Which of the following is TRUE of the process of conjugation in Paramecium?

Analyzing the question: It asks which statement is true of conjugation in Paramecium, a ciliate protozoan with two kinds of nuclei: a large macronucleus (controls everyday metabolism) and one or more small micronuclei (involved in reproduction).

Concept: the steps of conjugation

1. Two compatible Paramecia (different mating types) come together and fuse at the oral groove region.
2. The macronucleus disintegrates in each partner.
3. The micronucleus divides by meiosis, giving 4 haploid micronuclei. This is two successive divisions, so the micronucleus divides twice.
4. Three of the four haploid micronuclei disintegrate. One remains and divides by mitosis into two: a stationary one and a migratory one.
5. The partners exchange migratory micronuclei, which fuse with the stationary ones to restore a diploid zygote nucleus (synkaryon).
6. The partners separate (now called ex-conjugants). The new nucleus divides to form new micronuclei and macronuclei.

Checking each option

• A. Micronucleus disintegrates. Not accurate as a general statement. It is the macronucleus that disintegrates, and only three of the four micronuclei degenerate. Incorrect.
• B. Each ex-conjugant divides only once. Ex-conjugants typically divide more than once to restore the normal nuclear arrangement. Incorrect.
• C. Macronucleus undergoes division. The macronucleus breaks down during conjugation rather than dividing. Incorrect.
• D. Each micronucleus divides twice. The micronucleus undergoes meiosis, which involves two divisions and produces four haploid nuclei. This matches the standard description. Correct.

Exam tip: Remember "Macro Melts, Micro Meiosis". The macronucleus disintegrates, and the micronucleus undergoes meiosis (two divisions, four haploid nuclei). Conjugation is sexual, so it involves exchange of genetic material, not an increase in numbers.