Who proposed that the structure of dna is a double helix with two polynucleotide chains running in opposite directions and held together by hydrogen bonding between pairs of nitrogenous bases?

James Watson and Francis Crick

Explanation:

James Watson and Francis Crick with few other scientists proposed that DNA is made up of a double polynucleotide chains that are coil/twisted around each other to forming a helical structure. The two DNA chains runs anti-parallel in opposite direction to each other.

Each strand of the DNA is made up of long chain of nucleotides. Each nucleotide contains a sugars (D-ribose), a phosphate group and a nitrogen base forming the backbone of each DNA strand.

They are both DNA strands are held together by hydrogen bond between pairs of nitrogenous bases.

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

hope this .. .

the dark variety, as they are not visible on the dark tree barks

explanation: