Differential Uptake of Tritiated Thymidine into Hetero- and Euchromatin in Melanoplus and Secale

Abstract

Grasshoppers of the species Melanoplus differentialis were injected with tritiumlabelled thymidine. At intervals thereafter autoradiographic stripping film was applied over Feulgen squashes and sections. In this species during early prophase of meiosis the sex chromosome forms a heterochromatic block large enough to be resolved in tritium autoradiographs. A study of the squash preparations reveals that the sex chromosome is synthesizing DNA at a different period of time from the euchromatic autosomes. Since there is a developmental sequence of spermatocyte cysts along the testicular tubes it is possible from the sections to show that the heterochromatin synthesizes DNA later than does the euchromatin. To find out whether the results obtained in Mdanoplus were characteristic of heterochromatin in general, young seedlings of rye were grown in a tritiated thyrnidine solution and Feulgen squashes were made as for Mdanoplus. In rye leaf nuclei there is a large block of heterochromatin constituted by the proximal regions of the chromosomes and a euchromatic one formed by the median and distal regions of the same chromosomes. Here also the heterochromatin synthesizes DNA at a different period of time from the euchromatin. It is concluded that in rye the asynchrony of synthesis occurs within each chromosome. Counts of silver grains over the two types of chromatin in nuclei of Melanoplus and Secale disclosed that the number of grains per unit area was two to three times higher over the heterochromatin. To check the DNA content, Feulgen photometric measurements were made of Mdanoplus nuclei at the same stage. The Feulgen and grain counts agree in showing that the heterochromatin contains two to three times more DNA per unit area than the euchromatin.

Asynchronous Duplication of Chromosomes in Cultured Cells of Chinese Hamster

“…Later Gall (5) reported a sequence in the macronucleus of Euplotes in which synthesis of DNA began at both ends of an elongated nucleus and proceeded to the middle. Lima-de-Faria (6) reported an asynchrony in the duplication of the chromosomes of a species of grasshopper in which the sex chromosome duplicated later than the remainder of the complement.…”

DNA synthesis in rye chromosomes

“…The heavy labelling in terminal segments is no doubt associated with the prominent heterochromatic telomeres in rye chromosomes (Lima-de-Faria, 1959;Darlington and Haque, 1966). There is controversy as to whether the centromere region, also heavily labelled, is strictly heterochromatic (Lima-de-Faria, 1959) or merely precocious in coiling during prophase (Darlington and Haque, 1966) and thereby denser in DNA content at metaphase. We shall return to this subject in a later section.…”

“…Particularly marked is the asynchronous synthesis of DNA in heterochromatin as compared with euchromatin. In the former the DNA is synthesised later, a fact first reported by Lima-de-Faria (1959) in Melanop Itis and rye.…”

Differential Replication of the Chromosomes of <i>Pallavicinia longispina</i>, Liverwort

“…This apparent variability in the replication of heterochromatin suggests that late replication (Brown 1966. Lima-de Faria and Jaworska 1968, Lima-de-Faria 1969 may not be the basic characteristic of heterochromatin replication.…”