Heredity and Development Second Edition (1972) / Chapter Skim
Currently Skimming:
2 The Cellular Basis of Inheritance
2 The Cellular Basis of Inheritance
Pages 19-48
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 19... ...
Following the realization that some cells are present in some organisms, the next conceptual advance was to establish the hypothesis that all organisms are composed solely of cells or cell products. This took nearly a century and a half.
|
From page 20... ...
I Took a good clear piece of Cork, and with a Pen-knife sharpen'd as keen as a Razor, I cut a piece of it off, and thereby left the surface of it exceeding smooth, then examining it very diligently with a Microscope, me thought I could perceive it to appear a little porous; but I could not so plainly distinguish them, as to be sure that they were pores, much less what Figure they were of: But judging from the lightness and yielding quality of the Cork, that certainly the texture could not be so curious, but that possibly, if I could use some further diligence, I might find it to be discernable with a Microscope, I with the same sharp Penknife, cut off from the former smooth surface an exceeding thin piece of it, and placing it on a black object Plate, because it was it self a white body, and casting the light on it with a deep plano-convex Glass, I could exceeding plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular; yet it was not unlike a Honey-comb in these particulars. 2–1 The drawings and part of the text from Hooke's observations on cork (R.
|
From page 21... ...
2–2 Some of Schwann's drawings of cells (Th.Schwann, Mikroscopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen, Berlin, 1839)
|
From page 22... ...
The latter structure was apparently first recognized as an important and characteristic cell structure by Robert Brown in 1833. It had been observed much earlier, however.
|
From page 23... ...
Some observers held that the nucleus disappears during cell division and each daughter cell produces a new one. If this is so, there can be no connection between the nuclei of different cell generations.
|
From page 24... ...
A new nucleus appears to form in each of the daughter cells. The difficulty in making observations, coupled with the fact that methods of fixation and staining were poorly developed, makes it easy to understand why cytologists believed what their eyes told them: that the nucleus disappears during cell division.
|
From page 25... ...
He agreed with Schneider that the nucleus persisted during cell division. Fol, the third investigator to describe cell division in 1873, thought that the nucleus entirely disappeared during division and was re-formed in the daughter cells.
|
From page 26... ...
. He was outstanding, first in selecting excellent material, namely, the epidermal cells of larval salamanders; second, in being careful to check in living cells all things that he observed in fixed and stained preparations; and third, in employing hitherto unsurpassed technical methods.
|
From page 27... ...
Nevertheless when type 2 cells are fixed and stained, a nucleus that in all respects is identical in appearance to the nuclei of fixed and stained cells of type 1, can be seen. Since the stained nuclei of cell types 1 and 2 have the same appearance, and the treatment is the same in both cases, the most reasonable hypothesis is that a nucleus is present, though invisible, in living cells of type 2.
|
From page 28... ...
a. Resting stage.
|
From page 29... ...
In living resting stage cells the nucleus does not seem to have any internal structure. After fixation and staining, an irregular network of strands and granules, named chromatin, can be detected.
|
From page 30... ...
The nuclear membrane, asters, spindle, and centrioles are not shown (W.Flemming, Zellsubstanz, Kern und Zelltheilung, 1882)
|
From page 31... ...
As a result, each daughter cell contains a group of chromosomes. Eventually it becomes impossible to see the chromosomes; the cell has entered the resting stage once more.
|
From page 32... ...
At this time the precise role of the sperm was not understood. In 1854, George Newport conducted experiments that suggested the sperm actually penetrates the ovum.
|
From page 33... ...
A single nucleus is present. This is the zygote nucleus that will undergo a series of mitotic divisions to form all of the nuclei of the individual (O.Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung und Theilung des thierischen Eies,' Morph.
|
From page 34... ...
Shortly after fertilization, the paternal pronucleus would be noticed close to the outer membrane of the ovum. At later times it would be found progressively closer to the maternal pronucleus, and eventually fused with it.
|
From page 35... ...
The maternal chromosomes have undergone the second meiotic division. This resulted in a maternal pronucleus with 2 chromosomes and a second polar body with 2 chromosomes.
|
From page 36... ...
It was clear from the work of van Beneden, Boveri, and others that each parent transmits an equal number of chromosomes to the zygote. So far as one could tell the chromosomes in the maternal pronucleus were morphologically the same as those in the paternal pronucleus.
|
From page 37... ...
The ovum of Ascaris remains diploid until it has been released from the ovary and entered by a sperm. The ovum nucleus then undergoes a series of two meiotic divisions that leads to each of the resulting cells having the haploid number of chromosomes.
|
From page 38... ...
The second polar body has formed and it contains 2 chromosomes. The egg nucleus also contains 2 chromosomes (Th.Boveri, ‘Die Bildung der Richtungskörper bei Ascaris megalocephala und Ascaris lumbricoides,' Jenaische Zeit.
|
From page 39... ...
As a consequence, in mitosis the chromosome number remains constant from one cell generation to the next; in meiosis the two meiotic divisions form cells with the haploid number of chromosomes. With full realization that the nuclear events associated with maturation and fertilization were important biological phenomena, cytologists examined many species of animals and plants.
|
From page 40... ...
HEREDITY AND DEVELOPMENT: SECOND EDITION 40 2–10 Meiosis in Ascaris males. The diploid chromosome number in Ascaris is 4.
|
From page 41... ...
It is conceivable, therefore, that Haeckel's hypothesis of nuclear control of of the 2 pairs of synapsed chromosomes forms a tetrad, as shown in c. At the first meiotic division the 2 tetrads enter the spindle (d)
|
From page 44... ...
In 1884–85 four German scientists, working independently, came to the conclusion that the physical basis of inheritance must lie in the chromosomes. They were Oskar Hertwig, Edouard Strasburger, Rudolf Kölliker, and August Weismann (1834–1914)
|
From page 45... ...
The chromosomes, on the other hand, go through a complicated mitosis which results in each of the daughter cells receiving exactly the same number of chromosomes. It seemed to Hertwig and the others that the significance of this complicated process might be that the nucleus was the basis of inheritance: why should the chromosomes, alone among the cell structures, be duplicated and then divided equally unless they were of great importance in inheritance?
|
From page 46... ...
It made known the significant fact that in all the ordinary forms of cell-division the nucleus does not divide en masse but first resolves itself into a definite number of chromosomes; that these bodies, originally formed as long threads, split lengthwise so as to effect a meristic division of the entire nuclear substance. It proved that fertilization of the egg everywhere involves the union or close association of two nuclei, one of maternal and one of paternal origin.
|
From page 47... ...
3. In most instances it is far easier to see cell structures in fixed and stained cells than in living cells.
|
From page 48... ...
14. Do you believe that the chromosomes maintain their essential structure during the resting stage?
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.