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Nucleic Acids

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Nucleic Acids Nucleic acids are found in all living cells and form the genetic material, thus being responsible for storage and replication of genetic information. – PowerPoint PPT presentation

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Title: Nucleic Acids


1
  • Nucleic Acids
  • Nucleic acids are found in all living cells and
    form the genetic material, thus being responsible
    for storage and replication of genetic
    information.
  • Nucleic acids are also closely associated with
    protein synthesis and enhance the expression of
    genetic information .
  • Nucleic acids are high molecular weight polymers
    composed of structures called nucleotides as
    repeating units.
  • Nucleotides are joined by covalent bond to form
    a long molecule (polynucleotide). This
    joined nucleotide make a nucleic acids.
  • Both DNA and RNA are nucleic acid or
    polynucleotide.

2
  • A nucleotide is made up of three small
    molecules
  • A phosphate group or phosphoric acid
  • A pentose sugar either deoxyribose or ribose
  • A nitrogenous base, either thymine, adenine,
    guanine, cytosine or uracil.
  • If the nucleotides contain ribose, the resulting
    polynucleotide is ribonucleic acid (RNA)
  • If the nucleotides contain deoxyribose, the
    resulting polynucleotide is deoxyribonucleic
    acid (DNA)
  • DNA molecules are much longer than RNA molecules
    and quite enormous.

3
  • RNA (Ribonucleic acid)
  • Found in nucleus, mitochondria, chloroplast,
    cell wall
  • RNA nucleotide may contain several thousand
    nucleotides.
  • The four possible bases of RNA are adenine,
    guanine, cytosine, uracil and they never contain
    thymine.
  • Types of RNAs
  • Broadly classified as genetic RNA and non-genetic
    RNA.
  • Genetic RNA In most of the plant viruses, the
    genetic material is RNA as DNA is absent.
    Genetic RNA contain information which is
    normally found in DNA in higher organism( RNA
    replaced DNA)
  • Categories of genetic RNA
  • Double stranded RNA- generally follow rule of
    base paring as in case of DNA
  • Single stranded RNA in covalently closed circle
  • Single stranded RNA in a linear form

4
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5
  • Non-genetic RNA in organism where DNA is
    present, RNA does not serve as genetic material.
    The non-genetic RNA is synthesized on DNA.
  • Types of non-genetic RNA
  • Ribosomal RNA (rRNA)
  • Found in ribosomes of the cell,
  • Most stable type
  • Up to 80 of the total cellular RNA
  • In 70s type of ribosome, there are three
    molecules of RNA
  • In 80s type of ribosome, four molecules of RNA
    are found
  • rRNA exposed at the surface of the ribosome is
    double stranded the portion associated with
    internal proteins remains single stranded
  • Participate in translation translocation
    process of protein synthesis

6
  • 2) Messenger RNA (mRNA)
  • Consist of less than 5 of total cellular RNA
  • RNA has been isolated from many plants
  • Variable stability
  • Single stranded has no definite organisation
  • Carries the information needed for synthesis of
    protein from the nucleus to the sites of protein
    synthesis, hence called mRNA
  • Synthesized in the nucleus during transcription
    from one strand of DNA in the presence of enzyme
    RNA- polymerase.
  • Then transported through nuclear pore to the
    cytoplasm which spread on the surface of
    ribosomes participate in protein synthesis.
  • in eukaryotic cell, mRNA is not formed directly
    from DNA, rether it is processed through hnRNA

7
  • 3) Transfer RNA (tRNA)
  • consist of 15 of total cellular RNA
  • Is the smallest of RNA molecules being composed
    of 75-80 nucleotides.
  • Transfer RNA function as amino-acid carrying
    molecules during protein synthesis.
  • There is at least one specific tRNA for each of
    the twenty amino acids.
  • Freely present in cytoplasm
  • All tRNA contain several unusual bases due to
    methylation of normal bases.
  • tRNA looks like an L-shaped molecule in three
    dimensional view.
  • tRNA has two ends, one end to link with amino
    acid (acceptor end) the other anticodon end to
    link with mRNA.

8
  • DNA (Deoxyribonucleic acid)
  • DNA molecules contain two poly nucleotide
    strands. The strands are held together by
    hydrogen bonds between the bases.
  • Watson Crick in 1953 proposed that DNA
    molecule was double helix with two right handed
    polynucleotide chains coiled around a common
    axis.
  • The four possible bases of DNA are adenine,
    guanine, cytosine and thymine and DNA molecules
    never contain uracil.
  • Hydrogen bond can be formed only between
    complementary pairs cytosine with guanine and
    adenine with thymine.
  • This complementary base paring is the basis of
    the way in which information on DNA can be
    copied, either to be passed on to a new
    generation or to be used for building proteins.

9
DNA Nucleotide
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13
  • Because of highly polar phosphate located on the
    outside of double helix,DNA is a polyanion and
    readily binds various cations to maintain charge
    neutrality.
  • Phosphate group bind monovalent and divalent
    cations, various organic molecules such as amines
    and histones in higher cell chromosomes.
  • Different types of DNA
  • B-DNA Z-DNA. The alternative form differs in
    bases per turn spacing between base pairs along
    the helical axis.
  • Similarities
  • both have double helical organisation
  • Two strands in both run antiparallel
  • In both G C parings are found

14
Z- DNA B-DNA
It shows left handed helical sense It shows right handed helical sense
Phosphate backbone follows a zig- zag course Phosphate backbone is regular
The sugar molecules show alternating orientation so that repeating unit is dinucleotide Orientation of sugar molecules is not alternating the repeating unit is mononucleotide
One complete turn of DNA has 12 base pairs or 6 repeating dinucleotide One complete turn has only 10 base pair or 10 repeat units
The angle of twist per repeat unit is 60 The angle of twist per repeat unit is 34
The length of one complete turn is 45Å The length of one complete turn is 34Å
The diameter of DNA molecule is 18 Å The diameter of DNA molecule is 20 Å
15
Z
B
16
  • Enzymes and Co-enzymes
  • Mayrback (1952)- enzymes are simple or compound
    proteins acting as specific catalysts.
  • organic substances capable of catalysing
    chemical reactions in the living systems
  • Dixon and Webb (1964)- ENZYME is a protein with
    catalytic properties due to its power of specific
    activation.
  • enzymes are catalyst. catalyst are able to
    increase the rate of a chemical reaction without
    a increase in temperature. Catalyst must function
    by lowering the level of activation energy. When
    the product is formed, the catalyst is
    regenerated unchanged and can be used again.
  • very small amount of enzyme catalyze the
    conversion of large molecules of substrate (the
    substance in which the enzyme acts and which is
    thereby activated.

17
  • Nature of Enzymes
  • Enzymes are biocatalysts produced in the
    protoplasm.
  • Intra-cellular enzymes/endoenzymes which acts
    within the cell
  • Extra-cellular enzymes/ exoenzymes diffuse out
    of the cell to act upon some outside substrates
  • Properties of Enzyme
  • Enzymes are colloidal in nature- being colloidal
    in nature provides large surface area for a
    reaction to take place. Being colloidal in
    nature, enzymes are hydrophillic in nature anf
    form hydrosol in free state.
  • Enzymes can reacts with both acidic and alkaline
    substances (amphoteric in nature)
  • Enzyme show sensitivity
  • Enzyme molecules are larger than substrate
    molecules.

18
  • e. Enzyme are thermolabile (heat sensitive)
  • rise in temperature results in an increase in
    the rate of enzymic reactions until a temperature
    is reached when the rate falls sharply.
  • when the temperature increases, the rate at
    which enzyme-substrate collisions increases.
    Hence, the rate of reaction will also increases.
  • But, high temperature denatured the enzyme.
    High temperature cause hydrogen bonds and
    hydrophillic interactions to break in the enzyme
    molecules so that it begins to lose its shape.
  • optimum temperature the temperature at which
    the rate of reaction can take place fastest.
    Optimum temperature for plants 25C
  • f. enzymes are specific enzymes are specific in
    their actions, that is they react with only
    particular substrate. This type of nature of
    enzyme is called the specificity of enzyme.

19
  • g. Enzyme activity is controlled by pH
  • pH changes can affect the structure of an enzyme
    molecule and so it affects its ability to bind
    and act on its substrate.
  • - changes in pH affects ionic bonds that are
    holding the enzyme in shape, and may also affect
    the R- groups in the active site which form
    temporary bonds with the substrate.
  • Most enzymes act over only a very narrow range of
    pH. The optimum pH is normally around 7.
  • h. Enzyme lowers down the activation energy of
    reactions.

20
  • Structure and compositions of enzyme
  • enzyme are proteinaceous in nature
  • Enzyme may entirely consist of protein or may
    contain a non-protein part.
  • if an enzyme consists only of protein, it is
    called simple protein enzyme.
  • if it contains another group it is called
    conjugated protein enzyme.
  • many enzymes require the presence of another
    non-protein, non-catalyst substance in order to
    function. These substances are called coenzyme or
    cofactors.
  • biologist use cofactors for simple molecules and
    coenzyme for complex organic molecules.
  • Both coenzyme and cofactors works by binding with
    enzyme.
  • Coenzymes, which are tightly bound to the enzyme
    molecule is called prosthetic groups.
  • a true prosthetic group is one which remains
    attached to one enzyme throughout its whole
    catalytic activity.
  • AMP, ADP, ATP, NAD, NADP, FAD

21
Classification of enzymes Classified into six
major groups based on chemical reaction that they
catalyze 1. Oxidoreductases Enzyme
that catalyze oxidation-reduction reactions and
so are closely related to respiratory processes
in the cell. eg dehydrogenases, oxidases,
reductases 2. Transferases Catalyse the
transfer of variouss groups such as one-carbon
groups, glycosyl groups, or the phosphate groups
from substrate to acceptor molecule.
eg transaminases, kinases 3. Hydrolases
These enzymes catalyse a wide range of
hydrolytic reaction. They cause addition of water
to a variety of bonds and results in cleavage of
the substrate.
22
4. Lyases results in direct removal of
groups from substrate nonhydrolytically.
eg fumarase, carboxylase, aldolase 5.
Isomerases enzyme that bring about
isomerizations. It includes the racemerases and
epimeraces which catalyse the
interconversion of sterioisomers of amino acids
and sugars respectively. 6. Ligases/ synthetases
these enzyme catalyse the synthesis of
different types of bonds such as C-C,
C-N, C-S, C-O eg pyruvate
carboxylase
23
  • Mode of Enzyme action
  • Key-Lock theory
  • Proposed by Fischer but later advanced by Paul
    Fildes D.D. Woods.
  • As a particular lock can be opened by a
    particular key, in the same way a particular
    enzyme acts on a particular substrate. The
    substrate molecule gets fitted upon an enzyme and
    decomposes into products.
  • The shape of the active site (binding site) of
    the enzyme, matches the shape of the substrate.
    Allowing the two molecules to bind during the
    chemical reaction.

24
  • Induced fit-theory
  • By Michaelis-Menton
  • active site of the substrate and enzyme fit into
    each other and they may combine to form an active
    complex.
  • all enzyme has active sites. In most of the
    enzymes, when substrate slots into active site,
    the shape of the whole enzyme changes slightly so
    that it can accommodate the substrate better.
    This is called induced fit.
  • The arrival of the substrate molecules causes or
    induces, a change in the shape of enzyme which
    makes the substrate fit perfectly into the active
    site.
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