BIOCHEMICAL EVOUTION
LIFE: the inherent capacity of an organism to maintain and reproduce itself
BIOCHEMICAL (MOLECULAR) EVOLUTION: The changes that occur at the
molecular level in organisms over a period of time. These range from deletions,
additions, or substitutions of single nucleotides, through the rearrangement of
parts of genes, to the duplication of entire genes or even whole genomes.
Such mutations may result in functional changes to the proteins encoded by
the genes, or even the evolution of novel genes and proteins.
EVOLUTION: (origin: L. evolvo) orderly change from one condition to other to
suit the environment. It means to unfold or reveal hidden potentialities.
CHEMOGENY
ORIGIN OF LIFE
HISTORY OF LIFE
BIOGENY
EVOLUTION OF
LIFE
CHEMOGENY
It is widely believed that life originated from inanimate matter. The theory of
abiogenesis is the only one that provides an explanation and can be tested.
Naturalistic theory or the Theory of Chemical Evolution was given by AI Oparin
and JBS Haldane.
Aleksandr Ivanovich Oparin (Russian biochemist) and JBS Haldane (British
scientist) put forward the concept that the first living organism originated from
a non-living thing. In his book “Origin of Life”, Oparin stated “abiogenesis first
but biogenesis ever since”. His theory is known as Primary Abiogenesis.
Oparin and Haldane suggested what the sequence of events might’ve been.
The chemical evolution is divided into four phases:
I.
ATOMIC PHASE
Early Earth consisted of elements essential for the formation of the protoplasm
(hydrogen, oxygen, carbon, nitrogen, sulphur, phosphorus, etc.). Atoms were
segregated in three concentric masses based on their weights:
• Heaviest elements like iron, nickel and copper were found in the centre
• Medium weight elements like sodium, potassium, chlorine and fluorine
were found in the outer core
• Lightest elements like nitrogen, oxygen and hydrogen found in the
primitive atmosphere
II. ORIGIN OF MOLECULES AND INORGANIC COMPOUNDS
Free atoms combine to form molecules and simple inorganic compounds.
Hydrogen atoms are most reactive and numerous in primitive atmosphere.
Primitive atmosphere was a reducing atmosphere as H atoms first combined
with all O atoms to form H2O, leaving no free oxygen. H atoms also combined
with N to form NH3. Thus, water and ammonia were the first compound
molecules of primitive earth.
III. ORIGIN OF SIMPLE ORGANIC COMPOUNDS
The primitive atmosphere contained gases like CO2, CO, H2, N2, etc. nitrogen
and carbon in the atmosphere combined with metallic atom forming nitrides
and carbides. Water vapors and metallic carbide reacted to form the first
organic compound – CH4. Further, HCN was formed.
Torrential rains might’ve caused water to rush down that dissolved and carried
salts and minerals with it to accumulate in oceans. The early compounds
reacted and produced simple organic compounds like simple sugars (ribose,
deoxyribose, glucose), nitrogen bases (purines, pyrimidines), amino acids,
glycerol, fatty acids)
Some external sources such as solar radiations, energy from electrical
discharges like lightening, high energy radiation must have provided energy for
the reactions.
There was no ozone layer in the atmosphere. The oceanic water rich in mixture
of organic compounds was termed as “hot dilute soup” or prebiotic soup by
JBS Haldane. Thus, the stage was set for combination of various chemical
elements. Once formed, they accumulated in water because their degradation
was slow in the absence of catalysts.
IV. ORIGIN OF COMPLEX ORGANIC COMPOUNDS
A variety of amino acids, fatty acids, hydrocarbons, purine and pyrimidine
bases and simple sugars are present in ancient sea. Electrical discharge,
lightening, solar energy, ATP and polyphosphates might have provided the
source of energy for polymerization reactions of organic synthesis. Thus, small
simple organic molecules combined to form large complexes.
• Amino acids combined to form polypeptides and proteins
• Simple sugars combined to form polysaccharides
• Fatty acids and glycerols combined to form fats
i.
ii.
iii.
iv.
v.
vi.
CO2 – CO + [O]
CO2 + 2[O] – CH2O + H2O
CO +NH3 – HCN + H2O
CH4 + NH3 – HCN + 3H2
HCN + NH3 + CH2O – NH2-CH2-CN + H2O
NH2-CH2-CN + 2H2O – NH2-CH2-COOH + NH3
MILLER’S EXPERIMENT
BIOGENY
CONDITIONS FOR ORIGIN OF LIFE
• A supply of replicators or self-producing molecules
• Mutations during the copying of replicators (high temperature must
have played a major role)
• Continuous supply of free energy and partial isolation from the general
environment.
ORIGIN OF PREBIOTIC MOLECULES
Partial isolation has been attained with aggregates of artificially formed
prebiotic molecules. These aggregates are called protobionts, which can
separate combinations of molecules from the surroundings. They maintain an
internal environment but are unable to reproduce.
Two important protobionts are:
Coacervates. Oparin observed that if a mixture of large proteins and a
polysaccharide is shaken, coacervates are formed. They show simple form of
metabolism and as they don’t have lipid outer membranes, they cannot
reproduce.
Microspheres. When mixtures of artificially produced organic compounds are
mixed with coal water, microspheres are formed. Sydney Fox heated a mixture
of 18 amino acids at 130 - 180°C and obtained stable proteins like
macromolecules, which he named proteinoids.
STRUCTURAL PROPERTIES OF PROTENOID MICROSPHERES
Under an electron microscope, concentric double layered boundaries around
them have been observed through which diffusion of material occurs. They
have the ability of motility, growth, binary fission and a capacity of
reproduction by budding and fragmentation.
EVOLUTION OF ENZYME SYSTEM
Enzymes that control the types and rates of reactions within organisms are
proteins. These proteins are synthesized by a process that begins with the
transcription of information from DNA to mRNA.
rRNA has catalytic properties along with informational properties, due to
which the first genetic code was based on RNA catalyzed its own replication as
well as catalyzing other chemical reactions. The accumulated products of RNAcatalyzed reaction and form structures. For example, RNA could have catalyzed
the formation of lipid-like molecule that could have in turn formed plasma
membrane and proteins. Further, this protein could have been successful to
catalyze the synthesis of ither proteins.
Eventually, the proteins might have taken over most enzymatic functions
because they are better catalysts than RNA and are capable of more diverse
specific activities. If the first cells used RNA as their hereditary molecule, which
later evolved into DNA. DNA probably did not evolve as a hereditary molecule
until RNA-based life became enclosed in membrane. Once cells evolved, DNA
probably replaced RNA as genetic code for most organisms.
GENETIC REGULATION MECHANISMS
In cells of body, all genes not active all the time. A gene became active only
when the product of the genes required by the cell. It means a mechanism
controls the rate of polypeptide synthesis, amount of polypeptide and time at
which a gene should be active and which a gene should be inactive. This
control over polypeptide synthesis is known as gene regulation. It is studied
with the help of operons.
GENE REGULATION IN PROKARYOTES
Lactose operon in E.coli
Operon synthesis process was proposed by Jacob and Monad.
R
P
O
Z
Y
A
Operon. It is a part of genetic material which acts as a single regulated unit
having one or more structural genes – a regulator gene(R), a promoter gene(P),
an operator gene(O), a repressor and an inducer or compressor (structural
genes).
Operons are of two types – inducible and repressible.
INDUCIBLE OPERON
Switches on in response to the presence of a chemical. It consists of the parts:
1.
Structural Genes: synthesize mRNA. An mRNA controls metabolic activity of
cytoplasm through the formation of protein or enzymes over the ribosomes.
An operon can have one or more structural genes lac-operon has 3- Z,Y,A.
They transcribe polycistronic mRNA molecules that helps in synthesis of
three enzymes:
• β-galactosidase. For hydrolysing lactose to galactose
• Lactose or galactose permease. For allowing the entry of lactose from
outside the cell
• Galactose acetylase or transacetylase. Helps the bacteria in utilisation of
glucose by bacteria
2.
Operator Genes: directly controls the synthesis of mRNA.
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