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1 Origin of Life Hypotheses (or where did the first organism come from?) Darwin was one of the first advocates of the primordial soup hypothesis, supplemented by work in the 1920s and then the 1950s The idea is that sunlight, lightning, or geothermal heat supplied the energy necessary to turn inorganic molecules into organic ones Some experiments have shown you can make both amino and nucleic acids (the building blocks of proteins and DNA) in a test tube (even under less reducing conditions than those of Miller) The most puzzling aspect of this hypothesis is how could these simple molecules l have assembled themselves into the complex proteins and self-replicating DNA characteristic of living organisms? Idea 1: reactions occurred on the surfaces or even within oil droplets Idea 2: reactions occurred in interstitial spaces of various minerals see the chemoautotrophic hypothesis 1

2 The Panspermia Hypothesis Carl Sagan is one of the best known advocates for life or at least organic molecules being carried to Earth on meteorites or comets Builds on comets as the source of Earth s oceans h hypothesis Some support from the highly contentious Murchison meteorite depending on whether you believe these objects are fossilized cells or earth derived contaminants Furthermore it doesn t advance our understanding di of how life originated i maybe just where 2

3 The chemoautotrophic hypothesis This is basically a slight variation on the primordial soup hypothesis Plants that use the energy from the sun for photosynthesis are called photoautotrophs Organisms that still photosynthesize but use chemical energy are called chemoautotrophs The advocates of this theory contend that cells came before life the tiny cavities in iron sulphide minerals served as incubators, concentrating the early organic molecules and protecting them while millions of years of random combinations/re-combinations in a hot/high pressure environment eventually resulted in proteins and self- replicating DNA There is some genetic support for this idea 3

4 A phylogenetic tree or cladogram (Fig 2.21) These tree shaped diagrams are used to illustrate evolutionary relationships among species As you follow along the cladogram, it will split (a node.) A node represents a speciation event This cladogram illustrates the 3 d domains of fl living organisms: bacteria, archaea and eucaryotes 4

5 Eucaryotes organelles Eukaryotic organisms are those that t have cellular l inclusions i or organelles The most likely (but not universally accepted) hypothesis for the emergence of eucaryotes is endosymbiosis A slightly larger cell engulphs a smaller one (phagocytosis phagocytosis) which rather than being digested, continues to live inside the host eventually leading to a Here a mutually satisfactory relationship (a amoeba) type of symbiosis) Here a protoza (an amoeba) engulphs a bacteria 5

6 The advantage of having organelles do some of your work! Charles Darwin and Alfred Wallace were both influenced by Malthus (human population growth will outstrip resources leading to death and social upheaval) They both also recognized that: organisms compete for the resources needed to survive and reproduce organisms produce more offspring than can survive whichever characteristics gave specific individuals an advantage wrt surviving and reproducing had to be inherited for the tendency to become more prevalent in future generations If individuals with organelles derive an advantage from their presence, they will leave more individuals in the next generation see Table 3.1 (KR#8B) 6

7 Evolution by means of natural selection This is (as KR# 8 contends) a simple concept, but astonishingly powerful! All the organisms on Earth (living or fossil) came about through evolution Individuals with various traits that promote reproduction (and the number of offspring produced) will come to dominate the population over time Individuals without these adaptations will disappear (eventually) This is what we mean by survival of the fittest Where fitness is defined by the number of offspring particular individuals put into the next generation. 7

8 Since adaptations have to be heritable, they have to be coded for by DNA DNA: deoxyribonucleic acid is double-stranded d d The two strands are held together by bonds between the nucleic acid rungs There is only one type of DNA Reasonably stable structure DNA can actually repair itself deoxyribose nucleic acid(s) 8

9 Nucleic Acids or nucleotides (A,T,C,G) (the rungs of the ladder) There are only four nucleic acids (nucleotides): adenine, thymine (uracil uracil), guanine and cytosine and they can only pair in a specific order (A and T can bond, as can C and G) However we only find T in DNA while we find U in RNA 9

10 All species in the 3 domains and each of the five kingdoms (monera monera, protista, plants, animals and fungi) have both DNA and several kinds of RNA That all living organisms have the same 4 nucleotides (5 if you count uracil) constitutes powerful evidence of the relatedness of all living organisms Many viruses have no DNA and are comprised solely of RNA Since RNA is relatively unstable and incapable of repairing itself, viral mutations are frequent. Furthermore, RNA cannot replicate itself so RNA viruses cannot reproduce on their own they must enter a cell (moneran moneran, protistan, plant, animal or fungal), take over that host s DNA and re-direct it to viral reproduction. 10

11 Where do we find DNA? Inside the nucleus of cells packaged as chromosomes In spite of the fact that humans (and other multicellular organisms) have lots of different types of cells (hair cells, kidney cells, heart cells, muscle cells, ovary/testes cells, etc. etc. etc.)......the DNA in every cell is virtually genetically identical to that in every other cell. 11

12 Chromatin, chromosomes and genes represent a hierarchy: h progressively smaller segments of deoxyribonucleic acid, DNA Chromatin can be broken into specific segments we call chromosomes Chromosomes can be broken into smaller segments we call genes. A gene is just a region of DNA on a chromosome that codes for or regulates a particular trait or characteristic. These traits could be overt (e.g. eye, hair or skin colour) or they could be more covert (e.g. propensity to take risks, addiction to alcohol, l susceptibility to diseases like cancer, diabetes, Alzheimer s, etc.). 12

13 There are long segments of DNA that do not code for anything this is the so-called junk or epigenetic DNA We now know that this epigenetic material is not junk at all it regulates gene expression (when genes turn off and on) It also uses fractal geometry to combine gene regions And just as genes are heritable, so too are epigenes 13

14 If we ve got just one long strand of DNA, how do we know where different chromosomes begin and end, where coding vs non-coding sequences are located (and once we get to the level l of the individual id chromosome, how do we know where one gene ends and another starts)? Hierarchy! 14

15 These nucleotides are like letters in an alphabet that go together to make a series of 3 letter words called codons AUG is the word (or codon) )forstart start, i.e. gene Y starts here. Somewhere along the strand the codons TAA, (or TAG or TGA) signal stop, i.e. that s the end of gene Y. There are also very small markers called methyl groups that identify DNA that is epigenetic T (U) A G A T A 15

16 Slightly oversimplified since DNA doesn t directly code for all these things DNA actually codes for single-stranded stranded RNA (ribonucleic acid) DNA is transcribed (copied) on to RNA It s RNA, referred to as m-rna, for messenger RNA, that actually codes for proteins or translates the genetic code into a protein 16

17 What s a gene doing when its working? Remember that genes and chromosomes are only operationally not physically discrete. They exist as a long strand of DNA (chromatin) with billions of pairs of nucleotides m-rna The chromatin (DNA) needs to untwist, open up at the right spot (for the function of a particular gene), translate itself as m-rna which transcribes (via r-rna) into a protein (data suggest that multicellular organisms can add nucleotides at a rate of about 50 per second, while bacteria can transcribe at 500 nucleotides per second). 17

18 Yes, but not very often on genomic DNA Data suggest: Do mistakes happen? DNA subsitutions occur about once every billion DNA nucleotides (maybe once a month per cell) DNA repair proteins patrol DNA repairing mistakes (usually substitution btitti of an incorrect nucleotide) However this apparent consistency to rates of change in DNA turns out to be useful as a molecular clock Some mistakes (really just changes) do not get repaired and we have a new version of the gene, one with a different sequence of nucleotides. Different forms of the same gene are called alleles. In the fruit fly, there is one allele of the eye colour gene that codes for white and another for red. In humans there is an allele for blue eyes and a different one for brown but we all have a gene for eye colour! 18