How DNA Works: the library of life.
Many of us have heard of the following three magic letters: DNA. Often times it is used in the context of genetics, hereditary diseases or even cancer. Other times, it is used colloquially to refer to something that is within us or makes a part of our identity. Whether you have used this abbreviation for the former or the latter purpose, you were partly correct. But what is actually DNA? why is it so important for living and nonliving (i.e viruses) organisms? and how does DNA work? The following section will give you a brief explanation of DNA and its importance in everyday cellular processes within your body. Deoxyribonucleic acid: structure, function, and relation to your health. Structure of DNA: The abbreviation DNA refers to deoxyribonucleic acid which hints at the structure of DNA. A strand of DNA is composed of several building blocks known as nucleotides. A nucleotide is itself composed of a sugar (in the case of DNA: deoxyribose), a phosphate group (-PO) and a nitrogenous base (in the case of DNA: either adenine, thymine, cytosine or guanine). Adenine and Guanine are commonly labeled as A/G and referred to as purines. On the other hand, Cytosine and Thymine are commonly labeled as C/T and referred to as pyrimidines. Finally, A and T bond to each other within a DNA strand using two double bonds, while C and G use three. This bonding will become more clear to you in the following paragraphs. It is important to point out that, unlike the nucleotides in RNA (ribonucleic acid), the nucleotides in DNA are missing one hydroxyl group (-OH) at carbon number two (C2). Hence, the term “deoxy” is used to describe the fact that this group is missing, which has a tremendous effect on how deoxyribose interacts with other molecules. The term nucleic refers to the fact that DNA is found in the nucleus of the cell and the term acid simply refers to its acidic properties. The figure below gives a better sense of what one nucleotide in DNA and RNA looks like: RNA will also become more important as we discuss how DNA is used to code for the making of proteins and other important cellular structures. As mentioned before, DNA is not one single nucleotide but rather a collection of nucleotides joined together to form a strand. In fact, these nucleotides are bound together by their phosphate groups through a type of chemical bond known as a phosphodiester bond. However, DNA has composed of two strands of nucleotides joined together and twisted into a helical structure. The process described above would only yield one strand. For this reason, strands of DNA bind to each other through the nitrogenous bases on their nucleotides using chemical bonds known as hydrogen bonds. Once again, the picture below will provide a better of the idea of what DNA strands look like at the molecular level: The outer portion of DNA composed of deoxyribose sugars bound to each other through phosphodiester bonds is referred to as the sugar-phosphate backbone. It is also crucial to point out that the base pairs section indicates that the nitrogenous bases on the inside portion of DNA are binding together through those hydrogen bonds we discussed previously. Also, only A pairs with T and C pairs with G, no exceptions. If miss-pairing occurs, this is considered a mutation and the cell uses specific mechanisms to repair this issue. If not repaired early, the cell will detect the issue at a later time during cell division and will direct a self-destruction/controlled suicide process known as apoptosis. If none of the above occurs, this may lead to serious consequences including uncontrolled cell division which is also known more commonly as cancer. As you may notice the DNA strands run in opposite directions. This is because they are complementary and are actually read by cellular mechanisms as such. Finally, because DNA is composed of billions of nucleotides, a raw DNA strand may extend for miles at a time. That does not seem to be ideal if DNA must be stored within the nucleus of one single microscopic cell, right? Well, no need to panic, nature’s got it handled. DNA is further coiled through a process known as supercoiling which uses proteins called histones to hold the DNA tightly packed together. This not only allows DNA to fit within the nucleus without a problem but also protects it from unwanted physical or chemical damage. Keep in mind, everything we have just discussed applies to one out of trillions of cells within your body and should give you an idea of the complexity and beauty of the processes that occur every second without your awareness. The function of DNA and its health implications: DNA is, in simple terms, the memory card of the cell. It stores all information regarding every single cellular process and without its permission, no process proceeds. DNA dictates everything from the color of your eyes to the curliness of your hair, your height and even your propensity to suffer from heart disease. Nonetheless, not every bit of information is set in stone. In fact, DNA is extremely dynamic and interacts with the environment on a frequent basis. The branch of genetics that deals with how the environment influences DNA is known as epigenetics. Basically, it suggests that environmental events whether internal (inside of your body) or external (outside of your body) influence the expression of those innate traits coded in your DNA. This dynamic relationship between the environment and DNA is at play from the moment of procreation all the way to death. But how does DNA actually work? The central dogma of biology is a theory that suggests that DNA within the nucleus is converted into RNA through a process known as transcription. This process occurs within the nucleus of the cell and is beyond the scope of this excerpt. RNA is then converted into functional proteins through a process known as translation, which is also way beyond what we will be discussing today. Despite the complexity of this process, the central dogma of biology is all you need to understand the basics of how DNA works and why it is so important. During transcription, the two DNA strands are split apart and one strand of DNA (known as the leading strand) is used as a template to create a complementary strand of RNA. This single complementary strand of RNA is prepped to then leave the nucleus and migrate towards ribosomes (protein factory of the cell) to be converted into proteins. The RNA strand is read by the ribosome in sets of three nitrogenous bases known as codons (i.e. AUC; UAC; AGU). Each codon codes for one amino acid and at the end of the translation process, multiple amino acids are put together through peptide bonds to form a peptide. Multiple peptides can join together to form a polypeptide structure and different polypeptides can join together to form a functional protein. The reason why DNA is so important is due to its crucial role in dictating what types of proteins are produced when they are produced and in what amount. As we previously discussed, DNA also interacts with the environment and this influences the way these proteins are` produced as well. For example, changes in temperature may inhibit the production of certain proteins while promoting the production of others. Cytokines produced by one cell may influence the DNA of other cells nearby to express certain genes (sections of DNA). This is surprising how cells go from being undifferentiated (stem cells) to differentiated (having a specific job; i.e. cardiac cells, lung cells, neurons, etc.) during gestation. Another great example is the ability for steroids (both natural and synthetic) to actually enter the cell, travel to the nucleus and interact with the actual DNA strands to suppress or promote the expression of certain genes. To reference a more common occurrence, a piece of bread introduces glucose into your body, which in turns interacts with certain receptors on your cells and leads to an increased genetic expression of an important protein known as insulin. Insulin helps glucose enter the cells to be used in the cellular respiration process which produces ATP, the main energy currency of living organisms. If defective DNA leads to the inability to produce insulin, this could lead to the development of Type I Diabetes Mellitus (T1DM). On the other hand, too much insulin may lead to insulin tolerant cells and the genes within your cell will not react to glucose as expected. This can lead to Type II Diabetes Mellitus (T2DM). Even exercising can influence the way your DNA is expressed. Regularly exercising can suppress the expression of pro-inflammatory proteins that over time could lead to heart disease as well as other tissue damage. This is thought to be one of the main mechanisms by which exercise prevents atherosclerosis (hardening of the blood vessels) and subsequently reduces the risk of heart disease. On the other hand, certain genes within the genome (total DNA within a cell) can be irreversibly defective or code for certain products that lead to devastating illnesses such as Huntington’s disease. Although the complexity of DNA’s function is daunting for those who study it, all we must remember is that DNA turns into RNA within the nucleus, which then is converted into proteins. Proteins are the most functional macromolecules in living organisms and are the workforce of every cellular process within your body. If DNA is defective, the proteins produced (or not produced for that matter) will be defective as well. This is how DNA works and maintaining its integrity is crucial to your health.
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