Three-day-old embryos of red-eyed treefrog species Agalychnis callidryas. These embryos have external gills that protrude toward the surface of their eggs, where oxygen is most concentrated. This adaptation allows for high metabolic rates and accelerated development.
If by life you mean “human life,” this is a very complicated question that has no real answer. What you’re about to read is a combination of scientific fact and my respective interpretation of it.
Fertilization is the point at which two haploid gametes merge to become a diploid cell. This new diploid cell, or zygote, contains all the genetic material it needs to develop into a mature organism. Fertilized eggs are indeed considered living cells, so with that said, it’s not entirely unreasonable for one to infer that some sort of “life” begins there. However, all zygotes previously existed as two individual living gametes - split as an egg and a sperm before fertilization. Scientifically speaking, egg and sperm cells are living cells, so they are alive. They are not, however, in themselves humans, and thus do not constitute “human life.”
Why not? The capacity to develop into something does not constitute its existence as that thing. In biology, there are cells called “totipotent stem cells” that, given the right conditions, can develop into individual organisms. Totipotency is a complicated concept, but an example of it is cutting a piece of a plant, replanting it, and watching it grow into a brand new organism (no seed involved). While the totipotent cells involved helped make the plant cutting develop into an individual, they in themselves are not individual plants - it’s their product that is. Now, back to the zygote: a fertilized egg is classified as a totipotent stem cell, and I would say that it is no more a human being than a regular, single stem cell in your brain is your “other” brain.
That may be a bit confusing, so let me use another example – monozygotic, or identical twins. Following fertilization of an egg, there is about a 16 day period where the zygote retains its totipotency. During this time frame, if the zygote splits into two, you result in identical twins. If you classify the zygote as a human being, or introduce individuality or the concept of a “soul” to the zygote from the moment of fertilization, that would mean that a human being can be split in two, and that identical twins are the same human being. Life doesn’t work that way - it’s complicated, versatile, and most importantly, a scientific concept.
Life may seem like a magical phenomenon, but while truly beautiful, it is not magic. Two or three years ago, scientists were able to create the first self-replicating synthetic life form. That’s right – we are now in an age where humans are able to create life. As technology progresses, it is very likely that a combination of neuroscience and molecular genetics will be able to map out literally everything that you are as a human being. We’re getting to the point where there is no room for magic.
Several paragraphs later, we return to your original question. I believe that the question, “When does life begin?” is scientifically, an arbitrary question in itself – any satisfying answer would have to be philosophical in nature. Based on current biology, there is no singular definition of life - Life is more of a characteristic. The progression of a zygote into a human being is a developmental process, and there is no one point in development where we can label a blastocyst or embryo, or even fetus as an actual human being. What we can do, however, is use tangible markers such as when a fetus’ heart starts beating, or its nervous system begins to develop, or it first begins responding to external stimuli to really get a sense of what the life form really is.
Bacteriophages (bacteria-infecting viruses) target a single bacterial cell. The viruses anchor themselves to the bacteria’s cellular surface using proteinaceous tails before injecting their genomes into the bacterial host.
Using the natural evolutionary specificity of bacteriophages could contribute to medical biology by targeting and curing increasingly resistant bacterial infections such as tuberculosis and streptococcus.
Above is a popular scanning electrom micrograph of numerous HIV-1 virions (green) emerging from a cultured white blood cell. HIV has caused millions of deaths since the early 80s, but some individuals are actually immune to contracting the strain of the virus depicted above. Note:This isn’t a recommendation for risky behavior to see if you’re immune; HIV/AIDS is devastating and incurable.
The HIV-1 virus operates by detecting a protein on the surface of T-lymphocytes called CCR5. In some individuals, a 32 base pair deletion in the gene leads to defective CCR5 proteins that are absent from the white blood cell surfaces. If there is no CCR5 protein to bind to, the HIV virus cannot begin its attack on the immune system. Thus, these individuals will be resistant to HIV-1.
Above is an electron micrograph of a cannabis sativa leaf.
Disclaimer: When conducting and even reading scientific research, it’s important to do so removed of pre-conceived social and political biases - take the science at face value.
Research on the psychoactive drug, cannabis, or marijuana, has been ablaze for some time. An article published by Robert Melamedeon the online Harm Reduction Journal draws the distinction between tobacco smoke (that contains the highly addictive compound, nicotine), and cannabis smoke (which contains the psychoactive, THC). At this point in time, it’s medically established that marijuana, even when smoked, has less severe adverse effects on the human body than tobacco. Yet the question remains - what are the degree of the detrimental effects that cannabis does have, and are there any medically beneficial effects?
Some research points to cannabis killing a variety of cancer types, including lung, breast and prostate, leukemia, lymphoma, skin, and glioma cancers. At the same time, however, a German study found that low THC doses encouraged lung cancer in in-vitro cells. Seemingly contradicting results, no? Just keep in mind that while nicotine and THC are chemically similar, their actual receptors in the human body vary in cell type distribution, which is what ultimately determines the effects on the human body.
… cannabis typically down-regulates immunologically-generated free radical production by promoting a Th2 immune cytokine profile. Furthermore, THC inhibits the enzyme necessary to activate some of the carcinogens found in smoke. In contrast, tobacco smoke increases the likelihood of carcinogenesis by overcoming normal cellular checkpoint protective mechanisms through the activity of respiratory epithelial cell nicotine receptors. Cannabinoids receptors have not been reported in respiratory epithelial cells (in skin they prevent cancer), and hence the DNA damage checkpoint mechanism should remain intact after prolonged cannabis exposure.
I highly recommend this article, which you can read fully here. It gives great insights into cell biology within a biomedical context.
Scanning electron micrograph (SEM) depicting a Giardia lamblia protozoan undergoing binary fission, creating what appears to be a microscopic “heart.” This flagellated protozoan parasite inhabits and reproduces in the lumen of the small intestine and is responsible for the diarrheal infection giardiasis.
Model for the DNA binding and cleavage fragment of yeast topoisomerase II (blue/red spheres) bound to a separated DNA strand (green spheres). One protomer is shaded darker than the other to highlight the protein’s two subunits. A second DNA (viewed end-on, yellow sticks) is modeled into a large internal hole present in the enzyme. This figure represents a likely conformational and substrate-bound intermediate of this region of the topoisomerase during its duplex DNA passage reaction.