Chloroplast genetics What is the approximate size of the tobacco (Nicotiana tabacum) chloroplast genome?
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A55 kilobase
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B155 kilobase
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C325 kilobase
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D375 kilobase
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E15 kilobase
Answer
Correct Answer: 155 kilobase
Explanation
Introduction / Context:Chloroplast genomes are circular DNAs encoding photosynthetic and gene expression functions. Tobacco is a classic model for plastid transformation and genome mapping.
Given Data / Assumptions:
- Chloroplast genomes in higher plants are typically in the 120–170 kb range.
- Tobacco plastome is one of the best-characterized reference plastomes.
Concept / Approach:Recall the canonical tobacco plastid genome size used in transformation vectors and maps. The commonly cited size is about 155 kb, featuring large and small single-copy regions separated by inverted repeats.
Step-by-Step Solution:
Eliminate implausible sizes (too small 15–55 kb; too large >300 kb).Select 155 kb as the standard tobacco chloroplast genome size.Verification / Alternative check:Plastome maps used in biolistic transformation and marker insertion are based on ~155 kb tobacco chloroplast DNA.
Why Other Options Are Wrong:
- 55 kb and 15 kb: far below known plastome sizes.
- 325 kb and 375 kb: exceed higher-plant plastome sizes.
Common Pitfalls:Confusing bacterial genome sizes or mitochondrial genome variability with the relatively conserved plastid genome range.
Final Answer:155 kilobase