Biology Exam II- Ch. 4

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Which statement accurately summarizes a difference between ribonucleotides and deoxyribonucleotides?

Ribonucleotides have a hydroxyl group bonded to their 2′ carbon; deoxyribonucleotides have an H at the same location.

What are the components of a nucleotide?

a sugar, a phosphate group and a nitrogenous base

The condensation reaction that forms nucleic acid polymers occurs between a _____ group on one nucleotide and a _____ group on a second nucleotide.

phosphate, hydroxyl

By convention, nucleotide sequences are always written from the _____.

5′ end to the 3′ end

How are nucleotides linked together to form a polynucleotide chain?

In DNA and RNA, a phosphodiester linkage connects the 3′ carbon of one nucleotide and the 5′ carbon of another.

What is the difference between a ribonucleotide and a deoxyribonucleotide?

Ribonucleotides have a hydroxyl group on the 2 carbon of their sugar subunit.

What is/are the variable structure(s) of a nucleotide?

the sugar and the base

Which of the following includes all of the pyrimidines found in RNA and DNA?

cytosine, uracil, and thymine

When nucleotides polymerize to form a nucleic acid _____.

a covalent bond forms between the sugar of one nucleotide and the phosphate of a second

Some viruses consist only of a protein coat surrounding a nucleic acid core. If you wanted to radioactively label the nucleic acids separately from the protein, you would use radioactive _____.


What feature of single nucleotides provides the energy needed for polymerization when nucleic acids are formed?

their phosphate groups

Use the following information when answering the corresponding question.

Shu-ichi Nakano reported in Science in February 2000 that "The RNA enzyme (ribozyme) from hepatitis delta virus catalyzes self-cleavage of a . . . [chemical] . . . bond." This reaction is inherently slow due to the formation of an unfavorable negative charge on the ribozyme. The author goes on to show that this self-cleavage is made faster by joining it with an acid-base reaction that neutralizes the negative charge. Nakano’s model describes a positively charged cytosine base on the ribozyme acting as the acid, and magnesium hydroxide acting as the base. Specifically, the cytosine donates its proton (H+) to neutralize the unstable negative charge that would have formed on the ribozyme.

Which of the following chemical bonds do you think Nakano is referring to in his quote?


DNAase is an enzyme that catalyzes the hydrolysis of the covalent bonds that join nucleotides together. What would first happen to DNA molecules treated with DNAase?

The phosphodiester bonds between deoxyribose sugars would be broken.

Which of the following are pyrimidine nitrogenous bases?

cytosine and uracil

If a DNA sample were composed of 10% thymine, what would be the percentage of guanine?


The difference between the sugar in DNA and the sugar in RNA is that the sugar in DNA _____.

contains one less oxygen atom

If cells are grown in a medium containing radioactive 32P-labeled phosphate, which of these molecules will be labeled?


Which of these is a difference between a DNA and an RNA molecule?

DNA is usually double-stranded, whereas RNA is usually single-stranded.

In a nucleotide, the nitrogenous base is attached to the sugar’s _____ carbon and the phosphate group is attached to the sugar’s _____ carbon.

1′ … 5′

Nucleic acids are assembled in the _____ direction.

5′ to 3′

In a DNA double helix an adenine of one strand always pairs with a(n) _____ of the complementary strand, and a guanine of one strand always pairs with a(n) _____ of the complementary strand.

thymine … cytosine

If a strand of DNA has the nitrogen base sequence 5′-ATTTGC-3′, what will be the sequence of the matching strand?

3′-TAAACG-5′ Adenine pairs with thymine and guanine pairs with cytosine.

If a DNA double helix is 100 nucleotide pairs long and contains 25 adenine bases, how many guanine bases does it contain?

75 100 nucleotide pairs are a total of 200 nucleotides. Because of base pairing, if there are 25 adenine there must also be 25 thymine. This leaves 200-50 = 150 nucleotides to be divided evenly between guanine and cytosine.

The two strands of a DNA double helix are held together by _____ that form between pairs of nitrogenous bases.

hydrogen bonds

A nucleotide is composed of a(n) _____.

phosphate group, a nitrogen-containing base, and a five-carbon sugar

Complementary base pairing is possible only if two DNA strands align in antiparallel—instead of parallel—fashion. Which statement best explains why?

Only antiparallel strands create the geometry that allows complementary base pairs to form via hydrogen bonding.

In a DNA double helix, what kind of chemical bonds form between the complementary nitrogenous bases?

hydrogen bonds

If a DNA double helix contains 28% T nucleotides, then what is the percentage of A nucleotides?


Which of the following best describes DNA’s secondary structure?

double antiparallel helical strands

Which of the following did Watson and Crick already know when they were trying to determine the structure of DNA? The number of _____.

purines is always the same as pyrimidines

Franklin and Wilkins analyzed DNA by bombarding DNA crystals with X-rays. Their analysis yielded two numbers that sparked interest, 3.4 nm and 0.34 nm. What is the significance of these numbers?

These numbers tell us there are ten rungs, or steps, on the DNA "ladder" for every turn of the helix.

What is the structural feature that allows DNA to replicate?

complementary pairing of the nitrogenous bases

Enzymes that break down DNA catalyze the hydrolysis of the covalent bonds that join nucleotides together. What would happen to DNA molecules treated with these enzymes?

The phosphodiester linkages of the polynucleotide backbone would be broken.

Compare proteins to nucleic acids. Which of the following is true?

Both have primary and secondary structure.

Which statement is true of nucleic acids?

DNA carries information for making RNA.

The information in DNA tells a cell how to make …

Proteins, DNA, and tRNA.

RNA nucleotides contain __________ than DNA nucleotides.

more oxygen

Which statement helps to explain how DNA stores hereditary information?

DNA contains four kinds of nitrogenous bases.

DNA and mRNA differ with respect to …

the kind of sugar they contain.

How are the structures of RNA and DNA similar?

Their sugar-phosphate backbones are formed by phosphodiester linkages.

An RNA molecule that can catalyze chemical reactions is called a_____.


Why is it that RNA can catalyze reactions but DNA cannot?

The sugar of RNA is much more reactive than the sugar of DNA.

Both DNA and RNA _____.

are information-containing molecules

Which of the following statements about the 5′ end of a polynucleotide strand of RNA is correct?

The 5′ end has a phosphate group attached to the number 5 carbon of ribose.

If 14C-labeled uracil is added to the growth medium of cells, what macromolecules will be labeled?


Researchers hypothesize that the first self-replicating molecule may have been ____. This is known as the ______world hypothesis.


The chemical evolution theory states that the first living molecule needed these two attributes:

It could serve as a replication template and could catalyze reactions.

In the context of chemical evolution, DNA’s structure is interesting because it suggests a possible copying mechanism. What about DNA’s structure facilitates copying?

The strands of the double helix are complementary.

DNA is the main hereditary material of all life. However, DNA lacks one important characteristic for the first forms of life. Why have researchers rejected the idea that DNA was found in the first life form?

It does not function as a catalyst.

Recall the Bartel lab group experiment to create a ribozyme from scratch. If they had created exact copies of each ribozyme (rather than creating copies with a few differences), the ribozymes would have _____.

stayed the same with each round of selection

The work of Bartel’s group on the ribozyme RNA replicase supports which conclusion?

An RNA world could produce molecules that could self-replicate.

What functional groups define the two different ends of a DNA strand?

a free hydroxyl group on the 3′ carbon a free phosphate group on the 5′ carbon

What determines the primary structure of a DNA molecule?

the sequence of deoxyribonucleotides

If nucleotides from the DNA of a human were quantified and 30 percent of them consisted of thymine, what percentage of cytosine nucleotides would be present?


What would be the sequence of the strand of DNA that is made from the following template: 5′-ATCGAGCTA-3′ (Your answer must be written 5′ → 3′.)


How would this sequence be different if RNA were made from this DNA template?


DNA molecules with a high percentage of guanine (G) and cytosine (C) are particularly stable. Why?

A G-C base pair has three hydrogen bonds, while an A-T base pair has two.

Many RNA molecules exhibit tertiary structure, whereas most DNA molecules do not. Which statement best explains why?

RNA’s secondary structure can leave bases free for interactions that produce tertiary structure; DNA’s secondary structure generally does not.

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