IB Biology HL D1.2.12 Hl Directionality Questions

Explore how RNA polymerase and ribosomes use strand direction to synthesize RNA and translate mRNA in IB Biology HL D1.2.12.

Exam points

Explain that RNA polymerase reads the DNA template 3′→5′ while synthesizing RNA 5′→3′.
Explain that ribosomes read mRNA codons 5′→3′, so the start codon is at the 5′ end and enters first.
Use strand labels and ribosome-order evidence to identify the correct direction and start codon in a representation.

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Study D1.2.12 (HL)—Directionality conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

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The electron micrograph shows part of a cell in the pituitary gland of a rat. Ribosomes appear as dark granules. Some of the ribosomes are arranged in a linear array and some are in circles. The diagram shows how the ribosomes in a circular array are connected.

Figure for Question IB Biology HL D1.2.12 Hl Directionality Questions question 1 — IB Biology HL

Using the diagram, identify, with a reason, whether X or Y is the start codon.

Y is the start codon because translation goes from 5' to 3'
OR
Y is the start codon because it is in the $5^{\prime}$ end of the mRNA
OR
Y is the start codon because it is the first to enter the ribosomes;

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IB Biology HL D1.2.12 Hl Directionality Questions question 1