During meiosis, a tetrad is formed when homologous chromosomes pair up. Which of the following best describes what a tetrad consists of?

Introduction / Context:
Meiosis is the special cell division process that produces gametes. In the first division of meiosis, homologous chromosomes pair up in a process called synapsis. During this pairing, structures known as tetrads are formed. Understanding what a tetrad is helps in grasping how genetic recombination and independent assortment occur. This question asks for the best description of a tetrad during meiosis.

Given Data / Assumptions:

• The term tetrad is used in the context of meiosis, not mitosis.
• Options mention cells, gametes, chromatids, and haploid cells.
• We assume knowledge that each duplicated chromosome consists of two sister chromatids.

Concept / Approach:
Before meiosis begins, DNA is replicated so that each chromosome consists of two sister chromatids. During prophase I of meiosis, homologous chromosomes pair. Each homologous chromosome has two chromatids, so when one pair of homologous chromosomes aligns, there are four chromatids grouped together. This structure of four chromatids is called a tetrad. Crossing over, where chromatids exchange segments, occurs within these tetrads, contributing to genetic variation.

Step-by-Step Solution:
Step 1: Recall that a single duplicated chromosome consists of two sister chromatids. Step 2: Understand that homologous chromosomes come in pairs, one from each parent, and they pair up during prophase I of meiosis. Step 3: When both homologous chromosomes are duplicated and paired, the group contains a total of four chromatids. Step 4: Recognise that this set of four chromatids is called a tetrad, so the description mentioning four chromatids is the correct one.

Verification / Alternative check:
Diagrams of meiosis in textbooks show homologous chromosomes pairing in prophase I and label the resulting group as a tetrad. Each tetrad is drawn as four chromatids side by side. These same diagrams show crossing over occurring in chiasmata within the tetrad, reinforcing that the tetrad is a four chromatid structure, not a single cell or a pair of gametes.

Why Other Options Are Wrong:

• A single diploid cell: This describes the state of the cell before meiosis begins, but a tetrad is specifically a structure formed by chromosomes inside that cell.
• Two gametes: Gametes are the final haploid cells after meiosis II, not structures present in prophase I.
• A single haploid cell: This again refers to a cell at the end of meiosis, not the four chromatid structure during prophase I.

Common Pitfalls:
A common confusion is between chromatids and chromosomes, and between cells and chromosome groupings. It helps to remember that tetra means four, and in a tetrad you are counting chromatids. Visual aids showing the stages of meiosis can make it easier to keep track of how many chromatids are present and how they are organised at each stage.

Final Answer:
A tetrad in meiosis is best described as four chromatids aligned together as a pair of homologous chromosomes.