Our Friend DNA

DNA

Some DNA Basics… DNA stands for Deoxyribonucleic Acid DNA’s basic function is to store and transmit genetic information. This information tells an organism’s cells what proteins to make and when to make them. Proteins form cell structures and control cell chemical processes

DNA Structure… DNA is composed of 2 chains of repeating nucleotides A nucleotide = sugar + phosphate + nitrogen containing base

DNA Structure… Deoxyribose is a sugar molecule. The phosphate is a phosphorus surrounded by oxygen atoms. The nitrogen containing base is just that, a base containing nitrogen atoms.

Nitrogen Containing Bases… In DNA, the nitrogen containing bases come in 4 varieties. Adenine (__A__) Guanine (__G__) Cytosine (__C__) Thymine (__T__)

Purines vs. Pyrimidines Adenine and Guanine are Purines (2 carbon rings). Cytosine and Thymine are Pyrimidines (1 carbon ring). Purine Pyrimidine

The Double Helix… Watson and Crick are credited with the discovery of the Double Helix structure of DNA. “…In 1951, the then 23-year old biologist James Watson traveled from the United States to work with Francis Crick, an English physicist at the University of Cambridge. Crick was already using the process of X-ray crystallography to study the structure of protein molecules. Together, Watson and Crick used X-ray crystallography data, produced by Rosalind Franklin and Maurice Wilkins at King's College in London, to decipher DNA's structure.” “…Working with nucleotide models made of wire, Watson and Crick attempted to put together the puzzle of DNA structure in such a way that their model would account for the variety of facts that they knew described the molecule. Once satisfied with their model, they published their hypothesis entitled "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" in the British journal Nature (April 25, 1953. volume 171:737-738.).”

In the words of Watson & Crick… "...This (DNA) structure has two helical chains each coiled round the same axis...Both chains follow right handed helices...the two chains run in opposite directions. ..The bases are on the inside of the helix and the phosphates on the outside..." "The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases... The (bases) are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side...One of the pair must be a purine and the other a pyrimidine for bonding to occur. ...Only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine)." "...in other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined." "...It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." **Source for material on this page: Pamela Peters, Ph.D., Access Excellence, Genentech, Inc. http://www.accessexcellence.org/AE/AEC/CC/DNA_structure.html

Translating all that… 1. The DNA “backbone” consists of alternating Sugar (deoxyribose) and phosphate molecules. 2. Nitrogen containing bases face inward and are perpendicular to the backbone. 3. By facing to the center, the bases from one chain of DNA face bases on a second chain. 4. The bases on one chain bond to the bases on the second chain by hydrogen bonding. 5. The overall DNA molecule (the one found in most living organisms) is twisted with each turn occurring after 10 base pairs.

Let’s Draw DNA Rasmol Animation                                        

Complementary Base Pairing… (Chargaff’s Rules) DNA bases pair in a standard manner. The 2 base pairing rules are: Adenine pairs with Thymine Guanine pairs with Cytosine As mentioned, these bases are connected by hydrogen bonds.

Complementary Base Pairing… Cytosine and Guanine connect by 3 hydrogen bonds. Adenine and Thymine connect by 2 hydrogen bonds.  The fact that the way bases pair together consistently suggests that DNA has the ability to copy itself. This process is called replication.

Base Pairing Practice A T A T T C C C C C C T C C G C C G C A T G T A A G G

Base Pairing Practice… A T T A A T T A T A C G C G C G C G C G C G T A C G C G G C C G C G G C C G A T T A G C T A A T A T G C G C