Nucleic acids
All the information needed to build and maintain a living organism is written in an alphabet of just four letters, repeated billions of times in a molecule that fits inside the nucleus of a microscopic cell. This section explains the chemical unit of that alphabet, the nucleotide, and how its arrangement in a complementary double helix makes it possible to copy and pass on that information with extraordinary fidelity.
The nucleotide: the structural unit
When purified nucleic acids undergo mild chemical hydrolysis in the laboratory, three products are always obtained in a strict 1:1:1 molar ratio: phosphoric acid, a five-carbon sugar and nitrogenous heterocyclic bases. How do these three simple chemical building blocks assemble to form the fundamental unit of genes?
What three components form a nucleotide? State how a DNA nucleotide differs from an RNA nucleotide.
What three components form a nucleotide? State how a DNA nucleotide differs from an RNA nucleotide.
Look at the diagram of a nucleotide and identify the three structures labeled with the numbers 1, 2 and 3, stating the chemical name of each.
Look at the diagram of a nucleotide and identify the three structures labeled with the numbers 1, 2 and 3, stating the chemical name of each.
Nitrogenous bases: four letters, two similar alphabets
Name the nitrogenous bases found in DNA and RNA, and state which base is unique to each of them.
Name the nitrogenous bases found in DNA and RNA, and state which base is unique to each of them.
The double helix and complementary base pairing
In 1950, Erwin Chargaff analyzed the DNA of different species and discovered a remarkable universal pattern: in every sample, $\%\text{A} = \%\text{T}$ and $\%\text{G} = \%\text{C}$ (for example, in human thymus the values are nearly equal: approximately $30.9\%\text{ A}$ and $29.4\%\text{ T}$, $19.9\%\text{ G}$ and $19.8\%\text{ C}$). What three-dimensional geometry forces two bases of such different sizes to always appear in identical proportions?
Explain the essential features of Watson and Crick's double-helix model: arrangement and direction of the two chains, position of the nitrogenous bases, and types of…
Explain the essential features of Watson and Crick's double-helix model: arrangement and direction of the two chains, position of the nitrogenous bases, and types of bonds holding the structure together.
In the DNA fragment in the diagram, only the bases of one of the strands are shown. Write the bases that should occupy the positions marked with ? on the complementary…
In the DNA fragment in the diagram, only the bases of one of the strands are shown. Write the bases that should occupy the positions marked with ? on the complementary strand and state the rule you applied.
If a strand of DNA has the sequence 5'-ATGCCGTA-3', write the full sequence of the complementary strand indicating its polarity (5' and 3' ends) and explain what it…
If a strand of DNA has the sequence 5'-ATGCCGTA-3', write the full sequence of the complementary strand indicating its polarity (5' and 3' ends) and explain what it means that the two strands are antiparallel.
In one strand of a DNA molecule, adenine accounts for $28\ \%$ of the bases and thymine accounts for $12\ \%$. Calculate the percentages of each nitrogenous base in the…
In one strand of a DNA molecule, adenine accounts for $28\ \%$ of the bases and thymine accounts for $12\ \%$. Calculate the percentages of each nitrogenous base in the complementary strand and in the entire DNA molecule, reasoning each step using Chargaff's rules and explaining why the breakdown between guanine and cytosine within each strand separately cannot be known.
DNA and RNA: two molecules, different functions
Within the framework of astrobiology, an informational polymer isolated from a meteorite is analyzed. The compound is a double-stranded nucleic acid made of…
Within the framework of astrobiology, an informational polymer isolated from a meteorite is analyzed. The compound is a double-stranded nucleic acid made of deoxyribonucleotides joined by phosphodiester bonds. One of the scientists proposes that this molecule could have independently originated a primitive extraterrestrial metabolism, acting simultaneously as a genetic store and as a primeval metabolic catalyst, analogously to Earth's "RNA world" hypothesis.
- a) Refute the scientist's proposal: justify, based on the molecular differences between RNA and DNA (presence of the 2'-OH group on the ribose and single-stranded vs. double-stranded structure), why this double-stranded DNA is much less suited than RNA to adopting conformations with catalytic activity (such as ribozymes).
- b) Explain why the discovery that RNA possesses enzymatic activity resolved the classical paradox of the origin of life regarding which macromolecule arose first (nucleic acids or proteins).
- c) If RNA exhibits greater functional versatility, reason what evolutionary advantages double-stranded DNA provided so that natural selection fixed it as the universal genetic material of cellular living organisms.
The phosphodiester bond, polarity and the functions of RNA
Within each chain, nucleotides are joined by 3′–5′ phosphodiester bonds: the phosphate links the OH on carbon 3′ of one pentose to carbon 5′ of the next. It should not be confused with the hydrogen bonds that pair bases on different strands.