X-Ray Crystallography Analysis

The following picture was the X-ray picture of the B-form DNA (Picture 51) that

Rosalind Franklin produced.

1. Describe at least three features that you observed from this picture. It

would be useful for you to work out the DNA structure.

X-Ray Crystallography Analysis

2. You will be given some hints to analyse the X-ray picture. Try your best to

interpret the results and share them with your colleagues. Summarize

your results and you should decide whether you want to publish it or not.

2.1 You are provided with a laser pointer, which resemble an X-ray source.

You can strike the laser on different patterns in the transparency. The

laser would be diffracted and the diffraction pattern can be viewed with a

clear background. Observe the diffraction pattern and compare it with

the X-ray photo from Franklin. What conclusion can you make?

Distinctive regular “X” pattern means that DNA should have a *helical/

non-helical structure.

* Delete the inappropriate one.

Draw at least three possible structure of a helix.

X-Ray Crystallography Analysis

2.2 Because the X-ray pattern is so regular, the dimensions of the helix must

also be consistent. Which of the following(s) is/are supported by this

finding?

� the diameter of the helix stays the same;

� the helix twist at regular interval;

� the helix is continuous;

� it should be a double helix;

� it should be a triple helix;

From the above information, which of the following structure(s) may represent

the DNA structure?

A B C D

E F

X-Ray Crystallography Analysis

2.3 In an X-ray diffraction photo, the closer the spots, the larger the actual

distance. So the horizontal bars actually correspond to helical turns.

The vertical distance between the bars - 34Å - is a measure of the height

of one helical turn. (1 Å = 1X10-10m or 0.1 nm)

Indicate the length of one complete turn in DNA in the following drawing:

X-Ray Crystallography Analysis

2.4 The distance from the middle of the X-ray pattern to the top is measurable

at 3.4 Å. This corresponds to the distance between two stacked bases.

Since, we know the height of one helical repeat – 34 Å – and we know the

distance between stacked bases -3.4 Å, there must be ___________________

nucleotides per one complete helical turn.

X-Ray Crystallography Analysis

2.5 The presence of equaldistant layer-lines and their symmetric pattern

suggest that the helix makes a twist at regular intervals.

Which of the following helical structure(s) is/are supported by this finding?

A B C D

E F

X-Ray Crystallography Analysis

2.6 The fourth layer-lines in all four “appendages” are missing. This happens

because the X rays diffracted by the two strands of the helix where they

cross each other essentially cancel each other out. The missing smears

hint at DNA’s double helix structure, with major and minor grooves

between the strands.

Which of the following helical structure(s) is/are supported by this finding?

A B C D

E F

X-Ray Crystallography Analysis

2.7 The X-ray photograph also showed that the DNA unit cell has a two-fold

axis of symmetry. That means that the molecule looks the same whether

it is right way up or upside down.

This has important implications for the two strands of DNA. To see this,

think about two pencils side-by-side with the points down and the erasers

on top. If you turn the two pencils upside down they look very different.

Now the tips are pointing upward. However, if you line up the two pencils

side-by-side with the tip of one pointing up and the tip of the other pointing

down, when you flip the pair upside down they look the same. It means

the two strands of DNA must be anti-parallel.

Which of the following structure would be consistent with this finding?

P Q R S T

X-Ray Crystallography Analysis

2.8 Summarize what you can conclude from the X-ray photo from DNA?

Since the sugar and phosphate form the backbone in the double helix, the

nucleotides in the two anti-parallel strands should form base pairs. You

should now work on how the nucleotides form pair inside the helix.

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