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.

Explain what a nucleotide is and name its three components. Which of them varies from one nucleotide to another and constitutes the "letter" of genetic information?

The two strands of DNA are antiparallel: one runs along the double helix in the 5' to 3' direction and the other in the 3' to 5' direction. Explain with an analogy what it means for them to be antiparallel and what the 5' and 3' marks indicate.

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 the historic Hershey and Chase experiment (1952) with bacteriophages:

1. a) Predict what results would have been obtained in the bacterial pellet and in the supernatant liquid if the prevailing hypothesis of the time (that proteins constituted the hereditary material) had been correct.
2. b) Imagine that a student proposes repeating the experiment by labeling the viruses with nitrogen-15 ($^{15}\text{N}$, a stable isotope) instead of using phosphorus ($^{32}\text{P}$) and sulfur ($^{35}\text{S}$). Justify why this experimental design would be completely useless for determining whether the genetic material is DNA or protein.

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.

DNA replication occurs before the cell divides by mitosis. Explain why it is essential that it happens in that order and what would happen to the genetic information of the daughter cells if a cell divided without having copied its DNA first.

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?

A human metaphase chromosome measures about $5\ \mu\text{m}$, but the DNA strand it houses would measure about $5\ \text{cm}$ when stretched out (a $10\,000$-fold packaging).

1. a) If you apply an inhibitor of the enzyme RNA polymerase (responsible for transcription) to a cell in metaphase, you will see that it has barely any effect because RNA synthesis is already halted naturally. Justify this fact by relating it to the degree of chromatin compaction.
2. b) Explain why it is essential for the cell to achieve such a level of packaging during mitosis, and why it would be lethal for it to maintain that same degree of condensation permanently during interphase.

Compare how DNA is organised in a bacterium and in a eukaryotic cell: where it is found, how many chromosomes there usually are and what shape they have.

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.