What makes cellular inheritance possible

An exception to this rule are the sex cells egg and sperm , also known as gametes , which only have one set of chromosomes each they are haploid. However, in sexual reproduction the sperm cell combines with the egg cell to form the first cell of the new organism in a process called fertilisation.

This cell the fertilised egg has two sets of 23 chromosomes diploid and the complete set of instructions needed to make more cells, and eventually a whole person. Each of the cells in the new person contains genetic material from the two parents. This passing down of genetic material is evident if you examine the characteristics of members of the same family, from average height to hair and eye colour to nose and ear shape, as they are usually similar.

If there is a mutation in the genetic material, this can also be passed on from parent to child This is why diseases can run in families. How is sex determined? The sex of an individual is determined by the sex chromosomes called the X chromosome and the Y chromosome. Females have two X chromosomes XX.

Males have an X chromosome and a Y chromosome XY. Female gametes eggs therefore always carry an X chromosome. Male gametes sperm can carry either an X or a Y. When an egg joins with a sperm containing an X chromosome, the result is a girl. When an egg joins with a sperm containing a Y chromosome, the result is a boy.

What is a genotype? The genotype is a description of the unique genetic makeup of an individual. It can be used to describe an entire genome or just an individual gene and its alleles.

The genotype of an individual influences their phenotype. The first polar body has pinched off from the egg. It contains 2 dyads. The egg likewise contains two dyads.

The first polar body with its chromosomes is beneath one of the egg membranes. It can be seen in all of the remaining figures except h. In e the dyads are rotating prior to their separation. The second polar body has formed and it contains 2 chromosomes. The egg nucleus also contains 2 chromosomes Th. Half of each tetrad, or a dyad, goes to each pole of the spindle. It will be noticed that the spindle is not in the center of the cell but instead it is at the periphery.

Inasmuch as the cell will divide across the equator of the spindle, the result will be two cells of very unequal sizes. The large cell resulting from the division is the ovum and the small cell is the first polar body. The chromosomes that enter the first polar body are morphologically and numerically equivalent to those that remain in the ovum. The Second Meiotic Division of the Ova. In d the first polar body is well separated from the ovum and the two dyads within the ovum are.

At this division the chromo- somes do not duplicate themselves. Consequently the dyads are divided and as a result two chromosomes go to each pole of the spindle. This second mei- otic division divides the cell unequally, as did the first, the result being a large ovum and a tiny second polar body. At the end of the second, and last, meiotic division there are only two chromosomes in the Ascaris ovum.

The nuclear membrane forms around these two chromosomes, the haploid num- ber, and in this manner the maternal pronucleus is produced. Meiosis in the Male.

The observation that the paternal pronucleus was haploid, yet the male diploid in its body cells suggested that a process similar to that just described must also occur in the male.

A study of sperm formation in Ascaris showed this to be the case Fig. The last two cell divisions before a sperm forms are meiotic divisions. As in the egg, the four chromo- somes form two pairs and each chromosome duplicates itself. The result is two tetrads each composed of four chromatids. During the first meiotic divi- sion the tetrads are divided and half of each goes into each of the daughter cells.

Not only is nuclear division equal but cell division is also equal, which is in contrast to the situation in the ova. At the next division the dyads are divided between the two daughter cells, which are again of equal size. Thus, from one cell with four chromosomes, and by means of two meiotic divi- sions, four cells each with two chromosomes are formed.

Each of these four haploid cells develops without further division into a sperm cell. The essential difference between meiosis and mitosis is this: in mitosis there is one duplication of every chromosome for each cell division; in meio- sis there is only one duplication of every chromosome for the two meiotic divisions. As a consequence, in mitosis the chromosome number remains constant from one cell generation to the next; in meiosis the two meiotic divi- sions 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. It was found that the reduction divisions leading to haploid pronuclei occur throughout the animal and plant king- doms. In short, another principle of almost universal application a few excep- tions were found had been discovered.

The facts as outlined in this section were generally, though not universally, believed by The diploid chromosome number in Ascaris is 4. The cells of the testis that will later form the sperm are diploid as shown in a.

The 4 chromosomes are undergoing synapsis. As meiosis continues each chro- mosome becomes shortened until it forms a tiny sphere. During this process each chromosome splits. As a result each. The conceptual gap between observing an orderly sequence of chromosomal behavior and a general the- ory of inheritance is awesome.

Few minds are capable of being the first to bridge a gap of this magnitude. Various alternatives were proposed, were subjected to the merci- less scrutiny of the scientific community, and judged inadequate. But at least some progress was made: by the end of the nineteenth century, it was gener- ally agreed that inheritance, however it was effected, was related to the nucleus. We might have expected this to be the case when we realize that cytologists, in a decade of uparalleled discovery, had worked out the essentials of mitosis, fertilization, and meiosis.

The data available to Haeckel in were not sufficient to test this hypothesis. If a lucky guess of this sort had been made by some obscure scientist, it is probable that its influence on subsequent events would have been negligible. But Haeckel was a leader in the field of biology in his day. An idea of his, no matter how slight the factual basis, would have been noticed. At the first meiotic division the 2 tetrads enter the spindle d and are divided, half of each tetrad a dyad going to each pole as shown in e.

As a result of the first meiotic division 2 cells are formed f, g. Each of these con- tains 2 dyads. In the second meiotic division h, i, j, k the dyads of the 2 cells are pulled apart. At the end of this division there are 4 cells l, m, n, o. Each of these contains 2 chromosomes, the haploid number. There is no further division of these 4 cells and they develop directly into sperm A. The idioplasm was thought to be an invisible chemical network that extended throughout the cell and from cell to cell.

He invented it to account for inheritance. He did not regard it as a highly stable material, but as one that might change during development, or as the result of nutrition or other external conditions. In any event it must return to the original condition in the embryo. His hypothesis was nearly impossi- ble to test, and hence it could be of no real usefulness in directing efforts to profitable experimentation.

Early Evidence for the Nuclear Control of Inheritance. The first three were primarily laboratory scientists. For at least a decade they had been leaders in the analysis of problems concerned with the nucleus. Weismann, on the contrary, is remembered largely for his theoretical work. These four men believed that the chromosomes were the physical basis of inheritance for the following reasons.

Even though inheritance was not well understood, it seemed that both parents have an equal share in transmitting their characteristics to the offspring. The soundest support for this belief came from work on plant hybrids. The obvious conclusion was that both parents contributed equally to the characteristics of the offspring. What is the physical basis of this equality? It was know, of course, that the only links between parent and offspring are the ovum and sperm.

These two cells are about as different as any two cells could be. Usually the ovum has a mass thousands or millions of times the mass of the sperm. Ova usually contain a large quantity of cytoplasm, whereas sperm contain almost none. This would suggest that the cytoplasm was not the basis of inheritance because, if it were, it might be expected that the. The only parts of the sperm and ova that seemed to these four scientists to be equivalent were the nuclei. The sperm pronucleus and the egg pronu- cleus were identical so far as one could tell.

Perhaps this equivalence of structure was the basis of the equivalent importance of the two gametes in inheritance. During cell division, the cytoplasm and its formed structures seem to be divided passively. 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?

The complex chromosomal changes during meiosis were understand- able in terms of keeping the chromosomes constant from generation to generation. There was no similar phenomenon for any other cell struc- ture. Since inheritance was an intergeneration phenomenon and the chromosomes seemed to be the only cell structures that were transmitted in an exact way from one generation to another, perhaps the chromo- somes were of importance in inheritance.

Finally, there was a more direct test of nuclear function in regenerating protozoa. The forms selected for this work were single-celled organisms with one nucleus.

It was possible to cut the animals into two parts, one part containing cytoplasm and the other cytoplasm and the nucleus. Both parts healed. The part without a nucleus lived for some time, but it was unable to regenerate to form a whole animal, and it was incapable of reproduction.

The part with the nucleus could regenerate a whole animal and could reproduce normally. The fact that chromosomes appeared to be the only cell structure that remained constant from cell to cell, and from generation to generation, could mean that inheritance was by way of the chromosomes.

It is of interest, therefore, to summarize the advances that those cytologists interested in heredity. Such a summary was given retrospectively by E.

Wilson in The work of cytology in its period of foundation laid a broad and substantial basis for our more general conceptions of heredity and its physical substratum. It demonstrated the basic fact that heredity is a consequence of the genetic con- tinuity of cells by division, and that the germ-cells are the vehicle of transmis- sion from one generation to another.

It accumulated strong evidence that the cell-nucleus plays an important role in heredity. 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. It demonstrated that when new germ-cells are formed each again receives only half the number characteristic of the body- cells.

It steadily accumulated evidence, especially through the admirable stud- ies of Boveri, that the chromosomes of successive generations of cells, though commonly lost to view in the resting nucleus, do not really lose their individual- ity, or that in some less obvious way they conform to the principle of genetic continuity. From these facts followed the far-reaching conclusion that the nuclei of the body-cells are diploid or duplex structures, descended equally from the original maternal and paternal chromosome-groups of the fertilized egg.

Continually receiving confirmation by the labours of later years, this result gradually took a central place in cytology; and about it all more specific discov- eries relating to the chromosomes naturally group themselves. All this had been made known at a time when the experimental study of hered- ity was not yet sufficiently advanced for a full appreciation of its significance; but some very interesting theoretical suggestions had been offered by Roux, Weismann, de Vries, and other writers.

While most of these hardly admitted of actual verification, two nevertheless proved to be of especial importance to later research. This fruitful suggestion pointed out a way that was. The article by Coleman, listed below, provides an excellent introduction to the history of cytology as related to inheritance.

The Evolution of the Microscope. New York: Pergamon Press. A History of Cytology. New York: Abelard-Schuman. MARK, E. Croonian Lecture: The bearing of cytological research on heredity. Questions 1. Do you detect any element of sameness among these diverse objects that suggests all should be put into the same class, that is, be identified as cells?

Had Hooke seen these objects do you believe that he would have regarded them as equivalent to the struc- tures he observed in cork? The nucleus of a cell does disappear before cell division.

What could have been the reason, therefore, why some cytologists believed that the nucleus had an unbroken continuity from one cell generation to another? In most instances it is far easier to see cell structures in fixed and stained cells than in living cells. That being the case, why did Flemming lay such stress on the importance of observing structures in living cells?

Would it be possible to work out the sequence of events in mitosis from a study of fixed and stained material only? Would it be necessary to assume any relation between a nucleus in the resting stage and a metaphase? Prevost and Dumas performed experiments that led them to believe that the sperm and not the seminal fluid is the active agent in fertilization.

Can you suggest how such experiments might have been done? The meiotic mechanisms described for Ascaris are almost universal in animals and very similar in plants. Can you devise other ways of halving the number of chromosomes? You probably already know something about DNA and its role in inheri- tance.

Why cannot scientific ideas be accepted more promptly? In terms of what you know of cell biology, how would you evaluate these statements of Virchow Readings, Chapter 2? Can you suggest why the phenomenon of mitosis was not discovered in the eighteenth century? Do you believe that the chromosomes maintain their essential structure during the resting stage? We are living in an age when scientific knowledge is of the utmost concern to all mankind.

The proper use of scientific knowledge can result in unparalleled benefits to mankind and a misuse can lead to unimaginable disasters. Heredity and Development: Second Edition describes the progress of genetics as it took place and in so doing evaluates some of the problems facing scientists who are working on unknown phenomena. The principal purpose is to show how ideas in these two fields were formulated and studied.

The intellectual history of the two has been quite different. Therefore, the report provides a foundation of the data and concepts in the field of genetics and an understanding of the manner in which science develops.

Emphasizing the manner in which hypotheses and observations lead to the conceptual schemes that allow us to think in an orderly and satisfying way about the problems involved, Heredity and Development explores the subsciences of genetics and embryology detailing a range of topics from Darwin's Theory of Pangenesis, and Mendelism to DNA structure and function, and differentiation. Used chiefly in college biology and genetics courses, the text is essential to decision makers, including those without a scientific background.

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Page 20 Share Cite. Page 21 Share Cite. Page 22 Share Cite. Page 23 Share Cite. Page 24 Share Cite. Page 25 Share Cite. Page 26 Share Cite. Page 27 Share Cite. Page 28 Share Cite. Page 29 Share Cite. Page 30 Share Cite. Page 31 Share Cite. When an egg and a sperm cell unite, the resulting fertilized egg cell contains DNA from both parents.

Any variants that are present in that DNA will be present in the cells of the child that grows from the fertilized egg. Because non-inherited variants typically occur in somatic cells cells other than sperm and egg cells , they are often referred to as somatic variants. These variants cannot be passed to the next generation. Non-inherited variants can be caused by environmental factors such as ultraviolet radiation from the sun or can occur if an error is made as DNA copies itself during cell division.

Topics in the Variants and Health chapter What is a gene variant and how do variants occur? How can gene variants affect health and development? Do all gene variants affect health and development? What kinds of gene variants are possible? Can a change in the number of genes affect health and development? Can changes in the number of chromosomes affect health and development?

Can changes in the structure of chromosomes affect health and development? Can changes in noncoding DNA affect health and development? Can changes in mitochondrial DNA affect health and development?