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12.1 The Genetic Material -- Part 2
When a crystal is X-rayed, the diffracted beam reflects the pattern of the molecule in the crystal.
The beam is diffracted farther from the center if the structures are closer together.
The pattern of DNA produced by a person.
The dark portions at the top and bottom of the X pattern told investigators that some features are repeated over and over.
The hydrogen-bonded bases were determined to be this feature.
A double helix is shown in the X-ray pattern of DNA.
The cross pattern in the center of the photograph shows the shape of the helical.
There are dark portions at the top and bottom of the picture.
Wilkins showed one of Franklin's crystallographic patterns to James, who immediately grasped its significance.
The English biophysicist Francis H. Crick was part of one of the teams.
They received a prize for their work.
There are two strands of the molecule.
The strand is said to be 5' to 3' from top to bottom.
The 3' end of a strand has a -OH group, while the 5' end has a phosphate group.
They deduced that DNA is a double helix with sugar-phosphate backbones on the outside and pairs of bases on the inside by constructing models.
This arrangement is similar to the one provided by Franklin's X-ray data for the spacing between the base pairs and for a complete turn of the double helix.
The 5' phosphate of one nucleotide is linked to a free OH located at the 3' position on the sugar of the preceding nucleotide, giving the molecule directionality.
The model agrees with Chargaff's rules that A is hydrogenbonded to T and G is hydrogenbonded to C. The bases are oriented properly so that they can interact with each other.
The spacing between the two strands of the DNA was detected by Franklin's X-ray diffraction pattern because two pyrimidines and two purines are too wide.
The 5' to 3' direction is where the information must be read.
In a 5' to 3' direction, a DNA strand is replicated.
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