DNA and RNA
Each of your cells keeps, coiled up in its nucleus, a molecule almost two metres long that fits into a microscopic space. That molecule is DNA, and it contains the complete instructions for building you. Understanding its structure is the first step towards understanding how inheritance works.
Nucleotides, the building blocks of DNA
When DNA is chemically analysed in the laboratory, it is always found to break down into three different substances: phosphoric acid, a simple five-carbon sugar and nitrogen-containing compounds. How are these microscopic pieces organised to build the molecule of heredity?
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 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.
Name the four nitrogenous bases of DNA and explain the base-pairing rule followed by the double helix.
The double helix and complementary base pairing
In 1950, Erwin Chargaff analysed the DNA of various organisms and discovered a constant mathematical rule: in any sample, the amount of adenine always equals that of thymine, and the amount of guanine equals that of cytosine. What is the reason for this exact correspondence between pairs of bases?
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…
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.
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.
Explain what a gene is and describe its relationship with DNA, chromosomes and proteins.
RNA, DNA's helper
DNA safeguards the instructions for making proteins inside the cell nucleus, but the machinery that produces them is outside, in the cytoplasm. Which molecule acts as a go-between messenger to carry that information without putting the original copy at risk?
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…
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…
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: $30\ \%$…
A laboratory analyzes the nitrogenous base composition of the genetic material extracted from three different viruses, obtaining the following results:
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Virus 1: $30\ \%$ adenine, $30\ \%$ thymine, $20\ \%$ guanine, $20\ \%$ cytosine.
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Virus 2: $30\ \%$ adenine, $20\ \%$ thymine, $22\ \%$ guanine, $28\ \%$ cytosine.
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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.