Nucleic Acids Structure and Functions
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Transcript of Nucleic Acids Structure and Functions
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Nucleic Acids Structure and FunctionNucleic Acids Structure and Function
Dr. Atif H. Khirelsied
Department of Biochemistryp yFaculty of Medicine
International University of Africay
atifkhirelsied@gmail
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The learning objectives
By the end of this session the student should be able to:
g j
By the end of this session the student should be able to:
1 Outline the molecular structure of DNA and RNA1. Outline the molecular structure of DNA and RNA.
2. Explain the significance of specific base pairing.
3 D fi th i ti f DNA3. Define the main properties of DNA.
4. Define the central dogma of molecular biology.
5. Distinguish between single‐ stranded and double‐stranded nucleic acids.
6 E l i h i f d i d li f DNA6. Explain the meaning of denaturation and re‐annealing of DNA.
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The deoxyribonucleic acid (DNA)
• DNA: is the store of the genetic information
The deoxyribonucleic acid (DNA)
• DNA: is the store of the genetic information.
• It is the principal regulator of the cell physiology• It is the principal regulator of the cell physiology.
• DNA contains the instructions for creation and functioning
of the organism it acts as;of the organism, it acts as;
Template for replication.
Codes for proteins.
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The central dogma of molecular biology
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The Watson‐Crick structure of DNA
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The Watson‐Crick structure of DNA
1. It is a double‐stranded right‐handed helixhelix.
2. Each strand is is a polymer of 2‐. ach strand is is a polymer ofdeoxyribonucleotides linked by 3'‐5'phosphodiester links.
3. The nitrogenous bases in the nucleotides of DNA are A G C Tnucleotides of DNA are A,G,C,T.
4. The bases in the two complementary p ystrands are specifically paired and heldtogether by hydrogen bonds.
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The Watson‐Crick structure of DNA
5. The two strands are polar and anti‐parallelparallel .
6. The bases are planner and 6. The bases are planner andperpendicular to the axis of the DNA molecules.
7. The width of the DNA molecules is 2 nm (20 A°) and the height of the helical(20 A ), and the height of the helical turn is 3.4 nm (34 A°).
8. The genetic information resides in the sequence of bases in the DNA strands.
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The Watson‐Crick structure of DNA
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The Watson‐Crick structure of DNA
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The Watson‐Crick structure of DNA
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The structure of DNA
• DNA exists as circular molecule in some organisms such as
bacteria, bacteriophages, and many DNA‐containing animal
virusesviruses.
Circular DNA molecule of bacteriaChromosome (DNA) of eukaryotic
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DNA denaturation of
Denaturation is the melting of double stranded DNA t t t i lDNA to generate two single strands.
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The denaturation of DNA
• Denaturation may result from:
– Heating above its Tm.
– High pH
– Organic solvents (dimethyl sulfoxide).
– Lowering the salt conc. of the DNA solutiong
• Viscosity of DNA solutions decreases on denaturation.y
• Hyperchromicity (increased absorbance of UV on• Hyperchromicity (increased absorbance of UV on denaturation)
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The Reannealing of DNA
R li i th f ti f th d bl t d d• Reannealing is the reformation of the double stranded DNA.
• Viscosity of DNA solutions increases on reannealing.
• Hyporchromicity (decreased absorbance of UV on reannealing)
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The RNA structure differs from that of DNA
The RNAs are polymers of ribonucleotides linked together by
3'‐5' phosphodiester bonds.
RNAs share many structural features with DNA but have some
specific characteristics, these are:
it is usually single rather than double stranded– it is usually single rather than double stranded
– it contains ribose rather than deoxyribose
it contains the base uracil rather than thymine– it contains the base uracil rather than thymine
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The types of RNA
1 M RNA ( RNA )
yp
1. Messenger RNAs (mRNAs).
2. Transfer RNAs (tRNAs). Prokaryotes and ( )
3. Ribosomal RNAs (rRNAs).eukaryotes
4 ll l RNA ( RNA)4. small nuclear RNA (snRNA).
5. microRNA (miRNA). Only in eukaryotes
6. small interfering RNA (siRNA)
Only in eukaryotes
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The messenger RNA (mRNA)
• Heterogeneous in size and stability.
g ( )
• Carries the codons specifying amino acid sequences in proteins
• Act as templates for protein synthesis.
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The messenger RNA (mRNA)
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The table of genetic code
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Eukaryotic mRNAs have several unique properties
1 Th 5' t i l i d b 7 th l i
y q p p
1. The 5'‐terminal is capped by 7‐methylguanosine triphosphate.
2. Has poly(A) “tail” in the 3'‐terminals.
7‐mGA‐A‐A‐A‐A‐A‐A
mRNA
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Eukaryotic mRNAs have several unique propertiesy q p p
Eukaryotics is synthesized as pre‐mRNA (heteronuclear RNA) which contain intervening regions (Introns).
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Transfer RNA (tRNA)
• They are the smallest RNAs• They are the smallest RNAs.
• Fold into tertiary structure byFold into tertiary structure by formation of intra‐molecular base pairs .
• Have clover leaf shape (hair‐pin).
• Transfer amino acids into the ribosomesribosomes.
• Act as an adaptormolecule.Act as an adaptor molecule.
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Ribosomal RNA (rRNA)
• Have structural and catalytic functions.Have structural and catalytic functions.
• They have a ribozyme function, the 28S rRNA catalyze the synthesis of peptide bonds (peptidyl transferase).
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Ribosomal RNA (rRNA)
• It is a nucleoprotein particle.
• The mammalian ribosome, has a sedimentation velocity of 80S (Svedberg units).
C i f l 60S d• Consists of large 60S and small 40S subunits.
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Small nuclear RNA (snRNA)
• A large number of highly conserved small stable molecules of RNA found in eukaryotic cells.
• The majority of them exists as nucleoproteins distributed in both the nucleus and the cytosol.
• They are involved in mRNA processing and gene regulation.
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microRNA (miRNA)
• Are short RNA molecules found in eukaryotic cells.
• Have very few nucleotides (an average of 22) compared with hother RNAs.
• Have multiple roles in regulation of gene expression• Have multiple roles in regulation of gene expression.
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Small interfering RNA (siRNA)
Also known as short interfering RNA or silencing RNAg g
Are double‐stranded RNA molecules
Have 20‐25 nucleotides in length
They interfere with the expression of a specific gene.