DNA and RNA Structure Basics
This portion of the AP Biology curriculum delves further into the DNA and RNA structure in various species. We will begin by discussing the basics of nucleic acids (which were covered in more detail in section 1.6). Furthermore, we will examine how, similar to protein structures, the structures of DNA and RNA can be subdivided into primary, secondary, tertiary, and quaternary structures. We’ll examine a few of the purposes that these diverse cellular structures provide (such as tRNAs and ribozymes). Then, the functional and structural differences among circular and linear chromosomes would be examined. Finally, we’ll look at how prokaryotic species may move crucial genes across cells using small circular DNA units called plasmids.
DNA and RNA Structure Overview
Almost all life on Earth uses the identical basic DNA and RNA molecules to accumulate and distribute genetic data, including single-celled bacterial cells which occupy every inch of the planet; multicellular fungi which act as nutritional recyclers in different ecosystems; plants of all sizes and shapes; and animals of all sizes and shapes.
DNA and RNA Molecules
Although the differences between DNA and RNA are minor, understanding the molecular origins of heredity requires understanding their structure. Genes are stored by prokaryotes and eukaryotes using essentially identical molecules. There are significant structural changes in transmission techniques used by different species. The AP Exam will undoubtedly include this content. So stick with us as we go through all there is to know about DNA and RNA structure!
Let’s examine the general objectives and reasons for the existence of DNA and RNA structures before we discuss the specific structures. We addressed most of these concepts in depth in our video on section 1.6, so this will be a very brief overview.
The two strands of DNA are held together by hydrogen bonding with nitrogenous compounds. RNA, on the other hand, is a single strand. This distinction is due to a number of functional reasons. The double-stranded structure of DNA, which may aid in its preservation, allows it to store genetic data for an extended period. The double-stranded structure also aids in the detection of damage since the base pairing between strands is disturbed.
In most species, RNA, in contrast, has a short lifespan and is often not the primary provider of heritable data. RNA can efficiently transcribe from DNA and leaves the nucleus with a single strand prepared to be converted into a protein. Certain viruses violate this rule. storing their genetic information in RNA as a major storage molecule.
At the molecular level, there are just a few structural distinctions between DNA and RNA. In the sugar-phosphate backbone, ribonucleic acid employs ribose, while deoxyribonucleic acid uses deoxyribose. The only functional and structural distinction between the two sugar molecules is the existence of a solitary oxygen atom. It has a significant effect.
Due to the presence of oxygen in ribose, the sugar component is substantially larger and more electronegative. which causes RNA to be more highly reactive than DNA, one of the factors contributing to its reduced lifetime. This cannot simply affect the distinctive form of the sugar-phosphate foundation and also renders it extremely challenging for RNA to create a double-helix structure until exact base pairs are generated (including tRNA molecules).
The nucleotide monomers needed to create a strand are the sole additional difference between DNA and RNA. Although they are almost identical, DNA utilises thymine while RNA employs uracil. This distinction aids the organism in distinguishing between DNA and RNA because uracil is somewhat less energy-intensive to manufacture, allowing the cell to save energy.
Let’s take a deeper look at the structure of DNA and RNA before moving on to how these molecules store genetic information. The sequence of nucleotides inside each molecule stores information in DNA and RNA. During RNA transcription, this sequence is replicated, culminating in a molecule of RNA with similar data. A ribosome converts this RNA molecule into a protein once it exits the nucleus. Within the ribosome, codons, which are made up of three nucleotides, are paired with anticodons on tRNA molecules.
Every tRNA molecule contains a unique anticodon and transports a particular amino acid. The fact that all creatures employ the same fundamental code to inform ribosomes how to create proteins is one of the reasons we believe all life on Earth had a common ancestor. This codon is “read” in series through the initial nucleotide to the next, subsequently to the third, to determine which amino acid is incorporated into the growing polypeptide chain. In all species, for example, a codon of GAA instructs the ribosome to add glutamic acid to the developing polypeptide chain.
DNA and RNA Structure
DNA and RNA, like proteins, have a primary, secondary, tertiary, and even quaternary structure. The main form is made up of a particular nucleotide sequence. Base-pairing, whereby a strand bends back on its own, it forms a helix among two strands or a stem-loop, is the most common way a secondary structure is produced. Its tertiary forms of DNA and RNA are formed by base matching and specific connections between different nucleotides as well as the sugar-phosphate backbone. As DNA or RNA molecules combine with additional nucleic acids or proteins to create larger compounds, a quaternary structure is created.
Let’s take a deeper look at the process of base-pairing, which is responsible for many of these structures. When you remember the role of hydrogen bonds from Unit 1, you will recall that hydrogen bonds are formed between the positive and negative charges of two polar molecules. Certain pyrimidines establish hydrogen bonds with a certain purine (due to their double-ring structure) or with a single-ring structure.
Due to the fact that every nucleotide base carries a different charge, it can only attach to the nucleotide base that is complementary to it. Consequently, G constantly connects to C and A usually attaches to T (in the presence of RNA, Uracil). Similar charges repel each other if the bases approach each other in any other way. As a result, for the regular double-helix shape of DNA and the many types of folded RNA, proper base pairing is required (such as tRNA and rRNA).
This base pairing is a crucial process that regulates the structure of DNA and RNA. Base pairing is the primary mode of DNA replication in a DNA molecule. As the molecule expands, any fresh nucleotide base creates a link between the nucleotide in front of DNA polymerase, connecting it to the sugar-phosphate backbone. This procedure splits a single helice into two double helices.
Though RNA molecules are made in the same way, the ribose sugar backbone as well as uracil nucleotides create hydrogen bonds between base pairs that are less robust. The molecule of RNA may subsequently leave the nucleus as a single strand. Some sequences, including those that eventually developed into tRNA molecules, contain a specific nucleotide motif that, when folded into the appropriate form, may base-pair with each other. RNAs with tertiary structures like this have a variety of functions inside cells.
Let’s have a look at the higher-level structure of DNA and RNA – chromosomes. Many genes make up chromosomes. Furthermore, every gene contains a substantial quantity of nucleotide base pairs (anything from a few thousand to several million, based upon the gene). Exons (coding sections that convey genetic information) and introns make up each gene (regions without codons that divide exons). The function of introns is unknown, but they contain many more nucleotides than exons in many genes. If these genes are translated into RNA and then treated into mRNA, the introns will be removed.
Every one of these genes is connected by a continuous chain. This is an unusually lengthy strand of DNA, considering the human genome contains approximately 20,000 genes and 3.2 billion base pairs. These genes would be almost 6 feet long if they were stretched end-to-end.
On the other hand, nucleosomes are made up of DNA wrapped around protein complexes called histones. This lowers the length of each chromosome dramatically, similar to wrapping a ball of yarn.
Furthermore, these nucleosomes create a complicated arrangement which further compacts DNA to form a chromatin fibre. Individual chromosomes may be seen during mitosis and meiosis because it is possible to pack chromatin into an extremely dense structure. The chromosomes relax into a loose shape during the interphase of the cell cycle, permitting DNA to be converted into RNA and then replicated in readiness for the succeeding cell division.
Linear chromosomes are chromosomes that employ histones to compact themselves and are found predominantly in eukaryotic species. Most prokaryotes, on the other hand, have a considerably smaller circular chromosome. A straight chromosome is several folds larger than a circular chromosome, and it frequently includes several more genes.
The X-shape is also often used to represent chromosomes, but this is not necessarily true. To begin with, the X-shape denotes a duplicated chromosome that consists of two identical sister chromosomes joined at the centromere. In addition, the centromere of linear chromosomes is not always located in the middle; it may be located at the apex or nearer to one end than the other.
Let’s conclude by examining the genomes of prokaryotes. While the majority of prokaryotes spawn asexually and contain a single circular chromosome, certain prokaryotes produce offspring (that limits genetic diversity). They may also exchange plasmids, which are tiny circular DNA units.
A plasmid is a small DNA fragment that may carry a single gene, making it substantially smaller than a prokaryote’s primary circular chromosome. These plasmids might include genes that aid bacterial survival. Since numerous bacterial cells profit by existing in communities, which may create biofilms as well as other defensive barriers, it is advantageous for them to do so. bacterial cells that can assist other bacteria to survive and acquire a measure of fitness. Through the parasexual reproduction technique of bacterial conjugation, bacterial cells may share the genes contained in plasmids.
The plasmid is reproduced at this step. Then it’s transmitted between the two bacterial cells via a channel. Numerous essential genes, like those that confer resistance to antibiotics or an enzyme necessary to breakdown a specific source of food, are missing or are known to be carried by plasmids. Through passing on certain genes to a different bacterial cell, the bacterium may considerably speed up the formation of a colony while also defending itself.
Plasmids, on the other hand, may be used to insert specific genes into bacterial cells. Scientists may insert practically any gene into a bacterial colony by altering a plasmid to accommodate fragments of foreign DNA (called recombinant DNA since it comes from another species). The plasmids are made, the bacteria are heated and chilled to assist the plasmid slide past the cell membrane, and certain bacteria acquire the plasmid and effectively display the genes it carries. They may pass this new gene on to their offspring when they reproduce.
In one basic example, scientists were able to insert a gene that generates fluorescence in jellyfish into regular E. coli bacteria. The whole colony lights up green when this plasmid is effectively introduced into it. Human insulin (an important protein hormone utilised in the treatment of diabetes) is produced by bacteria modified with recombinant DNA, thus this may be utilised for more than merely making bacteria shine.