The key difference between Barr body and polar body is that Barr body is the inactive X chromosome in a somatic cell of a female while the polar body is one of the three haploid gametes that do not become an egg at the end of oogenesis.
Barr body and polar bodies are two different structures found in females. Barr body is an inactive chromosome while the polar body is a small haploid cell formed during oogenesis. Therefore, the Barr body is a chromosome while the polar body is a haploid cell. Both are unique to females. However, Barr bodies are found in somatic cells, while polar bodies are found during the sexual reproduction process.
CONTENTS
1. Overview and Key Difference
2. What is Barr Body
3. What is Polar Body
4. Similarities Between Barr Body and Polar Body
5. Side by Side Comparison – Barr Body vs Polar Body in Tabular Form
6. Summary
What is a Barr Body?
Barr body is the name given to the X chromosome that is inactive during the expression of the genes of somatic cells of females. Barr bodies are absent in normal males. Murray Barr discovered this inactive X chromosome in female somatic cells. Barr body is in the state of heterochromatin which is a transcriptionally inactive structure while the other copy, active X chromosome, is in the euchromatin state. Once the Barr body is packaged into heterochromatin, it is not easily accessible to molecules involved in transcription.
Since all females have two X chromosomes, X inactivation or lyonization is important in order to prevent them from having twice as many X chromosome gene products as males. In short, Barr body production ensures that only the necessary amount of genetic information is expressed in females, rather than doubling it. Therefore, throughout the cell’s whole life, one X chromosome of all somatic cells remains silent.
What is Polar Body?
In females, female gametes or eggs are produced by a process called oogenesis. Though this process starts during the embryonic development, the completion occurs after puberty. After puberty, every month, an ovum is produced. During the ovum production, three polar bodies are also produced at each cycle. Thus, polar bodies are the three haploid cells that result during the oogenesis process. They are not capable of undergoing fertilization with sperms.
Figure 02: Polar Bodies
Oogenesis begins from a diploid cell, and ovum production takes place via meiosis cell division. Meiosis I produces a primary oocyte and the first polar body. At fertilization, meiosis II process commences and produces a secondary oocyte, the second polar body and third polar body. At the end of meiosis, a mature oocyte (ovum) and three polar bodies are produced. Structurally, polar bodies are small cytoplasmic inclusion bodies containing a nucleus, ribosomes, Golgi, mitochondria and cortical granules.
What are the Similarities Between Barr Body and Polar Body?
- Barr body and polar bodies are found only in females.
- Both are important structures.
What is the Difference Between Barr Body and Polar Body?
Barr body is the inactive X chromosome in female somatic cells while the polar body is a small haploid cell originated during oogenesis. So, this is the key difference between Barr body and polar body. Besides, another significant difference between Barr body and polar body is that the Barr bodies are formed during the X inactivation process while polar bodies are formed during the oogenesis.
Barr body formation is important in order to prevent females from having twice as many X chromosome gene products as males. Meanwhile, polar body formation indicates the completion of meiosis during the egg cell formation in a female. They are also used as tools for clinical diagnosis of human diseases and as metrics of embryonic potential. Furthermore, another difference between Barr body and polar body is the fate of each structure. Barr body remains inactive throughout the cells’ whole lifetime while polar body disappears or degenerates fast.
Summary – Barr Body vs Polar Body
Barr body is the inactive X chromosome in female somatic cells. X inactivation maintains the expression of only the necessary gene products in females. Hence it prevents the doubling of X chromosome gene products in females compared to males. Meanwhile, the polar body is one of the three small haploid cells produced during the ovum production or the oogenesis. Polar bodies are not eligible for the fusion with sperm or fertilization. In short, the Barr body is an inactive chromosome in a female somatic cell while the polar body is a small haploid cell resulting during the oogenesis. So, this is the summary of the difference between Barr body and polar body.
Reference:
1. “Barr Body.” Biology Dictionary, 28 Apr. 2017, Available here.
2. “Oocyte Development.” Embryology, Available here.
Image Courtesy:
1. “Gray5” By Henry Vandyke Carter – This is a retouched picture, which means that it has been digitally altered from its original version. Modifications: vectorization (CorelDraw).(Public Domain) via Commons Wikimedia