The nucleus of a single one of your cells holds about two meters of DNA, coiled up until it fits into a space only a few microns across. When a lab at INMEGEN analyzes a sample, this molecule is what it reads. How is it built, and how does it manage to fit into such a tiny space without losing the information? We will find out on this page.
Nucleotides: the unit of DNA
DNA (deoxyribonucleic acid) is the molecule that stores the hereditary information of organisms. In eukaryotic cells most of it is in the nucleus; there is also a little in mitochondria and chloroplasts. In prokaryotes it is in the cytoplasm (the nucleoid). It is made of just a few ingredients: phosphoric acid, a sugar, and four nitrogenous bases.
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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.
Why is it always a purine with a pyrimidine? Because that keeps the distance between the two strands constant (a purine with another purine would be too wide; two pyrimidines, too narrow) and the helix keeps a uniform diameter. This helps with the diagram in the exercise below: the markers point out, on one side, the sugar-phosphate backbone, and on the other, the hydrogen bonds between bases.
Try it with this sequence: complete the complementary strand.
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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 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.
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:
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.
The relationship is a chain of levels: DNA → gene → protein → trait. Each different gene contains the information for a specific protein. In sexual reproduction, each parent contributes half of the chromosomes; that is why children resemble their parents without being identical to them.
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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.
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.
The DNA of a human cell is about two meters long, but the nucleus is only about $6\ \mu\text{m}$ in diameter. How does it fold without breaking or tangling?
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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.…
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.
a) Predict which two molecular processes essential for the cell will be immediately blocked and justify why at the molecular level.
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?
Back to the beginning: the two meters of DNA fit in the nucleus because they wind around histones, form nucleosomes, and supercoil into chromatin, with several levels of folding. In total, two meters ($2{,}000{,}000\ \mu\text{m}$) in a nucleus of about $6\ \mu\text{m}$ imply a compaction on the order of $300{,}000$ times in length. The figure of $10{,}000$ times in the previous example corresponds only to a single chromosome in metaphase.
Summary
Let's practice! 3 medium exercises in this section