DNA structure, Functions and properties
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DNA Structure, Functions and
Properties
Biochemistry for Medicswww.namrata.co
1Biochemistry for Medics
By- Professor(Dr.) Namrata ChhabraS.S.R.Medical College,University of Mauritius,Mauritius
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DNA
•DNA - a polymer of deoxyribo nucleotides
•found in chromosomes, mitochondria and chloroplasts
• carries the genetic information
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Components of a nucleotide
Base
Sugar
Phosphate
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Nucleotide
Nucleoside
Base
Phosphate
Sugar
X=H: DNAX=OH: RNA
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Basic structure ofpyrimidine and purine
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Pyrimidines
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Purines
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Nomenclature of Nucleic Acid Components
Base Nucleoside Nucleotide Nucleicacid
PurinesAdenine Adenosine Adenylate RNA
Deoxyadenosine Deoxyadenylate DNAGuanine Guanosine Guanylate RNA
Deoxy guanosine Deoxyguanylate DNA PyrimidinesCytosine Cytidine Cytidylate RNA
Deoxycytidine Deoxycytidylate DNAThymine Thymidine Thymidylate DNA
(deoxythymidine) (deoxythymidylate)Uracil Uridine Uridylate RNA
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5-Fluorouracil 6-Mercaptopurine
Anticancer agents
Azidothymidine Dideoxyinosine
Antiretroviral agents
Nucleoside and base analogs can be used as anti-cancer and anti-virus drugs
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DNA structure
Primary structure
Secondary structure
Tertiary structure
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The primary structure of
DNA is the sequence5’ end
3’ end
5’
3’
Phosphodiesterlinkage
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Traditionally, a DNA sequence is drawn from 5’ to 3’ end.
A shorthand notation for this sequence is ACGTA
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The secondary structure of DNA is the double helix
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The secondary structure of DNA
Two anti-parallel polynucleotide chains wound around the same axis.
Sugar-phosphate chains wraparound the periphery.
Bases (A, T, C and G) occupy thecore, forming complementary A · Tand G · C Watson-Crick base pairs.
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hydrogen
bonding;
base stacking
The DNA double helix is held
together mainly by- Hydrogen
bonds
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Hydrogen bond
a chemical bond in which a hydrogen atom of one molecule is attracted to an electronegative atom, especially a nitrogen, oxygen, or fluorine atom, usually of another molecule.
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Two hydrogen bonds between A:T pairsThree hydrogen bonds between C: G paired
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Base Stacking
The bases in DNA are
planar and have a
tendency to "stack".
Major stacking forces:
hydrophobic interaction
van der Waals forces.
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Structural forms of DNA
Property A-DNA B-DNA Z-DNA
Helix Handedness
Right Right Left
Base Pairs per turn
11 10.4 12
Rise per base pair along axis
0.23nm 0.34nm 0.38nm
Pitch 2.46nm 3.40nm 4.56nm
Diameter 2.55nm 2.37nm 1.84nm
Conformation of Glycosidic bond
anti anti Alternating anti and syn
Major Groove Present Present Absent
Minor Groove Present Present Deep cleft19Biochemistry for Medics
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Helical sense: right handed
Base pairs: almost perpendicular to the helix axis; 3.4 Å apart
One turn of the helix: 36 Å; ~10.4 base pairs
Minor groove: 12 Å across
Major groove: 22 Å across
Normally hydrated DNA: B-form DNA
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In eukaryotic cells,
DNA is folded into chromatin
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DNA Tertiary Structure
•DNA DOUBLE HELICAL STRUCTURE COILS ROUND HISTONES.
•DNA BOUND TO HISTONES FORMS
NUCLEOSOMES (10nm FIBRES)
•NUCLEOSOMES CONTAIN 146 NUCLEOTIDES
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Nucleosomes
any of the repeating globular subunits of chromatin that consist of a complex
of DNA and histone
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Structure of nucleosome core
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Compaction of DNA in a eukaryotic chromosome
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Supercoil = coil over coil
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DNA melting and annealing
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Reversible denaturation and annealing of DNA
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Melting point (tm) of DNA
The temperature at the mid-point of the transition
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The tm of DNA depends on:
size of DNA pH ionic strength
Content of G·C base pairs
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31Biochemistry for Medics
Functions of DNA and summary of structure
DNA consists of four bases—A, G, C, and T—that are held in linear array by phosphodiester bonds through the 3' and 5' positions of adjacent deoxyribose moieties.
DNA is organized into two strands by the pairing of bases A to T and G to C on complementary strands. These strands form a double helix around a central axis.
The 3 x 109 base pairs of DNA in humans are organized into the haploid complement of 23 chromosomes.
DNA provides a template for its own replication and thus maintenance of the genotype and for the transcription of the roughly 30,000 human genes into a variety of RNA molecules.