Home / IB DP Biology- A1.2 Nucleic acids – IB Style Questions For SL Paper 2

IB DP Biology- A1.2 Nucleic acids - IB Style Questions For SL Paper 2 - FA 2025

IBDP Biology Paper 2 - All Chapters

Question

Watson and Crick’s model of DNA as a double helix led to major advances in how we interpret the role of genes.
(a) Name the basic building blocks that form DNA.
(b) Explain two key structural differences between DNA and RNA.
(c) Research on pea plants (Pisum sativum) showed that the allele for short stems (\(t\)) is recessive. Describe why recessive alleles do not always appear in an organism’s visible traits.

Most-appropriate topic codes :

• TOPIC A1.2: Nucleic acids — parts (a), (b)
• TOPIC D3.2: Inheritance — part (c)
▶️ Answer/Explanation
Detailed solution

(a)
Nucleotides .

(b)
• Strands: DNA is double-stranded, while RNA is single-stranded .
• Sugar: DNA contains the sugar deoxyribose, while RNA contains ribose .
• Base: DNA contains the base thymine (\(T\)), while RNA contains the base uracil (\(U\)) instead .

(c)
An organism’s phenotype (observable trait) is determined by its genotype (the combination of alleles). A diploid organism has two alleles for each gene. A recessive allele is only expressed in the phenotype if the organism is homozygous recessive (e.g., \(tt\)). If a dominant allele (e.g., \(T\) for tall stems) is present, it will be expressed. In a heterozygous genotype (e.g., \(Tt\)), the dominant allele masks the effect of the recessive allele, so the recessive trait (short stems) is not expressed.

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