Nucleic Acid Free Diagram: Fill & Download for Free

Your headline question is a fairly easy one to answer, but in the context of your question details, this is one of the great unanswerable questions of science! We will probably never know for sure, but there are definitely some aspects of these great questions that basic chemistry and biochemistry can help us with. My biochemistry is quite rusty so I hope I don't make any mistakes - but the fundamentals will be correct for the purposes of this question.And in terms of your question "How did those first protein molecules get encouraged to form and replicate?", I hope you will see that no encouragement was required beyond the natural circumstances that existed on prehistoric Earth.How did carbon atoms form in complex patterns to begin creating complex molecules?This is fairly elementary chemistry. Although we talk about "carbon-based" life, it's important to note that what we're talking about is multi-element compounds that have carbon as their backbone. To really understand this, you have to look at the notion of valency.You're probably aware that an atom is a nucleus of neutrons (no charge) and protons (positive) surrounded by orbiting electrons (negative). The number of protons and electrons are equal (except in ions, which are positively or negatively charged atoms or groups of atoms looking to gain or lose an electron). The number of protons (and therefore electrons) is the element's periodic number, so Hydrogen (H, number 1) has one electron, Helium (He, number 2) has two electrons, and so on. Carbon (C) is number 6 and Oxygen (O) is 8.Now these electrons are arranged in shells around the nucleus. The inner shell is "full" when it has two electrons, and further electrons will fill into the next shell. The second shell is "full" when it has eight electrons (and so on). For the sake of this answer we don't need to look at subsequent shells. If an atom's outer shell is full, it is a non-reactive noble gas (such as Helium, which fills the first shell with its two electrons, or Neon, which has ten electrons and thus naturally has both shells full). But if the atom doesn't have its outer shell full, it will be highly reactive, "needing" to lend or borrow electrons through a 'covalent bond'. The number of electrons it's looking to lend or borrow is called its valence. A covalent bond is the standard chemical bond; a line between two atoms drawn in a chemical diagram, and always represents two electrons, one from each atom.How is this relevant? Well, Carbon, as element number six, has a full inner shell of two electrons, plus a half-full outer shell, giving it a valency of 4 which is as large a valency as you can get for such an abundant and light element. This leads to some pretty complicated possibilities when Carbon is covalently bonded to other elements. It can lead to single bonds, double bonds, even triple bonds, hexagonal, square or pentagonal molecules, and very long chains (polymers). If you study organic chemistry you'll start with hydrocarbons, which are just chains of carbon atoms with all the spare valency electrons shared with hydrogen atoms.Methane is the most simple hydrocarbon (one carbon and four hydrogens). (Image source: http://en.wikipedia.org/wiki/Valence_electron)Further information on outer shells: http://en.wikipedia.org/wiki/Electron_shellLook at the following simple hydrocarbons. In each case, you will see that there is one bond (electron pair) to each Hydrogen, giving it a full outer shell of two, and four bonds to each Carbon (with double-bonds counting twice), giving it 4+4=8 electrons in its outer shell (or alternatively, 4-4=0, in which case we look at the next shell down, which still has two electrons, so is full). Each of these compounds is stable.(Image source: Encyclopaedia Britannica)So wherever there is an abundant supply of carbon alongside hydrogen, and some degree of heat, pressure, movement or electrical charge, you will find hydrocarbons forming, and the more of these effects, the more complex the hydrocarbons. Add some Oxygen, Phosphorous and Nitrogen, and you have the basis for most organic molecules found in life on Earth.Hydrogen bondingOne other point before I go further. I mentioned that hydrogen atoms share their one electron in a covalent bond. This means most of their negative charge is biased towards the 'inside' of the molecule, where the bond is, while the proton that forms the hydrogen atom's nucleus is pointed outwards. This creates a localised area of positive charge on the surface of the molecule. Similarly, as oxygen (number 8) has 8-2=6 valence electrons, it is looking to share two of its electrons in bonds, whereas the remaining four are not shared, but instead form two pairs on the side of the atom pointed away from the bonds. So the oxygen atom forms a localised area of negative charge.This means that when a hydrogen atom in one molecule comes near an oxygen atom in another molecule, there is an attraction which is called hydrogen bonding. This is not a chemical bond but a physical link created by electromagnetic attraction. It's a bit like the way a balloon will stick to you if you rub it on a woollen jumper - the balloon is not chemically bonded to you, it's just attracted by an imbalance of electrons.Hydrogen bonding is the phenomenon that explains the wetness of water (which, as H2O, is all positive and negative poles) and a great many other phenomena. It is critically important in biochemistry.How did it go from random carbon connections, to ones that made 'sense'?It's hard to say when molecules start 'making sense', since this is something we subjectively impose on the complex chemical agglomerations we call life. But I think the best place to start is the spontaneous formation of nucleic acids and amino acids.In the famous 1953 Miller-Urey experiment, Stanley Miller and Harold Urey mixed water, methane, ammonia and hydrogen (all of which we could expect to find in the primordial planet Earth) in a simple set of flasks, applying heat, cold and electricity to simulate volcanic discharges (such as at thermal vents on the ocean floor), lightning strikes and so on. Within two weeks, 10-15% of the carbon in the system had formed organic compounds, including all the amino acids currently utilised by life, and sugars such as ribose (a component of ribonucleic acid and deoxyribonucleic acid, RNA and DNA).(Image source: http://people.chem.duke.edu/~jds/cruise_chem/Exobiology/miller.html)Subsequent versions of this experiment have added simple chemicals we have good reason to believe existed on the early Earth, such as hydrogen sulphide and carbon dioxide. These experiments demonstrated spontaneous formation of nucleic acids such as adenine.So this demonstrates that, given the right natural conditions and elements, complex organic compounds can form and accumulate naturally from simpler compounds.For more information: Wikipedia: Miller-Urey experimentSelf-Reproducing moleculesEvery molecule has different properties, and changing a single atom can subtly or dramatically change the properties of a compound. Nucleic acids have a couple of properties that are particularly remarkable from our perspective, but when you actually study the chemistry, are inevitable. One of these properties is reproduction. Now we're going to use some of the above chemical theory to explain very approximately how nucleic acid polymers reproduce.First, this diagram shows the chemical structure of a single strand of DNA. There are four nucleotides in this diagram, with the oxygen atom at the bottom of each one (3-prime end) covalently bonded to the Phosphate group at the top of the next (5-prime end). You'll notice the 'dangling' structures on the right are all different, as this diagram illustrates the four different nucleic acid nucleotides used in all life on Earth today: cytosine, guanine, adenine and tyrosine (represented as C, G, A and T). (Image source: http://www.chem.ucla.edu/harding/IGOC/N/nucleic_acid.html) Due to the nature of organic chemistry, it is assumed the reader will know that any 'corner' on this diagram is a carbon atom, and it is assumed you will be able to figure out where the hydrogen atoms are - only the 'exceptions' are drawn in.Place two single-stranded lengths of DNA together and something marvelous happens - due to hydrogen bonding, different subunits start to stick together.As in the above diagram (by Madprime, from Wikipedia), you can see how guanine subunits stick to cytosine (and vice versa), and adenine sticks to tyrosine (and vice versa). The dotted lines are hydrogen bonds. This means that the strands are complementary, and from one you can deduce what the other half should look like.Now what happens if you break the hydrogen bonds, tearing part of the double helix into two individual strands? What happens is that any free nucleotide ions in the solution (the liquid the strand is surrounded by) will stick to their complementary nucleotide bases, so that a copy of the separated strand will start to form spontaneously. As these free nucleotide bases have unbonded 5-prime and 3-prime ends looking for an atom to covalently bond with, they will chemically bond, gluing the single strand together. Where once you had one copy of the DNA double-helix, you will soon have two identical copies (barring random 'mutations').So the mystery of reproduction is not really a mystery. The mystery is how the first cleavage fork formed!Now we come to your next question:Was there some form of natural selection even at this atomic / molecular level?As soon as you have a nucleic acid double helix that sometimes forms cleavage forks, you have reproduction. And provided there are occasional 'errors' and that some of these forms are destroyed ('death'), then you have the basis of natural selection.It's tempting to think of evolution/natural selection as some big, God-like force that permeates complex life, but in reality it is the banal and obvious observation that the bits of stuff that reproduce more get more common, and the ones that die before reproducing aren't around anymore. So the information contained in DNA and RNA is really just a record of what has successfully reproduced in the past.One of the other traits of nucleic acids is that in some structures and combinations they cause surrounding amino acids to form into sequences (proteins) that perform a practically limitless range of functions, not least being triggering or regulating cleavage forks. (I am skipping some steps here for brevity and because my biochemistry is very rusty!)Now here I want to admit that the incredibly complex combinations of RNA, DNA and protein we observe in living things today are much more sophisticated than our intuition tells us to expect to have developed randomly. This is why creationists often zoom in on abiogenesis; as we don't have a definite theory of how it happened, and as life as we know it today is very complicated, "intelligent design" creationists argue that evolution may be observed in the short term, but God must have created the first life. They are certainly entitled to hold that view if they wish, although it doesn't seem to me to make the whole thing any easier to understand.Without wanting to create a great debate, I will explain why I personally don't find this complexity to be so impossible to believe as some people seem to.Once nucleic acids were reasonably abundant on prehistoric earth, strands would have begun multiplying and rudimentary natural selection would have created increasingly 'successful' combinations, with mutations accumulating over time. From time to time, some self-replicating DNA or RNA would have been trapped in phospholipid bubbles, forming a proto-cell (discussed in my answer to How did the first cell know how to divide itself?). All this would have been happening in countless places on shorelines, around thermal vents on the ocean floor and so on, completely spontaneously and without any need for guidance and intervention, through simple chemistry. This is a lot of matter, given that there are 6.023 x 10^23 (602,300,000,000,000,000,000,000) atoms in a mere 12 grams of carbon (Avagradro's Constant). And they would have been bumping up against each other, reacting and bonding, every second of every day.Many billions of such DNA molecules would have been reproducing in parallel around the world, and sometimes exchanging genetic material (see the earlier parts of my answer to this question about the development of sexual reproduction). Given we have evidence of cellular life from 3.5 billion years ago, this means it took one BILLION years for even the first "proper" cells to develop. While experiments such as the Miller-Urey experiment created the building-blocks of life within a fortnight but did not create life itself, planet Earth had a lot more time to play with. Not just ten times as long, but ten orders of magnitude as long. There are 25,000,000,000 fortnights in a billion years!So it seems to me that the development of complex, reproducing molecules from carbon and other simple elements was not only believable, but almost inevitable given the sort of space, time, material resources and physical effects present on prehistoric earth, without any need for guidance or encouragement.Now I admit this has been a very loooong answer, but then this is a big question. My best understanding of how complex, evolving life developed from carbon and other simple elements involves a lot of chemistry and a lot of steps, as you can see, and any shorter answer would have left gaps that might have been difficult to follow without a grounding in biochemistry. I hope this answer went some way to answering your question, and didn't cause your eyes to glaze over!Thank you to Daniel Super for the A2A, and apologies in advance to any chemists reading this for any errors I might have made!

2 months are more than enough to pass MBBS 1st Professional examinations.. If you are weak in Anatomy then go through Gray’s Anatomy for preparing Anatomy. It has diagrams drawn in a full elaborative way that you can easily memorize. Follow the following routine;First give 10 days to Anatomy to prepare Upper Limb, Lower Limb and Thorax. Then give 10 days to Physiology and then 5 days to Biochemistry. Then go to the minor subjects like Embryology (5 days would be enough) and 3 days would be enough for Histology. So you still have 1 month left for revision. Now revise accordingly and you will pass your examinations with ease. Don’t worry. Best of Luck to you.Note: This whole routine is in accordance with the syllabus of MBBS 1st Year observed in Pakistan. Here we have in First YearGross Anatomy: Upper Limb, Lower Limb and ThoraxPhysiology: Respiration, Blood, Nerve And Muscle, Cell, Heart, Circulation and Space physiologyBiochemistry: General concepts of Proteins, Lipids, Carbohydrates, Nucleic Acids along with their classifications and properties plus Acid Base Balance.Embryology: Development of Human Embryo upto 3rd Week and some topics from special Embryology including Musculoskeletal Development, Development of Limbs and Integumentary SystemHistology: Epithelium, Bone, Muscles, Connective Tissue, Respiratory System, Integumentary System, Nervous System, Vascular SystemsIf you want detailed routine including division days for various topics then feel free to askGood Luck

I want to focus on just one aspect of the question. Why proteins?They have several virtues that make them an excellent choice:Proteins can be easily created by linking units called amino acids.Chains of amino acids can be very long making it possible to produce structures that naturally fold up into complicated shapes that work as tiny machines.Some of the building blocks of proteins, amino acids, were naturally available in the primordial soup that life began in.By combining amino acids having diverse chemical properties, organisms can create proteins having diverse properties and abilities. That’s especially valuable for building catalysts (enzymes) that push biochemical reactions in ways that allow us to live.Proteins can be put together and broken apart using a modest amount of energy.Proteins can be very stable. Some substances made from them, like silk, hair, and leather, last for many years without breaking down. Some alternatives, like RNA, aren’t as stable.Are there alternatives to proteins?Sure.In the beginning, when the earth was without life and darkness was upon the face of the deep, and an oxygen-free atmosphere moved upon the face of the waters,[1][1][1][1] there were RNA molecules that replicated somehow. Scientists call that the RNA World and their best guess is that RNA molecules did the things that proteins do now.These molecules are called ribozymes. A few persist even now. This one is called the hammerhead ribozyme and it’s used by a plant parasite to help replicate RNA.[2][2][2][2]Ribozymes aren’t able to catalyze as wide a variety of reactions because they only have four possible units compared to the 20 amino acids. Some theorists think that there was an intermediate state in which there were RNA-Amino acid hybrid molecules in which RNA molecules (black in the diagram) had amino acids attached (purple) to them so as to enhance their ability to catalyze reactions.Nature uses many other polymers when appropriate.Plants use cellulose and lignin to make wood. And insects use chitin to make their hard shells. Our bodies use long hydrocarbon chains to store energy as fat. Bacteria create a great many polymers. Some are based on carbohydrates, like alginate, xanthan, hyaluronic acid, etc, and other resembling plastics, like Polyhydroxy-alkanoates.[3][3][3][3]Those alternative polymers work well for structure, energy storage, or some other purposes but they don’t have the chemical reactivity that catalysts need.You can get a sense of that by looking at the structure of cellulose. Each unit is identical, so there is no way to create a region in that molecule that will do something different from the rest of the molecule.Contrast that monotonous structure with this short peptide. The repeating portion of the chain is highlighted in yellow. The red portion shows a few of the different chemical groups amino acids can have. Those chemical groups allow proteins to manipulate cell chemistry in many ways that carbohydrates can’t do.Are there other possibilities?Maybe. I describe alternatives to RNA in these answers. Some of these might partially substitute for proteins.Israel Ramirez's answer to Could DNA/RNA analogues exist?Israel Ramirez's answer to Of so many sugars present in nature Why is only ribose present in DNA/rna?More to readThis article introduces the basics of amino acids and proteins:Introduction to ChemistryI describe the abilities of RNA based enzymes here:Israel Ramirez's answer to How exactly is a ribozyme created? Can it catalyze the formation of other ribozymes? Is the fact that some chemical groups on their nucleotides are highly reactive the determining factor between a ribozymes and plain, non-catalytic RNA?Footnotes[1] Bible Gateway passage: Genesis 1 - King James Version[1] Bible Gateway passage: Genesis 1 - King James Version[1] Bible Gateway passage: Genesis 1 - King James Version[1] Bible Gateway passage: Genesis 1 - King James Version[2] Israel Ramirez's answer to How exactly is a ribozyme created? Can it catalyze the formation of other ribozymes? Is the fact that some chemical groups on their nucleotides are highly reactive the determining factor between a ribozymes and plain, non-catalytic RNA?[2] Israel Ramirez's answer to How exactly is a ribozyme created? Can it catalyze the formation of other ribozymes? Is the fact that some chemical groups on their nucleotides are highly reactive the determining factor between a ribozymes and plain, non-catalytic RNA?[2] Israel Ramirez's answer to How exactly is a ribozyme created? Can it catalyze the formation of other ribozymes? Is the fact that some chemical groups on their nucleotides are highly reactive the determining factor between a ribozymes and plain, non-catalytic RNA?[2] Israel Ramirez's answer to How exactly is a ribozyme created? Can it catalyze the formation of other ribozymes? Is the fact that some chemical groups on their nucleotides are highly reactive the determining factor between a ribozymes and plain, non-catalytic RNA?[3] Bacterial polymers: biosynthesis, modifications and applications[3] Bacterial polymers: biosynthesis, modifications and applications[3] Bacterial polymers: biosynthesis, modifications and applications[3] Bacterial polymers: biosynthesis, modifications and applications