Exercises: DNA and RNA

Explain what DNA is, what type of information it contains, and in which part of a eukaryotic cell most of this molecule is found.

Name the four nitrogenous bases of DNA and explain the base-pairing rule followed by the double helix.

The diagram shows a segment of a DNA double helix. State what markers 1 and 2 represent, and complete the missing bases on the rungs marked with ?, knowing that A always pairs with T and G with C.

State three differences between DNA and RNA: the sugar they contain, the number of strands and one of their nitrogenous bases.

Explain what a gene is and describe its relationship with DNA, chromosomes and proteins.

Before dividing, the cell copies all of its DNA through replication. Explain what this process consists of and why it is said to be semi-conservative: which strands form each daughter DNA molecule and what role complementary base pairing plays in making an exact copy.

An astrobiology team analyzes the genetic material of an extraterrestrial microorganism. They discover that it consists of two facing strands, but they observe two anomalies compared to terrestrial DNA: 1) the diameter of the double helix is not constant, but instead shows continuous widenings and narrowings; 2) both strands run in the same direction (both in the $5' \to 3'$ direction). Using the principles of purine/pyrimidine base complementarity and antiparallelism proposed by Watson and Crick, reason what type of chemical pairings would be causing these thickness irregularities and predict what physical or enzymatic difficulty a parallel double helix would present during cellular replication.

A laboratory analyzes the nitrogenous base composition of the genetic material extracted from three different viruses, obtaining the following results:

• Virus 1: $30\ \%$ adenine, $30\ \%$ thymine, $20\ \%$ guanine, $20\ \%$ cytosine.

• Virus 2: $30\ \%$ adenine, $20\ \%$ thymine, $22\ \%$ guanine, $28\ \%$ cytosine.

• Virus 3: $30\ \%$ adenine, $30\ \%$ uracil, $20\ \%$ guanine, $20\ \%$ cytosine.

Determine with reasoning for each virus: what type of nucleic acid it possesses (DNA or RNA) and whether its strand is single-stranded or double-stranded. Furthermore, justify why virus 2 does not contradict the laws of chemistry even though its percentages of adenine and thymine do not match.

In a laboratory, an experimental drug is tested on a cell culture. The compound binds irreversibly to histones, preventing the DNA from unwinding from the nucleosomes.

1. a) Predict which two molecular processes essential for the cell will be immediately blocked and justify why at the molecular level.
2. b) If an already replicated cell enters mitosis and manages to condense its chromosomes, but the drug prevents it from decondensing them at the end of telophase, what consequences will this have for the viability of the daughter cells during interphase?