Download Structure of Nucleic Acids

DH. DALIA AL-KHATEEB
LACTURE 4
BIOCHEMISTRY/ NUCLEIC ACID
Nucleic Acid Definition
Nucleic acids are essential large biological molecules for all forms of life.
The nucleic acids include the DNA and the RNA. They are the hereditary
determinants of living organisms. They are present in most living cells
either in free state or bound to proteins as nucleoproteins. The nucleic
acids are biopolymers with mononucleotides ad their repeating units. The
monomers are known as nucleotides, they are made up of three units: a
sugar, an amine and a phosphate group.
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The nitrogenous bases found in DNA are adenine, guanine, cytosine
and thymine.
Adenine and guanine are purine bases while cytosine and thymine
are pyrimidines.
In RNA, the thymine bases id replaced by uracil which is also a
pyrimidine.
The nucleotides are linked with phosphodiester bonds.
They are linked by a phosphate group on the 5th position of sugar
residue becomes linked to 3' hydroxyl group of the proceeding
sugar molecule.
The order of the strand is in opposite directions, i.e., from 5' to 3'
direction in one and 3'to 5' direction in the other.
The bases stacked in the center of the helix as they interact with
each other through weak hydrogen bonds.
Hydrogen bonds are weaker than covalent bonds.
In the double stranded nucleic acids, the adenine form hydrogen
bonds only with thymine (or uracil) molecule. While cytidine will
hydrogen bonds with guanine.
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DH. DALIA AL-KHATEEB
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LACTURE 4
BIOCHEMISTRY/ NUCLEIC ACID
The base pairs form a flat plain in the helix, the adenine forming
two hydrogen bonds with thymine, and the cytidine forming three
bonds with guanine.
Using the concept of base pairing, all the enzymes and substrates
necessary, the two DNA strands when copied separately,
wherever there is adenine in the original strand, the duplicated
strand will have thymidine, and guanine would be matched to
cytosine.
After replication, each original strand (parent strand), is paired with
a new (daughter) strand. This type of replication is known as semiconservative mode of duplication.
The double stranded DNA can be denatured by alkaline conditions
or heat.
Types of Nucleic Acids;Nucleic acids are of two types DNA and RNA.
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DH. DALIA AL-KHATEEB
LACTURE 4
BIOCHEMISTRY/ NUCLEIC ACID
DNA (deoxyribonucleic acid)
DNA is one of the macromolecules, they are essential to all living
forms .Deoxyribonucleic acid contains the genetic information, it is used
in the development and functioning of all living organisms. The DNA
segments carry genetic information are called the genes. Other DNA
segments have structural functions or regulate the genetic
information. DNA are made of two chains made of polymer units of
nucleotides. The backbones of DNA are made of sugar and phosphate
groups which are joined by ester bonds. The two strands of DNA are antiparallel, they run in opposite directions .Each sugar molecule is attached
to one of the four nucleobases. The nucleobases encode genetic
information, that is read using the genetic code. Inside the cell, the DNA
are arranged in long structures called chromosomes. The chromosomes
are duplicated in the process of DNA replication, during cell
division. Each cell has its own one complete set of chromosomes.
RNA (ribonucleic acid)
The functions of ribonucleic acid is to convert genetic information from
genes into amino acid sequences of protein. In some viruses, RNA
contains the genetic information.
RNA Types;They are tRNA (transfer RNA), mRNA (messenger RNA) and rRNA
(ribosomal RNA).Messenger RNA, as the name suggests acts a
messenger. It carries genetic information sequences between DNA and
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DH. DALIA AL-KHATEEB
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BIOCHEMISTRY/ NUCLEIC ACID
ribosomes, and it also directs protein synthesis. rRNA is a major
component of the ribosomes, they catalyze the formation of peptide bond.
The tRNA act as a carrier molecule for the amino acids that are used in
protein synthesis. The tRNA are also responsible for decoding the
mRNA.
RNA directs synthesis of proteins.
1-m-RNA takes genetic message from RNA.
2-t-RNA transfers activated amino acid, to the site of protein synthesis.
3-r-RNA are mostly present in the ribosomes, and responsible for
stability of m-RNA.
Structure of Nucleic Acids
Structure of nucleic acids DNA and RNA are similar. The structure is
divided into four different levels, primary, secondary, tertiary and
quaternary.
Primary Structure:
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Primary structure of nucleic acids is a linear sequence of nucleotides,
which are linked to each other by phosphodiester linkages. Nucleotides
are made up of three components - Nitrogenous base, 5-carbon sugar and
phosphate groups.
Nitrogenous
base
are
purines
(adenine,
guanine) and
pyrimidines {cytosine, thymine (present in DNA only) , uracil (present
in RNA only)}. The 5-carbon sugar is deoxyribose for DNA and and
ribose sugar in RNA. The purine bases, form glyosidic bond between
their 9' nitrogen and the 9' - OH group of the sugar molecule.
The pyrimidine bases, they form glyosidic bond between 1' nitrogen and
the 9' -OH of the deoxyribose. In both purine and pyrimidine bases the
phosphate group forms a bond with the sugar molecule between one of its
negatively charged oxygen groups and the 5' -OH of the sugar.
Nucleotides forms phosphodiester linkages between the 5' and 3' carbon
atoms, these form the nucleic acids. Nucleotides sequences are
complementary to one another.
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DH. DALIA AL-KHATEEB
LACTURE 4
BIOCHEMISTRY/ NUCLEIC ACID
Example of complementary :-
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sequence AGCT is TCGA.
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Secondary Structure .
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Secondary structure is the interaction between the bases. This structure
shows parts of which strands are bound to each other. The two strands of
DNA in the double helix of the DNA are bound to each other by
hydrogen bounds. The nucleotides on one strand base pairs with the
nucleotides of the other strand. The secondary structure of the DNA is
predominantly the base pairing of the two polynucleotide strands forming
a double helix.
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DH. DALIA AL-KHATEEB
LACTURE 4
BIOCHEMISTRY/ NUCLEIC ACID
Tertiary Structure
Tertiary structure is the three dimensional shape into which the entire
chain is folded. Tertiary structure arrangement differs in four structural
forms:
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2.
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Left or right handedness.
Length of the turn of the helix.
Number of base pairs per turn.
The difference in size between major and the minor groove.
Quaternary Structure
Quaternary structure is the higher-level of organization of the nucleic
acids. This structure refers to the interactions of the nucleic acids with the
other molecules. The most commonly seen organization is the form of
chromatin which shows interaction with small proteins histones.
Functions of Nucleic Acids;Functions of nucleic acids are:
1-The main functions is store and transfer genetic information.
2-To use the genetic information to direct the synthesis of new protein.
3-The deoxyribonucleic acid is the storage for place for genetic
information in the cell.
4-DNA controls the synthesis of RNA in the cell.
5-The genetic information is transmitted from DNA to the protein
synthesizers in the cell.
6-RNA also directs the production of new protein by transmitting genetic
information to the protein building structures.
7-The function of the nitrogenous base sequences in the DNA backbone
determines the proteins being synthesized.
8-The function of the double helix of the DNA is that no disorders occur
in the genetic information if it is lost or damaged.
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