Question

Assertion: The complex formed by a pair of synapsed homologous chromosome is called a bivalent or a tetrad. Reason: The first two stages of prophase I are relatively short lived when compared to the next stage that is pachytene.

Short answer

The exercise's assertion that a pair of synapsed homologous chromosomes is referred to as a bivalent or a tetrad is correct. The reason for this assertion, which states that the first two stages of prophase I are shorter than the pachytene stage, is also correct.

Step-by-step solution

Understand Key Terms

Knowing the definitions of key terms will aid in understanding this exercise. Homologous chromosomes are a pair of chromosomes, one from each parent, that are similar in shape and size. The term 'bivalent' or 'tetrad' refers to this pair when they come together during the first stage of meiosis. 'Pachytene' refers to a stage in prophase I of meiosis during which the homologous chromosomes synapse or come together.

Verify the Terms

The next step is to verify the assertion that a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. This is accurate. During meiosis, homologous chromosomes pair up and synapse, creating a structure known as a bivalent or tetrad.

Validate the Reason

The last step is to validate the reason given for the assertion. The reason suggests that the first two stages of prophase I are relatively short when compared to the pachytene stage. Indeed, in the process of meiosis, the pachytene stage is longer than the previous stages (leptotene and zygotene) due to the important genetic exchanges occurring at this moment.

Key concepts

Homologous chromosomes

Imagine you inherited a cookbook from each of your parents. Although the cookbooks contain similar recipes—your mother's apple pie and your father's apple pie—the ingredients may differ slightly. Homologous chromosomes are just like these cookbooks: they are similar but not identical pairs of chromosomes that an individual receives from each parent. Each homologous chromosome carries the same types of genes, which determine individual traits. However, the exact version of each gene, called an allele, might differ between the two chromosomes.

For example, the gene for eye color is located at the same position on both chromosomes, but one chromosome might have the allele for brown eyes and the other for blue eyes. During meiosis, the homologous chromosomes have a crucial role during the exchange of genetic information which leads to variations in the offspring, a fundamental principle in biology known as genetic diversity.

Bivalent or Tetrad

When we talk about a high school prom, the image of dancing couples comes to mind. Similarly, during the early phase of meiosis, homologous chromosomes pair up so intimately that they could be considered dance partners in the cellular equivalent of a prom. This twosome formation is known as a bivalent or tetrad. Why two names, you might wonder? This tightly paired complex gets the name bivalent because it involves two chromosomes, and tetrad because it contains four chromatids—two from each homologous chromosome. Standing side by side, each member of the pair brings its own unique set of genetic information to the floor—just as prom partners might wear different colored outfits but move together as a unit. This pairing sets the stage for genetic recombination, a process where parts of these chromatids are exchanged, shuffling the genetic deck and setting the stage for the genetic diversity seen in sexually reproducing organisms.

Pachytene

Think of a complex dance move that takes time and practice to perfect. Similarly, in meiosis, the pachytene stage represents an intricate point in Prophase I where the homologous chromosomes—already paired up in bivalents or tetrads—begin a critical process. The chromosomes are tightly aligned and pressed together in a process called synapsis.

At this juncture, segments of genetic material can be swapped between the chromatids in a process known as crossing over. This meticulous exchange of DNA fragments extends the duration of the pachytene stage, making it one of the longest and most complex stages of Prophase I. It's like the moment in which dancers exchange handkerchiefs, symbolizing the exchange of gifts or traits. It is during pachytene that the foundation for genetic variation among offspring is laid down.

Prophase I

Prophase I is akin to the overture of an opera—it sets the stage for the drama of meiosis to unfold. This initial phase of meiosis I is both intricate and critical, involving several stages ending with the cells being primed to divide. Prophase I is traditionally divided into five subsequent stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

During this phase, chromatin condenses into visible chromosomes, homologous chromosomes pair up to form bivalents, and crossing over occurs, mixing up the genetic information and ensuring each gamete—the reproductive cells, like sperm or eggs—will be unique. Prophase I culminates with the breakdown of the nuclear membrane, the disappearance of the nucleolus, and the spindle fibers beginning to form. It is an elegant and carefully orchestrated set of events that ensures the proper distribution of genetic material to the resulting cells.