Different models of structure of chromosome

• One micron of a metaphase chromosome contains about 8000 µ of DNA helix.

a. Typical early model:

• This is one of the earliest model of chromosome structure based on light microscope.
• According to this model, the basic component of chromosome structure is chromonema, which is composed of chromatin and contain genes.
• The variation in chromosome length and thickness is proposed to be due to coiling and uncoiling of chromonemata.
• Each chromatid of a chromosome may contain two or more chromonemata, which runs across through the centromere.
• In case of metaphase chromosome, chromonemata are surrounded by an amorphous matrix, the outer side of which is enclosed in the membrane called pellicle.
• However, the electron micrographs of metaphase chromosomes don’t show any evidence for the existence of either matrix or pellicle. So, this model of chromosome structure is inadequate and of historical interest only.

b. Multistranded model:

• According to this model, each chromatin fibre is 100 A in diameter.
• Each chromatin fibre is composed of two strands, each strand being 35-40 A in diameter.
• Each strand consists of single DNA double helix and the associated histone and non-histone proteins.
• Thus, a chromatin fibre contains two DNA double helix and associated protein.
• Four chromatin fibre coil around each other to form a quarter chromatid, which is smallest subunit of the chromosome visible under light microscope.
• Association of two quarter chromatids give rise to one half chromatid of 800 A diameter, which is composed of DNA double helix.
• Finally, two half chromatids coil around each other to produce one chromatid which is 1600 A in diameter and is made up of 32 DNA double helix. Thus, a metaphase chromosome has 64 DNA double helices and would be 3200 A in diameter.
• The variation in chromosome thickness would be due to the differences in coiling of chromatids.

c. Folded-Fiber model:

• This model was proposed by DuPraw in 1965.
• According to this model, chromosome are made up of chromatin fibres of about 230 A diameter.
• Each chromatin fiber contains only one DNA double helix, which is in coiled state and is coated with histone and non-histone proteins.
• Thus, 230 A chromatin fibre is produced by coiling of a single DNA double helix.
• The coils of DNA molecule are stabilized by proteins and divalent cations.
• The DNA of chromatin fiber replicates during interphase producing two sister chromatin fibres.
• It remains unreplicated in the centromeric region so that the two sister chromatin fibres remain joined in this region.
• During cell division, the two sister chromatin fibers after replication undergo extensive folding separately in an irregular manner to give rise to two sister chromatids.
• This cause reduction in length and increase in their thickness and stainability.
• Then, this structure further undergo supercoiling which further reduces the length and increase thickness of chromosome.