In this lecture, we'll consider DNA and genetics. What was life like in the Proterozoic or afterwards? First of all, how did the Earth escape the cataclysm of the snowball episode that lasted from about 2.4 to 2.1 billion years ago? Basically, the Earth was still a volcanic active planet and gradually those volcanoes produce carbon dioxide and melted through some of the layers of ice, especially near the equators. The gradual rise in carbon dioxide caused by volcanism eventually reversed the Snowball Earth effect, melted the ice. Which then let more sun's radiation be absorbed, and so the positive feedback went in the other direction, raising the temperature of the earth. Eventually, the earth was reheated to more habitable and comfortable temperature. Prokaryotic and eukaryotic single-celled life thrived and diversified on the now warm earth. During this time, the area of the earth's surface covered by continents and dry land steadily increased. The earth had a primordial super continent and plate tectonics started to cause the continents to separate and move around in an early version of the continental drift that we still see in the present day earth. At this time, the ozone layer was thin because the oxygen content was not as large as it is today, and so the ultraviolet flux on the land was fatal for the still fragile organisms. The ocean was still the safe environment for life on earth. It probably took two billion years, from about 3.8 billion years ago to 600 million years ago, for life to develop beyond the first cell to different variations of these first two types of cells. Life, of course, exists and thrives by reproduction, which is one of the seven fundamental traits of living organisms. Prokaryotes, the first cells without nuclei, reproduce asexually. The parent cell simply splits into daughter cells that are genetic clones of the parent. Sexual reproduction only emerges in the Proterozoic era. In sexual reproduction, eukaryotic cells take DNA, deoxyribonucleic acid, from two genetically distinct individuals and combine it to produce genetically differentiable offspring. As we'll see, sexual reproduction is an innovation of biology that leads to further diversification of life on earth. First, we should look a little bit closer at DNA, deoxyribonucleic acid. It's the long chain molecule in the shape of a double helix that stores the genetic information for every individual organism. Each cell contains all of the information necessary to build the entire organism in its DNA. The structure is famously the double helix discovered by Crick and Watson in the early 1950s, or a twisted ladder, where the rungs are made up of pairs of nucleotides that bond in the middle. The nucleotides follow rules of base pairing, where cytosine always bonds to guanine and adenine bonds to thymine. The full sequence of base pairs along a strand of DNA is the genetic code. The base pairing and the rules that govern it for DNA is important because it allows information coded by these alternating sequences of nucleotides to be translated into a physical organism. The essence of the genetic code is that group of three base pairs form what's called a codon. A specific sequence of codons corresponds to a particular gene. Genes then give cells instructions for assembling proteins, and the proteins are sent out into the organism to carry out structural and regulatory processes that perform all the essential functions of an organism. The complete set of genes, corresponding to tens of thousands for a human being, for example, composes the genome. Sexual reproduction, as I mentioned, was an important innovation of life because it allowed the mixing of genetic material and new possibilities to result. Base pairing is important as a mechanism that allows a DNA sequence to be replicated and allows it to be passed from parent to offspring. The entire genome of DNA is condensed into what's called a chromosome. Female and male reproductive cells, called gametes, each contain half of the full number of chromosomes for that organism. The gametes fuse during reproduction and DNA from each gamete is exchanged in the process of genetic recombination. Each gamete contributes half of the number of chromosomes, resulting in an offspring that has the full number of chromosomes specific to that organism. Humans, for example, have 46 chromosomes, where each gamete contributes 23. So, the child inherits the full set of 46 chromosomes. Let's use an analogy of a book to consider the information content of DNA. The fundamental bit of information, to think of it in computer terms, is a single base pair, corresponding to about 10 atoms. That one bit of information might be like a single letter in the English alphabet. A codon is six bits of information corresponding roughly to a word and composed of about 100 atoms. A gene is about 10,000 atoms corresponding to about 1,000 bits of information. We could think of that as a sentence in the English language, and indeed, that's a good analogy because a sentence would be a description of something about you. Like, this person has a tendency for colon cancer or this person likes coffee or this person has blonde hair. Simple organism, like a bacterium, composed of about 10 million atoms and about one million bits of information, a megabit. It corresponds in language terms to a short book. It doesn't take much English to describe what a single bacterium can do. A human, a complex organism, consists of about three billion atoms in the genome. The information content in a human genome corresponds to about 6 billion bits, or one gigabyte. So, less than a DVD's worth. The language analog for that, where each letter is a single bit or base pair, would be perhaps an encyclopedia. So, for a complex organism like a human being, it does indeed take a book's worth of information to describe everything about that organism. Genetic complexity evolves over time. The larger the amount of information content coded by DNA, the more different organs can be created, the more specificity of function the organism has, and the better it adapts to the environment. Sexual reproduction is metabolically more expensive than asexual reproduction. So there has to be some selective advantage that would gain from this reproduction. That selective advantage is, of course, genetic diversification. Longer genomes contain more instructions, or genes, than shorter genomes. A larger genome also, however, has more room for mistakes. So, DNA replication step of sexual reproduction is a safety net to allow along genome to be conveyed without mistakes. That, indeed, was one of the reasons in the evolutionary process on earth that RNA was replaced by DNA in the cell's nucleus. RNA is an effective information carrier in the human cell or any cell of a complex organism, but it's not quite as robust to reproduction and transcription as DNA. The fidelity of DNA, locked in the vault of the cell's nucleus, is what maintains the robustness of reproduction of such a large amount of information in a complex organism. So, the advantage of a multi-cellular organism with specificity of function is that these abilities are passed on through the complex genome between generations. Simple eukaryotic cells in the Proterozoic era housed the potential for complexity in their DNA, by sharing genetic material through sexual reproduction. After a billion or so years of relative quiet in the earth's climate and history, another snowball episode awaited. The evidence for this is better because it happened only 700 to 600 million years ago. But there's fairly good evidence that glaciers extended all the way down from the poles to the middle latitudes, and possibly all the way to the equator, at the depth of the cooling episode. Glacial deposits have been found on all seven continents, showing this was indeed a global event. Within the global glaciation there were warming and cooling cycles. The northern and southern extents of the oceans were likely frozen, with a very thick layer of ice. The surface temperature looks like it was between minus 45 and minus 20 degrees C. However, new fossils of life appear during this episode, so even during the snowball earth evolution continues. For example, fossils of sponges are seen first doing the glaciation and multi-celled animals appear for the first time during this cold period. Life had potential, in terms of the DNA content, of course, but biodiversity was probably limited by these harsh conditions. However, at the end of the snowball earth, multi cellular life exploded on the planet, leading to an era in the evolution of life called the Cambrian explosion. Life on earth was unicellular and restricted to living in the oceans for the first two billion years in its history. DNA is the molecule that stores genetic information for each living organism. It encodes what's called the genome, the sum of all the information for the function of that organism. These are the instructions that describe how proteins are made and the proteins carry out all the essential functions. Sexual reproduction, an innovation of life with multi-celled organisms and cells with nuclei, allows organisms to maintain longer genetic codes and more information, while minimizing the mistakes in DNA reproduction. The genetic code is very similar for all organisms. Every living creature on the earth uses DNA for its genetic code.
