Double Helix: DNA Trivia Quiz

Answer: Hershey and Chase

Friedrich Miescher was the first to isolate DNA (from pus, no less). Griffith discovered the presence of a "transforming factor" in bacteria, but did not identify it as DNA (Avery and his colleagues later identified the transforming factor as DNA). But it was Hershey and Chase who first conclusively demonstrated that DNA is the genetic (hereditary) material, using bacteriophages. Watson and Crick, of course, are the two scientists widely credited for the discovery of the structure of DNA.

Answer: nucleosides do not contain phosphates

Nucleosides are usually named by combining the name of the nitrogenous base found in them with the suffix "sine" (if the base is a purine) or "dine" (if the base is a pyrimidine). If the nucleoside contains deoxyribose instead of ribose, the prefix "deoxy-" is added.

For example, adenosine, cytidine, deoxyguanosine, and thymidine are all nucleosides. Note that it is common to name individual NUCLEOTIDES as if they were nucleosides and simply specifying how many phosphates are attached. For example: adenosine triphosphate (ATP) is a nucleotide, but can be thought of as a nucleoside (adenosine) with three phosphates attached (triphosphate).

Answer: to maximize hydrophobic interactions between adjacent base pairs

While all the nitrogenous bases found in DNA contain polar functional groups that make the bases soluble in water, the skeletal ring structures of the bases are largely non-polar and hydrophobic. If DNA were kept unwound, there would be sufficient empty space between adjacent base-pairs for water to fill, which is thermodynamically unfavorable since hydrophobic substances want to stay away from water.

Therefore, by twisting DNA into a double-helix, the space between adjacent base-pairs is minimized and water gets "squeezed out". If you can't see why twisting DNA moves the base-pairs closer to one another, you can try it out yourself using crude models - it really does work!

Answer: carbon 2' in ribose is bonded to one hydroxyl group while in deoxyribose that carbon is not bonded to any hydroxyl groups

The "deoxy-" prefix in deoxyribose pretty much sums up the difference between deoxyribose and ribose. Deoxyribose is missing an oxygen! Specifically, the lone hydroxyl (-OH) group at carbon-2' in ribose is replaced with a hydrogen in deoxyribose. Note that deoxyribose can't possibly be missing the oxygen at carbon-1' because losing that particular oxygen would mean that deoxyribose would not be an aldehyde, and therefore wouldn't be a sugar (it would just be a polyhydroxyalcohol).

Answer: thymine

Thymine has a methyl group (-CH3) and two carbonyl groups (C=O). Adenine has a single amine group (-NH2). Both guanine and cytosine have an amine group (-NH2) and a carbonyl group (C=O). Uracil is identical to thymine except it is missing the methyl group.

Answer: beta-glycosidic linkage at carbon-1'

The bases are always linked to the sugars at the 1' carbon. The beta just means that the bond is angled upward from the rough plane of the sugar's carbon ring if the sugar is positioned such that the carbon numbers increase clockwise when looking DOWN on the plane of the ring.

Answer: 3'-5' phosphodiester bonds

Between each sugar in DNA and RNA is a single phosphate group linked to the 3' carbon of one sugar and to the 5' carbon of the other. A phosphate group can be considered an acid (it is the completely deprotonated form of H3PO4 - phosphoric acid) so the bonds are ester bonds (an ester bond is defined as a bond between a carbon and an acid).

Answer: from the 5' end to the 3' end

It DOES matter which way you read a strand of DNA, and it is completely OK to read just one strand (that's what transcription enzymes do). A common misconception is that in order to tell which end of a DNA strand is the 5' end and which end is the 3' end, you just look at which end has a phosphate; the end with the phosphate has to be the 5' end.

This isn't necessarily true. While it is certainly the norm in biological systems (since it's a consequence of how DNA is replicated), there is no fixed rule saying that the 5' end HAS to have a phosphate and the 3' end HAS to be un-phosphorylated. Also, if you want to read a section of DNA in the middle of a strand (more often than not this is the case) there are phosphates at BOTH ends.

The only foolproof method to determine which end is which is to look at the orientation of the deoxyribose sugar; the oxygen that is part of the sugar ring always points toward the 5' end.

Answer: TGACCT

DNA strands in the same helix are complementary and anti-parallel. So if you know the base sequence of one strand read 5' to 3', the base sequence of the other strand also read 5' to 3' is composed of the complementary bases (consequence of complementary structure) but in the REVERSE order (consequence of the anti-parallel structure).

Answer: B form

The famous biologists Watson (got his PhD without passing organic chemistry...lucky man...) and Crick described the B form structure of DNA. A-DNA is a more tightly-wound form of B-DNA, with the base pairs inclined at a greater angle respective to the front-back axis. Z-DNA twists in the opposite direction (left-handed helix vs. right-handed for A-DNA and B-DNA) and its structure repeats every 2 nucleotides instead of every 1 nucleotide. Only A, B, and Z-DNA have been found in biological systems, but there are many other possible forms of DNA (including C-DNA).