The structure of DNA
DNA is a double-stranded helix.
The strands are made up of sugar groups (deoxyribose), phosphate groups and four different bases –adenine,
thymine, cytosine and guanine. These groups join together in a condensation reaction to form a long chain of alternating sugar-phosphate groups. One of the four bases is joined to each sugar residue.
Each sugar phosphate base group is called a nucleotide. The two strands in DNA are held together by hydrogen
bonds between pairs of bases. Between A and T there are 2 hydrogen bonds and between G and C there are 3 hydrogen bonds.
Adenine always pairs with thymine, and cytosine with guanine.
This is known as complementary base pairing.
• Base paring allows a DNA molecule to copy itself before cell division –a process called replication
• It allows a strand of DNA to make a complimentary copy of another nucleic acid called messenger RNA, mRNA which it does during the process of transcription
Scientists once thought that proteins were more likely to transmit genetic data because proteins have more amino acids than the bases/nucleotides in DNA
Let's see how the scientists in the past thought about DNA:
She investigated the structure of DNA using X-ray diffraction. She noticed that the diffraction pattern of DNA suggested that the DNA chains were in closely packed helical form, with the phosaphate group near the outside.
Crick and Watson
The pair decided that the sugar phosphate backbone was in the centre of the molecule, and that there were three chains twisted together, held by doubly-charged cations such as Mg2+. They eventually realised that it is the sugar phosphate helix that surrounds horizontally positioned base pairs holding them together.
Pauling proposed a 3-helix modelof phosphate groups in the middle, with sugars around them and bases on the outside.
Drafted by Eunice (Chemistry)