AP Biology : Understanding Differences Between DNA and RNA

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Dna And Rna Structure

Which of the following characteristics is NOT seen in both DNA and RNA?

Possible Answers:

A double helix

Read in the 5'-to-3' direction

A pentose sugar

Adenine

Correct answer:

A double helix

Explanation:

DNA and RNA share many characteristics. They are both composed of nucleotide monomers and are read in the 5'-to-3' direction. They also share the same complementary base pairs, except RNA uses uracil in place of thymine; both contain adenine.

RNA does not present in a double helix structure, and is typically single stranded.

Example Question #1 : Dna And Rna Structure

A section of mRNA is composed of 28% guanine bases. What percentage of the RNA strand is cytosine bases?

Possible Answers:

More information is needed in order to answer this question.

Correct answer:

More information is needed in order to answer this question.

Explanation:

Remember that mRNA is not a double helix like DNA; RNA is only one single strand of nucleotides. This means that we are unable to say that there are just as many cytosine bases as guanine bases, even though they would be able to form nucleotide pairs in DNA. For all we know, there could be zero cytosine bases! More information is needed before we can make a conclusion as to how many cytosine bases are in the RNA section.

Example Question #3 : Dna And Rna Structure

Which of the following statements is correct about the differences between DNA and RNA?

Possible Answers:

DNA is not present in prokaryotes, while RNA is

RNA is synthesized from DNA during transcription, but DNA can never be synthesized from RNA

RNA contains the same bases as DNA, except uracil is present instead of guanine

The sugar molecule in RNA has one more hydroxyl group than the sugar molecule in DNA

DNA is present as a single-stranded molecule while RNA is double-stranded

Correct answer:

The sugar molecule in RNA has one more hydroxyl group than the sugar molecule in DNA

Explanation:

The only correct statement here is the one regarding the types of sugar in the two molecules. RNA stands for "ribonucleic acid," which is a simple way to remember that it contains the sugar ribose. DNA, on the other hand, stands for "deoxyribonucleic acid." Its sugar is deoxyribose, which is identical to ribose except it is missing a hydroxyl (-OH) group on its second carbon. In total, RNA contains three hydroxyl groups, while DNA contains only two.

In RNA, uracil replaces thymine, not guanine. DNA is generally double-stranded and RNA is generally single-stranded (though both can exist in either form). Prokaryotes contain both DNA and RNA. Finally, DNA is transcribed to RNA in most biological organisms, but RNA can be reverse transcribed to DNA by the protein reverse transcriptase, which is found in some viruses.

Example Question #4 : Understanding Differences Between Dna And Rna

Which of the following statements is false?

Possible Answers:

DNA is typically double-stranded, while RNA is typically single-stranded

DNA uses thymine, while RNA uses uracil

DNA and RNA utilize different sugars in their sugar-phosphate backbones

DNA is a polyanion, while RNA is a not

Correct answer:

DNA is a polyanion, while RNA is a not

Explanation:

Both DNA and RNA are polyanions. This is just a fancy way of saying that they are polymers of negatively charged molecules. The phosphate groups in the sugar-phosphate backbone account for this, as phosphate groups generally carry a charge of negative three.

DNA uses thymine and deoxyribose sugar, while RNA uses uracil and ribose sugar. While DNA is usually molded into a double-stranded helix, RNA is usually single-stranded, which allows for the binding of anticodons during translation.

Example Question #1 : Dna And Rna Structure

Which of the following is characteristic of RNA?

Possible Answers:

RNA is less stable than DNA

RNA is a permanent molecule in most cells

RNA has a hydroxyl group on the 2’ carbon of ribose

RNA is typically double-stranded

Correct answer:

RNA has a hydroxyl group on the 2’ carbon of ribose

Explanation:

RNA and DNA are very similar in composition, but differ in structure and function. DNA is used to code for genetic material, while RNA is used to generate protein products. Since DNA has a long-term goal of storing information and RNA has a short-term goal of increasing production, it makes sense that DNA is a permanent molecule and RNA is transient. Soon after translation, mRNA is degraded by ribonuclease (RNase). The transient nature of RNA is also linked to its stability. DNA must be very stable to avoid problems with gene storage. DNA is double-stranded to help enhance stability. In contrast, RNA can afford to be less stable and is easily degraded, partially due to its single-stranded structure.

Another key difference between DNA and RNA is the sugar component of the nucleic acid backbone. DNA uses deoxyribose, which lacks a hydroxyl (-OH) group on the 2' carbon. RNA uses ribose, in which this hydroxyl group is present on the 2' carbon.

Example Question #3 : Understanding Differences Between Dna And Rna

Which of the following is not a true characteristic of an RNA molecule?

Possible Answers:

It can be single stranded

It contains deoxyribose sugar

It contains the nucleotide uracil

It can be double stranded

Correct answer:

It contains deoxyribose sugar

Explanation:

RNA molecules are most commonly found in a single stranded form (e.g. mRNA), but they can also be found in a double stranded form (e.g. viruses). It has four base pairs—uracil, adenine, guanine, and cytosine—and a ribose sugar-phosphate backbone. DNA molecules contin a deoxyribose sugar in their sugar-phosphate backbone; thus, this answer is incorrect.

Example Question #6 : Understanding Differences Between Dna And Rna

Which of the following types of RNA is double stranded?

Possible Answers:

rRNA

tRNA

Viral RNA

mRNA

Correct answer:

Viral RNA

Explanation:

Many types of RNA exist within cells, some are single stranded and others are double stranded. The following are among the types of single stranded RNA: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). Messenger or mRNA carries genetic information from DNA to ribosomes for protein synthesis. Transfer or tRNA carries amino acids to ribosomes that match with mRNA codons. Last, ribosomal or rRNA codes for ribosomes, which are necessary for protein synthesis. An example of a double stranded RNA molecule is viral RNA. Viral RNA is the genetic material of many viruses and has a structure of two complementary strands.

Example Question #5 : Dna And Rna Structure

Which of the following choices is an element common to both DNA and RNA molecules?

Possible Answers:

Both are often single-stranded

They can both be found in the nucleus

Both contain the nucleotide uracil

There is deoxyribose sugar in both of their sugar-phosphate backbones

Correct answer:

They can both be found in the nucleus

Explanation:

DNA is a macromolecule that has a double helical form. It has four nitrogenous base pairs (thymine, adenine, cytosine, and guanine) and a deoxyribose-phosphate backbone. The two strands of the helix are joined by hydrogen bonds. On the other hand, RNA molecules can be double-stranded (e.g. viruses) or single-stranded (e.g. mRNA), but RNA most commonly exists in a single-stranded form. RNA contains four base pairs (uracil, adenine, cytosine, and guanine) and a ribose sugar-phosphate backbone. Despite their differences, these molecules share one similarity: both DNA and RNA molecules can be found in the nucleus. DNA molecules reside solely within the nucleus. RNA is transcribed in the nucleus and, once targeted and post-transcriptionally modified, can leave via nuclear pores.

Example Question #2 : Dna And Rna Structure

Which of the following are correct regarding the central dogma of biology?

Possible Answers:

RNA encodes the information for the assembly of ribosomes, which are the sites of protein production

DNA directs the synthesis of mRNA, which in turn directs the production of polypeptides

RNA signals DNA to activate protein production

RNA assists with the replication of DNA which encodes information for proteins

Correct answer:

DNA directs the synthesis of mRNA, which in turn directs the production of polypeptides

Explanation:

DNA encodes information for the production of messenger RNA which then interacts with the cell's protein-synthesizing machinery to produce proteins. Ribosomes are the sites of polypeptide synthesis but are not coded for by RNA. The central dogma of biology is DNA  RNA  protein

Example Question #5 : Understanding Differences Between Dna And Rna

Which of the following are properties of RNA?

Possible Answers:

Deoxyribose, thymine, and a linear structure

Ribose, uracil, and a linear structure

Deoxyribose, uracil, and a globular structure

Ribose, thymine, and a linear structure

Deoxyribose, ribose, and uracil

Correct answer:

Ribose, uracil, and a linear structure

Explanation:

Remember that the sugar for DNA is deoxyribose, and that for RNA is ribose. The nitrogenous bases adenine, guanine, and cytosine in DNA and RNA are the same. However, DNA which contains thymine and RNA that contains uracil. The structure of DNA is a double helix. There are three different structures of RNA: linear, clover-leaf, and globular. Also note that RNA is single stranded and DNA is double stranded

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