Top 2 Models of Chromosomes | Genetics

The following points highlight the top two models of chromosomes. The models are: 1. Multiple Strand Models 2. Single Strand Models.

Model # 1. Multiple Strand Models:

a) Simple Multi-Stranded Model:

According to this model the chromosome consists of 64 double helices of DNA arranged in a parallel number, and twisted together like the strands of a rope. The difficulty with this model is that replication of DNA would involve a large amount of untwisting and untan­gling.

b) Ris’ Multistrand Model:

Accor­ding to this model the 20Å wide DNA molecule is associated with histone (protein) to form a 40Å DNA-_histone (nucleo-protein) fibril. Two 40Å nucleoprotein fibrils make up a 100Å fibril called the elementary chromosome fibril (Ris, ’60, ’61). Two 100Å fibril wrap around each other to form a 200Å fibril.

Thus each 200Å fibril contains four parallel DNA-histone double helices. The 200Å fibrils are associated to form the chromatid of the chromosome. Each chromatid apparently has 16-32 elementary chromosome fibrils. Later work by Ris’66 led him to give up this model.

Comments:

Most of the evidences now indicate that chromosomes are not multi- stranded. This statement applies to normal chromosomes. A type of giant chromosome, called polytene chromosome, may have sever­al hundred strands. These strands are, howev­er, formed by division of chromatids.

Model # 2. Single Strand Models:

a) Taylor’s Centipede Model:

Taylor (1957) postulated that the chromosomes con­sist of a long protein backbone from which DNA coils branch off just like the legs of a cen­tipede. He explained replication by assuming that the protein backbone is two-layered, and that these layers can be pulled apart.

Each layer would have one strand of the DNA helix on separation. A new chromatid could then be built up on each separated chromatid. This model does not reconcile with the fact that the genes are arranged in a linear sequence.

b) Freese-Taylor Model:

A second model was proposed by Taylor (1959) based on one suggested by Freese (1958). According to this model there are two protein spines instead of one. The DNA chains stretch between them like the steps of a ladder. In effect, the DNA molecules are kept in position by the protein-linkers.

If the linkers become closely apposed they would form the axis of the chromosome, and the DNA would be in the form of lateral loops. This model can sat­isfy the concept that the genes are arranged in a linear order.

c) Coiled Coil Model:

According to Nebel and others the chromosome is made up of only a single fibril. This fibril is highly coiled and the coil is thrown into secondary coils.

d) Ris’ Modified Model:

Ris (1967) suggested a modified model according to which the chromosome fibril is folded to form the chromosomes. The 20Å wide DNA helix is associated with the histone (protein) to form a 100Å diameter of nucleohistone fibril.

Folding of the 100Å fibril takes place because of calcium bridges to form a 250Å basic fibril undergoes still further folding to form the chromosome. Electron microscope pictures of 100Å and 200Å fibrils were obtained by Ris by spreading chromosomal material at an air- water interface.

e) Folded Fibre Model of Dupraw:

Dupraw (1965,’66) studied whole mounts of human leucocytes under the electron micro­scope. He found that sister chromatids consist of irregularly folded fibres 200-500A in diam­eter. Each chromatid consists of a single fibre (Type fibre) folded both longitudinally and transversely. Autoradiography experiments have also indicated the existence of a single fibre.

The 20Å DNA double helix 56 µ long, is spirally packed in protein to form a fibril. This fibril, if coiled, would form a fibre 80 to 100Å in diameter and 7 to 8 p long. This fibre is called the Type A fibre. The DNA is packed inside the type A fibre. The Type A fibre is then in turn coiled to form a type B fibre 200 to 250Å in diameter.

The 200 to 250 Å type B fibre is extensively folded to form the chro­matid. This has been confirmed by McDermott (1968), Abuelo and Moare (1969), Lampert (1969) and Fedric (1969). Lampert (1971) supports the folded fibre model. Kiblman (1971) also supports the model, stating that “The findings indicate that a mitotic chromosome is formed by coiling and folding of fibre consisting of a long single DNA molecule associated with protein”.

Comments:

Dupraw’s folded fibre model has been accepted by many authors including M. J. D. White. In his book The Chromosomes’ (1973) White writes, “Two general hypotheses of chromosome structure which are certainly incorrect are the ‘folded fibre’ model of Dupraw and the same author’s extraordinary idea that the DNA of all the chromosomes is really continuous, i.e. present in the form of a ring.

The ‘folded fibre’ model is certainly incompatible with the constant, sequence of chromosome constriction. Heterochromatic segments etc. along the length of the chromosomes, as well as with all observations on the giant lamp-brush and polytene chromosomes and the whole theory of structural chromosomal rearrangements.”