What is the Difference Between a Tetrad and a Chromatid Pair

The main difference a tetrad and a chromatid pair is that the tetrad includes the four sister chromatids of the bivalent, whereas the chromatid pair includes the two sister chromatids of a single chromosome after undergoing DNA replication. Furthermore, homologous chromosomes pair during the prophase I of the meiosis I, while chromatid pair emerges after undergoing DNA replication during the S phase of the interphase of the cell cycle. Moreover, a tetrad can undergo homologous recombination while chromatid pair does not undergo homologous recombination. 

Tetrad and chromatid pair are two terms used to describe different combinations of chromatids of the bivalent. Generally, bivalent is a pair of homologous chromosomes, which pair during meiosis. Also, they have already undergone DNA replication.  

Key Areas Covered 

1. What is a Tetrad
     – Definition, Formation, Role
2. What is a Chromatid Pair
     – Definition, Formation, Role
3. What are the Similarities Between Tetrad and Chromatid Pair
     – Outline of Common Features
4. What is the Difference Between Tetrad and Chromatid Pair
     – Comparison of Key Differences

Key Terms 

Chromatid Pair, Homologous Chromosomes, Homologous Recombination, Sister Chromatids, Tetrad

What is a Tetrad 

All chromosomes undergo DNA replication, and thus, synthesize the exact copy of the DNA molecules, which make up each chromosome. It occurs during the S phase of the interphase of the cell cycle. Here, the S phase of the interphase is responsible for preparing the cell for subsequent cell division. Furthermore, the homologous chromosomes pair during the prophase I of the meiosis I. And, these two chromosomes of the homologous pair are known as the bivalent. Tetrad is the four sister chromatids of the two chromosomes of the homologous pair (or the bivalent). 

Figure 1: Chromatid Pair

Here, the chromatids, which belong to the opposite chromosomes of the homologous pair, can undergo recombination through chromatid crossing over. Note that the chromatids that are capable of undergoing crossing over are the non-sister chromatids. Importantly, the homologous recombination is responsible for exchanging parts of chromosomes, and these exchanged parts pass into different gametes. Hence, it is the main process of producing genetic variation in successive generations.  

What is a Chromatid Pair 

Chromatid pair is the two chromatids of a chromosome that has undergone DNA replication. Basically, the two chromatids formed from the same chromosome are sister chromatids. As mentioned above, DNA replication occurs during the S phase of interphase, which prepares the cell for subsequent cell division. In this process, the replicated chromosomes condense during the metaphase I of the meiosis I. And, these are visible under the light microscope during this step as thread-like structures.

Figure 2: Chromatid Pair

Moreover, the DNA replication occurs in both types of cell divisions; the mitosis and meiosis, in order to double the amount of DNA in the nucleus. Further, the sister chromatids thus produced are held together by cohesin proteins throughout the subsequent steps of cell division until they get separated during the anaphase of the mitosis and anaphase II of the meiosis II.  

Similarities Between Tetrad and Chromatid Pair 

• Tetrad and chromatid pair are two combinations of sister chromatids of homologous chromosomes.  
• Here,  the sister chromatids are produced after undergoing DNA replication to prepare the cells for nuclear division during the growth phase of the cell cycle.  
• So, both pair during the prophase of meiosis. 

Difference Between a Tetrad and a Chromatid Pair 

Definition 

Tetrad refers to the four-part structure that forms during the prophase of meiosis. It consists of two homologous chromosomes, and each composed of two sister chromatids. Meanwhile, chromatid pair refers to one-half of two identical threadlike strands of a replicated chromosome.  

Occurrence 

Tetrad only occurs during the prophase of the meiosis I after pairing of homologous chromosomes. Meanwhile, chromatid pair occurs after undergoing DNA replication during the S phase of the cell cycle. 

Number of Sister Chromatids 

Tetrad includes the four sister chromatids of the bivalent, while chromatid pair includes the two sister chromatids of a single chromosome of the bivalent.  

In the Bivalent 

Tetrad includes the sister chromatids of both chromosomes of the homologous pair, while chromatid pair includes only one chromosome of the homologous pair. 

Origin 

Tetrad has sister chromatids with both maternal and paternal origin, while a chromatid pair either has a maternal or paternal origin.  

Homologous Recombination 

The tetrad can undergo homologous recombination, while chromatid pair does not undergo homologous recombination. 

Holding Together 

Chromatids of the tetrad are held together by the formation of synaptonemal complexes, while sister chromatids of the chromatid pair are held together by cohesin proteins.  

Conclusion 

Tetrad is the four sister chromatids of the two chromosomes of the homologous pair. Generally, homologous chromosomes pair during the prophase of meiosis I. Also, a pair of sister chromatids, which belong to each of the two homologous chromosomes, can undergo homologous recombination. Meanwhile, the chromatid pair is the two sister chromatids in a particular chromosome that has undergone DNA replication during the S phase of the interphase. Here, the cohesin proteins are responsible for holding the two sister chromatids together. Therefore, the main difference between a tetrad and a chromatid pair is the type of sister chromatids and their role. 

References:

1. “Meiosis I | Biology for Majors I.” Lumen, Available Here.

Image Courtesy:

1. “0330 Homologous Pair of Chromosomes.”By OpenStax (CC BY 4.0) via Commons Wikimedia   
2. “Figure 11 01 02” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia