Biochemistry : Biochemistry

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #2 : Dna And Rna

Which of the following is a difference between DNA and RNA?

Possible Answers:

The sugar of DNA is glucose and the sugar of RNA is fructose

DNA has a  as opposed to a  on carbon two

RNA contains sulfur and DNA contains phosphate

DNA is only found in prokaryotes and RNA is found in eukaryotes

Correct answer:

DNA has a  as opposed to a  on carbon two

Explanation:

DNA's sugar is deoxyribose, which involves the lack of a hydroxyl group on the second cabon. RNA's sugar is ribose. DNA and RNA is found in both prokaryotes and eukaryotes and both contain phosphate and neither contain sulfur.

Example Question #131 : Biochemistry

What enzyme involved DNA replication has a 3'-5' proofreading exonuclease activity to help ensure fidelity in replication of DNA?

Possible Answers:

DNA polymerase III

DNA polymerase I

DNA ligase

Primase

Helicase

Correct answer:

DNA polymerase III

Explanation:

DNA polymerase III is the only DNA replication enzyme with proofreading (3'-5' exonuclease) capabilities. Ligase links Okazaki fragments. Helicase unwinds the two DNA strands. DNA polymerase I contains 5'-3' exonuclease activity, but this is involved in primer removal, not proofreading. Primase is a type of RNA polymerase.

Example Question #1 : Dna And Rna

The primosome remains that are associated with the lagging strand in DNA replication are known as which of the following?

Possible Answers:

Primase fragments

DNA polymerase I

DNA polymerase III

Okazaki fragments

DNA ligase

Correct answer:

Okazaki fragments

Explanation:

Okazaki fragments are found in the lagging strand, and are linked by DNA ligase. These short fragments of DNA are formed because DNA polymerase III (the main polymerizing enzyme complex) can only add nucleotides the the 3' end of a DNA strand. Since DNA strands are antiparallel, this is unavoidable. 

Example Question #21 : Nucleic Acid Structures

Meselson and Stahl described DNA replication as __________.

Possible Answers:

dependent

independent

conservative

dispersive

semiconservative

Correct answer:

semiconservative

Explanation:

The semiconservative model of DNA replication describes the daughter strand containing one new and one old strand. Dispersive and conservative models were both rejected from their experiment. Independet and dependent do not pertain to any of the scientist's theories of replication.

Example Question #132 : Biochemistry

Which of the the following DNA motifs is held together by hydrophobic interactions along the length of an amino acid side chain, to form a coiled-coil? 

Possible Answers:

Helix-turn-helix

Two-stranded beta sheet

Zinc finger

Leucine zipper

Helix-loop-helix

Correct answer:

Leucine zipper

Explanation:

The helix-turn-helix motif has two helices at a particular angle, with one of them, the recognition helix, fitting into a major groove. Zinc fingers have sheets and helices held together via zinc complexes. Beta sheets have hydrogen bonds along their strand backbones. The helix-loop-helix motif has one helix folded and packed against another. In the leucine zipper, two helices are coiled up to where they are "unzipped" to form a Y.

Example Question #23 : Nucleic Acid Structures

The backbone of a strand of DNA is comprised of which of these?

Possible Answers:

Sugars only

Nucleotides only

Nucleotides and phosphates

Sugars and nucleotides

Sugars and phosphates

Correct answer:

Sugars and phosphates

Explanation:

The backbone of DNA is made up of alternating phosphate groups and sugar groups, linked together via phosphodiester bonds. The nitrogenous bases jut off of the backbone and form bonds with nitrogenous bases on other strands of DNA to become double stranded. A nucleotide consists of a sugar, nitrogenous base, and one or more phosphate groups.

Example Question #24 : Nucleic Acid Structures

What is the anticodon?

Possible Answers:

The region of tRNA that recognizes DNA

The region of a tRNA that recognizes mRNA

The region of mRNA that recognizes tRNA

The region of mRNA that recognizes DNA

The region of DNA that recognizes mRNA

Correct answer:

The region of a tRNA that recognizes mRNA

Explanation:

The anticodon is a part of tRNA that is capable of finding its complementary codon on mRNA. This allows the tRNA to carry its specific amino acid to a ribosome when necessary in the production of proteins.

Example Question #15 : Dna And Rna

What types of bonds hold together a tRNA molecule?

Possible Answers:

Disulfide bonds

Ionic bonds

Hydrogen bonds

Covalent bonds

The tRNA molecule is not held together by bonds

Correct answer:

Hydrogen bonds

Explanation:

The clover-like structure of tRNA is held together by hydrogen bonds between nitrogenous bases of the molecule. Without them, this tertiary structure would not be possible.

Example Question #25 : Nucleic Acid Structures

In RNA, which base pairs with adenine?

Possible Answers:

Adenine

Guanine

Cytosine

Uracil

Thymine

Correct answer:

Uracil

Explanation:

In RNA the four bases are: adenine, uracil, guanine, and cytosine. The bases in DNA are similar, except uracil is replaced with thymine. In RNA, adenine will always pair with uracil, and guanine will always pair with cytosine. Remember, a purine (adenine, guanine) will always pair with a pyrimidine (cytosine, uracil or thymine). 

Example Question #16 : Dna And Rna

Suppose that a portion of the coding strand in a given gene reads as follows:

What would the mRNA encoded by this gene read?

Possible Answers:

Correct answer:

Explanation:

This question presents us with a portion of the coding strand of a gene, and asks us to determine the corresponding sequence of nucleotides in the mRNA that would be transcribed from this gene.

To begin, it is very important to recognize that the sequence given to us in the question stem is the coding strand. This is to distinguish it from the template strand.

For any given gene, there is a template strand and a coding strand. These two strands are complementary to one another and run in opposite directions. The template strand is the one that is transcribed to give rise to a complementary mRNA molecule that will go on to be translated into a polypeptide. Hence, it is called the template strand because it provides the template for the production of mRNA, and subsequently a protein.

The coding strand, on the other hand, is not transcribed. Instead, the coding strand gets its name because its sequence is identical to the sequence of the pre-mRNA (before introns are removed). The exception to this is that the coding strand contains thymine nucleotides, whereas the pre-mRNA contains uracil in place of thymine. The reason why these two have the same sequence is because both of them are complementary to the template strand. Sometimes, the template strand is also referred to as the antisense strand, while the coding strand is referred to as the sense strand.

It is important to realize this distinction because, without it, one might arrive at the incorrect answer. Because this is the coding strand (sense strand), the mRNA product will have an identical sequence, with the exception of uracil in place of thymine.

Coding strand:

mRNA strand:

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