What is DNA?
As we established in the previous article, DNA contains information about almost every single process in a cell. We also discussed that it controls the cell’s metabolism by giving instructions about the production of protein. For that, it uses a code that can be translated and later formulated to a task-specific molecule. Let’s discuss how exactly the CEO of the cell functions.
The structure of the DNA
The full name of DNA is deoxyribonucleic acid, which may sound strange at first, but as we will discover later, it does make sense. First of all, in eukaryotic cells (so most non-bacterial cells), DNA forms a double helix, composing of two strands. Each strand is made of nucleotides, connected with each other and nucleotides on the opposite sides. The chain of each strand is made of two major components- an inorganic component of phosphoric acid connected to deoxyribose- a kind of simple sugar. Then, deoxyribose connects one of four nitrogenous bases.
Nitrogenous bases are simple molecules that together form a genetic code. In DNA, there are four different kinds: Cytosine (C), Guanine (G), Adenine (A), and Thymine (T). Their detailed structure isn’t important; we need to remember that each base can only make a pair with one other kind because of their structure. For example, cytosine is always connected to Guanine, and Adenine always pairs with Thymine. The base determines the kind of nucleotide – like, a nucleotide with adenine is called an adenine nucleotide. For simplicity, each nucleotide has its own letter (the capital first letter of its name). The bonds between bases hold the double helix together. The order of nucleotides on a strand is called a sequence.
The genetic code
Because nucleotides always form the same pairs (A with T, G with C), knowing the sequence on one strand enables us to determine the opposite sequence (in biology, we will call it compatible) strand. Nucleotides make up a kind of four-base code, but what’s its meaning? To determine that, let’s start with establishing that proteins (molecules essential for metabolic processes and capable of many tasks) are made of 20 kinds of amino acids. Of course, their structure is quite complex, but we can break its base down to a sequence of amino acids.
Exactly that happens in the genetic code. Every three nucleotides form a codon, which encrypts one amino acid. Molecules in the cell can crack that code and form a basic amino acid strand needed to produce a certain protein. So the genetic codon chart makes it easy for us to crack a code (the three-letter phrases mean different amino acids, Stop codons are a signal for the cell to stop the process of cracking the code). Don’t worry; a chart is a tool, and you don’t need to remember what each codon means. You can simply read it from the chart when necessary.
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Characteristics of the Genetic Code
There are a few important principles that will help you understand the nature of the genetic code. First of all, you may wonder what happened to Thymine (T) and what the U is on the chart. The chart is made for RNA (which we will discuss later), and in that molecule, Thymine is replaced by Uracil (U). As we discussed, a codon (a sequence of 3 nucleotides) defines one amino acid. A specific codon always defines the same amino acid. Therefore the genetic code is universal for every kind of living organism. However, one amino acid can be encrypted by several codons (for example, glutamine is both CAA and CAG).
Synthesis of Protein- Transcription
At this point, you’re probably wondering why DNA switched to RNA in the genetic code. That happened because DNA is always in the cell’s nucleus, but the instructions for making protein are needed in various places in the cell at once. Therefore, to extract a piece of code needed for a certain process, it can be transcripted into a single strand of RNA- ribonucleic acid. It is quite similar to the first molecule, but instead of deoxyribose, it has ribose as a sugar part, and Thymine (T) is replaced by Uracil (U). It is also mostly single-stranded.
When instructions for creating a certain process are needed, cell machinery assembles. The DNA strands part a bit, and the synthesis of RNA starts. An RNA polymerase molecule creates an RNA strand compatible with one of the strands of DNA and identical to the other (with the replacement of T by U). The synthesis always goes in one direction because of signaling parts. The process continues until a specific molecule called CPSF cuts the strand. Afterward, the new RNA strand is properly signaled and formed (for example, additional non-coding instructions are removed). Then it can travel out of the nucleus. The synthesis of the protein continues in other parts of the cell.
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In this article, we learned that deoxyribonucleic acid is a molecule made of two strands in the shape of a double helix. Each strand consists of nucleotides made of an acidic component, a sugar, and one of four bases (A, T, G, C). The strands are connected by bonds between bases, A always with T and G always with C. Every three nucleotides make up a codon, which encrypts one amino acid. Proteins are made of amino acids, and they are necessary for most metabolic processes of the cell. Therefore, the chart of the genetic code enables reading the amino acid behind every codon of RNA.
RNA is slightly different from DNA-it is made of a different sugar (ribose), and T is replaced with U. RNA t is made during the process of transcription. First, DNA strands part a bit, then a molecule of RNA compatible to one strand and identical to the other is synthesized. Afterward, it leaves the nucleus. The next article will discuss how protein is further synthesized in the cell and how DNA multiplicates.
Senior Author at SOU. I am a science student, utterly fascinated by the world from atoms to galaxies. I learn something new every day and aspire to share my passion and knowledge, whether it’s related to our Earth or space conquest and the future of humanity. My hobbies include science fiction, swimming, reading, and makeup.