lecture7

Types of chromosomes based on position of centromere, based on structure and function: normal and special chromosomespolytene, lamp brush, B chromosomes, ring and isochromosomes

Types/Classification of Chromosomes:

   Chromosomes can be classified in different ways. The various criteria which are usually used for classification of chromosomes include,

• Position of centromere

• Number of centromeres

• Shape at anaphase

• Structure and appearance

• Role in heredity essentiality

• Role in sex determination

• Structure and function.

A brief classification on the bases of these criteria is presented below.

Some tissues of certain organisms contain chromosomes which differ significantly from normal chromosomes in terms of either morphology or function. Such chromosomes are referred to as special chromosomes. The following are included under this category:

• Giant chromosomes or polytene chromosomes

• Lamp brush chromosomes

• Accessory chromosomes

• Isochromosomes

• Ring Chromosomes

• Allosomes / sex chromosomes

Giant chromosomes or polytene chromosomes:

• These were first discovered by E. G. Balbiani in 1882 in Dipteran salivary glands and hence commonly called salivary gland chromosomes.

• These chromosomes replicate repeatedly but the daughter chromatids do not separate from one another and the cell also does not divide. This phenomenon is known as endomitosis or endoreduplication.

• It results in the formation of many stranded giant chromosomes known as polytene chromosomes and the condition is known as polyteny.

• Their size is 200 times or more than the normal somatic chromosomes (autosomes) and very thick. Hence, they are known as giant chromosomes.

• These chromosomes are somatically paired and their number in the salivary gland cells always appear to be half of that in the normal somatic cells.

•  Along the length of chromosomes, a series of dark bands are present alternate with clear bands known as interbands. These bands have greatly helped in mapping of the chromosomes in cytogenetic studies.

• In the dark band region, the DNA is tightly coiled while in the interband region, DNA is less tightly coiled.

• The morphological expression of such sites is represented by local enlargements of certain regions called puffs.

• These puffs are also known as Balbiani rings. Puffs are the sites of active RNA synthesis.

Lamp brush chromosomes:

• These were first observed by W. Flemming in 1882 and were described in detail in oocytes of sharks by Rukert in 1892.

• They occur at diplotene stage of meiotic prophase in oocytes of all animal species.

• Since they are found in meiotic prophase, they are present in the form of bivalents in which the maternal and paternal chromosomes are held together by chiasmata at those sites were crossing over has previously occurred.

• Each bivalent has four chromatids, two in each homologue. The axis of each homologue consists of a row of granules or chromomeres, each of which have two loop like lateral extensions, one for each chromatid.

• Thus, each loop represents one chromatid of a chromosome and is composed of one DNA double helix.

• One end of each loop is thinner than other which is known as thickened. There is extensive RNA synthesis at thin ends of the loop while there is little or no RNA synthesis at the thick ends.

Accessory chromosomes:

• In many species some chromosomes are found in addition to normal somatic chromosomes. These extra chromosomes are called accessory chromosomes or B-chromosomes or supernumerary chromosomes.

• These chromosomes are broadly similar to normal somatic chromosomes in their morphology, but have some peculiar functional aspects.

• For instance, presence of several such chromosomes often leads to reduction in vigour and fertility in males.

• These chromosomes are generally smaller in size than the normal somatic complement.

• They are believed to be generally inactive genetically. However, they may not be completely devoid of genes. Origin of these chromosomes in most species is unknown.

Isochromosomes:

•  An isochromosome is the one in which two arms are identical with each other in gene content and morphology.

• Such a chromosome is in as sense a reverse duplication with centromeres separating the two arms.

• Every isochromosome is metacentric. The attached ‘x’ chromosome of Drosophila is a classic example of an isochromosome.

• However, its origin is uncertain. There is no evidence that isochromosomes had any evolutionary significant.

Ring Chromosomes:

• A ring chromosome is an aberrant chromosome whose ends have fused together to form a ring.

• Ring chromosomes were first discovered by Lilian Vaughan Morgan in 1926. 

• A ring chromosome is denoted by the symbol r in human genetics and R in Drosophila genetics.

• Ring chromosomes may form in cells following genetic damage by mutagens like radiation, but they may also arise spontaneously during development.

• In order for a chromosome to form a ring, both ends of the chromosome are usually missing, enabling the broken ends to fuse together.

• In rare cases, the telomeres at the ends of a chromosome fuse without any loss of genetic material, which results in a normal phenotype.

• Complex rearrangements, including segmental microdeletions and microduplications, have been seen in numerous ring chromosomes, providing important clues regarding the mechanisms of their formation.

• Small supernumerary rings can also form, resulting in a partial trisomy.

• Ring chromosomes are unstable during cell division and can form interlocking or fused rings.

Allosomes / sex chromosomes:

• Chromosomes differing in morphology and number in male and female are called allosomes.

• They are responsible for determination of sex. E.g. X and Y chromosomes in human beings and Drosophila.

• Chromosomes which have no relation with determination of sex and contain genes which determine somatic characters of individuals are called autosomes and are represented by letter ‘A’.

Chromosome models:

   Chromosome fibres are the basic units of chromosome structure. Chromosome model refers to organization of chromatic fibres in a chromosome. Two models namely folded fibre model and nucleosome solenoid model are widely accepted to explain chromosome structure and organisation if chromatic fibre in a chromosome. These models are briefly described below:

1. Folded fibre model:

     This model was proposed by DuPraw in 1965. According to this mode, chromatin fibres are about 230 Å in diameter. Each chromatid consists of single chromatin fibre, which is made up of single chromatin fibre, which is made up of single DNA double helix. The folding and super coiling of very long chromatin fibre causes reduction in length and increase in thickness of the chromosome.

 2. Nucleosome – solenoid model:

       This model was proposed my Kornberg and Thomas in 1974. Chromatin is composed of DNA, RNA, histones and other proteins. Chromatin fibres are 300 Å in diameter. The nucleosomes are sub units of chromatin and have bead like appearance. Each nucleosome is composed of a histone octamer and 146 base pairs (bp) of DNA. Each nucleosome consists of a core particle and linker or spacer DNA. The core particle has two copies each of H2B, H3 and H4 histones molecules: Thus, it has a histone octamer. The core particle is about 100 Å in diameter and 60 Å in height. A duplex DNA strand is tightly wound around this core particle making two circles. Spacer of liner DNA has four base pairs. One molecule of histone H1 is connected with linker DNA. The super coiled nucleosome fibre is known as solenoid.

        According to this theory, a very long molecule of DNA (146 bp) is packed into a single unit of nucleosome and several units of nucleosome constitute chromatic fibre. The chromatic fibre of 300 Å which is visible under electron microscope at metaphase develops from the nucleosome fibres as a consequence of super coiling of latter. This model is universally accepted as a model of chromatin fibre organization.