MCB

1. First it is important to realize that Hydrogen bonds are weak chemical bonds. When the DNA is in its double stranded helical form the complementary base pairs are basically held in a position for maximum hydrogen bonding to occur between them. When the phosphate backbone is broken, the nucleotides at the sticky ends are no longer physically held in place by the backbone and the weak hydrogen bonds can break (many hydrogen bonds in unison can be strong, but the small amount in the sticky overhangs of 4-10 base pairs is not very significant). The sticky ends of a double stranded DNA that has been cut by restriction enzymes can easily come back together to hydrogen bond too. In fact, when ligase is added to cloning reactions the number of DNA molecules that just re-ligate back together is much greater than those that actually receive the foreign DNA insert. Ligase is needed any time the phosphate backbone is broken. 2. I believe it is possible. For the most part though i think most restriction enzymes act as dimers and recognize palindromic DNA sequences to cut dsDNA. 3. Restriction enzymes and other sequence specific DNA binding proteins recognize the specific sequence through chemical interactions between the DNA and amino acid side chains in the DNA binding motif of the protein. Usually, the protein recognizes certain features in the Major groove of the DNA in which it can distinguish between every type of base pairing! really exciting stuff There's really a LOT more to all of these things you're inquiring about but you'll learn all of that in a molecular genetics course if you so choose that path!

Okay, so i know that full siblings share on average 50% of their DNA. What i can't seem to find an answer to is why is this only on average? Can the DNA shared differ to amounts greater than or less than 50% in full siblings (i.e. a brother and a sister actually share only 42% of genes)? How much can full siblings dna differ from 50% of genes shared, and what is the reason that siblings may share more or less than the average 50%? I am asking about regular siblings and not identical twins.