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Structure of dna, types of dna forms, turners syndrome

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DNA as a Genetic Material
DNA as a Genetic Material
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Structure of dna, types of dna forms, turners syndrome

  1. 1. STRUCTURE OF DNA, TRANSFORMATION, TYPES OF DNA FORMS, TURNER’S SYNDROME By Kripa Justine
  2. 2. • DNA as an acidic substance present in nucleus was first identified by Friedrich Meischer in 1869. • He named it as ‘Nuclein’. • Due to technical limitation in isolating such a long polymer intact, the elucidation of structure of DNA remained elusive for a very long time. • James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but famous Double Helix model for the structure of DNA. • based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine STRUCTURE OF DNA
  3. 3. • DNA is a polymer containing chains of nucleotide monomers. • Each nucleotide contains a sugar, a base, and a phosphate group. • The sugar is 2′-deoxyribose which has five carbons named 1′ , 2′, etc. • Four types of base: adenine and guanine have two carbon–nitrogen rings and are purines; thymine and cytosine have a single ring and are pyrimidines. • Bases are attached to the 1′ carbon of the deoxyribose. • A sugar plus a base is termed a nucleoside. • A nucleotide has one, two or three phosphate groups attached to the 5′ carbon of the sugar. • Nucleotides occur as individual molecules or NUCLEOTIDE
  4. 4.  The base pairing confers a very unique property to the polynucleotide chains.  They are said to be complementary to each other, and therefore if the sequence of bases in one strand is known then the sequence in other strand can be predicted.  If each strand from a parental DNA acts as a template for synthesis of a new strand, the two double stranded DNA (daughter DNA) thus, produced would be identical to the parental DNA molecule.  The genetic implications of the structure of DNA became very clear.
  5. 5. • Nucleotide triphosphates of the four bases are joined to form DNA polynucleotide chains. • Two phosphates are lost during polymerization and the nucleotides are joined by remaining phosphate. • A phosphodiester bond forms between the 5′ phosphate of one nucleotide and the3′ hydroxyl of the next nucleotide. • The polynucleotide has a free 5′ phosphate at 5′ end and a free 3′ OH at 3′ end. • The sequence of bases encodes the genetic information. • It is read 5′→3′. Polynucleotides are extremely long. It is possible to have 4n different sequences. DNA polynucleotide
  6. 6. i. Two polynucleotide chains, where the backbone is sugar-phosphate and bases project inside. ii. They have anti-parallel polarity. If one chain has polarity 5'3', the other has 3'5'. iii. The bases in two strands are paired through hydrogen bond forming base pairs. iv. Adenine forms two hydrogen bonds with Thymine from opposite strand v. Guanine is bonded with Cytosine with three H-bonds. The salient features of the Double-helix structure of DNA are as follows:
  7. 7. vi. Always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix. vii. The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. The distance between a bp in a helix is approximately equal to 0.34 nm. viii.The plane of one base pair stacks over the other in double helix. This, in addition to H- bonds, confers stability of the helical structure ix. The helix has a major groove which interacts with proteins and a minor grove.
  8. 8. TRANSFORMATION Transforming Principle • In 1928, Frederick Griffith, in a series of experiments with Streptococcus pneumoniae witnessed transformation in the bacteria. • During the course of his experiment, a living organism had changed in physical form. • When Streptococcus pneumoniae bacteria are grown on a culture plate, some produce smooth shiny colonies (S) others produce rough colonies (R). • S strain bacteria have a mucous/polysaccharide coat, while R strain does not. • Mice infected with the S strain die from pneumonia infection but mice infected with the R strain do not develop pneumonia.
  9. 9. • Griffith was able to kill bacteria by heating them. • He observed that heat-killed S strain bacteria injected into mice did not kill them. • When he injected a mixture of heat-killed S and live R bacteria, the mice died. • He recovered living S bacteria from the dead mice. He concluded that the R strain bacteria had been transformed by the heat-killed S strain bacteria. • Some ‘transforming principle’, transferred from the heat-killed S strain enabled R strain to synthesise a smooth polysaccharide coat and become virulent. • This must be due to the transfer of the genetic material. • The biochemical nature of genetic material was not defined from his experiments.
  10. 10. Biochemical Characterisation of Transforming Principle • Genetic material was thought to be a protein. • Oswald Avery, Colin MacLeod and Maclyn McCarty, worked to determine the biochemical nature of ‘transforming principle’ in Griffith's experiment. • They purified biochemicals (proteins, DNA, RNA) from heat-killed S cells to see which could transform live R cells into S cells. • They discovered that DNA alone from S bacteria caused R bacteria to become transformed. • Proteases and RNases did not affect transformation, so transforming substance was not a protein or RNA. • Digestion with DNase inhibited transformation • They concluded that DNA is the hereditary material, but not all biologists were convinced.
  11. 11. TYPES OF DNA FORMS
  12. 12. TYPES OF DNA A-DNA • A-DNA is one of the possible double helical structure which DNA can adopt along with other two biologically active helix structure(B- DNA,Z-DNA). • It is rare. • Right handed double helix . • Short and fat compared to B-DNA. • Occur only in dehydrated sample of DNA , those used in crystallographic experiments. • A-DNA was originally identified by X-ray diffraction analysis of DNA fibres at 75% relative humidity. • Deep major groove and flat minor groove. • The bases are more tilted (to 20A°). • The base pairs per turn is 11. • Rise per base pair is 2.3A°.
  13. 13. B-DNA • Described by James D. Watson & Francis crick. • Commonly found in DNA. • DNA molecule consists of 2 helical polynucleotide chains coiled around common axis. • 2 helices are wound in such a way so as to produce 2 interchain spacing or groove – • Major/wide groove(width 12A°,depth 8.5A°) • Minor /narrow groove(width 6A°,depth 7.5A°) • These grooves provide surface with which proteins,chemicals,drugs can
  14. 14. C-DNA • C-DNA formed at 66% relative humidity (low)in presence of Li+ or Mg2+. • Right handed ,with axial rise of 3.32A° per base pair . • 9.33 base pairs per turn. • Helical pitch 3.32A°×9.33°A=30.97A°. • Base pair rotation=38.58°. • Has diameter of 19A°,smaller than that of A-&B- DNA. • The tilt of base is 7.8°
  15. 15. Z-DNA • One of the many possible DNA double helix structure. • Left handed double helix structure winds to left in zig- zag manner so they are termed as Z-DNA. • Discovered by Rich, Nordheim &Wang in 1984. • It has antiparallel strands as B-DNA. • It is long and thin • adjacent sugar have alternating orientation • Thermodynamically relatively unstable.  Purines: bases & sugar are near & on same side  pyramidines: bases & sugar are distant ,on opposite sides • Only one deep helical grooves. • There are 12 base pairs per turn with axial rise 3.8A°,& angle of twist 60°. • Transition to Z-DNA facilitated by cytosine methylation at C5
  16. 16. RELATIONSHIP OF B-DNAAND Z- DNA
  17. 17. • Helix has a higher‐energy form. • This leads to a greater distance (0.77 nm) • segment of B‐DNA consisting of GC pairs can be converted into Z‐DNA when the bases are rotated 180 degrees. • The modification of DNA by methylation of cytosine is frequent in certain regions of the DNA of eukaryotes. • Z‐DNA has biological roles. • Sequences favoring the formation of Z‐DNA occur frequently near the promoter region, where Z‐DNA stimulates transcription
  18. 18. • Condition that affects only females • One of the X chromosomes is missing or partially missing. • Turner syndrome can cause a variety of medical and developmental problems, including short height, failure of the ovaries to develop and heart defects. TURNER’S SYNDROME
  19. 19. The genetic alterations of Turner syndrome may be one of the following: Monosomy. The complete absence of an X chromosome generally occurs because of an error in the father's sperm or in the mother's egg. This results in every cell in the body having only one X chromosome. Mosaicism. An error occurs in cell division during early stages of fetal development. This results in some cells in the body having two complete copies of the X chromosome. Other cells have only one copy of the X chromosome. CAUSES
  20. 20. X chromosome abnormalities. Abnormal or missing parts of one of the X chromosomes can occur. Cells have one complete and one altered copy. This error can occur in the sperm or egg with all cells. Y chromosome abnormalities In a small percentage of Turner syndrome cases, some cells have one copy of the X chromosome and other cells have one copy of the X + Y chromosome material. These individuals develop biologically as female, but the presence of Y chromosome material increases the risk of developing gonadoblastoma.
  21. 21. • Turner syndrome may be suspected in pregnancy during an ultrasound test. • This can be confirmed by prenatal testing - chorionic villous sampling or amniocentesis - to obtain cells from the unborn baby for chromosomal analysis. • If a diagnosis is confirmed prenatally, the baby may be under the care of a specialist pediatrician immediately after birth. • Diagnosis is confirmed by a blood test, called a karyotype. This is used to analyze the chromosomal composition of the female. DIAGNOSIS
  22. 22. Symptoms • Wide or weblike neck • Low-set ears • Broad chest • High, narrow roof of the mouth • Arms that turn outward at the elbows • Fingernails and toenails that are narrow and turned upward • Swelling of the hands and feet, especially at birth • Slightly smaller than average height at birth • Slowed growth • Cardiac defects • Low hairline at the back of the head • Receding or small lower jaw • Short fingers and toes
  23. 23. TREATMENT • There is no cure for Turner syndrome. However, much can be done to minimize the symptoms. • Growth hormone, either alone or with a low dose of androgen, will increase growth. • Estrogen replacement therapy like birth control pill, to promote development of secondary sexual characteristics. • Estrogens are crucial for maintaining • good bone integrity • cardiovascular health • tissue health. • Surgery for removing webbed neck. • Women with Turner syndrome who do not have spontaneous puberty and who are not treated with estrogen are at high risk for osteoporosis and heart conditions.
  24. 24. CONCLUSION • In females with mild signs and symptoms of Turner syndrome, the diagnosis is delayed until the teen or young adult years. • Girls and women with Turner syndrome need ongoing medical care from a variety of specialists. • Regular checkups and appropriate care can help most girls and women lead healthy, independent lives.
  25. 25. Thank You

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