What structure forms during meiosis that contains four chromatids?

During meiosis, the structure that forms containing four chromatids is known as a tetrad. This happens during prophase I of meiosis when homologous chromosomes pair up closely together. Each homologous chromosome is made up of two sister chromatids, and since there are two homologous chromosomes in the pair, the combination results in four chromatids overall.

The formation of tetrads is crucial for the process of genetic recombination, or crossing over, which occurs during this stage. Crossing over increases genetic diversity by allowing the exchange of genetic material between homologous chromosomes. This process is fundamental to the formation of gametes, as it mixes the genetic material from the two parent organisms, ultimately leading to unique combinations in offspring.

The other options do not represent a structure with four chromatids during meiosis. A chromosome consists of a single thread of DNA and can be made up of two chromatids but does not refer to the pairing of two homologous chromosomes. A chromatid is one half of a chromosome that gets pulled apart during cell division, while a zygote is the result of fertilization when a sperm and an egg combine, not a structure found during meiosis itself.