DNA Structure: The Molecule of Heredity Worksheet

Let’s go step by step to label the DNA diagram correctly.

We’re working with a standard DNA double helix structure, shown in three views:
- Left: full twisted ladder (double helix)
- Middle: zoomed-in side view showing one strand and part of the other
- Right: even closer look at one nucleotide pair

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Step 1: Circle and label a nucleotide

A nucleotide is made of 3 parts:
→ A sugar (pentagon shape)
→ A phosphate (circle or square attached to sugar)
→ A nitrogen base (rectangle sticking out from sugar)

In the middle diagram, pick any one complete unit — for example, the top-left one that has:
- Pentagon (sugar)
- Square attached to it (phosphate)
- Rectangle labeled “T” (base)

Circle all three together and write “nucleotide”.

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Step 2: Label the sugar and phosphate molecules

Look at the backbone on either side of the DNA ladder.

- The sugar is always the pentagon-shaped piece.
- The phosphate is the small circle or square connected to the sugar.

Label several sugars as “sugar” and several phosphates as “phosphate” along both backbones.

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Step 3: Label the bases that are not already labeled

You see some bases already labeled: T, G, C.

Remember base pairing rules:
- A pairs with T
- G pairs with C

So if you see a “T”, the base across from it must be “A”.
If you see a “G”, the base across must be “C”.
If you see a “C”, the base across must be “G”.

In the middle diagram:

- Top row: T → so opposite base = A (label it)
- Second row: G → opposite = C (already there? Check — if not, label it)
- Third row: blank base opposite blank — but we can infer from context. Actually, let’s use the bottom one:

Bottom row shows “C” on right → so left base must be “G”

Also, in the far-right close-up box: one base is unlabeled, paired with another unlabeled — but since they’re connected by 3 dots (hydrogen bonds), that means it’s G-C pair. So label them accordingly.

Actually, looking again — in the middle diagram, the third base pair from top has two blanks. But below that, there’s a pair with 2 dots → that’s A-T. Above that, 3 dots → G-C.

Wait — better approach: count hydrogen bonds.

- 2 dashed lines between bases = A-T pair
- 3 dashed lines = G-C pair

So:

Top pair: T on left → must have A on right (and 2 bonds) → label right base “A”

Second pair: G on left → must have C on right (3 bonds) → label right base “C” (if not already)

Third pair: both blank → but has 2 bonds → so A-T → label left “A”, right “T”? Wait — no, direction matters.

Actually, in DNA strands run antiparallel, but for labeling purposes, just match partners.

Looking at the diagram carefully:

From top to bottom in middle panel:

Row 1: Left base = T → Right base = ? → should be A → label it “A”

Row 2: Left base = G → Right base = ? → should be C → label it “C”

Row 3: Both blank → but connected by 2 dashes → so A-T → which is which? Since left strand goes down, and right strand goes up, but for simplicity, assume left is A, right is T? Not necessarily.

Actually, in most diagrams, the left strand is written 5’ to 3’ downward, so bases are read that way.

But for this level, just pair them correctly.

Since Row 4 has a base on right labeled “C” → then left must be “G”

And Row 5: both blank, 3 bonds → G-C → label left “G”, right “C”? Or vice versa?

Wait — in the bottom of middle diagram, there’s a box around a nucleotide on left that includes a base — it’s unlabeled, but connected to a base on right that’s also unlabeled, with 3 bonds → so G-C.

But actually, in the very bottom, there’s a “C” labeled on the right strand → so the base opposite it (on left) must be “G”

Similarly, above that, a pair with 2 bonds → A-T

Above that, a pair with 3 bonds → G-C

Above that, G on left → C on right

Top: T on left → A on right

So let’s list rows from top to bottom in middle diagram:

1. Left: T → Right: ___ → label “A”
2. Left: G → Right: ___ → label “C”
3. Left: ___ → Right: ___ → 2 bonds → so A-T → but which side? Since the strand continues, and no label yet, we can assign based on common convention — but actually, look at the connections.

Notice that in row 3, the left base is connected to a sugar that is part of a nucleotide that also has a phosphate above it — same as others.

To avoid confusion, let’s use the bond count:

- 2 bonds = A-T
- 3 bonds = G-C

So:

Row 1: T (left) + ? (right) + 2 bonds → ? = A → label right base “A”

Row 2: G (left) + ? (right) + 3 bonds → ? = C → label right base “C”

Row 3: ? (left) + ? (right) + 2 bonds → so one is A, one is T. Which is which? In DNA, the bases are complementary, but without knowing orientation, we can choose. However, typically in such diagrams, the left strand is the "template" or reference. But for accuracy, note that in row 4, we have:

Row 4: ? (left) + C (right) + 3 bonds → so left must be G → label left base “G”

Row 5: ? (left) + ? (right) + 3 bonds → G-C → but we don’t know which is which yet.

Wait — in the bottom of the middle diagram, there’s a nucleotide circled on the left side — it includes a base that is unlabeled, and it’s paired with a base on the right that is also unlabeled, with 3 bonds → so G-C.

But just above that, there’s a “C” labeled on the right strand → so the base directly opposite it (on left) must be “G”

That’s row 4: right = C → left = G

Then row 5: below that, both blank, 3 bonds → so another G-C pair. We can label left “G”, right “C” — or doesn’t matter as long as they match.

Actually, to be precise, let’s number the rows from top to bottom in the middle diagram:

Row 1: Left base = T → Right base = A (label it)

Row 2: Left base = G → Right base = C (label it)

Row 3: Left base = ? , Right base = ? , 2 bonds → so A-T. Now, since the left strand is continuous, and we have T, G, then next could be A or C, etc. But logically, after G, it could be anything. However, in many textbook diagrams, they show a sequence like T-G-A-C-G or something.

But here’s a better way: look at the far-right close-up box. It shows one nucleotide with a base, and its partner. The base on the left in that box is unlabeled, and the one on right is unlabeled, with 3 bonds → so G-C. And since it's a magnified view of one pair from the main diagram, likely it's from row 5 or similar.

Perhaps it's easier to label based on what's missing.

In the middle diagram, the only bases not labeled are:

- Right side, row 1: should be A
- Right side, row 2: should be C
- Left side, row 3: should be A or T? Let's say we'll put A on left, T on right for row 3? But wait, row 3 has 2 bonds, so A-T.

Actually, I think I made a mistake earlier. Let me re-express:

Standard base pairing:

Adenine (A) always pairs with Thymine (T) via 2 hydrogen bonds.

Guanine (G) always pairs with Cytosine (C) via 3 hydrogen bonds.

In the diagram:

- Wherever you see "T", the opposite base is "A"
- Wherever you see "G", opposite is "C"
- Wherever you see "C", opposite is "G"
- For unlabeled pairs, use the number of bonds to decide.

In the middle diagram:

Top pair: left = T, bonds = 2 → right = A → label "A"

Second pair: left = G, bonds = 3 → right = C → label "C"

Third pair: both unlabeled, bonds = 2 → so A-T. Now, which is A and which is T? In DNA, the strands are antiparallel, but for labeling, we can assign arbitrarily as long as they pair. However, typically, if the left strand is going down, and we have T then G, the next might be A, so left = A, right = T. But to be safe, let's look at the fourth pair.

Fourth pair: right = C (labeled), bonds = 3 → so left = G → label left "G"

Fifth pair: both unlabeled, bonds = 3 → G-C. Again, we can label left "G", right "C" or vice versa. But since the right strand has C in row 4, and this is below, it could be C or G. However, in the sequence, it might be consistent.

Actually, in the bottom of the middle diagram, there is a nucleotide on the left that is boxed — it includes a base that is unlabeled, and it's paired with a base on the right that is unlabeled, with 3 bonds. Also, just above that, there is a "C" on the right, so the base opposite it is G on left.

For the fifth pair (bottom), since it's 3 bonds, and no label, we can label the left base "G" and right "C", or if we want to match the pattern, but it's fine.

To simplify for the student, I'll specify:

Label the following:

- In row 1 (top): right base = "A"
- In row 2: right base = "C"
- In row 3: left base = "A", right base = "T" [since 2 bonds]
- In row 4: left base = "G" (since right is "C")
- In row 5: left base = "G", right base = "C" [for 3 bonds, and to have variety]

But row 5 might be intended to be different. Perhaps row 3 is T-A, but let's check the close-up.

In the far-right box, it shows a single nucleotide with a base, and its partner. The base on the left in that box is a rectangle, unlabeled, and the one on right is also rectangle, unlabeled, with 3 bonds between them. This is likely a G-C pair. And since it's magnified, it corresponds to one of the pairs in the main diagram, probably the bottom one.

So for consistency, in row 5, label left "G", right "C".

But in row 4, we have right "C", so left "G" — that's fine, two G-C pairs in a row is possible.

Alternatively, perhaps row 3 is A-T with A on right, T on left? But in row 1, T is on left, so maybe the left strand has T,G,T,G,... but not specified.

I think for educational purposes, as long as the pairing is correct, it's acceptable.

So let's finalize:

Unlabeled bases to label:

- Right strand, top: "A" (paired with T)
- Right strand, second: "C" (paired with G)
- Left strand, third: "A"
- Right strand, third: "T" (so A-T pair)
- Left strand, fourth: "G" (paired with C on right)
- Left strand, fifth: "G"
- Right strand, fifth: "C" (G-C pair)

But in the diagram, the fifth pair might be the one in the close-up, so yes.

Now, moving on.

Step 4: Label a base pair

A base pair is two bases connected by hydrogen bonds — one from each strand.

For example, the top pair: T and A together form a base pair.

Draw a bracket or circle around any one pair of bases (like T and A) and label it "base pair".

Step 5: Label the sugar-phosphate backbones

The backbone is the outer rails of the DNA ladder, made of alternating sugar and phosphate groups.

On both sides of the DNA, the chain of sugars and phosphates forms the backbone.

Label the entire left vertical chain as "sugar-phosphate backbone" and similarly for the right side.

Or, you can label a segment, like "this is the sugar-phosphate backbone".

Step 6: Label the hydrogen bonds

Hydrogen bonds are the dashed lines between the bases.

Each dashed line represents a hydrogen bond.

Label one or more of these dashed lines as "hydrogen bond".

For example, between T and A, there are two dashed lines — label one of them "hydrogen bond".

Between G and C, three dashed lines — label one "hydrogen bond".

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Now, to summarize what to do on the diagram:

1. Circle one complete nucleotide (sugar + phosphate + base) and label it "nucleotide".
2. Label several pentagons as "sugar" and several circles/squares as "phosphate".
3. Label the missing bases:
- Opposite T: label "A"
- Opposite G: label "C"
- For the pair with 2 bonds and no labels: label one "A" and the other "T" (e.g., left "A", right "T")
- Opposite C: label "G"
- For the last pair with 3 bonds: label one "G" and the other "C" (e.g., left "G", right "C")
4. Draw a bracket around any one pair of bases (like T and A) and label it "base pair".
5. Label the left and right chains of sugars and phosphates as "sugar-phosphate backbone".
6. Label one or more dashed lines between bases as "hydrogen bond".

This completes the task.

Final Answer:
Follow the steps above to label the diagram: circle and label a nucleotide; label sugars and phosphates; fill in missing bases using A-T and G-C pairing; label a base pair; label the sugar-phosphate backbones; and label hydrogen bonds.