What is life? Can you define it? You can probably point out things that are living and things that are not but there are grey areas. Is a virus alive? What about the prions that cause mad cow disease? Trying to describe what life is can be difficult and in fact there is no perfect definition. Instead scientist rely on a set of characteristics that define something as being alive, these are:
- Homeostasis – The ability to regulate your internal environment. Like sweating when your body gets too hot.
- Organization – Being composed of at least one cell.
- Metabolism – Being able to produce energy from your surrounding environment.
- Growth – Being able to grow in size
- Adaptation – Being able to change over time in response to your environment, or evolve.
- Respond to stimuli – For example leaves turning to face the sun.
- Reproduction – Ability to produce offspring
If something can satisfy all these definitions, then they are considered to be living. By these characteristics, viruses and prion are not considered to be alive. But now we come across another question, what makes something alive? What are the necessary components that must be in place for something to be alive? This is a question that is tough to answer but a group of scientists from California are trying to answer it using bacteria.
Using a group of bacteria called mycoplasmas, the scientists set out to determine which of the genes in its entire genetic code are necessary for it to maintain life. One particular species of mycoplasma, Mycoplasma genitalium, has one of the smallest known genomes (complete set of an animal’s DNA), only 525 genes. By comparison, a human has between 20,000-25,000 genes and a water flea has around 31,000 genes, the most in the animal kingdom. After selecting the most optimal bacteria to use, the researchers set out to mutate the genes to see which ones were essential for life. If they mutated a gene and it killed the bacteria, then it was clearly necessary for life. After they discovered which genes were essential for life they constructed an artificial genome and implanted it into a bacterial shell. This is where it gets interesting. The artificial genome only contained 473 genes and grew at 5 times the speed of the normal bacteria. Two things about this were interesting:
- The 473 genes are more than they thought the bacteria would need. In preliminary analysis it was suggested that around 250 genes would be essential for life. This is because across different species of mycoplasma there are 250 genes that are in common for each of them. Also, about 30% of the genes have no known function. This suggests there are functions that are essential to life that we don’t yet know about. Exciting!
- The fact that these bacteria grow at 5 times the rate implies that there is some sort of mechanism in the cell to slow down growth so that they don’t use up all the nutrients or develop harmful mutations. These ‘braking’ genes aren’t essential for life but the lack of them could prove detrimental later in the cells life cycle.
The 473 genes that are essential for life can be categorized into the following functions:
- Metabolism – Not surprising since this is one of our characteristics of life. Cells need energy to live and so need a working metabolism to get it.
- Cell Membrane – The membrane is essential for keeping good stuff in the cell and bad stuff outside the cell. It also stops the vital proteins in the cell from floating into free space. Kind of like how a plastic bag keeps all groceries from going all over the car.
- Preservation of genome information – Essentially, preventing mutations in the genome from killing the cell. Very important.
- Expression of genes – These are the pieces of the machinery that turns your genes into proteins so you can make hair, repair wounds, grow taller. Without them, genes are useless.
- Unknown – Still need to discover what these genes do and why they are so darn important.
So what does this research do for us as a human race? If we understand what the basic requirements for life are then we can understand how they get messed up in things like spontaneous abortions. We can also learn how they change as we get older and how to prevent them from failing in an effort to increase our longevity. In addition, understanding how life originates and survives is one of the fundamental questions that biology has yet to solve. We could also use this technology to design cells to break down toxic waste (oil spills, plastic waste, radioactive spills) or use it to create a cell that can turn the suns light into electricity to improve on solar energy. These are very exciting times.