SAT II Biology E : Molecular Biology

Study concepts, example questions & explanations for SAT II Biology E

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Dna Structure And Function

There is a certain type of chemical bonding between the paired nucleotides on each strand of DNA which helps maintain the double-helix structure of DNA by attracting each strand to the other. What type of bonding is responsible for this?

Possible Answers:

Phosphodiester

Covalent

Hydrogen

Peptide

Ionic

Correct answer:

Hydrogen

Explanation:

The correct answer is hydrogen bonding, and each nucleotide attracts its pairing mate because they have corresponding number of hydrogen bonds. Adenine is attracted to thymine to create two hydrogen bonds, and cytosine is attracted to guanine to form three hydrogen bonds. While phosphodiester bonds are very important in creating the strand of DNA, they are not the bond that keeps the two strands in the double helix structure.

Example Question #61 : Sat Subject Test In Biology

With respect to DNA, the terms 3' and 5' (pronounced 3-prime and 5-prime, respectively) are used in order to refer to one strand or the other. What do these two terms signify?

Possible Answers:

3' refers to the unbound hydroxyl group and 5' refers to the unbound phosphate group at the end of each DNA strand.

3' refers to the unbound sulfhydral group and 5' refers to the unbound phosphate group at the end of each DNA strand.

3' refers to the unbound methyl group and 5' refers to the unbound hydroxyl group at the end of each DNA strand.

3' refers to the unbound hydroxyl group and 5' refers to the unbound carboxyl group at the end of each DNA strand.

3' refers to the unbound phosphate group and 5' refers to the unbound hydroxyl group at the end of each DNA strand.

Correct answer:

3' refers to the unbound hydroxyl group and 5' refers to the unbound phosphate group at the end of each DNA strand.

Explanation:

When nucleotides bond together and form DNA strands, the first and last nucleotides in the strand have slightly different structures than the rest of the nucleotides between them. On one end of the strand, the nucleotide has an exposed hydroxyl group bound to the third carbon in the carbon ring: this end of the strand is thus called 3'. On the opposite end of the strand, the nucleotide has a phosphate group attached to the 5' carbon in the carbon ring, and is thus called the 5' end. These two groups are exposed because they are used in the bonding of nucleotides to one another to form the strand, but each strand ends with one nucleotide that only is bound on one side: thus, leaving either the hydroxyl or phosphate group exposed (depending on which end you are observing).

These terms are useful because they allow us to discuss the directionality of DNA-related events- if we didn't have terms for directionality the concept would be much more confusing. Example: "DNA polymerase synthesizes the new DNA strand in the 5'-3' direction." Without 3'/5' how would we determine which way the reaction occurs?

Example Question #1 : Dna Structure And Function

If a structural gene in an organism's genome is comprised of 33% adenine nucleotides, what percentage of the gene is comprised of cytosine nucleotides?

Possible Answers:

It cannot be determined from the given information.

17%

33%

22%

34%

Correct answer:

17%

Explanation:

According to Chargaff's rule, DNA nucleotides pair in a 1:1 ratio. Therefore, if we know how much of the particular gene is made up of one nucleotide, we can extrapolate that known variable to find the other three unknown variables.

To do so, you must remember that adenine pairs with thymine, and cytosine pairs with guanine (A-T, C-G), and that since the ratio between each pair is 1:1 then a gene with 33% adenine must also have 33% thymine. Combine these numbers and subtract from 100: the number leftover is the % of total cytosine and guanine in the gene.

100% - 66% = 34%

Finally, since we know that 34% of the DNA is both C and G, and that the ratio between C-G is 1:1, C and G must both be 17%.

Example Question #41 : Molecular Biology

Which type of bond makes up the backbone of DNA strands by linking together adjacent nucleotides?

Possible Answers:

Glycosidic bond

Ester bond

Hydrogen bond

Peptide bond

Phosphodiester bond

Correct answer:

Phosphodiester bond

Explanation:

DNA and RNA nucleotides are linked together through phosphodiester bonds. A strong covalent bond (ester bond) forms between the 3' carbon atom of the sugar pentose of one nucleotide and a phosphate group, and a second ester bond forms between the phosphate group and the 5' carbon atom of the sugar pentose of another nucleotide. This alternation of sugar and phosphate groups forms a strong backbone and is also the reason why DNA is antiparallel and forms in the 5' to 3' direction.

Example Question #21 : Dna, Rna, And Proteins

When synthesizing a strand of double-stranded DNA, which of the following could be a plausible combination of nitrogen bases?

Possible Answers:

48% Adenine, 52% cytosine, 52% guanine, 48% thymine

28% Adenine, 22% cytosine, 22% guanine, 28% thymine

23% Adenine, 23% cytosine, 27% guanine, 27% uracil

24% Adenine, 24% cytosine, 26% guanine, 26% thymine

None of these

Correct answer:

28% Adenine, 22% cytosine, 22% guanine, 28% thymine

Explanation:

DNA nucleotides all contain one of four possible nitrogen bases: adenine (A), thymine (T), cytosine (C), or guanine (G). In forming base pairs, an A must always pair with a T and a C must always pair with a G: [A-T], [C-G]. This means that for any DNA composition, the percent of adenine (A) must be equal to the percent of thymine (T) and, likewise, the percent of cytosine (C) must be equal to the percent of guanine (G). Looking across the answer choices, there is only one choice that satisfies this condition while also correctly summing to 100%. The choice with uracil can be eliminated immediately, since uracil only replaces thymine in RNA and is not present in DNA. 

Example Question #2 : Dna Structure And Function

A sample of DNA is sequenced and found to contain  guanine. What percentage of thymine does it contain?

Possible Answers:

Correct answer:

Explanation:

In DNA, guanine pairs with cytosine and adenine pairs with thymine. In RNA, which does not have thymine, adenine pairs with uracil. Thus, if a sample contains  guanine, it also contains  cytosine. Together, the two make up  of the  total. The remaining  is divided evenly between the paired adenine and thymine molecules, so the DNA sample contains  percent each of adenine and thymine.  is the correct answer.

Example Question #71 : Sat Subject Test In Biology

Which of the following is NOT found in DNA?

Possible Answers:

Thymine

Cytosine

Deoxyribose

Phosphate groups

Ribose

Correct answer:

Ribose

Explanation:

DNA and RNA are both made of sugar-phosphate backbones. Ribose is the sugar found in RNA; deoxyribose is the sugar found in DNA. DNA also contains the nucleic acid bases adenine, guanine, cytosine, and thymine. Both DNA and RNA contain phosphate groups as part of the backbone.

Example Question #1 : Rna Structure And Function

In terms of histone modification, what will result in a greater rate of gene expression?

Possible Answers:

Histone acetylation

Alternative RNA splicing

Addition of 3' Poly A Tail

Addition of 5' Terminal Cap

DNA methylation 

Correct answer:

Histone acetylation

Explanation:

Histone acetylation is the process of adding acetyl groups to positively charged lysine groups of histones. This process loosens the histone which allows for an easier initiation of transcription, which will lead to greater gene expression. DNA methylation does the opposite by adding methyl groups to DNA and lowering the rate of transcription. Alternative RNA splicing deals with RNA having certain introns and exons spliced out in a manner that produces different strands of mRNA from the same template strand of RNA. Addition of 5’ Terminal Cap and the addition of 3’ Poly A Tail relate to gene expression in that they both have to do with creating mature mRNA that is ready for translation into protein.

Example Question #41 : Molecular Biology

You find a specialized type of RNA in the nucleus but nowhere else in the cell, including the cytoplasm. What type of RNA is it?

Possible Answers:

rRNA

scRNA

snRNA

mRNA

tRNA

Correct answer:

snRNA

Explanation:

mRNA will be found in both the nucleus and in the cytoplasm because it is transcribed from DNA in the nucleus and then exported to the cytoplasm to go through translation. tRNA will be found in the cytoplasm because it is an integral part of translation in that it delivers amino acids to the ribosome. rRNA will also be found in the cytoplasm because it couples with ribosomal proteins to make up the ribosomes found in the cytoplasm. scRNA is also known as small cytoplasmic RNA and has a function that is still not very well known, but they are mostly only found in the cytoplasm. snRNA, or small nuclear RNA, are only found in the nucleus and are an integral part of splicing introns of RNA so it can go onto becoming mRNA.

Example Question #41 : Molecular Biology

In RNA, three nucleotide bases together determine the amino acid that is added to the growing polypeptide chain during translation. This three base grouping is known as a(n) __________.

Possible Answers:

polypeptide

gene

operator

codon

promoter

Correct answer:

codon

Explanation:

The three base grouping in RNA that determines the amino acid created in translation is known as a codon. Gene refers to the region on DNA that codes for a given trait. Operators and promoters are also located on DNA, and act as regulatory elements.

Learning Tools by Varsity Tutors