The Story of Life is one that scientists have been writing since the dawn of civilization. The great thing about this story is that it's an ongoing work of non-fiction, continually updated as technology advances. It is a narrative full of cooperation and deceit, sacrifice and selfishness, birth and death, survival and extinction. I have come across many stories that have stirred my imagination throughout my studies of biology. As most of these go untold to those outside of the scientific community, my aim in writing Stripped-Down Science is to make these fascinating stories accessible and comprehensible. This column will be a biweekly publication: the first post of the month will introduce a biological topic, the second post of the month will be an interview with a scientist working in that field. I hope to appeal to your curiosity about the world around you by helping you understand why and how life functions. Open your mind and come along for the journey!
I think the natural place to start is with the most basic unit of life: the cell. Every living organism on this planet is a cell- whether it is one cell like bacteria, or trillions of cells like humans. Each cell is surrounded by a membrane, which acts like a gate that only permits certain things to enter or exit, and contains DNA, the genetic material that stores the information (in the form of genes) for synthesizing and maintaining an organism. Cells carry out a multitude of processes: they proliferate, differentiate, respond to the environment, and communicate with one another. We will explore these processes in further detail and acquaint you with some of the vocabulary that will be used in the forthcoming months.
In order to make a copy of itself, a cell needs to divide (proliferate). Bacteria simply copy their DNA and divide in half, making identical clones of themselves. This process is called binary fission and can be as short as 15 minutes, allowing them to quickly multiple! In eukaryotes- organisms with a separate membrane inside their cells (a nucleus) that houses the DNA- cell division is a bit more complex. For example, in humans the genetic material is stored on two copies of 23 different chromosomes and all of this information needs to be copied perfectly each time a new cell is made. The process is called mitosis. The average human undergoes about 10,000 trillion cell divisions in a lifetime. These cells all descend from one single cell, the product of the fusion between your father's sperm and mother's egg.
So how do you go from a single cell to the complex being you are now? Through cellular differentiation, a process in which cells assume a unique identity and specialized function. The first type of cell produced after conception is an embryonic stem cell, which has the ability to give rise to any type of cell in the human body (this is why they are so promising for treating illnesses and mending injuries). As these stem cells divide, they begin to form a three-dimensional structure (the embryo) with three distinct layers. The cells in each layer start to gain their unique functions via the silencing of specific genes. The silenced genes will remain non-functional in that cell and it can only express (turn on) or repress (turn off) the unsilenced genes. This distinct combination of accessible genes is important, because the information stored in a gene will be read and translated into a protein. Proteins interact to carry out life processes, such as food digestion (provides our cells with energy), circulation (deliver oxygen to our cells and remove carbon dioxide from them), filtration (remove wastes from our blood), and immunological functions (protects our cells from harmful foreign invaders).
The aforementioned process of switching genes on and off is known as gene regulation. This is an essential process that allows cells to respond and adapt to their ever-changing environment. It would be a waste of energy for a cell to synthesize every protein in its genetic repertoire; it is much more efficient to turn on a gene to synthesize a specific protein only when it is needed. A relatable example: one wouldn't carry around a heavy jacket, umbrella, and a pair of shorts everyday to prepare for the snow, rain, or heat; instead, one can simply look out the window or use a thermometer to determine the weather and adjust his or her outfit accordingly. This is exactly what our cells do using receptors on their surface, which can determine the state and detect the presence of substances in their surrounding environment. For example, if a harmful bacterium is floating around in our bloodstream, then our blood cells’ receptors will detect it and pass the message on to the DNA, which will result in turning on a gene that can make a protein to attack the invader. This process of passing on information about the outside world to the inside of the cell in order to turn on/off certain genes to respond to the environment is known as signal transduction. It's kind of like the game of telephone: the children that pass the message on can be substituted with proteins in the cellular world; the language the children speak can be substituted with signaling molecules, like phosphate and ubiquitin (for a nice diagram on signal transduction, click here). Many cancers and diseases are the result of mutated (abnormal) signaling molecules that distort the message and cause cellular processes to malfunction. Isn't interesting to think of how in the game of telephone the message usually winds up distorted after a few rounds, yet our cells can faithfully pass messages on for decades without malfunctioning!
Cells communicate not only with their environment, but with other cells as well. This allows them to coordinate their behavior. Nerve cells deliver chemical messages from the brain to muscles cells, telling them to contract, resulting in movement. Hormones are another type of molecule used for cellular communication. Growth hormones are responsible for telling certain cells to proliferate, while insulin hormones cause cells to take up glucose in order to lower the amount of sugar in the blood. Cytokines are an additional class of communication molecules. Returning to our example above, when a bacterium invades a cell, the cell may release cytokines to warn neighboring cells that it is infected and to recruit immune cells that can fight the bacteria. Cytokines released by nearby cells may tell the infected cell to commit suicide, a process called apoptosis. Bacteria can also harness the power of cell-to-cell communication by counting how many of them are present and once a certain population density is reached, the bacteria will all turn on the genes for a coordinated behavior. This process is called quorum sensing. In the case of pathogens (disease-causing bacteria), quorum sensing allows them to release their attack only when enough members are present, providing them with the ability to overwhelm the immune system.
I hope to have sufficiently introduced you to the beautifully complex cellular world and some of the processes that cells undergo. You should notice that cells are subject to a very ordered way of life and the manner in which they function is determined by what is happening around them. They receive information in the form of signaling molecules and switch certain genes on or off, which results in the production of a protein that allows them to coordinate their behavior accordingly. We may not be able to see what is happening with the naked eye, but these microscopic “things” are constantly working together to keep us alive (for a really cool video produced by the BioVisions Team at Harvard University of how they imagine what the inner life of the cell looks like, check out this link: https://www.youtube.com/watch?v=wJyUtbn0O5Y However, it is not just the cells in our bodies that work like this… every cell on Earth functions in a similar fashion. We will build upon this knowledge of the cellular world in future posts and come to see how DNA has diversified over time to produce different organisms. We will explore their stories and the scientists that uncovered them. And through these stories, I hope that you will see the significance of science- which has provided the human species with the tools to answer questions about our existence and to write it down in The Story of Life- and that you will come to appreciate life and its interconnectedness.
Don’t forget to check back in two weeks for an interview with a scientist (who are we and what are we like inside/outside the laboratory?), followed up by a post on one of the greatest scientific accomplishments- the human genome project.