Tutorial: DNA: The Chemistry of Life: Answers

  1) In a sentence, give a definition of DNA?

DNA is a chemical/molecule which contains the genetic code for all living things. Note that viruses sometimes contain RNA, but it is controversial as to whether viruses are “living”.


2) Who discovered DNA and when?

First discovered by Fredrich Miescher, a Swiss physician, in 1869, who recovered a substance that he called nuclein from the nuclei of white cells. He did not realise however the importance of his discovery.

Friedrich Miescher
Friedrich Miescher







3) What does the acronym DNA stand for?

DeoxyriboNucleic Acid.


4) What is the basic structure and how does this relate to its’name?

DNA consists of sequential units called nucleotides which consist of a backbone of alternating sugars (Deoxyribose) and phosphate molecules, to which are linked one of the four bases (Guanine, Adenine, Thymine and Cytosine). For simplicity the nucleotides are often named after the base that is attached to them.


5) Why is DNA double stranded?

DNA is a genetic code. During the replication process, things can go wrong (mutations). Thus it is important for a cell to have a “back-up”.


6) What shape does the DNA usually take when in double stranded form? (Who discovered this?!).Bdna

Double helix, discovered by Watson and Crick in 1953.





7) Why are the two strands in double-stranded DNA complementary?

In the double helix the bases of one DNA strand bond with the bases fom the other strand. Take the 4 bases: guanine (G), adenine (A), thymine (T) and cytosine (C). Guanine will only bond to Cytosine, and Adenine will only bond to Thymine. Thus the two strands in the double helix are mirror images of each other (complementary).







8) In it’s most simple terms, how does DNA translate into protein?

The sequence of bases in a DNA strand are divided into sequential groups of three, each called a CODON eg CAT, CAG. Each codon codes for an amino acid. The DNA is converted to mRNA by RNA polymerase (known as TRANSCRIPTION), which is then converted into tRNA on ribosomes. tRNA contain amino acids and each codon sequence codes for a particular amino acid. The amino acids attach to the peptide chain according to the codon sequence. When the tRNA codes for STOP codon eg TGA, TAA, TAG then the process of transcription finishes and the peptide/protein chain is complete.


9) What is supercoiled DNA?

The DNA in the double helix makes a circle just over every 10 bases. During transcription and DNA replication, there is more stress in the direction of coling so the helix is “tighter”. Ie think of a spring. If you twist a spring in the same direction as its’ coiling, the spring will get tighter. If you twist it in the other direction, the coils will get looser. Enymes such as topoisomerases help to relieve this stress.


10) What are the differences between DNA and RNA?

From a chemical point of view, the main difference is in the sugar that is used to make the backbone. DNA uses deoxyribose and RNA uses ribose. In RNA the base thymine is usually replaced by uracil.

RNA is less susceptible to damage from Ultraviolet light, but more susceptible to alkali and also to attack from enzymes.


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