Special types of chromosome: Lampbrush chromosome

 Chromosomes of a special kind are found in a variety of primary oocyte nuclei in vertebrates (mainly amphibians) as well as in some invertebrates occur at the diplotene stage of meiotic prophase. These chromosomes, known as lampbrush chromosomes first observed by Walther Flemming (in 1882) in salamander (amphibian oocyte) and described in detail by R.

Ruckert (1892) in shark oocyte. The lampbrush chromosomes are not condensed, instead, they are very long and stretched out. These chromosomes sometimes become even larger than giant salivary gland chromosomes. The largest chromosome having a length up to 1 mm has been observed in urodele amphibians.

The lampbrush chromosome is present in the form of bivalent in which the maternal and paternal chromosomes are held together by chiasmata. Each bivalent has four chromatids, two in each homologue. The axis of each homologue consists of a row of chromomere from which lateral loops extend. Two loops are extended from each chromosome at a point, one from each chromatid.



One to nine loops may arise from a single chromomere. These pairs of loops in these chromosomes give them the characteristic lampbrush appearance. The size of loops varies with an average of 9.5μ in inter-chromomeric fibers. The number of pairs of loops gradually

increases in meiosis till it reaches the maximum in diplotene. As meiosis proceeds further, the number of loops gradually decreases and the loops ultimately disappear due to disintegration or reabsorption back into the chromomere. Each loop of the lampbrush chromosome is found to perform intense transcription of RNA. The loop is covered by a matrix made of RNA and protein.

Generally, one end of the matrix is thinner than the other end. RNA synthesis starts at the thinner end and progresses towards the thicker end.

Comments

Post a Comment

Popular posts from this blog

Morgan’s experiment: proof of chromosomal theory of inheritance

Image
 Morgan worked on the eye color of Drosophila. He observed that the pattern of transmission of the white eye gene was identical to that of the X chromosome of Drosophila. This prompted Morgan postulate that the gene for eye color is located on the X chromosome. This was the first demonstration of an association between a specific gene and a specific chromosome. His work gave direct evidence in support of the chromosomal theory of inheritance. The white-eyed male fly was crossed with the red-eyed female, all the F1 offsprings were red-eyed. This gives an indication that the white eye is controlled by a recessive allele. The predictions match the F1 phenotypes, but this set of phenotypes could also be explained by a gene that is not on the X chromosome since all the flies were red-eyed (regardless of sex). So, the real test comes when the F1 is mated to make the F2 generation. Something strange happened in F2: all of the female F2 flies were red-eyed, while about half of the male F2 ...
Read more

Chromosome number

Image
Chromosome number in every species is generally constant. In higher organisms, the number of chromosomes in a somatic cell is called somatic number irrespective of the ploidy level (number of basic chromosome set) and is represented by 2n. In the gametes, the chromosome number is reduced to half, it is known as the gametic number or haploid number and is represented by “n”. Some organisms including many plants have three or more sets of chromosomes and are referred to as polyploid. The basic chromosome number is denoted by “x” so that the chromosome number of a diploid cell or individual is expressed as “2x”. Diploid: 2n = 2x, n = x Tetraploid: 2n = 4x, n = 2x Hexaploid: 2n = 6x, n = 3x
Read more

Prokaryotic nucleoid

Image
 Recent work by a number of workers demonstrated that the network of threads consists of a single chromosome in the form of a ring. The exact three-dimensional arrangement by which 1100μ -1400μ long DNA chain, which forms 80% of the chromosome by mass (the remaining 20% being protein + RNA), is packed in an lμ long nucleoid, which could also be established now. At least two proteins, which bind DNA, resemble histones of eukaryotes and organize the DNA into structures comparable to nucleosomes of eukaryotes. It has been shown that the chromosome of E. coli is organized in about 45 loops, which radiate out from a dense proteinaceous scaffold, which is assumed to anchor the DNA. In each of the 45 loops, DNA is supercoiled and complexed with protein. The functional significance of looped domains is not clear but they do not represent units of transcription as in the case of lampbrush chromosomes.
Read more