculate the melting temperature of a DNA molecule.

Determine the % GC content of a DNA molecule given its melting temperature.

Compare the melting temperature of two DNA molecules.

Determine the maximum melting temperature for a DNA molecule of known length.

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Which of the following shows the correct pairing of DNA base pairs? 

Possible Answers:

A and T; G and C

A and U; G and C

T and U; G and C

A and G; T and C

A and C; G and T

Correct answer:

A and T; G and C

Explanation:

DNA is a double-stranded helix. The two sugar-phosphate backbones are connected by hydrogen bonds that for between nitrogenous bases. The DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). In DNA, adenine always pairs with thyine and cytosine always pairs with guanine. These pairings occur because of geometry of the base,s allow hydrogen bonds to form only between the "right" pairs. Adenine and thymine will form two hydrogen bonds, whereas cytosine and guanine will form three hydrogen bonds. This hydrogen bonding brings stability to the DNA. 

The double-helix structure of DNA was discovered using X-ray crystallography (Rosalind Franklin) and elucidated by Watson and Crick. Before this discovery, it was previously known that DNA is comprised of four nucleotide bases: adenine, thymine, guanine, and cytosine. Adenine always binds to thymine, while guanine always binds to cytosine.

You analyze a double-helix and determine that 17% of the helix is made up of the nucleotide adenine (A). What percentage of the same helix must be comprised of the nucleotide guanine (G)?

Correct answer:

Explanation:

Nucleotides that bind together will always be present in equal amounts in a sample of DNA. This principle is known as Chargaff's rule. Essentially, the percentage of adenine will be equal to the percentage of thymine, while the percentage of guanine will be equal to the percentage of cytosine.

Together, all of the percentages must add up to 100%.

We know that the sample is 17% adenine, which means it is also 17% thymine.

We can use this equation to solve for the percentage of guanine.

Guanine and cytosine pair together, which means they will be present in equal amounts. To find the percentage of guanine, we can simply divide by 2.

You can check your answer if needed:

In complementary base pairing of DNA, guanine always pairs with __________ and adenine always pairs with __________.

Possible Answers:

cytosine . . . uracil

cytosine . . . thymine

guanine . . . adenine

thymine . . . cytosine

Correct answer:

cytosine . . . thymine

Explanation:

The four nitrogenous bases of DNA are thymine, adenine, guanine, and cytosine. Guanine and cytosine are bound together by three hydrogen bonds; whereas, adenine and thymine are bound together by two hydrogen bonds. This is known as complementary base pairing. In RNA, the nucleotide thymine is replaced by the nucleotide uracil.

Which type of bond is responsible for holding complementary base pairs together?

Possible Answers:

Double covalent bond

Ionic bond

Hydrogen bond

Single covalent bond

Correct answer:

Hydrogen bond

Explanation:

DNA (Deoxyribonucleic acid) is a chain of two strands of nucleotide that are linked to one another through hydrogen bonding and arranged into a structure known as a double-helix. Nucleotides are molecular components of DNA that are composed of a deoxyribose sugar, a nitrogenous base, and a phosphate group. The backbone of DNA consists of phosphodiester bonds made up of interchanging phosphate groups and deoxyribose sugars. The four nitrogenous bases of DNA are thymine, adenine, guanine, and cytosine. Guanine and cytosine are bonded together by three hydrogen bonds; whereas, adenine and thymine are bonded together by two hydrogen bonds. This is known as complementary base pairing. One strand of the DNA double helix will be in a 5’  3’ direction in relation to the –OH group on the deoxyribose sugar. The other strand is oriented in the 3’  5’ direction.

The orientation of the two strands of the DNA molecule are considered to be __________.

Possible Answers:

transverse

antiparallel

parallel

perpendicular

Correct answer:

antiparallel

Explanation:

DNA is a chain of two strands of nucleotides that are linked to one another through hydrogen bonds and arranged into a structure known as a double-helix. Nucleotides are molecular components of DNA that are composed of a deoxyribose sugar, a nitrogenous base, and a phosphate group. The backbone of DNA consists of phosphodiester bonds made up of interchanging phosphate groups and deoxyribose sugars. The four nitrogenous bases of DNA are thymine, adenine, guanine, and cytosine. Guanine and cytosine are bound together by three hydrogen bonds; whereas, adenine and thymine are bound together by two hydrogen bonds. This is known as complementary base pairing. One strand of the DNA double helix will be in a 5’  3’ direction in relation to the  group on the deoxyribose sugar. The other strand is oriented in the 3’  5’ direction. This is why DNA is considered antiparallel.

You analyze a double-helix and determine that 17% of the helix is made up of the nucleotide adenine (A). What percentage of the same helix must be comprised of the nucleotide guanine (G)?

Correct answer:

Explanation:

Nucleotides that bind together will always be present in equal amounts in a sample of DNA. This principle is known as Chargaff's rule. Essentially, the percentage of adenine will be equal to the percentage of thymine, while the percentage of guanine will be equal to the percentage of cytosine.

Together, all of the percentages must add up to 100%.

We know that the sample is 17% adenine, which means it is also 17% thymine.

We can use this equation to solve for the percentage of guanine.

Guanine and cytosine pair together, which means they will be present in equal amounts. To find the percentage of guanine, we can simply divide by 2.

You can check your answer if needed:

DNA polymerase is used to crate a new DNA strand based on the following template strand.

5'-GCCTCATGA-3'

Which of the following shows the correct complementary strand?

Possible Answers:

5'-CGGAGTACT-3'

5'-AGTACTCCG-3'

5'-TCATGAGGC-3'

5'-UCAUGAGGC-3'

Correct answer:

5'-TCATGAGGC-3'

Explanation:

When determining the complementary strand, remember that it will be written in the opposite direction of the template strand. This means that the new strand's 5' end will begin at the 3' end of the template strand. The complementary strand will also be composed of the nucleotides that complete the base pairs found in DNA (A-T and C-G).

Template: 5'-GCCTCATGA-3'

Answer:     5'-TCATGAGGC-3'

To see these pairs match up, the 3' end of the answer must align with the 5' end of the template.

Template:       5'-GCCTCATGA-3'

Answer (3'-5'): 3'-CGGAGTACT-5'

Which of the following bases are purines?

Possible Answers:

Adenine and thymine

Guanine and cytosine

Adenine and guanine

Cytosine, thymine, and uracil

Adenine, thymine, and uracil

Correct answer:

Adenine and guanine

Explanation:

There are two classes of bases in DNA and RNA: purines and pyrimidines. The difference between these classes is the structure of the base. Purines have two rings in their structure, while pyrimidines have only one. Purines will pair with pyrimidines.

The purines are adenine and guanine, and the pyrimidines are thymine, cytosine, and uracil. You can remember that the bases that contain a "y" are pyrimidines (thymine and cytosine).

What is a common feature of all the nucleotides in DNA?

Possible Answers:

Each of the nucleotides can bind to any of the other ones

They are all used in equal amounts in a strand of DNA

They are the same nucleotides used in RNA

They are bound to a sugar backbone

They contain the same atoms in different structures

Correct answer:

They are bound to a sugar backbone

Explanation:

There are four nitrogenous bases in DNA: adenine, guanine, cytosine, and thymine. A nucleotide is composed of one of these bases bound to a deoxyribose sugar and a phosphate group. Polymers of nucleotides form strands of DNA, which adhere to one another by hydrogen bonding between the bases.

Each strand of DNA is unique and may contain any ratio of the nitrogenous bases, but strands of DNA will always be complementary to one another. The structure of the bases requires that adenine bind to thymine and cytosine bind to guanine to maintain the structural integrity of the DNA molecule. RNA does not contain thymine, and instead uses uracil.

The DNA of a eukaryotic cell is found to contain 30% cytosine. The DNA also contains __________.

Possible Answers:

30% adenine

30% thymine

20% guanine

20% adenine

Correct answer:

20% adenine

Explanation:

There are four bases in DNA: adenine, thymine, guanine, and cytosine. Adenine always pairs with thymine, so the number of adenine residues always equals the number of thymine residues. Guanine always pairs with cytosine, which means they are always present in equal amounts as well. If one strand contains three cytosine bases and five thymine bases, then the opposite strand must contain three guanine bases and five adenine bases.

The trick to this problem is remembering that the sum of all four bases by percentage must be 100%.

We know that 30% of the bases are cytosine. Since cytosine pairs with guanine, there is also 30% guanine.

That leave us with 40% of the bases being thymine and adenine.

Since adenine and thymine will be equal, each will represent 20% of the DNA composition.

All High School Biology Resources

Are there always going to be an equal number of guanine and cytosine in a DNA molecules How about adenine and thymine Why?

Is there always going to be an equal number of guanine and cytosine?

Why would you expect equal numbers of guanine and cytosine?

Why would you expect there to be equal numbers of guanine and cytosine in a molecule of DNA but not necessarily in a molecule of RNA?