Illustration of DNA replication and transcription, highlighting DNA strands, mRNA, and nucleotide bases.

Let’s solve this step by step.

We are looking at two main structures: DNA (on the left) and Messenger RNA (on the right). We also see four nucleotide building blocks at the bottom with letters G, A, C, T.

Step 1: Understand what each part is labeled.
In the DNA double helix on the left:
- “D” points to one of the nitrogenous bases (the shapes that pair in the middle — like puzzle pieces).
- “P” points to the phosphate group (part of the backbone, shown as circles connected to pentagons).

In the mRNA strand on the right:
- “P” again points to the phosphate group (same as in DNA — part of the sugar-phosphate backbone).
- “R” points to the ribose sugar (in RNA, it’s ribose; in DNA, it’s deoxyribose — but here we’re just labeling the sugar part).

Wait — let’s check the bottom diagram. It shows four nucleotides:
Each has:
- Circle = Phosphate (P)
- Pentagon = Sugar (Deoxyribose in DNA, Ribose in RNA)
- Shape with letter = Nitrogenous base (G, A, C, T)

But note: In RNA, there is no “T” — instead, there’s “U”. But here, the bottom diagram includes “T”, which suggests these are DNA nucleotides. However, the question is about labeling parts in both DNA and mRNA.

Looking back at the labels:

In DNA (left):
- D → points to a base (one of the paired shapes in the middle)
- P → points to the phosphate (circle on the outside backbone)

In mRNA (right):
- P → same as above, phosphate (circle on backbone)
- R → points to the sugar (pentagon shape attached to phosphate and base)

So now, match the labels to their correct names:

For DNA:
- D = Base (nitrogenous base)
- P = Phosphate

For mRNA:
- P = Phosphate
- R = Ribose (sugar in RNA)

But wait — in the bottom diagram, the sugar is shown as a pentagon, and in RNA it’s called ribose, while in DNA it’s deoxyribose. Since the label “R” is on the mRNA side, it must stand for “Ribose”.

Also, note that in the DNA structure, the sugar isn’t labeled — only D and P are labeled. So we don’t need to name the sugar in DNA unless asked.

Now, let’s write down what each letter stands for based on standard biology knowledge and the diagrams:

→ In DNA:
- D = Deoxyribose? Wait — look again. The arrow “D” is pointing to one of the bases (the inner shapes), NOT the sugar. Because in the DNA ladder, the rungs are bases, and the sides are sugar-phosphate backbones.

Actually, re-examining the image description (since I can’t see it, but from your text):

You said:
“D” and “P” are labeled on the DNA double helix.
On the mRNA, “P” and “R” are labeled.

And at the bottom, you have nucleotides with:
Circle – Phosphate
Pentagon – Sugar
Shape with letter – Base

So in the DNA diagram:
- “P” is pointing to the circle (phosphate) on the backbone.
- “D” is pointing to one of the bases (the inner shapes that pair up).

In the mRNA diagram:
- “P” is pointing to the phosphate (circle) on its backbone.
- “R” is pointing to the sugar (pentagon) — because in RNA, the sugar is ribose.

Therefore:

Final identification:

DNA:
- D = Nitrogenous base (or simply “Base”)
- P = Phosphate

mRNA:
- P = Phosphate
- R = Ribose (sugar)

But sometimes in such diagrams, “D” might be intended to mean “Deoxyribose”, but according to where the arrow is pointing — if it’s pointing to the base, then it’s base.

Wait — let me think again. In many textbook diagrams, when they label “D” on DNA, it often refers to “Deoxyribose”, especially if it’s pointing to the sugar part. But in your description, you said “D” is pointing to one of the shapes in the middle — which are the bases.

Actually, looking at common labeling:

In a DNA nucleotide:
- Phosphate (P)
- Deoxyribose sugar (often labeled S or D)
- Base (B or specific letter)

But in your case, since “D” is pointing to the base region (the paired shapes inside), it should be “Base”.

However, let’s consider the possibility that “D” stands for “Deoxyribose”. If “D” were pointing to the pentagon (sugar) in DNA, then it would be deoxyribose. But you described “D” as pointing to one of the inner shapes — which are bases.

To resolve this, let’s use logic from the bottom diagram.

The bottom diagram shows four DNA nucleotides (because they include T, not U). Each has:
- Circle: Phosphate
- Pentagon: Sugar (deoxyribose in DNA)
- Attached shape: Base (G, A, C, T)

In the DNA double helix drawing, the “P” label is on the outer edge — matching the phosphate (circle). The “D” label is on one of the inner shapes — matching the base.

Similarly, in mRNA, “P” is on the phosphate, and “R” is on the sugar (which in RNA is ribose).

Therefore:

Answer:

In DNA:
- D = Base (Nitrogenous base)
- P = Phosphate

In mRNA:
- P = Phosphate
- R = Ribose

But to be precise and match typical educational materials, sometimes “D” is used for “Deoxyribose”, but only if it’s pointing to the sugar. Since you specified “D” is pointing to the base area, we’ll go with Base.

Alternatively, perhaps there’s a mistake in interpretation. Let me double-check with standard conventions.

Upon second thought — in many diagrams, especially simplified ones, the labels are:

For DNA:
- P = Phosphate
- D = Deoxyribose (sugar)
- And bases are unlabeled or labeled with letters.

But in your case, since “D” is pointing to the base (as per your initial description: “D” points to one of the shapes in the middle of the DNA helix), then it must be the base.

I think the safest answer based on standard labeling and the context is:

Final Answer:

DNA:
- D = Nitrogenous base
- P = Phosphate

Messenger RNA:
- P = Phosphate
- R = Ribose

Yes, that makes sense.

Final Answer:
D: Nitrogenous base
P (in DNA): Phosphate
P (in mRNA): Phosphate
R: Ribose