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Biochemistry of meiosis
Studies of male mammals using special fractionation technique to
identify metabolic characteristic of prophase meiosis stage .
1-Prezygonema
There is a premeiotic DNA replication {s-phase} which is 100-200
times longer than that in somatic cells .However ,0.3-0.4 present of the
DNA remains un-replicated ,this is due to the reduction of initiation
points during DNA duplication and to the DNA remains un-replicated.
2-Zygonema
During this stage un-replicated DNA is synthesized, this is called ZDNA because it occurs at this stage in coordination with chromosome
pairing .This late replicating Z-DNA appears to be essential to synopsis
,but if it is inhibited
,the cells are arrested at Zygonema .Another
biochemical characteristic of this period is the appearance of a DNA
binding protein which is associated with a lipoprotein fraction and which
catalyzes the re-association of single –stranded DNA .This protein has
been called re-association or re-protein ,and it has been suggested that it
may play a role in the alignment of homologous stretched of DNA.
3-Pachynema
If protein synthesis is inhibited at late Zygonema the chromosome
pairs fall a part and are not reconstituted .During this stage there is also a
DNA stage which is of a smaller magnitude than that of Zygonema {PDNA}.
Energy
Energy is defined as the capacity to do work ,to bring about a
change. There are several different forms of energy : mechanical ,light,
electrical, and heat energy .Another form of energy is that which stored in
the bonds that hold atoms together .
Energy can be transformed from one form to another ,for example ,
chemical energy in food provides the energy to muscles to do mechanical
work .
Electrical energy is transformed into light energy within light bulb
,and the chemical bond energy of oil is transformed into electrical energy
at a power plant {i.e. mitochondria} ,the latter does not create energy ,lit
merely transforms it .
The first low energy states :
Energy can be transformed from one form into another ,but it
cannot be created or destroyed ,there is a system that has surrounding ,the
last supply the food and when it used to do work it results in heat that is
accepted by the surroundings .
The first energy low ,mean that if we put {x} amount of energy
into a system ,then eventually {x} amount of energy will leave the system
even if work has been done .Energy usually leaves in the form of heat
,besides the amount of useful energy has changed ,i.e. food energy is used
to power muscles this is no longer available because it has been converted
to heat .
The second energy low states :
When one form of energy is transformed to another form some
useful energy is always lost as heat ,therefore energy cannot be recycled .
The second energy low implies that only process that decrease the
amount of useful energy occur naturally or spontaneously ,for example
,after death organisms decay and eventually disintegrate ,in other words
,the amount of disorder{entropy} is always increasing in the universe ,so
there is a continual input of energy and eventually increases the entropy
of universe .The living things are continually taking in useful energy and
putting out heat just like the nonliving system .Living organism
represents stored energy in the form of chemical compounds .The useful
energy stored in one organism can be used by another to maintain its own
organization .The source which supplies all living things with energy is
the sun ,photosynthesizing organisms like plant and algae are able to
capture less than 2% of the solar energy that reach the earth .
Living things continually lose useful energy to the environment
,but a new supply comes to them in the for of organic food produced by
photosynthesizing organisms.
Metabolism involves energy exchanges
The food's organism eats becomes nutrients molecules for cells
,within cells ,these molecules take part in a chemical reactions termed the
metabolism {meta :change } of the cell .Nutrient molecules have to be
broken down {oxidized} i.e., removal of an electron either alone or in the
form of hydrogen atom ,as oxidation occurs energy is released .
The cell needs energy to maintain its structure and to grow ,i.e.,
some of the nutrient molecules are used to form the structure of the cell
,to make membrane ,amino acids {joined to form proteins} ,fatty acids
.There are also synthetic reactions in which reduction occurs ,it happens
when hydrogen atoms are added to a molecule .
There are thousands of reaction occurring in cells {organelles } ,these are
organized into metabolic pathways . Some of these are release energy ,ex.
, when CHO are broken down into mitochondria ,others that require
energy input when lipid are synthesized in smooth endoplasmic
reticulum.
Enzymes speed reactions
Every reaction within a metabolic pathway requires an enzyme
,it is organic catalysts ,usually globular protein molecules that speed
chemical reactions without being permanently changed .In addition some
RNA molecule particularly in the nucleus ,can be enzymes and these are
called riboenzymes .Every enzyme is very specific in its action and can
speed only one particular reaction or one type of reaction ,enzymes name
is formed by adding {ase} term to the name of its substrate ,ex., lipid as a
substrate { lipase} ,urea {urease} ,maltose {maltase} ,lactose {lactase}
,ribonucleic acid {ribonuclease} .Some enzymes are named for the action
they perform ,a dehydrogenase is an enzyme that removes hydrogen
atoms from its substrate ,almost no reaction can occur in a cell unless its
own enzyme is present and active .
Molecules don't react one another unless they are activated in
some way ,in the laboratory by heating the reaction increase the number
of effective collisions between molecules ,so this energy that must be
added to cause molecules to react with one another is called the energy of
activation {Ea} .Enzymes speed the rate of chemical reactions because
they lower the amount of energy required to activate the reaction ,ex., the
hydrolysis of casein {protein found in milk} requires an energy of
activation 20.600 kal/mole in the absence of an enzyme ,but only 12.600
kal/mole in the presence of an enzyme. Enzymes lower the energy of
activation by forming an enzyme –substrate complex .
Enzyme –substrate complexes
Enzymes{E} are proteins with one or more loci ,called active
sites ,to which the substrate {S} attached {i.e. the substrate upon which
the enzyme acts} .Enzyme and substrate seems like a key fits a lock
,active site undergoes a slight change in shape in order to accommodate
the substrate more perfectly ,this called the slight alteration to achieve an
optimum fit ,and the change in shape of the active site facilitates the
reaction . The substrate is chemically modified and converted into one or
more products {P} since this is generally a reversible reaction .
E+S
ES
E + P
ES is an intermediary enzyme-substrate complex .Enzymes accelerate the
reaction until equilibrium is reached .After the reaction has been
complete ,the active site returns to its original state .Only small amount of
enzyme is actually needed in a cell because enzymes are used repeatedly.
Moreover ,some enzymes actually participate in the reaction ,e.g., trypsin
digest protein by breaking peptide bonds ,the active site of trypsin
contains three amino acids with R group which interact with a membrane
of the peptide bond ,first by braking the bond and then to introduce the
components of water .This shows that the formation of enzyme –substrate
complex is very important in speeding up the reaction.
A very important feature of enzyme activity is the substrate specific
,i.e., a particular enzyme will act only on a certain substrate .Some
enzymes have nearly absolute specificity for a given substrate ,e.g.,
stereoisomer of the same molecule.