Help me with this Genetic Question!!?

Question

indicate what type of nucleic acid is most likely found in each species.
Is it double-stranded or single-stranded?
Explain your answer.

Species (1)

Adenine 30%
Guanine 20%
Thymine 30%
Cytosine 20%
Uracil 0%

Species (2)

Adenine 21%
Guanine 29%
Thymine 0%
Cytosine 29%
Uracil 21%

Species (3)

Adenine 29%
Guanine 21%
Thymine 20%
Cytosine 30%
Uracil 0%

Species (4)

Adenine 26%
Guanine 25%
Thymine 24%
Cytosine 25%
Uracil 0%

Species (5)

Adenine 18%
Guanine 30%
Thymine 0%
Cytosine 18%
Uracil 34%

The answer I came up with was the following:

(i) single stranded DNA
(ii) single stranded RNA
(iii) single stranded DNA
(iv) double stranded DNA
(v) single stranded RNA

The logic I used is when you match the A-T (or A-U in RNA) and G-C, they sum up to 50 if they are double stranded otherwise it is single stranded which base composition doesn't matter. Is that correct or am I getting it wrong?

Answer 1

Your logic is a little bit off. The answers should be as follows:

species 1: double stranded DNA
species 2: double stranded RNA
species 3: single stranded DNA
species 4: single stranded DNA
species 5: single stranded RNA

First of all, there is double stranded RNA. Second of all, in double-stranded DNA/RNA, A-T bonds do not necessarily make up 50% of the nucleotide contribution, they can be any random amount as long as:

percentage of A = percentage of T (for DNA) or U (for RNA)
AND
percentage of G = percentage of C

Because if it is double-stranded, there must be a T/U across from every A (and vice-versa), and a C across from every G (and vice-versa).

If this is not the case, then you know you are dealing with a single-stranded molecule, and whether it is DNA or RNA depends on whether U is present (ie: RNA) or T is present (ie: DNA).

Answer 2

No you are not getting it right. They do not have to add up to 50% because there is no requirement that the A-T content is half the content of the DNA. You are better off matching up A with a non-pairing base like C or G to come up with 50%.

So, that actually makes it as follows,

species 1 double stranded DNA
species 2 double stranded RNA
species 3 single stranded DNA
species 4 single stranded DNA
species 5 single stranded RNA

You can make the argument for species 4 that some weird A-A pairing is taking place and that it is therefore double stranded, but single stranded is a reasonable conclusion as well.

RNA can be double stranded and if your teacher disagrees, tell them that Geneticists say they are wrong. There are plenty of examples of double stranded RNA.

Answer 3

i think you got it right just remember that RNA (with uracil) is always single stranded