Question

(Q5-9) We have the following set of single-stranded nucleic acids that have been isolated from cells.

a    5'-AGGCTTAAGCTCGTAA-3'

b    5'-UUACGAGCUUAAGCCU-3'

c    5'-TTACGAGCTTAAGCCT-3'

d    5'-AGGCUUAAGCUCGUAA-3'

e    5'-UUACGAGCTTAAGCCT-3'

5. Which one(s) contain only deoxyribose?

A.        a

B.        b

C.        c

D.        d

E.         a and c

6. Which one(s) contain only ribose?

A.        a

B.        b

C.        c

D.        d

E.         b and d

7. Which one(s) are unstable in water solution?

A.        a

B.        b

C.        c

D.        d

E.         b and d

8. Which pair will make a complete double-stranded DNA based on the Watson-Crick base-pairing rule?

A.        a and b

B.        a and c

C.        b and d

D.        b and e

E.         a and e

9. Which one is most likely an Okazaki fragment containing both DNA and RNA?

A.        a

B.        b

C.        c

D.        d

E.         e

Answer 1

Ans 5: Correct option (E) a and c

Explanation: The deoxyribose sugar is present in the DNA. The base uracil is absent in the DNA and thymine is present in place of uracil. Among the above-mentioned sequence (a) and (c) does not have uracil in it so they are the sequences of DNA and have only deoxyribose sugar.

(A) a is an incorrect option because not only (a) but (c) is also a DNA sequence and they both have only deoxyribose sugar.

(B) and (D) are also incorrect options because of all these sequences have uracil in them and uracil is not present in DNA. Hence these are the sequences that do not have only deoxyribose sugar.

Option (C) c is not the correct option as although the sequence (c) is a DNA sequence but the sequence (a) is also a DNA sequence.

Ans 6: Correct option (E) b and d

Explanation: These are the RNA sequences as they have the uracil base( the base thymine is replaced by uracil in RNA). These RNA sequences have only ribose sugar in them.

Incorrect options:

(A) and (C)

Explanation: Because these are the  DNA sequences and DNA has deoxyribose sugar instead of the ribose sugar.

(B) (b)

Explanation: The sequence (b) and (d) both are the RNA sequences. Only ribose sugar is present in both of these sequences.

(D) (d)

Explanation: Although the sequence (d) is the RNA sequence having only ribose sugar but the sequence (b) is also an RNA sequence and the only sugar present in it is the ribose sugar.

Ans (7): Correct option (E) b and d

Explanation: b and d both are RNA sequences and RNA molecules are unstable in water solution because of the presence of the hydroxyl group at 2' position.

Incorrect options:

(A) a and (C) c

Explanation: Both are the DNA sequence which is more stable in water

Options (B) b and (D) d are also incorrect because both are the RNA sequences the sequence (b) along with (d) are less stable in water solution as compared to the DNA.

Ans 8: Correct option (B) a and c

Explanation: (a) and (c) are the DNA sequences after replication they will form complete double-stranded DNA.

According to the Watson and crick base-pairing rule Adenine pairs with thymine (with uracil in case of RNA) and cytosine pairs with guanine.

Incorrect options:

(A) a and b

Explanation: As sequence (a) is the DNA sequence and the sequence (b) is the RNA sequence.

(C) b and d

Explanation: Because they both are the RNA sequences they will not form DNA duplex.

(D) b and e

Explanation: Because the sequence (b) is the RNA sequence but sequence (e) is the Okazaki fragment.

(E) a and e

Explanation: Although the sequence (a) is the DNA sequence but the (e) is the Okazaki fragment.

Ans 9: Correct option (E) e

Explanation: During DNA replication one strand is synthesized continuously this is called the leading strand while the other strand is synthesized in fragments (Okazaki fragments) this is called the lagging strand. Following are the reasons for this

• DNA strands are antiparallel
• DNA polymerase is capable of synthesizing DNA only in 5' to 3' direction.

Okazaki fragment has RNA primer at the 5' end and in the sequence (e) the presence of uracil at the 5' end indicates the presence of RNA there. Thus it is most likely the Okazaki fragment.

Incorrect options:

(A) a and (C) c

Explanation: These are the DNA sequences. They don't have RNA.

(B) b

Explanation: It is the RNA sequence as at the 3' end uracil is present which is only present in the RNA. Okazaki fragment has a short RNA primer followed by DNA.

(D) d

Explanation:

As uracil is present near the 3' end it is less likely to be an Okazaki fragment.