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Because 15N has 1 more molecular weight than 14N, there are now 1000 base pairs in the DNA molecule, note that it is a base pair, that is to say, there are 1000 bases on a chain, and you must pay attention to the unit in this question. Therefore, the molecular weight of the bases in the newly synthesized daughter chain is 1000 less than the molecular weight of the original bases that were all labeled with 15N. Since DNA is semi-retained replication characteristics, reproduction 4 times is replication 4 times, and the final total number of DNA molecules is 16.

Among the 16 DNA molecules, there are two DNA bases on one strand that are labeled by 15N, and the bases on the other strand are labeled by 14N, then the method of dividing and then combining (can also be called "broken mirror recircle" method) can be used, so that the strands of these two DNA molecules are separated first, and then combined to obtain two DNA that are both 15N labels, and the molecular weight of such a DNA does not change compared with the original DNA, so the remaining 15 DNA, The molecular weight of each bar is 2000 less than the original, so the average molecular weight is reduced by (2000 15) 16=1875

The question is that the DNA molecule of 15 nitrogen elements is cultured in a nutrient solution of 14 nitrogen elements for 4 generations.

If not replicated, the average relative molecular mass is 15+x (x is the relative molecular mass of the other elements). Continuous propagation for 4 generations is equivalent to 4 replicates, yielding 16 single strands of DNA molecules, two of which have relative molecular masses of (15+x). The other 14 have a relative molecular mass of (14+x).

So the total average relative molecular weight is 14 (14 16) + 15 (2 16) = so the average relative molecular weight of the progeny molecule is reduced by 15+

The parental DNA contains 2000 deoxynucleotides, and each deoxynucleotide has 1 nitrogenous base (or a nitrogen element), which should be divided into two situations, 1. The average relative molecular weight of the DNA molecule containing the parent chain should be reduced by 1000, 2. The average relative molecular weight of the DNA molecule without the parent chain should be reduced by 2000

Start with 15 and end with 15*1 8+14*7 8 (because nitrogen-15 accounts for 1 8 and the answer is 7 8.)

Because RNA is the genetic material of the virus. The fused viral genetic material is A, which replicates the genetic material first, and then transcribes the capsid protein according to the genetic material, and then assembles it to form a new virus to spread. So it reproduces to form forma RNA and formaprotein.

c, because the genetic material of the tobacco virus is RNA that controls the production of proteins and the replication of genetic information.

From experiment C: red yellow red orange: yellow = 1 2 1 it can be judged that the color of citrus peel cannot be controlled by a pair of alleles; According to the title, the peel color of citrus should be controlled by two pairs of alleles (a, a and b, b).

Yellow is the recessive trait genotype is AAB, the red genotype is A-B-, and the orange genotype is AAB-A-BB.

According to the phenomenon of experiment D: orange red red orange: yellow = 3 4 1, the red in the parent must be AABB.