The X chromosome you receive from your mother is a mixture of your mother’s two X chromosomes. However, your father only has one X chromosome, and if you are a female this was passed on to you un-recombined. It is identical to your father’s single X chromosome. If you are male, you don’t get an X chromosome from your father, instead you receive his Y chromosome. This too has not been mixed with another chromosome. No individual has two Y chromosomes, and this means no Y chromosome ever gets mixed with another Y chromosome. However, very, very occasionally a Y chromosome may mix with a bit of DNA from another chromosome, such as an X chromosome or even another chromosome in a completely different chromosome pair, but when this happens it is an error in replication. On some of these rare occasions when it occurs, it can signal the beginning of an evolutionary journey that results in a new species, but that is a topic for another book.
The above paragraphs reveal that the self-assembly manual that is your genetic code is complex. We will discover why it is so complex later in the book when I discuss why sex evolved. For now, it is sufficient to appreciate that DNA is a large molecule, and the way it is inherited, passed from parents to offspring, with your genetic sequence being a mix of those of your parents, is complicated. Upon your eventual death, you will cease to be, but if you have children some of your genes will live on. Genes replicate, being passed from parents to offspring, but individuals die.
Biologists distinguish the reproductive cells that can fuse with one another to produce the first cell of a new individual in the next generation from the non-reproductive cells that have the sole purpose of helping to keep individuals alive long enough for them to reproduce. The reproductive cells make up the germ line, while the non-reproductive cells are collectively called the disposable soma. What is truly remarkable about life on Earth is how complicated the disposable soma has evolved to be in higher animals like you and me. Evolution has created genetic sequences to self-assemble organisms with arms, legs, eyes, brains, skin, teeth, toenails, tonsils and sentience. These attributes maximize the chance that the DNA sequences in germ lines that instruct how these body parts are produced are passed on to the next generation. DNA is a truly remarkable molecule. It replicates, making copies of itself, but it does this by providing self-assembly manuals to build species as diverse as cats, cabbages, cockroaches and chlamydia.
Although not as complicated as some species, the way that single-celled bacteria, like chlamydia, make a copy of their DNA still requires many steps. There are nine enzymes that are essential for bacterial DNA replication, and many others that play less crucial roles. Skipping over the details, most bacteria species store their double helix DNA molecule in a loop. Replication takes one loop and produces two, and it happens just before a bacterium splits to become two bacteria. The process begins with the double helix being separated by enzymes at a single location on the loop. Other enzymes steadily unzip the double helix, with the point at which the strands are separated called the fork. Yet more proteins latch on to each of the separated strands. On one strand, enzymes follow close behind the moving fork, reading the nucleobases and producing a continuous mirror-image strand of DNA. On the opposite strand things are more complicated. The enzyme that moves along DNA making a mirror-image copy can only move in one direction, and in this case, it is away from the moving fork. On the second, or lagging, strand, the enzymes join close to the fork and move away from it, producing small strings of nucleotides called Okazaki fragments that then need to be stitched together by yet more proteins to produce a continuous strand of DNA. Even DNA replication in simple species, and I use the word ‘simple’ with a touch of irony, is very involved and did not simply spring into existence.
Replication is not the only complicated bit in life’s definition. Making proteins from the DNA code is significantly more involved than I have space to explain, and it relies on another key chemical of life called ribonucleic acid, or RNA as it is abbreviated. I won’t say much about RNA, but I will introduce it in a little detail as it may have played a key role in the emergence of life. Only single-stranded, with a backbone of alternating ribose and phosphate molecules, and using a nucleobase called uracil instead of thymine, it is a close cousin of DNA. Ribose has one more oxygen atom than deoxyribose, but is otherwise very similar, while uracil is simpler than thymine, and in particular lacks a group of carbon and hydrogen atoms that form the hydrogen bonds that keep DNA’s two strands bound together.
Like DNA replication, and the way proteins are synthesized from the genetic code, metabolism is also fiendishly complicated, and it did not arise in a single step. Human metabolism works by taking energy in the form of sugars such as glucose from our food and using them to create a proton gradient, by concentrating more protons on one side of a molecular membrane than on the other. Protons are one of the building blocks of atomic nuclei, and were described in an earlier chapter. The proton gradient is then used to make a chemical called adenine triphosphate, or ATP, from a molecule of adenine diphosphate, or ADP, and a molecule of phosphate. ATP is important because it is the fuel that life uses to run. In animals, like me and you, the proton gradient used to make ATP is inside structures found inside all your cells called mitochondria. These organelles, the word that scientists use to describe structures within cells, are surrounded by a double membrane, and the proton gradient is created by moving protons into and out of the space between these two membranes – the intermembrane space. Mitochondria, and their inter-membrane space, are critical for life because that is where ATP is produced.
If left to their own devices, and assuming they could cross the membrane unhindered, the protons would settle to an equilibrium where the same concentration of protons would exist on either side of the membrane. But that does not happen. The protons are instead being pushed from a lower concentration inside the mitochondrion to a higher concentration in the intermembrane space. To do this, electrons are passed along a chain of chemicals called, appropriately enough, an electron transfer chain, and as they move along molecules in the chain, they change those molecules’ electric charge, and this pushes protons across the membrane. Life uses the electromagnetic force to run. By the end of the process, two electrons have pushed ten protons across the membrane through special channels before the electrons are used, along with a molecule of oxygen (O2) and four other protons, to make two molecules of water. You’ll eventually pee or sweat these out of your body.
The proton gradient is then used by the cell to create ATP. The positively charged protons in the intermembrane space readily attach to a negatively charged part of a protein called ATP synthase that sits in the mitochondrion’s inner membrane. The protons then pass through a channel in this protein, and as they do so they spin part of it that looks like a tiny molecular water wheel, and as this happens ADP and a phosphorus ion are combined to form a molecule of ATP. Ten protons pushed into the intermembrane space by the electron transfer chain produce 2.5 molecules of ATP. The ATP is then used to fuel the creation of new proteins and to create gradients of other compounds, including sodium and potassium ions, across membranes.
Once again, the structure of a protein, this time ATP synthase, is critical for life. The spinning water wheel that is at the heart of the protein, is a structure central to making ATP, the molecule that powers life. Life is about building stable molecular structures that facilitate chemical reactions, including metabolism, that enable DNA replication. The DNA being replicated is itself structured in a way such that it contains instructions on how to assemble the molecular structures central for life.