Nucleic Acids Basics
Nucleic acids are the subject of the last part of “Unit 1: The Chemistry of Life.” This section looks at the relevance of DNA and RNA in cells, as well as the structure of each of these molecules, to see why they serve various purposes. We’ll look at how DNA and RNA are made, as well as the various functions they play in cells.
Nucleic Acids Overview
In this area of the AP Biology curriculum, the larger picture is that heritable information ensures life’s continuance. The term “heritable” refers to anything that may be handed down from generation to generation, while “continuity of life” refers to the continual process of organisms developing, duplicating their DNA, and producing a new generation of organisms with the same DNA. This section focuses on DNA and RNA in more detail. We’ll look at how they’re alike and distinct, as well as the numerous functions they play in cells.
Watson and Crick were the first to discover the structure of DNA in 1953. The team utilised x-ray crystallography photos created by Rosalind Franklin, who died of cancer before the rest of the team was given the Nobel Prize. Her way of seeing the structure of DNA using X-rays eventually led to the model we use today.
DNA stands for “deoxyribonucleic acid,” which refers to the sugar ribose as well as the nucleotide bases that make up every DNA molecule. The term “deoxy” refers to the fact that deoxyribose contains one fewer oxygen atom than regular ribose. The sugar-phosphate backbone and hydrogen bonds created between complementary nucleotide bases are the two fundamental properties that give DNA its structure at the molecular level.
A phosphodiester bond is formed between each nucleotide in the sequence, which is formed by a dehydration process aided by the enzyme DNA polymerase. The hydroxyl group of the pentose sugar molecule joins the phosphate group of the new nucleotide in this connection. As a result, the new strand is built from the 5′ end (which contains a phosphate group) to the 3′ end (with a hydroxyl group). Because the two strands are antiparallel, the DNA polymerase molecule proceeds along the template strand in the opposite direction, from 3′ to 5′.
The nitrogenous bases protrude perpendicularly from the sugar-phosphate backbone in the structure of a DNA molecule. Because of the angle of the bonds that make up the sugar-phosphate backbone, it forms a helix. This molecule takes on a double-helix shape, sometimes known as a duplex, when nitrogenous bases make hydrogen bonds with the antiparallel strand.
Two grooves weave their way up the molecule in this duplex. The structure of the sugar-phosphate backbone creates the first groove, known as the main groove. Another groove is produced while the second strand is manufactured. The space between the two strands, which is filled with nitrogenous bases, forms the minor groove. These grooves are used by enzymes that interact with DNA to detect DNA molecules, connect to them, and complete their activity.
RNA molecules vary somewhat from DNA ones. Ribonucleic acid is referred to as “RNA.” Unlike DNA, RNA uses the sugar molecule ribose, which has an additional oxygen atom that deoxyribose lacks.
While the oxygen atom causes several modifications in the function of RNA, it is just one minor atomic alteration in the structure of a much bigger molecule. For starters, RNA is a less stable molecule than DNA. The oxygen atom is much more reactive than a single hydrogen atom, and it often participates in hydrolysis events that alter RNA structure. Second, because of the physical presence of the oxygen atom, RNA is most typically a single-stranded molecule. The big hydroxyl group formed by the oxygen atom bonding with the hydrogen lies immediately above the nitrogenous base from the nucleotide below. This prevents the nitrogenous bases from forming hydrogen bonds with one another.
Another distinction between RNA and DNA is that it employs Uracil rather than thymine. RNA may employ uracil for a variety of reasons, despite the fact that all other nitrogenous bases are the same. Uracil is less difficult to make, although it is highly similar to cytosine and degrades rapidly. This isn’t a concern since RNA is so short-lived. To help keep information accurate for lengthy periods of time, DNA needs a more stable base.
While DNA is almost always found as a duplex in nature, RNA may take on a variety of shapes. Other typical secondary structures include transfer RNA (tRNA) and ribosomal RNA, in addition to the single-stranded, single-helix structure most often observed as messenger RNA (mRNA) (rRNA).
Transfer RNA, or tRNA for short, is a kind of RNA that is used to add additional amino acids to a peptide chain as it grows. When a single-stranded RNA molecule folds back on itself to form tiny structures known as hairpins, tRNA is formed. Three nitrogenous bases are accessible on one side of a tRNA molecule. These bases will form a hydrogen bond with a codon on an mRNA molecule, indicating to a ribosome that it has chosen the correct amino acid. The amino acid on the other end of the tRNA is particular, and it can only bind to that amino acid. The tRNA molecule’s additional components guarantee that it can be digested by a ribosome.
While ribosomes are mostly constituted of protein, they also include an RNA component that intertwines with the protein structure—known as rRNA—and assists in the translation process. As the translation process develops, rRNA assists the ribosome in keeping mRNA and tRNA in place. It also aids in the dehydration process required for the formation of new peptide bonds between amino acids!
Scientists are continually uncovering novel functions for RNA inside cells, in addition to these kinds of RNA. There are also microRNAs that regulate genes inside the nucleus, RNAs that serve as enzymes in certain processes, and a slew of additional special-purpose RNAs that are constantly being found!
Except for a few viruses that utilise RNA as their primary information molecule, most organisms on Earth store information in DNA and convert it into proteins using RNA. Let’s look at each molecule’s structure to understand why they’re so good at what they do.
DNA’s duplex structure is very stable. Not only are hydrogen bonds between complementary bases utilised to hold the two strands together, but the sugar used (deoxyribose) is also considerably less likely to react with other molecules since it lacks the reactive oxygen atom found in RNA. This structure makes DNA robust and assures that it will not be damaged for a long period. Furthermore, DNA is robust enough to retain information in a complicated way.
It would take around 5 feet to stretch all the DNA stored in a single cell of your body. Nucleosomes, on the other hand, are made by wrapping DNA around storage proteins called histones. Nucleosomes may be securely packed into chromatin, which can subsequently be packed even more densely into a chromosome. This enables the storage of 6 billion nucleotides in the nucleus of a cell — only 1/500th of an inch!
RNA, on the other hand, is not a very stable molecule. However, it fulfils several functions in the cell that DNA could not. RNA polymerase, an enzyme that converts DNA into RNA, may swiftly produce an RNA transcript that can transport information out of the nucleus. RNA may form new proteins by interacting with ribosomes, and these molecules can fold into protein-like forms that function as enzymes and gene regulators in cells. RNAs degrade fast because they employ uracil and have an additional oxygen atom in the ribose sugar. That’s OK, since each RNA molecule is only required for a brief period of time, and more can be simply produced by transcribing the DNA code!