Parts of a Human Cell

The cell is the main building unit for all living organisms, from bacteria to humans. All complex organs and systems are made up from billions of individual cells working together, but also individually. In the human body, cells come in different shapes depending on the tissue they belong to. However, all cells have four parts in common: the plasma membrane, cytoplasm, ribosomes, and DNA.

The plasma membrane (also called the cell membrane) is a thin coat of lipids that surrounds the cell. It forms the physical boundary between the cell and its environment and therefore is very important for all interactions between the cell and the outside. Many proteins assist with these interactions by docking on the plasma membrane and binding to outside molecules. Furthermore, cells can send signaling molecules through their plasma membrane to communicate messages to other parts of the tissue or body.

Next, the cytoplasm refers to all of the cellular material inside the plasma membrane, other than the nucleus. Cytoplasm is gel-like in its texture and contains mostly water, salt and other molecules, as well as all organelles that assist with cellular function, such as ribosomes.

Ribosomes are structures in the cytoplasm where proteins are made. They consist of RNA molecules and proteins, and their main role is to translate genetic sequences to proteins. Each cell may have a large number of ribosomes which either exist as free particles in the cytoplasm, or are organized in larger structures.

Finally, the DNA is a nucleic acid that contains all the necessary genetic information for the cell to function. The DNA is usually tightly packed in the nucleus of the cell, where it is protected from the outside environment. If we were to measure the length of a DNA molecule, each cell alone contains about 6 feet of DNA. The human DNA contains about 25,000 gene sequences, as well as many sequences that do not encode proteins and are either evolutionary remnants or regulatory components.

What is RNA and What are its Types?

RNA is the second most well-known ribonucleic acid after DNA. When it comes to structure, it is highly similar to DNA, but has some important differences. For example, RNA has one distinct nucleotide called uracil (or U) instead of thymin (or T). Furthermore, RNA commonly exists in single-strand format, while DNA is almost always double-stranded.

Human DNA GraphicAs we discussed in the previous post, all human cells contain genetic information for protein expression in the form of DNA. Our DNA is tightly packed in the cell nucleus to ensure protection from outside factors. However, almost all processes that are required for protein generation are taking place outside the nucleus. For this reason, there are mechanisms in place for creating copies of genetic information that can traverse from the nucleus to other locations. This is where RNA comes into play, since it can be synthesized in a complementary manner to DNA and contain the same type of genetic information. Furthermore, RNA can interact with proteins and create necessary complexes for various cell processes.

The main types of RNA are the following:

  • mRNA: Also known as messenger RNA, mRNA is a single-stranded RNA molecule that contains information for generating a protein sequence. It is created in the nucleus through the process of transcription, during which the two DNA strands are temporarily separated to copy the information of one gene onto the new mRNA molecule. Then, mRNA can exit the nucleus and go to the ribosomes, where proteins are generated using its information.

  • tRNA: tRNA, or transfer RNA, is a small RNA molecule that is essential for protein synthesis. Proteins consist of amino acids, and each amino acid has a 3-letter code (or codon) in the DNA or mRNA that corresponds to it. This way a DNA or RNA sequence can be translated to a protein sequence. Codons are examined in series, and tRNA is responsible for bringing the correct amino acid depending on each codon. This is done by utilizing the complementary structure of one tRNA part to the codon triplet.

  • rRNA: Ribosome RNA (or rRNA) is an RNA molecule that is a fundamental component of ribosomes. As mentioned before, ribosomes are the machinery for protein synthesis in the cell. There are two rRNA subunits in each ribosome, a large one and a small one. Along with proteins and enzymes, they facilitate the process of translating an mRNA sequence into protein.

Honorable Mentions

There are some other RNA types in human cells that are not commonly known but are often used in molecular biology research. For example, micro RNA or miRNA is a small single-strand RNA molecule that can silence other types of RNA (such as mRNA) by binding to them using the rules of complementarity. Small interfering RNA or siRNA is a double-stranded RNA molecule that has a similar goal, but acts by triggering an mRNA degradation response called RNA interference. Overall, these molecules can affect protein expression levels by effectively silencing certain genes without affecting the DNA itself.

More sources on RNA:

— Vicky Kanta

How is DNA Organized in the Human Cell?

Our DNA contains the genetic code for creating every single protein in our bodies. All cells contain an almost identical copy of our DNA in their nuclei. However, since DNA molecules can be over 6 feet long if stretched out, they are packed and organized in a very specific manner in order to fit in the very small space of the nucleus. Here are the different levels of DNA organization:

  • Human DNA GraphicDouble helix DNA: This is the unpacked form of our DNA, in which two complementary strands are chemically linked together and are spiraling around counterclockwise, forming a “ladder-like” double-stranded molecule. Each “step” of the ladder consists of a pair of nucleotides, which are the basis of DNA sequences and are commonly known by their initials (A and T, G and C). In every cell, genetic information exists in two copies, with one coming from each of the two parents.

  • Nucleosome: This is the first step of “packaging” inside the nucleus. In a nucleosome, a segment of double helix DNA is wrapped around a set of proteins called histones. Usually, the length of DNA in each nucleosome is fixed to about 150 nucleotide pairs. When many nucleosomes are seen together in series, they have the appearance of “beads on a string”, which is a common term used for this level of organization.

  • Chromatin: In this next level, nucleosomes are tightly grouped together forming a fiber that is about 30 nm in diameter. There are two types of chromatin, namely euchromatin and heterochromatin, that differ in how compact they are and whether they allow unpacking for gene expression. These two types can be located on a microscope in different parts of the nucleus.

  • Chromosome: This is the final form of DNA organization inside the nucleus. In the chromosome form, chromatin is even more tightly packed. In the end, the entirety of the double helix DNA is packaged in 23 pairs of chromosomes, that fit in the 6μm wide cell nucleus.

It is worth mentioning here that DNA does not only exist in the cell nucleus. There is some amount of DNA in other organelles inside our cells called the mitochondria. Mitochondrial DNA is also organized in small chromosomes and contains genetic information for mitochondrial function. Unlike nuclear DNA, which is inherited from both parents, mitochondrial DNA is only inherited from the mother via the egg.

More DNA organization sources:

— Vicky Kanta

What is Herd Immunity?

Infectious diseases caused by microorganisms such as bacteria and viruses can spread widely within a community. In particular, densely populated areas like big cities allow rapid disease transmission, because people are frequently in close proximity. When a certain proportion of the community acquires immunity for a disease, then further spread is limited, because immune individuals usually cannot get re-infected or infect others. This is when we reach herd immunity for this particular disease.

There are two main ways to achieve herd immunity. The first way is for the pathogen to infect a large number of people, who then recover and become immune. The second way is through vaccination, which provides future immunity against the disease by stimulating the natural pathways that lead to it. Nowadays, many infectious diseases are either eliminated or controlled through vaccination, such as measles, chickenpox, and the seasonal flu. In fact, a vaccine is the safest way towards herd immunity, leading to less sick people and therefore a lower overall number of disease-related deaths and complications.

Depending on the size of the community, as well as the infectiousness of the disease, the percentage of people that must become immune before herd immunity is achieved can vary. For example, measles requires 93-95% vaccination rates, whereas polio requires around 80-86%. This is why timely vaccination of children at the recommended schedule is really important; skipping or delaying a dose does not only increase chances of disease susceptibility for the child, but also contributes to a decrease in herd immunity and therefore allows previously controlled diseases to reappear like the recent resurgence of the measles.

Herd immunity is highly important for our society, because it is an important tool to curtail or even eliminate infectious diseases. Furthermore, it protects some of the most vulnerable members in our communities from getting sick, such as the elderly, those who can’t have a vaccine for medical reasons, or the immunocompromised, as well as pregnant women and infants. For this reason, there is extensive research and funding dedicated towards the development of safe and effective vaccines.

More resources on herd immunity and vaccination:

What is Diabetic Nephropathy?

Diabetic nephropathy, or diabetic kidney disease, is a common condition associated with Type I or Type II diabetes. It is estimated that about 20-40% of diabetic patients develop some form of kidney disease. If undetected, diabetic nephropathy can be a debilitating and dangerous disease, which is why people with diagnosed diabetes have to be frequently monitored for kidney function.

The main role of the kidneys is to filter the blood and remove waste and harmful products, while maintaining proteins, water and other useful substances. More specifically, kidneys contain small blood vessels called glomeruli that are responsible for filtering a large volume of blood every day. There are two kidneys in the human body, working together in a complementary manner. However, if one kidney stops working, the glomeruli in the other kidney start filtering more blood to compensate for the loss of function. This is why people can live without issues if they have at least one functioning kidney.

In unregulated diabetes, blood sugar levels are elevated, which slowly damages the glomeruli. Furthermore, many diabetics also have high blood pressure, which also stresses kidney function over time. When glomeruli stop functioning properly, protein starts leaking in the urine and eventually harmful waste may stay in the bloodstream, causing severe problems.

Thankfully, diabetic nephropathy can be prevented in most cases by maintaining regulated blood sugar levels. However, even if a patient reaches the early stages of nephropathy, blood sugar regulation can slow down the progression of kidney disease. If left untreated, both kidneys may fail, in which case interventional treatments such as dialysis or even a kidney transplant may be necessary. Transplants can come from compatible organ donors that have fully functional kidneys and a compatible blood type to the recipient. Frequently, transplants come from living donors, which often are family members wanting to help their loved ones.

More resources on diabetic nephropathy

  1. National Institute of Diabetes and Digestive and Kidney Diseases.

  2. National Kidney Foundation.

  3. American Heart Association.–diabetes

— Vicky Kanta

The Promise of CRISPR-Cas

Our bodies are made up of various cell groups that can do different things depending on the combinations of proteins they contain. Proteins can do wonderfully complex things, from acting as oxygen sensors to molecular motors. All of the information for making these proteins is stored in our DNA. The DNA consists of four molecules (commonly called by the letters A,T,G,C) put together in long strings of specific sequences. Each part of our DNA containing the information for creating a single protein is called a gene, thus our DNA holds the “genetic code” for all the proteins our body can make. The “letters” in our genes are converted to another type of molecular-letter-string called mRNA, which is then converted into protein.

Courtesy of Genetic Alliance UK

We get our DNA from our parents, and have a separate paternal and maternal copy of the entire human genetic code. However, sometimes you can inherit a gene that leads to disease — your DNA creates a slightly altered or dysfunctional protein that can affect certain processes going on in your body. For example, people with sickle cell disease have a gene for hemoglobin that is one “letter” different from the “normal” code. Hemoglobin is the protein inside your red blood cells that lets your body transport oxygen. When the code has been mutated through this one-letter change, it gives your red blood cells a sickle-like shape that can lead to many harmful effects.

Fixing a single base-pair mutation in our DNA could drastically change the shape of our blood cells, preventing sickle cell disease. Courtesy of Megan Hoban, UCLA.

Since many diseases are caused by very small alterations of our genetic code, it would be great if there was a way to change those codes carefully and correctly. While many methods have been designed to change the sequence of DNA, none of them are as easy to apply or as broadly effective as CRISPR-Cas, or simply CRISPR systems. CRISPR systems are groups of  proteins that are able to cut nucleic acids, like DNA, at specific points, when they have “guide RNAs” attached to them. To guide a CRISPR system to a pre-determined point in our DNA, we take advantage of the base pairing code. DNA is double stranded, and the letters A and G interact with the letters T and C on the other strand, respectively. Let’s say our target gene has an A at a specific position. Based on this complementary matching of letters, if we make our guide RNA have a T at the equivalent spot, it can interact with that A, too. Now if all the letters in our guide RNA are complementary to a certain gene sequence, the Cas protein will be chemically attracted toward that spot in your DNA. When it gets there, it will bind to the DNA, similarly to a Velcro strap, and make a precise cut. This way, we can make targeted small changes to specific genes causing known diseases, without causing any wide effects to the rest of the genetic code.

CRISPR interacting with DNA. Courtesy of Mirus Bio.

The ability to make site-specific cuts is an incredibly powerful tool in genetic engineering. Is there a protein you don’t want a cell to make? If you know the sequence of that protein, and therefore the sequence of its gene, you can cut it out. You can even add new genes by making a cut and putting something new in the middle — similar to ripping a piece of cloth apart and stitching a new fabric in the middle. With tools for genetic editing getting better every day, this is going to be a reality soon.

Even though CRISPR is a very powerful tool, there is plenty of debate in the scientific community about the rules that should govern its use in medicine. Since this method can be used to potentially eliminate diseases but also add features to organisms, important ethical issues have come up regarding use in animals and humans. The topic reached the news worldwide when a Chinese scientist, He Jiankui, announced that he used CRISPR to genetically modify two babies that were born through in vitro fertilization. The researcher had not received any official approval for these experiments and he was eventually fired from his position and is now facing prison time.

CRISPR ethics is still a fairly new field, but many government agencies like the National Institutes of Health have started working with bioethicists to study the best approach for regulating genetic editing. Since CRISPR is already widely used in research and even human clinical trials, scientists should always work in coordination with funding and governing agencies to continue exploring its benefits in an ethical and safe manner.  

— Devin Bog & Vicky Kanta

What are the Types of Kidney Failure?

Normal kidney function is fundamental for our health, since kidneys are tasked with removing waste from our bloodstream while keeping important nutrients. When kidneys stop performing this process, we are faced with a serious condition called kidney failure. Kidney failure is very prevalent worldwide, with approximately 5-10 million people dying every year from it. Since the human body has two kidneys, patients can survive if only one of the two is functioning. However, many times the conditions that damage our kidneys end up affecting both, which is why medical intervention is important.

There are two main categories of kidney failure: acute and chronic. The main difference has to do with the disease onset and progression rate. However, there may also be different underlying factors in their causes.

Acute kidney failure
In acute kidney failure (also called acute kidney injury), kidneys stop functioning over a very short period of time, usually a few days. There are many possible causes, but some of the most common ones have to do with decreased blood flow to the kidneys, direct injury to the kidneys or blockage of the urine pathway. Many times, acute kidney failure is a result of other conditions that bring patients to the hospital, which is why many times it occurs while patients are already hospitalized. Some examples are heart attacks, liver failure, or specific infections such as hemolytic uremic syndrome. Acute kidney failure is usually treated in the hospital with hemodialysis and dietary changes, until the kidneys restore their proper function.

Chronic kidney failure
In chronic kidney failure (or chronic kidney disease), the progression is slow and kidney function worsens over time. Some of the most prevalent causes are diabetes, hypertension and some genetic conditions that are associated with a family history of kidney failure. There are different stages of chronic kidney failure, measured by the ability of the kidneys to filter the blood. Based on the stage in which it is detected, chronic kidney failure can be treated with dietary changes and controlling the underlying condition. However, in late stages, special medication or hemodialysis may be required. If nothing else works, doctors may recommend a kidney transplant, which if successful may restore healthy kidney function.

Overall, kidney failure is treatable if detected early. Markers of deteriorating kidney function can be found in a simple blood test. This is why regular check-ups are important, especially if someone belongs to a group with a higher risk for developing kidney failure, such as diabetics, people with hypertension and older individuals.

More sources on kidney failure

  1. American Kidney Fund.

  2. National Institute of Diabetes and Digestive and Kidney Diseases.

  3. National Kidney Foundation.

— Vicky Kanta

What is a Pandemic?

Whenever a new disease emerges, the term “pandemic” is very commonly heard on the news and social media. However, its definition is not always clear. Briefly, a pandemic occurs when a new disease spreads widely across multiple areas of the world. There are some key characteristics that separate a pandemic from other epidemiological terms, such as endemics and epidemics; experts declare a pandemic when a) the infectious agent (e.g. a virus) causing the disease is entirely new or sufficiently different from existing ones, b) the number of cases in each area exceeds what is expected based on the population (i.e., an outbreak), and c) there are increased occurrences in various parts of the world simultaneously.

virus 3d illustrationThere have been various pandemics throughout history, such as the plague, the Spanish flu, etc. Although pandemics can be caused by any highly infectious agent, most recent instances are associated with novel strains of the influenza virus, which causes the flu. The influenza virus mutates constantly and is highly infectious, which is why many pandemics are caused by it. For example, the 2009 H1N1 pandemic was caused by a new strain of the Influenza A virus. This strain had many differences when compared to the seasonal flu strains, which were sufficient to render the existing flu vaccine mostly ineffective, and required the development of a specialized vaccine.

Nowadays, most countries have emergency preparedness plans for the possibility of future pandemics. However, there are also some simple things we can all do to limit the spread of disease and infection, such as washing our hands frequently, avoiding contact with sick individuals, and covering our cough and sneeze using the inside of our elbow.

More pandemic resources

  1. World Health Organization.

  2. Centers for Disease Control and Prevention.

  3. – Department of Homeland Security

— Vicky Kanta

How Does a Defibrillator Work?

Defibrillators are medical devices used to restore the heart’s regular rhythm. They work by delivering an electrical current or shock close to the heart to help the heart regain its normal beating pattern. There are different types of defibrillators that are either used in emergency settings or as a treatment to a chronic heart condition.

A common type of defibrillator is the Automated External Defibrillator or AED. This is a portable battery-powered device commonly found in public areas to be used in case of a medical emergency. In fact, many states have laws in place about the existence of AEDs in places such as schools, gyms, government buildings, etc. When someone shows signs of cardiac arrest, use of a defibrillator can help save the person’s life until medical personnel arrive. It has been estimated that quick use of an AED can increase chances of survival after cardiac arrest by 5-40%. AEDs come with detailed instructions so that even untrained individuals can assist with defibrillation. An AED will automatically analyze the person’s heartbeat pattern through sticky electrode pads applied on the chest and deliver a shock if deemed helpful.

Another type of defibrillator is the Implantable Cardioverter Defibrillator or ICD. ICDs are implanted under a person’s skin and track the heart rate. If an abnormality is detected, the ICD will deliver a shock at the right time, similarly to an AED. ICDs are used in patients with certain heart conditions who are at risk for dangerous heart rhythm abnormalities. Examples of such conditions are certain previous heart attacks, cardiomyopathies, and others.

More sources on defibrillators

  1. National Heart, Lung and Blood Institute.

  2. U.S. National Library of Medicine, MedlinePlus.

  3. American Heart Association.–treatment-of-arrhythmia/living-with-your-implantable-cardioverter-defibrillator-icd

  4. American Red Cross.

— Vicky Kanta

What is Atrial Fibrillation?

Atrial fibrillation (also called AFib) is a common condition in which the heart beats irregularly. More specifically, AFib occurs when the heart’s upper chambers (the atria) do not coordinate with the lower chambers (the ventricles) during a heartbeat. This results in an insufficient amount of blood being pumped from the atria to the ventricles, which subsequently leads to the rest of the body not receiving enough blood.

Patients with AFib report feeling heart palpitations or an irregular heartbeat. They may also feel faintness, fatigue or breathlessness, which may result from the inadequate blood flow to the body. AFib can be detected during a regular physical examination, through an electrocardiogram or a Holter monitor.

There are different types of AFib, depending on the frequency of the symptoms and their response to treatment. In paroxysmal AFib, symptoms go away within a week with or without treatment. In persistent AFib, symptoms are continuously present for at least a week and require treatment to recede. Finally, in permanent AFib, the rhythm cannot be restored to normal and long-term use of medication or other treatment is required.

AFib is usually treated by a combination of diet and lifestyle changes, along with medications such as beta blockers, calcium channel blockers or anticoagulants. For more severe cases, more invasive interventions may be required to help restore a normal heart rhythm. Some examples of such procedures are catheter ablation, which is minimally invasive, or surgical ablation, which is a full surgical procedure.

More sources on AFib

  1. National Heart, Lung and Blood Institute.

  2. Centers for Disease Control and Prevention.

  3. American Heart Association.

— Vicky Kanta