Understanding DNA Structure I619 Structural Bioinformatics
Molecular Biology Basics + Scale total length of DNA in a human cell is about 2m DNA is compacted in length by a factor of 10000 the compaction could be higher (if DNA was a ball of string) DNA of one chromosome is very long and narrow (expanded scale: length = 30km, diameter = 2mm. DNA carries genetic material
Molecular Biology Basics - Chromosomes human DNA is stored in 46 chromosomes 22 homologous pairs plus X and Y chromosomes that determine sex fruit fly has 8 chromosomes (3 homologous pairs + X and Y) Chromosomes just after duplication before cell division, no pictures of chromosomes exist in their extended form
Molecular Biology Basics - Chromosomes Drosophila chromosomes in its extended form. Drosophila chromosomes further magnified. A chromosome is split into bands and interbands
Molecular Biology Basics - Chromosomes Scale: each DNA-protein spool is about 100Ǻ = 10 8 m. Wrapping twice about each spool reduces DNA length by about 6 times.
Molecular Biology Basics - DNA DNA double helix. Diameter of the helix is about 20 Ǻ Two separated strands of DNA. Each nucleotide is about 6 Ǻ wide.
Nucleotides, very basic Each nucleotide (about 20 atoms) contains: (a) sugar, (ii) phosphate; and a (iii) base. The phosphate group.
Nucleotides to Amino Acids
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Why a Helix? Let s start from scratch! Phosphates are very soluble in water. Sugars are very soluble in water. Bases are insoluble (different bases dissolve at different ph, but not ph = 7).
A ladder formed by two strands of DNA. Ladder
Skewed Ladder A skewed ladder formed by two strands of DNA.
Different ways of stacking Stacking of base-pairs. Stacking of typs (b) is sterically preferred and ultimately will be leading to helical conformation.
Helical Conformation Base pairs wrap around an imaginary cylinder of radius 9Ǻ. This, using simple geometry we can calculate that θ = 32.3 since P 11 is directly above P 0. Typical θ in practice ranges from 20 to 50, with the mean of 34.
Helical Conformation B DNA: 10 phosphates per turn; A DNA: 11 phosphates per turn; ZDNA: 12 phosphates per turn
What allows this flexibility? Flexibility of the sugar-phosphate chains is considered to be the origin of flexibility.
Watson-Crick Base Pairing
Hoogsteen Base Pairing
Other Pairings?
Back to Bases (and Nucleotides)
Connecting Nucleotides
Basic DNA and RNA Structure Components Sugar Base Phosphate 5 to 3 direction RNA ribose - extra OH at 2 of ribose DNA deoxyribose Numbering Voet, Donald and Judith G. Biochemistry. John Wiley & Sons, 1990, p. 792.
The 5 Bases of DNA and RNA The Nucleotides Purines Pyrimadines and Purines T->U in RNA Names Numbering Bonding character Position of hydrogen Tautomers Pyrimadines Neidle, Stephen. Nucleic Acid Structure and Recognition. Oxford University Press, 2002, p. 18.
Kinds of Double Helix driving force for helix formation is the property of bases to exclude water Base pairs typically found in propeller twists 6 degrees of freedom to move one base pair with respect to the other not all degrees are sterically allowed
Kinds of Double Helix most movements are well described by a twist, a roll and a slide.
Kinds of Double Helix
History 1946 DNA is the main constituent of genes (Avery) 1950 First X-ray pictures of DNA (Franklin) 1953 DNA structure revealed (Watson and Crick) 1970 onwards - Multiple conformations and structures 1973 X-ray structure confirms double helix (Rich) 1974 t-rna structure (Kim) 1980 Structure of first complete turn of B DNA (Dickerson)
History http://images.google.com/imgres?imgurl=http://www.achievement.org/achievers/wat0/large/wat0-001.jpg&imgrefurl=http://www.achievement.org/autodoc/photocredit/achievers/wat0-001&h=311&w=400&sz=37&tbnid=dc39azwoazql9m:&tbnh=96&tbnw=124&prev=/images%3fq%3djames%2bwatson%26um%3d1&start=1&sa=x&oi=images&ct=image&cd=1
References Majority of figures from Understanding DNA by Calladine et al. Some figures from Fundamental concepts of Bioinformatics: by Krane and Raymer Some slides made by Phil Bourne, UCSD