Structure of DNA for medical school
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V.S.RAVIKIRAN, MSc.
DEOXYNUCLEIC ACID (DNA)
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V.S.RAVIKIRAN, MSc., Department of Biochemistry,
ASRAM Medical college, Eluru-534005.AP, [email protected]
om
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NUCLEIC ACIDSV.S.RAVIKIRAN
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The “central dogma” of Molecular Biology
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NUCLEIC ACIDS: INFORMATIONAL POLYMER
• Nucleic acids are polymers of monomer units called nucleotides
• Nucleic acids store and transmit hereditary information: gene
• Two major forms of nucleic acid polymers: DNA and RNA
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DNA(Deoxyribonucleic Acid)
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DNA Structure
DNA is a nucleic acid.The building blocks of DNA are nucleotides,
each composed of:– a 5-carbon sugar called deoxyribose– a phosphate group (PO4)
– a nitrogenous base• adenine, thymine, cytosine, guanine
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DNA Structure
The nucleotide structure consists of– the nitrogenous base attached to the 1’ carbon of
deoxyribose– the phosphate group attached to the 5’ carbon of
deoxyribose– a free hydroxyl group (-OH) at the 3’ carbon of
deoxyribose
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DNA Structure
Nucleotides are connected to each other to form a long chain
phosphodiester bond: bond between adjacent nucleotides– formed between the phosphate group of one
nucleotide and the 3’ –OH of the next nucleotideThe chain of nucleotides has a 5’ to 3’
orientation.
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DNA Structure
Determining the 3-dimmensional structure of DNA involved the work of a few scientists:
– Erwin Chargaff determined that • amount of adenine = amount of thymine• amount of cytosine = amount of guanine
This is known as Chargaff’s Rules
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Chargaff
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DNA Structure
Rosalind Franklin and Maurice Wilkins– Franklin performed X-ray diffraction studies to
identify the 3-D structure– discovered that DNA is helical– discovered that the molecule has a diameter of
2nm and makes a complete turn of the helix every 3.4 nm
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Franklin
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DNA Structure
James Watson and Francis Crick, 1953– deduced the structure of DNA using evidence
from Chargaff, Franklin, and others– proposed a double helix structure
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DNA Structure
The double helix consists of:– 2 sugar-phosphate backbones– nitrogenous bases toward the interior of the
molecule– bases form hydrogen bonds with complementary
bases on the opposite sugar-phosphate backbone
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DNA Structure
The two strands of nucleotides are antiparallel to each other– one is oriented 5’ to 3’, the other 3’ to 5’
The two strands wrap around each other to create the helical shape of the molecule.
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Double Helix• 2 complementary strands of DNA
– sugar-phosphate backbone– nitrogenous bases stacked in the center– antiparallel
• 5’ end• 3’ end
– twists to right
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Base Pairing in DNA: The Watson-Crick Model
According to the Watson–Crick model, a DNA molecule consists of two polynucleotide strands coiled around each other in a helical, screwlike fashion.
The sugar–phosphate backbone is on the outside of this right-handed double helix, and the heterocyclic bases are on the inside, so that a base on one strand points directly toward a base on the second strand.
The double helix resembles a twisted ladder, with the sugar–phosphate backbone making up the sides and the hydrogen-bonded base pairs, the rungs.
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AT
GC
TA
CG
CG
GC
AT
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Chargaff’s Rules• base pairing
rules– A=T– C G
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Prentice Hall © 2007 Chapter Twenty Six 40
Hydrogen bonds connect the pairs of bases; thymine with adenine, cytosine with guanine.
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Structure of DNA
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DNA double Helix
Chargaff’s rules: the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine, and the total amount of purines equals the total amount of pyrimidine.
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Hydrogen bonds between complementary basesFORMAMIDE, ANNEALING
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The three helical forms of DNA (and RNA)
PhysiologicalDNA
Very unusualDNA
RNAHigh-salt
DNA
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Type Shape Helix Base pair per turn
Pitch per bp
Width Occurrence
A Broadest Righthanded
11 0.256 nm 2.3 nm High saltMedium
B Intermediate
Righthanded
10 0.338 nm 1.9 nm NormalForm
Z Elongated Lefthanded
12 0.571 nm 1.8 nm Some ofDNA
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B-form DNA consists of a right-handed double helix with antiparallel strands
34 Å (10 bp) per turn
major groove
minor groovemajor groove
minor
3.4 Å per bp
These dimensions are for DNA fibers. In solution, there are ~10.5 base-pairs per turn.
5’ 3’
5’3’
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Pharm201 Lecture 2 2007 50
V o e t, D o n a ld a n d Ju d ith G . B io c h e mis try.Jo h n Wile y & S o n s , 1 9 9 0 , p . 8 0 0 .
Canonical A DNA
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Pharm201 Lecture 2 2007 51
Z-DNA
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Summary of the main structural features of B-form DNA
•Right-handed helix
•Two antiparallel strands held together by
Watson-Crick hydrogen bonds
•Pitch (repeat length) = 34 Å (3.4 nm)
•36o rotation between residues
•Helix diameter of 20 Å (2.0 nm)
•Wide major groove, narrow minor groove
•Chargaff’s Rules: A = T; G = C
•Charged phosphates
•Bases in anti configuration
•The strands separate at high temperatures
•The solution structure is dynamic
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DNA
• Functions• 1. Storage of genetic information• 2. Self-duplication & inheritance.• 3. Expression of the genetic message.• DNA’s major function is to code for proteins.• Information is encoded in the order of the
nitrogenous bases.
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Figure 11.6
DNAdoublehelix(2-nmdiameter)
Metaphase chromosome
700nm
Tight helical fiber(30-nm diameter)
Nucleosome(10-nm diameter)
Histones
“Beads ona string”
Supercoil(200-nm diameter)
HISTONES
H1, H2A, H2B, H3, H4
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