Mitochondrial DNA in humans is
Answer: A
The circular form is a legacy of the bacterial ancestry, and the egg contributes essentially all of the mitochondria in a zygote. This makes the sequence useful for tracing maternal lineages. Population geneticists use it to follow descent through the female line.
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State whether the following is true or false.
A father passes his mitochondrial DNA to his children.
Answer: B
The sperm contributes almost no mitochondria, and the few that enter are destroyed, so inheritance is effectively maternal. This is why mitochondrial disease can pass from a mother to all her children but never from a father.
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Most of the proteins working inside a mitochondrion are encoded by
Answer: C
Over the course of evolution most of the original bacterial genes moved to the nucleus, leaving the organelle with a very reduced genome. The proteins are made in the cytosol and imported afterwards. Targeting sequences on each protein direct it to the correct compartment.
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Human mitochondrial DNA is about 16,500 base pairs and encodes 13 proteins. Roughly how many base pairs per encoded protein is that?
Answer: D
Dividing 16,500 by 13 gives about 1,270. The genome also encodes transfer and ribosomal RNAs, so the figure overstates how much is devoted to protein coding alone. It is one of the most compact genomes known, with almost no spare sequence.
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Mitochondria increase in number within a cell by
Answer: A
They divide in a manner reminiscent of bacterial fission, which is exactly what an endosymbiotic origin predicts. A cell that lost all of them could not rebuild them from its nuclear genes alone. Continuity of the organelle across generations is therefore unbroken.
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State whether the following is true or false.
Chloroplasts are found in all plant cells.
Answer: B
Root cells, for instance, live in darkness and contain colourless plastids instead. Chloroplasts develop chiefly in tissues exposed to light, which is why roots are white and leaves are green. The plastid family adapts to what each tissue actually needs.
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Chromoplasts, leucoplasts and chloroplasts are all
Answer: C
Plastids form a family that can interconvert, which is why a green tomato turns red as its chloroplasts become chromoplasts. Leucoplasts are the colourless storage form found in roots and tubers. A potato stores its starch in leucoplasts rather than in chloroplasts.
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Assertion (A): A tomato changes from green to red as it ripens.
Reason (R): Chloroplasts in the fruit are converted into chromoplasts.
Answer: D
Both are true and the reason gives the mechanism: chlorophyll is broken down while carotenoid pigments accumulate in the same plastid, so the colour change reflects a conversion rather than a replacement of the organelle.
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Rubisco, the enzyme that fixes carbon dioxide, is often described as inefficient because it
Answer: A
Its competing reaction with oxygen leads to photorespiration, which consumes energy and releases fixed carbon. Plants compensate by making enormous quantities of the enzyme. It accounts for a large share of the soluble protein in a leaf.
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C4 and CAM plants have evolved mechanisms that chiefly serve to
Answer: B
Raising the local carbon dioxide concentration suppresses the wasteful reaction with oxygen. C4 plants separate the steps in space and CAM plants separate them in time, but the aim is the same. Both adaptations are commonest in hot or dry climates where the waste is worst.
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