Topic 3 – Chemistry of Life Topic 3.3 – DNA Structure Topic 3 – Chemistry of Life

What is DNA? DNA (Deoxyribose Nucleic Acid) is the molecule that codes for protein. It is a polymer (chain) of nucleotides. Nucleotides are another type of biological molecule that is made up of three parts: A 5 carbon sugar called deoxyribose A phosphate group A Nitrogen containing ring structure called a nitrogenous base

Nucleotides Phosphate Base Sugar

Nucleotides Adenine (A) Cytosine (C) Guanine (G) Thymine (T) Not all nucleotides are the same. In DNA there are 4 different bases: Adenine (A) Cytosine (C) Guanine (G) Thymine (T)

Building DNA molecules The nucleotides can join together by a covalent bond between the phosphate group ( )of one nucleotide and the sugar ( ) of another nucleotide. This eventually forms a strand of nucleotides consisting of a sugar-phosphate “backbone” with the bases sticking out into the middle.

Building DNA molecules

A double helix Work done with X-rays by Rosalind Franklin in 1952 suggested that DNA is a double helix. This means that there are 2 strands of nucleotides wrapped around each other. The backbones coil around each other with the base pairs lined up in the middle. The nucleotides are arranged in an anti-parallel fashion (one goes up, the other goes down).

The DNA double helix

Chargaff’s rule In 1949 Erwin Chargaff researched the number of bases in the DNA of different organisms and got this data:

Chargaff’s rule What patterns do you see in this data? Source of DNA A T G C Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8 What patterns do you see in this data?

Chargaff’s rule Chargaff found that the number of T is roughly equal to the number of A and the number of G is roughly equal to the number of C. The only explanation is that T pairs with A and C pairs with G

A ? T T A G C C G

Building DNA molecules The base pairs are held together by more Hydrogen bonds which gives the whole structure more stability.

A C A T G T G C

Quick Question If a DNA sample is 20% A, what % will C be? Hint: 20% A therefore 20% must be T. A+T = 40% That leaves 60% for G and C. Half of this is G and half is C. So C must be 30%!

Building DNA molecules The two strands of nucleotides are said to be complementary. This means that the sequence on one strand determines the sequence on the other. This means you only need one strand to be able to make a copy the DNA. This is vital in mitosis and in protein synthesis… More of this in Topics 3.4 and 3.5!

More questions What type of bond holds the nucleotides together to form the backbone for each strand? What two parts of the nucleotides forms the backbone? What are the 4 nitrogenous bases called? How do they pair up? What is an “anti-parallel fashion”?

Story time… The story of the discovery of the structure of DNA is controversial to say the least. 2 groups of scientists (in London and Cambridge) were racing to discover the exact structure of DNA.

Kings College University (London) Rosalind Franklin and Maurice Wilkins X-ray diffraction photography Worked alone and did not want to share data Franklin’s photograph that proved DNA was a double helix Cavendish Laboratory (Cambridge) James Watson and Francis Crick Molecular model building Were shown Franklin’s photograph and calculations by her colleague Built the model of DNA and won a Nobel Prize

Who really won?

Fair or not? Is it fair that Franklin’s data was passed to Watson and Crick without her knowledge? It often takes collaboration between groups of scientists tackling the same problem from different angles to get an answer. Should Franklin have been working with Watson and Crick from the outset to get the structure quicker?

Fair or not? Why didn’t she work with them? Is it better for scientists to work together or individually? How much advancement is held back because of ideals of prestige, fame, money that are the foundation of non-collaboration? Could we have already found the cure for cancer and AIDS???

Fair or not? Read the article “Thinking about Science: Co-operation in Science”. Answer question 1 and 2 for homework.