Part 1
Humans have 30 trillion cells that make up the body, but how do they do their jobs creating us and keeping us alive?
Science has an answer for us.
A human cell has about 6 feet of condensed DNA that holds the information on how every single cell is supposed to look like and function in long segments called genes, which in turn is condensed onto ball shaped proteins called histones and curled into 46 chromosomes. Chromosomes and DNA keep you alive and growing, so they are extremely important to every single cell in the body, which is why they need to keep all the information for cell division.
Cells in the human body divide every 24 hours. Cells divide for three reasons: to replace dead cells, repair the body, or to grow. For about 22.5 hours of the day, cells are in interphase, where they perform normal cell functions (like cellular respiration) and replicate the DNA. The DNA is unwrapped from the chromosomes and then they go through a process formally known as "unzipping." This process is when the DNA is split into two strands by a protein called helicase, and you are left with long half-strands of DNA. DNA polymerase then goes over the half-strands and attaches spare nucleotides (DNA building blocks), onto both, effectively and perfectly duplicating the DNA. This DNA is then wrapped back around the histones and put back into chromosome form.
During the last hour of the cell cycle, the cell goes through the 7 stages of the mitotic phase, starting with the two phases of prophase. In the early prophase stage, two things happen: spindle fibers form in the cell, and the nuclear envelope (which protects the DNA) breaks down. In late prophase, the spindle fibers attach to the chromosomes and prepare them for metaphase.
During metaphase, the spindle fibers arrange the chromosomes in the middle of the cell, and anaphase pulls the replicated chromosomes toward the spindle poles to prepare for telophase and cytokinesis, where two nuclear envelopes form and the cells split, creating an identical cell. After nine months, the cells divide enough to create a human baby.
how do things like eyes and hair form? Well, the first cells in a body are called embryonic stem cells, and they have the power to transform into any kind of specialized cell, like a skin cell or a brain cell.
The embryonic stem cells disappear after birth, but we do have “adult” stem cells. Adult is a bit of a misnomer, as you can find adult cells throughout your entire lifespan. Adult stem cells can transform into multiple specialized cells, but the kinds of cells they can transform into are limited.
Well, stem cells use the DNA in their nucleus to activate or repress genes, which can be as long or short as the sequence demands, which we'll explain next.
Part 2
DNA is the blueprint of life, but if DNA is stuck in the nucleus, how do things get done? Meet proteins, the building blocks of the body. Proteins are instructed by DNA and control outward traits, like eye color, hair color, and almost everything else about the body. This process is called the central dogma, and it's a core principle of biology. Here's how it works. Your body is made of 40% proteins, and those proteins are responsible for getting anything done in the body, but they need instructions from DNA to function, so mRNA comes in to save the day.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
For this part of the article, we're going to focus on ALX1, a beak shape gene in birds found in chromosome 1, and we'll get into why later. The DNA has instructions on how anything in the body functions, and ALX1 is responsible for making sure that the sharp beak shape is made instead of a blunt beak. The ALX1 gene is recorded by mRNA, which is a single-stranded version of DNA. Using the base pairing rule, mRNA takes nucleotides and copies down the DNA's information in a process called transcription. Now that it has what it needs to begin creating ALX1 proteins, the mRNA leaves the nucleus and enters a protein-creating ribosome, which has rRNA, a type of RNA that has an amino acid attached to the top and a base pair stamp on the bottom, called an anticodon.
The anticodons are in groups of three nucleotides, and each set of three nucleotides translates to a different type of amino acid. This process is called translation, and a few of the three patterns have start and stop information to let the ribosome know where the gene begins and ends, and when to tie off the nucleotide string. Once the nucleotide string is finished, the new protein folds into a shape because of the hydrophobic and hydrophilic properties. A protein's shape dictates its function. For example, red blood cells are round and able to fit into narrow arteries, while transport proteins are more barrel-shaped.
Sometimes, due to a number of factors like evolution or incorrectly transcribing DNA or something else, mRNA has a different combination of amino acids, which creates a new protein that functions differently than a healthy gene. This is called a mutation, and it can possibly lead to no change, the death of the body or a new trait. A nonfunctional ALX1 gene occurs when even a single amino acid difference in the translation bonds to a whole different amino acid, namely proline to leucine, which changes the shape of the protein, rendering the protein nonfunctional. A bird with a nonfunctional ALX1 gene will not be able to regulate beak shape genes back in the nucleus, leading to a blunt beak, and potentially an inability to find food, if there are no available seeds and nuts, which a blunt beak excels in cracking.
If blunt beak finches were to find an area where blunt beaked food was readily available, they would be able to reproduce and pass on their genes to the next generation. Blunt beaks are considered a recessive trait, and if the bird were to breed with a sharp beaked bird, the offspring would have a sharp beak but still have the genes for a blunt beak to pass on. If the bird was to find and breed with another blunt beaked bird, they would likely have a blunt beaked baby. Siblings from the same birds can have different beak shapes due to their punnet square/ independent assortment recombonations, as one of them can have a blunt beak while the other has a sharp beak as well as the possibility of the children mutating a completely new beak type.
However, it is important to remember that the model of dominant and recessive is greatly oversimplified, because life is never that simple, and there are plenty of factors before, during and after an offspring is produced that would challenge what the simple inheritance model says.
Part 3
How did we get to “human” compared to other animals?
All animals evolved from a single celled organism, created at least 3.7 billion years ago, but how we got to what we did was not by chance. Traits evolved randomly, but animals with weaker traits, like underwater animals without gills, or darker skinned people in areas with less sunlight naturally died off in a process called natural selection. The better your trait was, the more likely you were to survive. As traits kept evolving, animals kept splitting on the evolutionary tree and becoming their own species, like humans and monkeys 7 million years ago. Prey animals kept evolving to hide from predators and being able to get away, while predators evolved hunting, sharp teeth and eyes that could spot prey, as a sort of 4-billion-year long arms race that is still in progress to this day, and humans keep changing too. Evolution is an extremely long process, but humans are undeniably different from the humans that first evolved in Africa. We know humans evolved from Africa for multiple reasons, including ancient fossils of early humans, as well as allele concentration, or years of mutation is found most common in Africa compared to the rest of the world. However, humans didn't simply split from monkeys and immediately become the people you see today, there was a lot of trial and error. We know that humans evolved and interbred along with at least 8 other extinct types of humans, and their genes remain within the human population, as Europeans are about one percent homo neanderthal. One of the things that makes humans so special is the ability to intermix with the local populations around the world. As humans left Africa and settled in other areas, they intermixed with the local population. This is the reason why the idea of an evolutionary tree doesn't work for humans, as it is instead more like a trellis, with constant mixing and sharing of genes.
Part 4
Was evolution an accident?
No, evolution was definitely not an accident. This is a common misconception, and while it can seem true, it discounts the billions of years of methodical change that got us to where we are now. Like we discussed in the previous section, alleles appear randomly, and evolution makes sure that the creature that is the most able to survive will be able to pass down their genes. For example, let's imagine that aliens came down to earth today, and forced every human on earth to play and win a game of basketball to survive for hundreds of years. Humans already have some very tall people, and because height is an advantage in basketball, the tallest humans would be the most likely to survive and reproduce. After a while, the entire human population would gradually get taller and more suited to play basketball. This is an absurd example, but it has similarities with very real instances of evolution, like Darwin's finches. In the Galapagos islands, finches migrated to each island, which each had very different conditions. Some islands had soft seeds and fruit, so there was no preference for Alx1 beak types, leading to no change in the population. On other islands, there were only hard seeds that big beaked birds could crack, leading to all the small beaked birds to die off, leaving only big beaked birds. Plants and animals grow to survive their environments in the most efficient way possible, by growing out allele branches, and seeing which ones survive as a constantly expanding tree.