During DNA replication in cells, which one of the following statements about the mechanism of replication is incorrect (false)?

Introduction / Context:
DNA replication is a fundamental process that ensures genetic information is accurately copied before cell division. Exam questions often present several statements about replication and ask you to find the one that is false. This tests your understanding of enzymes, base pairing rules, and the semiconservative nature of replication. Here you must identify which statement does not match the known mechanism of DNA replication.

Given Data / Assumptions:

• We consider standard DNA replication in cells.
• Statements mention DNA polymerase, unwinding, semiconservative replication, and base pairing rules.
• One statement must be incorrect, while the others should accurately describe replication.
• We assume normal double stranded DNA with A, T, G, and C bases.

Concept / Approach:
DNA replication requires several key features. DNA polymerase is the main enzyme that adds nucleotides to a growing DNA strand. The double helix unwinds as hydrogen bonds between complementary bases break, exposing template strands. Replication is semiconservative, meaning each daughter DNA molecule consists of one parental strand and one newly synthesized strand. Importantly, base pairing is complementary: adenine pairs with thymine, and guanine pairs with cytosine. Bases do not pair with identical bases (A with A); they pair with complementary partners. Therefore, any statement claiming that each base pairs with exactly the same base is false.

Step-by-Step Solution:
Step 1: Check statement about DNA polymerase. It is correct that DNA polymerase catalyses DNA synthesis. Step 2: Check statement on unwinding. During replication, helicase and other proteins unwind the double helix as hydrogen bonds break, which is true. Step 3: Check statement on semiconservative replication. Each new double helix retains one old strand and one new strand, which matches the definition. Step 4: Examine the statement that each base is paired with another base exactly like itself (A with A, T with T, etc.). This contradicts the known complementary base pairing rules. Step 5: Recall that proper pairing is A with T and G with C, not identical base with identical base. Step 6: Conclude that the identical pairing statement is the false one, while the other statements are true.

Verification / Alternative check:
Molecular biology texts and diagrams show the Watson Crick base pairing rules: A pairs with T through two hydrogen bonds, and G pairs with C through three hydrogen bonds. These complementary pairs are essential for accurate replication. Experiments on semiconservative replication, such as the Meselson and Stahl experiment, confirm that each daughter molecule contains one old and one new strand. No credible source teaches that A pairs with A or T with T during replication. This confirms that the statement about identical base pairing is false.

Why Other Options Are Wrong (as the false choice):
The enzyme that catalyses DNA strand synthesis is DNA polymerase: Correct, DNA polymerases are the main replication enzymes. Unwinding of the DNA molecule occurs as hydrogen bonds break: Correct, helicase and other proteins break hydrogen bonds to separate strands. Replication is called semiconservative because one old strand is conserved: Correct, each daughter molecule contains one parental and one new strand. Replication requires precise base pairing rules: Correct, fidelity depends on strict complementary base pairing.

Common Pitfalls:
Students sometimes misinterpret the idea that a duplicated strand looks similar to the original and mistakenly think identical bases pair with each other. Another mistake is to gloss over the word exactly and not notice that A with A contradicts complementary pairing. To avoid this, always remember the specific pairings A with T and G with C in DNA. If a statement describes any other pairing pattern as correct, treat it as false.

Final Answer:
The false statement is that DNA replication occurs as each base is paired with another base exactly like itself (A with A, T with T, etc.).