TOPIC 12 DAY 1 DNA STRUCTURE AND REPLICATION

In the diagram above, strands I and II represent the two complementary strands of a portion of a DNA double helix. The sequence of strand I is indicated below.

What is the sequence of strand II?

• C – T – A – C

• G – A – T – G

• G – A – A – C

• G – T – A – G

Answer explanation

The sequence of strand II is complementary to strand I. If strand I has A, T, C, G, then strand II will have T, A, G, C respectively. Therefore, if strand I is A – T – A – C, strand II is G – A – T – G.

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Harper is studying genetic information in different organisms in her biology class. She notices that almost all organisms translate the genetic information in DNA in a consistent way.

What inference can Harper draw from this observation?

• All living organisms share a common ancestor.

• All species can interbreed to form new species.

• Genetic information is transferred from one species to another.

• Genetic information does not vary from one organism to another.

Answer explanation

The consistent way organisms translate DNA suggests a shared mechanism, indicating that all living organisms likely evolved from a common ancestor, supporting the idea of common descent in evolutionary biology.

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DNA replication is the process by which DNA makes a copy of itself during the S phase of interphase before mitosis. DNA replication is performed by proteins working together.

Arrange the following proteins in the proper order in which they participate in DNA replication.

(1) Primase (2) Helicase (3) Ligase (4) DNA polymerase

• 1, 2, 3, 4

• 1, 3, 2, 4

• 2, 1, 4, 3

• 2, 3, 4, 1

Answer explanation

During DNA replication, helicase unwinds the DNA (2), primase synthesizes RNA primers (1), DNA polymerase extends the DNA strand (4), and ligase seals gaps between Okazaki fragments (3). Thus, the correct order is 2, 1, 4, 3.

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During DNA replication, the DNA molecule produces two new complementary strands. DNA replication is often described as semiconservative because each DNA molecule has one new strand and one original strand. Which enzyme begins the process of replication by unwinding the double helix?

• DNA polymerase

• Helicase

• Primase

• DNA ligase

Answer explanation

Helicase is the enzyme that unwinds the double helix of DNA during replication, allowing the formation of new complementary strands. This is why it is the correct answer.

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The diagram above shows DNA replication. Which of the following statements about the process shown is true?

• Two new molecules form that are identical to the original molecule.

• Two new molecules form that are different from the original molecule.

• The original molecule is rearranged into one new molecule.

• Two new molecules form, one of which is identical to the original molecule.

Answer explanation

During DNA replication, the original DNA molecule unwinds and serves as a template for the synthesis of two new strands. Each new molecule is identical to the original, resulting in two identical DNA molecules.

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The first step in DNA replication occurs when the enzyme helicase separates the DNA molecule into two strands. What is the next step in DNA replication?

• DNA polymerase joins the individual nucleotides.

• Primase adds an DNA primer to initiate replication.

• Primase adds an RNA primer to initiate replication 

• DNA polymerase “proofreads” each strand to make sure the sequence is correct.

Answer explanation

After helicase unwinds the DNA, primase adds an RNA primer to provide a starting point for DNA polymerase, which then begins synthesizing the new DNA strands.

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