Why the Sexual mode of reproduction ??
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The creation of two new cells from one involues copying of the DNA as bell as of the cellular apparatus. The DNA copying mechanism, as we have noted, cannot be absolutely accurate, and the resultant errors are a source of variations in populations of organisms. Every individual
organism cannot be protected by variations, but in a population,
variations are useful for ensuring the survival of the species. It would therefore make sense if organisms came up with reproductive modes that allowed more and more variation to be generated. While DNA-copying mechanisms are not absolutely accurate, they are precise enough to make the generation of variation a fairly slow
process. If the DNA copying mechanisms were to be less accurate, many of the resultant DNA copies would not be able to work with the cellular
apparatus, and would die. So how can the process of making variants be speeded up? Each new variation is made in a DNA copy that already has variations accumulated from previous generations. Thus, two
different individuals in a population would have quite different patterns of accumulated variations. Since all of these variations are in living
individuals, it is assured that they do not have any really bad effects.
riations from two or more individuais would thus create
new combinations of variants. Each combination would be novel, since it would involve two different individuals. The sexual mode of reproduction incorporates such a process of combining DNA from two different individuals during reproduction. But this creates a major difficulty. Ifeach new generation is to be the
combination of the DNA copies from two pre existing individuals, then each new generation will end up having twice the amount of DNA that the previous generation had. This is likely to mess up the control of the cellular apparatus by the DNA. Besides, ifeach generation doubles its DNA. Jairly soon there will be only DNA on earth, with no room for
anything else. How many tvays can we think of for solving this diffculty? We have seen earlier that as organisms become more complex, the specialisation of tissue increases. One solution that many multicellular organisms have found for the problem mentioned above is to have special lineages of cells in specialised organs which have only half the number of chromosomes and half the amount of DNA as compared to the non-reproductive body cells. Thus, when these germ-cells from two individuals combine during sexual reproduction to form a new
individual, it results in re-establishment of the number of chromosomes and the DNA content in the new generation.
If the zygote is to grow and develop into an organism which has highly specialised tissues and organs, then it has to have sufficient stores of energy for doing this. In very simple organisms, it is seen that the two
germ-cells are not very different from one another, or may even be similar. But as the body designs become more complex, the germ-cells also specialise. One germ-cell is large and contains the food-stores while the other is smaller and likely to be motile.Conventionally, the motile germ-cell is called the male gamete and the germ-cell containing the stored
food is called the female gamete. We shall see in the next few sections how the need to create these two different types of gametes give rise to differences in the male and female reproductive organs and, in some cases, differences in the bodies ofthe male and female organisms
organism cannot be protected by variations, but in a population,
variations are useful for ensuring the survival of the species. It would therefore make sense if organisms came up with reproductive modes that allowed more and more variation to be generated. While DNA-copying mechanisms are not absolutely accurate, they are precise enough to make the generation of variation a fairly slow
process. If the DNA copying mechanisms were to be less accurate, many of the resultant DNA copies would not be able to work with the cellular
apparatus, and would die. So how can the process of making variants be speeded up? Each new variation is made in a DNA copy that already has variations accumulated from previous generations. Thus, two
different individuals in a population would have quite different patterns of accumulated variations. Since all of these variations are in living
individuals, it is assured that they do not have any really bad effects.
riations from two or more individuais would thus create
new combinations of variants. Each combination would be novel, since it would involve two different individuals. The sexual mode of reproduction incorporates such a process of combining DNA from two different individuals during reproduction. But this creates a major difficulty. Ifeach new generation is to be the
combination of the DNA copies from two pre existing individuals, then each new generation will end up having twice the amount of DNA that the previous generation had. This is likely to mess up the control of the cellular apparatus by the DNA. Besides, ifeach generation doubles its DNA. Jairly soon there will be only DNA on earth, with no room for
anything else. How many tvays can we think of for solving this diffculty? We have seen earlier that as organisms become more complex, the specialisation of tissue increases. One solution that many multicellular organisms have found for the problem mentioned above is to have special lineages of cells in specialised organs which have only half the number of chromosomes and half the amount of DNA as compared to the non-reproductive body cells. Thus, when these germ-cells from two individuals combine during sexual reproduction to form a new
individual, it results in re-establishment of the number of chromosomes and the DNA content in the new generation.
If the zygote is to grow and develop into an organism which has highly specialised tissues and organs, then it has to have sufficient stores of energy for doing this. In very simple organisms, it is seen that the two
germ-cells are not very different from one another, or may even be similar. But as the body designs become more complex, the germ-cells also specialise. One germ-cell is large and contains the food-stores while the other is smaller and likely to be motile.Conventionally, the motile germ-cell is called the male gamete and the germ-cell containing the stored
food is called the female gamete. We shall see in the next few sections how the need to create these two different types of gametes give rise to differences in the male and female reproductive organs and, in some cases, differences in the bodies ofthe male and female organisms
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