Great DNA Experiments Trivia Quiz

Answer: An enzyme that breaks down proteins

Miescher was Swiss born and was trained in medicine before becoming partially deaf. From then on, Miescher turned towards physiology and biochemistry. The enzyme used to test whether or not nuclein was a protein or not was pepsin - one of many proteases which cleaves polypeptide chains at specific site (i.e. at specific amino acid residues).

Answer: Transformation

Griffith's experiment used two strains of bacteria - a dead virulent strain and a live non-virulent strain - and showed that a mixture of the two was capable of killing mice. This suggested that there was something which causes bacteria to be virulent which was transferred from the dead strain to the live strain, making the live strain virulent also.

The work was built upon by Oswald Avery, Colin MacLeod and Maclyn McCarty. This was the first method of gene transfer identified. The two other main methods of gene acquisition by bacteria are conjugation (directly from other bacteria) and transduction (from viruses). Both of the latter mechanisms were discovered by Joshua Lederberg.

Answer: Phosphorus

This experiment employed the technique of transduction, which had been discovered only a year before. Bacteriophages (viruses which infect bacteria) had their proteins labelled with sulphur-35 and DNA labelled with phosphorus-32. There was one component that was being transferred to the bacteria from the bacteriophage. Analysis of the transformed bacteria showed them to contain P-32, but not S-35.

The transforming material was therefore DNA, not protein.

Answer: Uracil is a purine

Cytosine, thymine and uracil (C, T and U) are all derived from the molecule pyrimidine, and so are called the pyrimidines. They are single-ringed structures. Purines are double ringed structure which include guanine and adenine (G and A). As well as the number of purines being equal to the number of pyrimidines, Chargaff stated that the number of C and G nucleotides were equal and that the number of A and T nucleotides are equal (U is not found in DNA).

This can be explained by the fact that C always pairs with G and A always pairs with T.

Answer: That mutations arise spontaneously

Luria and Delbruck tested this hypothesis by growing several cultures before exposing them to a selective media (e.g. a plate containing an antibiotic). If mutations were induced by the exposure to this media, then a similar number of colonies would survive on each plate.

However, there was much fluctuation between the number of surviving colonies on each plate, suggesting that antibiotic resistance mutations arose spontaneously.

Answer: Double helix, with bases pointing inwards and a sugar-phosphate backbone

The triple helix model was proposed by a group led by Linus Pauling in America, but this was obviously incorrect to the eyes of the groups in Britain, who favoured a double helix model. X-ray diffraction images (especially those from Rosalind Franklin) were proof that the double helix model was correct and that the bases pointed inwards.

The 1962 Nobel Prize was awarded to Watson, Crick and Wilkins.

Answer: Two "daughter" DNA molecules contain one strand from the "parent" DNA molecule

Meselson and Stahl showed this by growing cells on a medium containing nitrogen-15 (a heavy isotope). After several divisions, the DNA of these cells contained no nitrogen-14 (the usual isotope), but only the heavier nitrogen-15. By transferring these cells onto a medium containing nitrogen-14, the DNA was allowed one round of replication before the density of the DNA was measured.

The density was shown to be in between the DNA containing only nitrogen-15 and the DNA containing only nitrogen-14, supporting the semi-conservative hypothesis.

Answer: That three nucleotides code for one amino acid

It had already been established that there were 20 naturally occurring amino acids in living organisms, and four nucleotides found in DNA. The first attempt to explain this problem was a rather convoluted system involving an overlapping code in which two nucleotides coded for one amino acid.

The reality was much simpler - that three nucleotides code for one amino acid. The number of combinations of the four nucleotides into a triplet would mean 64 combinations, showing that some amino acids are coded for by more than one triplet (called a codon).

For example, tyrosine is coded for by UAU and UAC.

Answer: A nucleotide which terminated DNA elongation when added to a growing chain

Sanger used radiolabelled dideoxynucleotides, which, when added to a growing DNA chain, would prevent elongation. Only one form of dideoxynucleotide was used at a time (e.g. ddA, ddT, ddG or ddC). For example, if ddA was used, this meant that wherever a chain terminated, the nucleotide found at that point was an A nucleotide. By doing this with each type of dideoxynucleotide, the full sequence could be determined.

Answer: The Human Genome Project

The Human Genome Project (HGP) identified and sequenced every gene in the human genome, but did not sequence the entire sequence of DNA, as is often thought. Much of the human genome is non-coding/serves structural or unknown purposes, rather than to code for RNA or proteins. Several next generation techniques such as 454 pyrosequencing, Solexa, and SOLiD sequence by synthesis, and the reading is performed by computers.

This is in comparison to Sanger's technique of analysing DNA fragments on a gel.