what happens if a change occurred during DNA copying ? if these changes have passed to several generations can it form a new generations
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Answer:
DNA replication is a truly amazing biological phenomenon. Consider the countless number of times that your cells divide to make you who you are—not just during development, but even now, as a fully mature adult. Then consider that every time a human cell divides and its DNA replicates, it has to copy and transmit the exact same sequence of 3 billion nucleotides to its daughter cells. Finally, consider the fact that in life (literally), nothing is perfect. While most DNA replicates with fairly high fidelity, mistakes do happen, with polymerase enzymes sometimes inserting the wrong nucleotide or too many or too few nucleotides into a sequence. Fortunately, most of these mistakes are fixed through various DNA repair processes. Repair enzymes recognize structural imperfections between improperly paired nucleotides, cutting out the wrong ones and putting the right ones in their place. But some replication errors make it past these mechanisms, thus becoming permanent mutations. These altered nucleotide sequences can then be passed down from one cellular generation to the next, and if they occur in cells that give rise to gametes, they can even be transmitted to subsequent organismal generations. Moreover, when the genes for the DNA repair enzymes themselves become mutated, mistakes begin accumulating at a much higher rate. In eukaryotes, such mutations can lead to cancer.
Errors Are a Natural Part of DNA Replication
After James Watson and Francis Crick published their model of the double-helix structure of DNA in 1953, biologists initially speculated that most replication errors were caused by what are called tautomeric shifts. Both the purine and pyrimidine bases in DNA exist in different chemical forms, or tautomers, in which the protons occupy different positions in the molecule (Figure 1). The Watson-Crick model required that the nucleotide bases be in their more common "keto" form (Watson & Crick, 1953). Scientists believed that if and when a nucleotide base shifted into its rarer tautomeric form (the "imino" or "enol" form), a likely result would be base-pair mismatching. But evidence for these types of tautomeric shifts remains sparse.When Replication Errors Become Mutations
Incorrectly paired nucleotides that still remain following mismatch repair become permanent mutations after the next cell division. This is because once such mistakes are established, the cell no longer recognizes them as errors. Consider the case of wobble-induced replication errors. When these mistakes are not corrected, the incorrectly sequenced DNA strand serves as a template for future replication events, causing all the base-pairings thereafter to be wrong. For instance, in the lower half of Figure 2, the original strand had a C-G pair; then, during replication, cytosine (C) is incorrectly matched to adenine (A) because of wobble. In this example, wobble occurs because A has an extra hydrogen atom. In the next round of cell division, the double strand with the C-A pairing would separate during replication, each strand serving as a template for synthesis of a new DNA molecule. At that particular spot, C would pair with G, forming a double helix with the same sequence as its original (i.e., before the wobble occurred), but A would pair with T, forming a new DNA molecule with an A-T pair in place of the original C-G pair. This type of mutation is known as a base, or base-pair, substitution. Base substitutions involving replacement of one purine for another or one pyrimidine for another (e.g., a mismatched A-A pair, instead of A-T) are known as transitions; the replacement of a purine by a pyrimidine, or vice versa, is called a transversion.
Answer:
DNA replication is a truly amazing biological phenomenon. Consider the countless number of times that your cells divide to make you who you are—not just during development, but even now, as a fully mature adult. Then consider that every time a human cell divides and its DNA replicates, it has to copy and transmit the exact same sequence of 3 billion nucleotides to its daughter cells. Finally, consider the fact that in life (literally), nothing is perfect. While most DNA replicates with fairly high fidelity, mistakes do happen, with polymerase enzymes sometimes inserting the wrong nucleotide or too many or too few nucleotides into a sequence. Fortunately, most of these mistakes are fixed through various DNA repair processes. Repair enzymes recognize structural imperfections between improperly paired nucleotides, cutting out the wrong ones and putting the right ones in their place. But some replication errors make it past these mechanisms, thus becoming permanent mutations. These altered nucleotide sequences can then be passed down from one cellular generation to the next, and if they occur in cells that give rise to gametes, they can even be transmitted to subsequent organismal generations. Moreover, when the genes for the DNA repair enzymes themselves become mutated, mistakes begin accumulating at a much higher rate. In eukaryotes, such mutations can lead to cancer.
Errors Are a Natural Part of DNA Replication
After James Watson and Francis Crick published their model of the double-helix structure of DNA in 1953, biologists initially speculated that most replication errors were caused by what are called tautomeric shifts. Both the purine and pyrimidine bases in DNA exist in different chemical forms, or tautomers, in which the protons occupy different positions in the molecule (Figure 1). The Watson-Crick model required that the nucleotide bases be in their more common "keto" form (Watson & Crick, 1953). Scientists believed that if and when a nucleotide base shifted into its rarer tautomeric form (the "imino" or "enol" form), a likely result would be base-pair mismatching. But evidence for these types of tautomeric shifts remains sparse.
oday, scientists suspect that most DNA replication errors are caused by mispairings of a different nature: either between different but nontautomeric chemical forms of bases (e.g., bases with an extra proton, which can still bind but often with a mismatched nucleotide, such as an A with a G instead of a T) or between "normal" bases that nonetheless bond inappropriately (e.g., again, an A with a G instead of a T) because of a slight shift in position of the nucleotides in space (Figure 2). This type of mispairing is known as wobble. It occurs because the DNA double helix is flexible and able to accommodate slightly misshaped pairings (Crick, 1966).
A schematic diagram shows two nucleotide base-pairs with a wobble. At top, the chemical structure of the base thymine (red) is connected to the chemical structure of the base guanine (blue) by two hydrogen bonds. The hydrogen bonds are depicted as dashed red lines. Below, the chemical structure of the base cytosine (orange) is connected to the chemical structure of the base adenine (green) by two hydrogen bonds.
Figure 2: Wobble in mismatched nucleotide base pairs.
A shift in the position of nucleotides causes a wobble between a normal thymine and normal guanine. An additional proton on adenine causes a wobble in an adenine-cytosine base-pair.
© Nature Education Adapted from Pierce, Benjamin. Genetics: A Conceptual Approach, 2nd ed. All rights reserved. View Terms of Use
Replication errors can also involve insertions or deletions of nucleotide bases that occur during a process called strand slippage. Sometimes, a newly synthesized strand loops out a bit, resulting in the addition of an extra nucleotide base (Figure 3). Other times, the template strand loops out a bit, resulting in the omission, or deletion, of a nucleotide base in the newly synthesized, or primer, strand. Regions of DNA containing many
