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lemily516 lemily516
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11 years ago
Scientists seeking to determine which molecule is responsible for transmisson of characteristics from one generation to the next knew that the molecule must (1) copy itself precisely, (2) be stable but able to be changed and (3) complexz enough to determine the organism's phenotype

Select one of the criteria stated above and describe experimental evidence used to determine that DN is hereditary material.
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wrote...
11 years ago
The structure of DNA, due to its base pairing, does suggest that it could be replicated by copying of the individual strands. It is stable, as predicted from the structure and tested experimentally, but can be altered by changing the order of the bases. It can form extremely long molecules with an endless variation of the sequence of bases, so even without knowing the genetic code, one can infer that it is capable of transmitting a large amount of information.

The DNA has to produce its own copies through DNA replication. Stability of DNA is achieved by the hydrogen bonds between DNA strands. Its ability to be changed (due to base pairs undergoing changes) leads to mutations within the DNA strands.
wrote...
11 years ago
Hi,

DNA meets all the three above requirements.

1. DNA copies itself at a high speed and with high acuracy. There is error checking mechanism which ensures that all errors get corrected and it is highly energy consuming process. The process takes place in only one direction (from 5' to 3' direction).

The error rate is very low because it is not simple copying but a synthesis of complementary strand. i.e. there are four nitrogenous bases which form the backbone of the DNA strand. They are called adenine, thymine, cytosine and guanine, in short a,t,c and g. In the two strands of DNA, A will always bind with T by 2 hydrogen bonds and C will always bind with G with 3 hydrogen bonds. This is called pairing. During synthesis, the enzyme DNA polymerase substitutes A for T in the original strand and C for G in the original strand and so on... So if there is a error in the sequence, the original strand will not bind with the newly synthesised strand.

Besides there is a highly efficient checking and correcting system.

2. DNA is highly stable and is bound to histone proteins and tightly coiled. Only the portions needed for protein synthesis remain uncoiled. DNA does not undergo change during growth of an individual as mitosis is the only process involved during growth.

DNA undergoes a process called variation during meiosis which takes place when the reproductive cells are formed, ensuring the next generation does not possess the same combination of genes as the parent. Beside, the progeny receives two "Variated" sets of genes (from dad and mom). Thus further recombination occurs in the progeny ensuring "change"

3. A sequence of DNA forms what is called a "gene". Genes determine the phenotype of a person. Genes may be Dominant and Recessive.

For example, lets take hair color. Let's hypothesize that genes for black and red hair are dominant and genes for blond and silver are recessive. Then the various combinations will give rise to results as below.

Black gene + Blond gene = Black hair (only dominant is expressed)
Black gene + Silver gene = Black hair (only dominant is expressed)
Black gene + Red gene = Brown hair (both are dominant, so both get expressed forming brown hair)
Red gene + Silver gene = Red hair (only dominant is expressed)
Red gene + Blond gene = Red hair (only dominant is expressed)
Silver gene + Blond gene = Silver blond (both get expressed)

Thus they are complex enough to control phenotype.

Hope this helps!
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