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Karyogamy is?
A. The fusion of two germ cells
B. The Fusion of two gametic nuclei
C. The Fusion of somatic cells and reproductive cells
D. The Fusion of two somatic cells.

Answer
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Hint: The term Karyogamy comes from the Greek word 'karyo' means ‘nut’. Karyogamy is the final step of the fusion process. For Karyogamy to occur the cell membrane and cytoplasm of each cell must fuse by the process known as plasmogamy.

Complete answer:
-Karyogamy is the fusion of two gametic nuclei that takes place after the fusion of two haploid eukaryotic cells, each carrying one complete copy of the organism's genome. Once the cell membrane is joined the nuclei is referred to as pronuclei. Once the cell membrane, cytoplasm, and pronuclei fuse it results in the formation of a single diploid cell. This diploid cell is called zygospores (in case of fungi) or zygote in general.

Figure 1: Cell fusion


Additional information:
Karyogamy is a process usually seen during sexual reproduction involving the fusion of two haploid gametes. That results in the formation of a zygote. In the case of fungi, karyogamy is followed by meiosis to restore the haploid phase.

Note: The fusion of two protoplasts that is the fusion of two cell contents is known as Plasmogamy. A process that brings together two compatible haploid nuclei. The fusion of these haploid nuclei is known as Karyogamy that results in the formation of a diploid cell containing a single nucleus with two sets of chromosomes, one from each parent.