Structure Does DNA fit the requirements of a hereditary material? REQUIREMENT DNA Component Has biological y useful information to Genetic code: 3 bases code for 1 make protein amino acid(protein) Must reproduce faithful y and transmit Complementary bases are faithful: to of spring found in germ cel s Must be stable within a living organism Backbone is strong covalent : hydrogen bonds Must be capable of incorporating stable Bases can change through known changes mechanisms
THE EARLY EFFORTS • By the early 1900’s it was known that the chromosomes carry the genetic (hereditary) information • Chromosomes consist of DNA (deoxyribonucleic acid)
Gregor Mendel: Introduces the concept of heredity 1865 1909 1911 1929 1944 1950
Wilhelm Johannsen: Coins the term “Gene 1865 1909 1911 1929 1944 1950
Thomas Hunt Morgan: Discovers that genes are responsible for inheritance 1865 1909 1911 1929 1944 1950
Phoebus Levene: Discovers that DNA is made up of nucleotides, phosphates, sugars and 4 bases 1865 1909 1911 1929 1944 1950
Oswald Avery: Shows that DNA can transform the property of cells However, this idea was not universally accepted 1865 1909 1911 1929 1944 1950
Erwin Chargaff: Shows that: A + G = T + C = 50% 1865 1909 1911 1929 1944 1950
Franklin’s W TW o O r F k ORMS OF DNA In 1951 Rosalind Franklin discovers the Two Forms of DNA through her X-ray diffraction images. A – Dry Form B – Wet Form
SOON AFTER WWII THE RACE TO DISCOVER THE “SECRET OF LIFE” WAS ON. SCIENTISTS KNEW THIS WOULD BE THE DISCOVERY OF THE CENTURY AND WOULD GUARANTEE A NOBEL PRIZE.
The Watson-Crick Model of the Structure of DNA
On Feb. 28, 1953, Francis Crick walked into the Eagle pub in Cambridge, England, and, as James Watson later recalled, announced that "we had found the secret of life." Actually, they had. That morning, Watson and Crick had figured out the structure of deoxyribonucleic acid, DNA. And that structure — a "double helix" that can "unzip" to make copies of itself — confirmed suspicions that DNA carries life's hereditary information.
Watson and Crick’s • I W n 195 ork 1 James Watson traveled from the United States to work with Francis Crick at Cambridge University. • Watson and Crick used the “Model Building” approach. • They physically built models out of wire, sheet metal, nuts and bolts to come up with the structure of DNA. Why did they build models? “Sometimes the fingers can grasp what the mind cannot” (Biology the Science of Life)
• DNA consists of two chains of nucleotides in a ladder-like structure which is twisted (Double Helix) • Used data of M.H.F. Wilkins and Rosalind Franklin, early 50’s • Wilkins and Franklin studied the structure of DNA crystals using X-rays. • The X pattern suggested the structure of DNA was a helix.
• Used data of Erwin Chargaff, 1940’s and early 50's • Chargaff’s Rule: His data showed that in each species, the percent of A equals the percent of T, and the percent of G equals the percent of C. • Watson was shown this picture by Wilkins in early 1953. From the picture it was possible to calculate: 1) the distance between bases (3.4A) 2) the length of the period (34A) 3) the rise of the helix (36 degrees)
• Francis Crick and James Watson with Maurice Wilkins received the 1962 Nobel Prize for discovering the molecular structure of deoxyribonucleic acid (DNA). • Widely regarded as one of the most important discoveries of the 20th century it has led the way to the mapping and deciphering of all the genes in the human chromosomes
Watson and Crick Model: • The sides of the ladder are made up of alternating molecules of phosphate and deoxyribose. • The bases make up the rungs of the ladder are attracted by a weak chemical bonds called hydrogen bonds. • The DNA double helix is anti-parallel, which means that the 5' end of one strand is paired with the 3' end of its complementary strand (and vice versa). • 5'--------------->3‘ 3'<---------------5' • Two hydrogen bonds connect T to A; three hydrogen bonds connect G to C.
Watson and Crick’s Double Helix Model explained: 1. How replication of DNA during mitosis produces exact copies for the daughter cells. 2. How DNA acts as a code, specifying how proteins are made by the cell.
“Nature” Watson & Crick quickly published their Scientific Journal called “Nature” on April 25th 1953
The Nobel Prize • In 1962 Watson, Crick & Wilkins won the Nobel Prize for their discovery of the structure of DNA.
Different forms of DNA double helix 1. A-DNA 2. B-DNA 3. Z-DNA
A-DNA • A-DNA is one of the many possible double helical structures of DNA. • It is most active along with other forms. • Helix has left-handed sense, shorter more compact helical structure.
• It occurs only in dehydrated samples of DNA, such as those used in The A-DNA structure. crystallographic experiments.
Structure • A-DNA is fairly similar to B-DNA. • Slight increase in the number of bp/ rotation (resulting in a tighter rotation angle), and smaller rise/turn.
• deep major groove and a shallow minor groove. • Favoured conformation at low water concentrations. • In a solution with higher salt concentrations or with alcohol added, the DNA structure may change to an A form, which is still right-handed, but every 2.3 nm makes a turn and there are 11 base pairs per turn.
Function • A transition from B-DNA to A-DNA occurs during Transcription. A-DNA also plays a imp role in some processes that do not involve RNA. For Example: • In sporulating bacteria, there is a protein which can bind to DNA in the B-conformation & induce a change to the A-DNA helix • Also, Long terminal repeats (LTRs) of transposable elements, these regions often contains purine stretches which favour the A-DNA conformation.
B-DNA • Most common DNA conformation in vivo. • Favoured conformation at high water concentrations. • Also known as Watson & Crick model of DNA. • First identified in fibre at 92% relative humidity. The B-DNA structure
Structure • Narrower, more elongated helix than A.
• Wide major groove easily accessible to proteins & Narrow minor groove. • Base pairs nearly perpendicular to helix axis • One spiral is 3.4nm or 34Ǻ. • Distance between two H-bonds is 0.34nm or 3.4Ǻ.
Z-DNA • Z-DNA is one of the many possible double helical structures of DNA. • Helix has left-handed sense.
• It is most active double helical structure. • Can be formed in vivo, given proper sequence and super helical tension, but function remains obscure. The Z-DNA structure.
Structure • Z-DNA is a transient form of DNA.
• Narrower, more elongated helix than A or B. • Z-DNA was first discovered in 1979,certain proteins bind very strongly to Z-DNA. • Z-DNA plays an important biological role in protection against viral disease. • One turn spans 4.6 nm, comprising 12 base pairs. • The DNA molecule with alternating G-C sequences in alcohol or high salt solution tends to have such structure.
Function • While no definitive biological significance of Z-DNA has been found, it is commonly believed to provide torsional strain relief (supercoiling) while DNA transcription occurs. • Toxic effect of ethidium bromide on ttrypanosoma is caused by shift of their kinetoplastid DNA to Z-form. • Scientists have since discovered that certain proteins bind very strongly to Z-DNA, suggesting that Z-DNA plays an important biological role in protection against viral disease.
Direction of Helix
The helix axis of A-, B-, and Z-DNA. Helix sense : Right-handed Right-handed Left-handed Bp/turn : 11 10 12 Diameter : 23Ǻ 20Ǻ 18Ǻ Axial rise(nm) : 0.26 0.34 0.45
Factors involved for different DNA conformations: There are at least three factors on which the DNA conformation depends 1) Ionic or hydration environment 2) DNA sequences. 3) Presence of specific proteins In a living cell, DNA is a Mixture of A-& B-DNA conformation with a few regions capable of forming Z-DNA
REFERENCE Molecular Biology & Biotechnology by H. D. Kumar DNA Technology the awesome skills by I. Edward Alcamo The Molecular Biology A Structural Approach by C. U. SMITH Fundamentals of Molecular Biology by Jayanta.K.Pal Molecular Biology of the Gene by James.D.Watson