During meiosis in cell division, the structure formed when a pair of homologous chromosomes come together and consist of four chromatids is known as what?

Introduction / Context:
Meiosis is a specialized type of cell division that reduces the chromosome number by half and produces gametes. A key feature of meiosis is pairing of homologous chromosomes and genetic recombination. The structure that forms when homologous chromosomes pair and each consists of two sister chromatids is important for crossing over and proper segregation. This question asks for the term used for that paired structure.

Given Data / Assumptions:

• Homologous chromosomes are pairs of chromosomes that have the same genes at the same loci but may have different alleles.
• Each chromosome after replication consists of two sister chromatids joined at a centromere.
• During meiosis I, homologous chromosomes pair up.
• The options include common terms from genetics and cell division.

Concept / Approach:
When homologous chromosomes pair during prophase I of meiosis, each chromosome consists of two sister chromatids, so the paired structure has four chromatids in total. This four chromatid unit is called a tetrad or bivalent. It is the site where crossing over and recombination occur. Gametes are the final haploid cells produced by meiosis, not the paired structures. A chromatid is one of the two identical halves of a duplicated chromosome. A zygote is the diploid cell formed after fertilization, and a centromere is the region joining sister chromatids. Therefore, the term for the four chromatid structure is tetrad.

Step-by-Step Solution:
Step 1: Recall that before meiosis, chromosomes replicate, so each consists of two sister chromatids. Step 2: In prophase I, homologous chromosomes pair up side by side in a process called synapsis. Step 3: The paired homologous chromosomes now comprise four chromatids, two from each homolog. Step 4: This four chromatid unit is referred to as a tetrad because tetra means four. Step 5: Evaluate other terms and confirm that none of them specifically describe the paired homologous chromosomes with four chromatids.

Verification / Alternative check:
Textbooks and diagrams of meiosis label these paired structures as tetrads or bivalents during prophase I and metaphase I. Crossing over is illustrated as occurring between non sister chromatids within a tetrad. At metaphase I, tetrads align at the metaphase plate, and then homologous chromosomes separate during anaphase I, reducing chromosome number.

Why Other Options Are Wrong:
Option B, gamete, refers to the haploid reproductive cells such as sperm and egg produced at the end of meiosis, not the intermediate paired structure. Option C, chromatid, is a single strand of a duplicated chromosome, not the four chromatid structure. Option D, zygote, is a diploid cell formed by fusion of two gametes and is not part of the meiotic division in the same cell. Option E, centromere, is the region where sister chromatids attach, not the whole paired homologous unit.

Common Pitfalls:
Students may confuse tetrad with chromatid because the words are similar, or may think gamete is correct simply because meiosis produces gametes. It is important to focus on the stage when homologous chromosomes pair and the total number of chromatids involved in that structure.

Final Answer:
The structure formed by a pair of homologous chromosomes consisting of four chromatids is called a Tetrad.