6.2 DNA Replication AP Bio Worksheet - Rachel Taylor. youtu ... - Free Printable

Let’s go step by step through each question. We’ll use what we know about DNA replication — how cells copy their DNA before dividing.

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Question 1: Label the 5’ and 3’ ends of each strand.

In DNA, every strand has direction: one end is called 5’ (five prime) and the other is 3’ (three prime). The 5’ end has a phosphate group (P), and the 3’ end has an OH (hydroxyl) group.

Looking at the top diagram:

- Left side of left strand: starts with P → that’s 5’
- Bottom of left strand: ends with OH → that’s 3’
- Right side of right strand: starts with OH → that’s 3’
- Top of right strand: ends with P → that’s 5’

So label them like this:

Left strand:
Top = 5’, Bottom = 3’

Right strand:
Top = 5’, Bottom = 3’? Wait — no! Actually, in double-stranded DNA, strands are antiparallel. So if left strand goes 5’→3’ downward, then right strand must go 3’→5’ downward — meaning its top is 3’ and bottom is 5’.

Wait — let’s look again at the diagram:

Left strand:
- Top nucleotide has P on left → so that’s 5’ end
- Bottom nucleotide has OH on left → so that’s 3’ end

Right strand:
- Top nucleotide has OH on right → so that’s 3’ end
- Bottom nucleotide has P on right → so that’s 5’ end

✔ Final labeling for Q1:

Left strand: top = 5’, bottom = 3’
Right strand: top = 3’, bottom = 5’

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Question 2: Identify A, B, and C in the diagram.

Look at the dashed boxes:

- A surrounds a whole unit: sugar + phosphate + base → that’s a nucleotide
- B points to the bond between two bases (C and G) → those are held by hydrogen bonds
- C points to the line connecting the sugar of one nucleotide to the phosphate of the next → that’s the phosphodiester bond (or backbone bond)

✔ Answers:

A. Nucleotide
B. Hydrogen bond
C. Phosphodiester bond

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Question 3: Explain why each strand in DNA serves as a “template” for replication.

DNA is made of two complementary strands. That means wherever there’s an A on one strand, there’s a T on the other; wherever there’s a C, there’s a G.

When DNA replicates, the two strands separate. Each old strand acts like a mold or template. New nucleotides come in and match up according to base-pairing rules (A-T, C-G). So each original strand guides the creation of a new partner strand.

This ensures the new DNA molecules are exact copies of the original.

✔ Answer:

Each strand serves as a template because the sequence of bases on one strand determines the sequence on the new strand via base pairing (A with T, C with G). This allows accurate copying of genetic information.

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Now moving to the second big diagram (the replication fork):

We see:

- Two strands being copied
- RNA primers labeled
- Enzymes A and B
- Direction arrows (5’ to 3’)
- Letter D near where new DNA is being added

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Question 4: Identify the name of the process represented by the diagram above.

The diagram shows DNA unwinding, RNA primers being placed, and new DNA being synthesized in both directions — classic signs of DNA replication, specifically showing the replication fork.

✔ Answer:

DNA Replication

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Question 5: Identify the name and function of A.

Structure A is a large enzyme sitting at the replication fork, adding new nucleotides to the growing DNA chain. It moves along the template strand and builds the new strand in the 5’ to 3’ direction.

That’s DNA Polymerase.

Function: Adds DNA nucleotides to the growing chain during replication. Also proofreads and fixes mistakes.

✔ Answer:

Name: DNA Polymerase
Function: Synthesizes new DNA strand by adding nucleotides complementary to the template strand; also proofreads for errors.

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Question 6: Identify the name and function of B.

Structure B is ahead of the replication fork, cutting or relieving tension in the DNA helix as it unwinds. Without it, the DNA would get too twisted and break.

That’s Helicase? Wait — actually, helicase unzips the DNA. But structure B looks like it’s ahead of the fork, possibly relaxing supercoils.

Actually, looking closely — B is shaped like a clamp or ring around the DNA, and it’s helping hold the polymerase in place? Or maybe it’s topoisomerase?

Wait — standard diagrams show:

- Helicase = unzips DNA (often shown as wedge-shaped at fork)
- Topoisomerase = relieves twisting ahead of fork
- Sliding clamp = holds polymerase on DNA

But here, B is drawn as a gray oval surrounding the DNA just behind the fork — likely representing Sliding Clamp (like beta clamp in bacteria or PCNA in eukaryotes).

Function: Keeps DNA polymerase attached to the DNA so it doesn’t fall off while synthesizing long stretches.

Alternatively, some diagrams label B as Helicase — but helicase is usually at the very front opening the fork.

Looking again: In many textbook diagrams, the enzyme that opens the fork is helicase (often labeled near the Y-junction), and another enzyme (topoisomerase) works ahead.

But in this diagram, A is clearly the polymerase (adding nucleotides), and B is positioned right at the fork, appearing to help unwind or stabilize.

Actually — re-examining common AP Bio diagrams: Often, B is Helicase, which breaks hydrogen bonds between bases to separate the strands.

Yes — that makes sense. Helicase is responsible for unwinding the double helix at the replication fork.

✔ Answer:

Name: Helicase
Function: Unwinds the DNA double helix by breaking hydrogen bonds between base pairs, creating the replication fork.

*(Note: Some sources may vary, but based on position and typical labeling in such diagrams, B is most likely Helicase.)*

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Question 7: Explain why C is necessary for DNA replication.

C is labeled as “RNA Primer”.

DNA polymerase cannot start building a new strand from scratch — it needs a starting point with a free 3’ OH group to add nucleotides onto.

RNA primer provides that starting point. An enzyme called primase makes a short RNA segment complementary to the DNA template. Then DNA polymerase can attach to the 3’ end of the RNA primer and begin adding DNA nucleotides.

Without RNA primers, DNA replication couldn’t start.

✔ Answer:

C (RNA Primer) is necessary because DNA polymerase can only add nucleotides to an existing strand — it can’t start from nothing. The RNA primer gives DNA polymerase a 3’ end to begin synthesis.

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Question 8: Identify the name and function of D.

D is located at the end of the newly synthesized DNA strand, right after the RNA primer. It appears to be replacing the RNA with DNA.

That’s DNA Ligase? No — ligase joins fragments together later.

Actually, D is probably RNase H or FEN1 — enzymes that remove RNA primers — followed by DNA polymerase filling the gap.

But in simplified diagrams, often the enzyme that replaces RNA with DNA is still considered part of DNA polymerase activity, or sometimes labeled separately.

Wait — looking at the arrow pointing to D: it’s right at the junction between RNA primer and new DNA. And the box says “D” over the area where RNA is being replaced.

In many curricula, they say that DNA Polymerase I (in prokaryotes) removes RNA primers and replaces them with DNA.

Then DNA Ligase seals the nick.

But here, since D is shown actively working at the primer-DNA junction, and given context, D is likely referring to the enzyme that removes RNA and fills in DNA — so DNA Polymerase I (or simply “replacement polymerase”).

However, in many high school/AP contexts, they might just call it “enzyme that removes RNA primer and replaces with DNA”.

But let’s check the third small diagram at bottom right — it shows “DNA Polymerase” acting on a single strand with a primer — suggesting that D could be DNA Polymerase doing replacement.

Actually, in the main diagram, D is marked right where the RNA primer meets the new DNA — and there’s a little starburst indicating removal/replacement.

Standard answer expected here:

Name: DNA Polymerase (specifically Pol I in bacteria)
Function: Removes RNA primer and replaces it with DNA nucleotides.

Sometimes they accept “exonuclease” or “repair enzyme”, but best to stick with DNA Polymerase for simplicity.

✔ Answer:

Name: DNA Polymerase (or DNA Polymerase I)
Function: Removes the RNA primer and replaces it with DNA nucleotides to complete the strand.

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Question 9: Describe the specific reaction taking place at D.

At D, the RNA primer is being removed and replaced with DNA.

Specifically:

- Enzymes cut out the RNA nucleotides.
- DNA polymerase adds DNA nucleotides in their place, using the template strand.
- This creates a continuous DNA strand (though initially with a nick, which ligase will seal later).

So the reaction is: Removal of RNA primer and synthesis of DNA in its place.

✔ Answer:

At D, the RNA primer is excised (cut out), and DNA polymerase adds complementary DNA nucleotides to fill the gap, extending the new DNA strand.

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## ✔ Final Answers:

1. Left strand: top = 5’, bottom = 3’
Right strand: top = 3’, bottom = 5’

2.
A. Nucleotide
B. Hydrogen bond
C. Phosphodiester bond

3. Each strand serves as a template because the sequence of bases on one strand determines the sequence on the new strand via base pairing (A with T, C with G). This allows accurate copying of genetic information.

4. DNA Replication

5. Name: DNA Polymerase
Function: Synthesizes new DNA strand by adding nucleotides complementary to the template strand; also proofreads for errors.

6. Name: Helicase
Function: Unwinds the DNA double helix by breaking hydrogen bonds between base pairs, creating the replication fork.

7. C (RNA Primer) is necessary because DNA polymerase can only add nucleotides to an existing strand — it can’t start from nothing. The RNA primer gives DNA polymerase a 3’ end to begin synthesis.

8. Name: DNA Polymerase (or DNA Polymerase I)
Function: Removes the RNA primer and replaces it with DNA nucleotides to complete the strand.

9. At D, the RNA primer is excised (cut out), and DNA polymerase adds complementary DNA nucleotides to fill the gap, extending the new DNA strand.