What is the Difference Between Polytene and Lampbrush Chromosome

The main difference between polytene and lampbrush chromosome is that polytene chromosomes occur in the salivary glands and other tissues of insects whereas lampbrush chromosomes occur in the oocytes of vertebrates except for mammals and some invertebrates. Furthermore, polytene chromosomes are visible during the interphase and prophase of mitosis while lampbrush chromosomes are clearly visible during the diplotene stage of mitosis. Additionally, polytene chromosomes contain many longitudinal strands called chromomenta while lampbrush chromosomes contain a series of large, laterally extended loops. 

Polytene and lampbrush chromosome are two types of giant chromosomes which decondense during the interphase. Generally, they actively undergo transcription. 

Key Areas Covered 

1. What is a Polytene Chromosome
     – Definition, Structure, Transcription
2. What is a Lampbrush Chromosome
     – Definition, Structure, Transcription
3. What are the Similarities Between Polytene and Lampbrush Chromosome
     – Outline of Common features
4. What is the Difference Between Polyetene and Lampbrush Chromosome
     – Comparison of Key Differences

Key Terms 

Giant Chromosome, Lampbrush Chromosome, Mitosis, Polyetene Chromosome

What is a Polytene Chromosome 

A polytene chromosome is a type of giant chromosome with a high-level function in some tissues of insects such as salivary glands. It was first discovered by E.G.Balbiani in 1881. Generally, it is clearly visible under the compound microscope during the interphase and prophase due to their enormous size. Basically, polytene chromosomes are much larger than normal chromosomes and their size can reach up to 200 μm in length. Significantly, the duplication of chromonema without separation is the reason for their large size. Therefore, polytene chromosomes contain 1000 times more DNA than the normal somatic chromosomes. The name ‘polytene’ is given due to these many strands. 

Figure 1: Polytene Chromosomes: Salivary Glands of Nonbiting Midges Larvae (Chironomidae)

Moreover, chromocenter is the central point of a polytene chromosome. From this central point, 5 long and one short arms radiate. Its central point is formed by the fusion of all the centromeres in the nucleus. During the interphase, polytene chromosomes contain distinct thick and thin banding patterns. Generally, intensely stained segments are the heterochromatin, and they are genetically inactive while the less intensely stained segments are euchromatin, which is genetically active. Approximately, 80% of DNA occurs in the bands while the rest occurs interband.  

On the other hand, based on the development stage, the functional state of the chromosome structure differs. These expanding regions on the chromosome structure are called puffs, and they are the active chromosomal regions. Furthermore, due to the presence of polytene chromosomes, cell expands with the increase of the size of the nucleus. The presence of multiple copies of genes as a result of duplications in polyetene chromosomes provides a high level of gene expression.  

What is a Lampbrush Chromosome 

The lampbrush chromosome is the largest type of chromosomes that especially occurs in the growing oocytes of vertebrates except for mammals and in some invertebrates. The cells with lampbrush chromosomes have a high volume in the nucleus and the cytoplasm. Generally, the detailed structure of the lampbrush chromosomes can be observed during the diplotene stage. These chromosomes are visible as large loops of DNA, resembling a lampbrush. Moreover, the lampbrush chromosomes are visible under the light microscope. 

Figure 2: Lampbrush Chromosome from the Cell Nucleus of an Ovarian Egg from Triton, a Salamander Species

Furthermore, the cells with lampbrush chromosomes have a high volume in the nucleus and the cytoplasm. Generally, the detailed structure of the lampbrush chromosomes can be observed during the diplotene stage. These chromosomes are visible as large loops of DNA, resembling a lampbrush. Also, the lampbrush chromosomes are visible under the light microscope. On the other hand, transcription in the lampbrush chromosome can occur along the whole loop. Therefore, one loop resembles a long operon with repetitive cistrons. Moreover, these chromosomes show a high level of gene expression.  

Similarities Between Polytene and Lampbrush Chromosome

• Polytene and lampbrush chromosomes are two types of giant chromosomes. 
• They decondense during the interphase. 
• Moreover, they actively undergo transcription. 
• During metaphase, they are very long and thick. 

Difference Between Polyetene and Lampbrush Chromosome 

Definition 

Polytene chromosome refers to a giant, cross-banded chromosome, which results from multiple replications of its genetic material with the duplicated chromatin strands remaining closely associated while lampbrush chromosome refers to a greatly enlarged diplotene chromosome, which has apparently filamentous granular loops extending from the chromomeres. 

Discovered by 

The polytene chromosome was discovered by E. G. Balbiani in 1881 while the lampbrush chromosome was first discovered by Walther Flemming in 1882. 

Occurrence 

Moreover, the place of occurrence is the main difference between polytene and lampbrush chromosome. That is; the polytene chromosomes occur in the salivary glands and other tissues of insects, while the lampbrush chromosomes occur in the oocytes of vertebrates except for mammals and some invertebrates.  

Size 

A polytene chromosome is smaller than the lampbrush chromosome, while the lampbrush chromosome is the largest type of chromosomes. 

Visibility 

Furthermore, another important difference between polytene and lampbrush chromosome is that the polytene chromosomes are visible during the interphase and prophase of mitosis, while the lampbrush chromosomes are clearly visible during the diplotene stage of mitosis. 

Microscope 

Besides, the polytene chromosome is visible under the compound microscope while lampbrush chromosome is visible under the light microscope. 

Appearance 

While polytene chromosomes contain many longitudinal strands called chromomenta, lampbrush chromosomes contain a series of large, laterally extended loops. 

Method of Transcription 

The functional state of polytene chromosomes differs according to the development stage, while each loop of the lampbrush chromosome serves as a long operon. 

Conclusion 

A polytene chromosome is a type of giant chromosome that occurs in insects. It is clearly visible during the interphase and prophase. Also, these chromosomes contain many longitudinal strands. On the other hand, lampbrush chromosome is the largest type of chromosomes. However, it occurs in the oocytes of vertebrates except in mammals and in some invertebrates. In contrast, lampbrush chromosome contains laterally-extended loops. However, both polytene and lampbrush chromosomes are two types of giant chromosomes, which are actively undergoing transcription. Hence, the main difference between polytene and lampbrush chromosome is their structure and occurrence. 

References:

1. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Global Structure of Chromosomes. Available Here.

Image Courtesy:

1. “Polytene chromosomes (26 2 97) Salivary glands of nonbiting midges larvae (Chironomidae)” By Doc. RNDr. Josef Reischig, CSc. – Author’s archive (CC BY-SA 3.0) via Commons Wikimedia   
2. “O.Hertwig1906Fig5” By Dr. Oskar Hertwig (Public Domain) via Commons Wikimedia