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Dna structure and analysis
Dna structure and analysis
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Basics of DNA

The presentation includes about the basic knowledge of Deoxyribonucleic Acid or DNA. It involves the definition, structure, occurence, quantity, chemical composition, stability, variety, types, molecular weight, complementary of base pairs, absorbance, viscosity, ionic interactions, alternative forms and functions of DNA.

The presentation includes about the basic knowledge of Deoxyribonucleic Acid or DNA. It involves the definition, structure, occurence, quantity, chemical composition, stability, variety, types, molecular weight, complementary of base pairs, absorbance, viscosity, ionic interactions, alternative forms and functions of DNA.

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Basics of DNA

  1. 1. DNA (DeoxyriboNucleic Acid) - Genetic material in all organisms.
  2. 2. I. It duplicates its genetic information by replication during interphase and its copies are precisely distributed to the daughter cells during mitosis. This preserves the genetic information from generation to generation. II. It develops mutation (inheritable variation) in the genes and chromosomes occasionally. III. It expresses its genetic information by transcription of mRNA and synthesis of proteins. The latter, by acting as enzymes, control metabolic activities in cell. IV. DNA occurs in all cells and permits infinite variety. Its quantity is the same in all somatic cells of an organism and is half of that in the gametes. Hence, above properties of DNA make it fit to act as the genetic material.
  3. 3.  1869 – DNA was discovered by Swiss researcher Friedrich Miescher, he isolated a new molecule he called nuclein (DNA with associated proteins) from a cell nucleus.  1881 – Nobel Prize winner and German biochemist Albrecht Kossel, identified nuclein as a nucleic acid. He also isolated those five nitrogen bases that are now considered to be the basic building blocks of DNA and RNA: adenine (A), cytosine (C), guanine (G), and thymine (T) (which is replaced by uracil (U) in RNA).  Early 1900s – Theodor Boveri and Walter Sutton were independently working on chromosome theory, or the chromosomal theory of inheritance.  1944 – Oswald Avery first outlined DNA as the transforming principle, which essentially means that it’s DNA, not proteins, that transform cell properties .  1944 – 1950 – Erwin Chargaff discovered that DNA is responsible for heredity and that it varies between species. His discoveries, known as Chargaff’s Rules, proved that guanine and cytosine units, as well as adenine and thymine units, were the same in double-stranded DNA, and he also discovered that DNA varies among species.  1951 – Roslind Franklin’s work in X-ray crystallography began when she started taking X-ray diffraction photographs of DNA. Her images showed the, helical form of DNA which was confirmed by Watson and Crick nearly two years later.  1953 – Noble prize winner, Watson and Crick published on DNA’s double helix structure that twists to form the ladder-like structure we think of when we picture DNA.
  4. 4. 1. Griffiths & Avery’s Transformation Exp- - Griffith and Avery studied bacteria and mice. Their S and R experiment revealed that DNA stores and transmits genetic information from one generation of bacteria to another. - Griffith’s experiment was an experiment done in 1928 showing that bacteria can get DNA through a process called transformation.
  5. 5. 2. Bacteriophage Experiment - The Hershey-Chase experiment, which demonstrated that the genetic material of phage is DNA, not protein. The experiment uses two sets of T2 bacteriophages. In one set, the protein coat is labeled with radioactive sulfur (35S), not found in DNA. - Hershey and Chase showed that when bacteriophages, which are composed of DNA and protein, infect bacteria, their DNA enters the host bacterial cell, but most of their protein does not. Hershey and Chase and subsequent discoveries all served to prove that DNA is the hereditary material.
  6. 6.  The features of theWatson-Crick model of DNA deduced from the diffraction patterns are:  Two helical polynucleotide chains are coiled around a common axis. The chains run in opposite directions.  The sugar-phosphate backbones are on the outside and, therefore, the purine and pyrimidine bases lie on the inside of the helix.  The bases are nearly perpendicular to the helix axis, and adjacent bases are separated by 3.4 Å. The helical structure repeats every 34 Å, so there are 10 bases per turn of helix. There is a rotation of 36 degrees per base.  The diameter of the helix is 20 Å.  Each deoxyribonucleotide unit consists of – phosphate, deoxyribose sugar & nitrogenous bases.
  7. 7.  The ball-and-stick model is a molecular model of a chemical substance which is to display both the three- dimensional position of the atoms and the bonds between them.The atoms are typically represented by spheres, connected by rods which represent the bonds.  The DNA content of a cell has 109 nucleotide pairs and that of a single gene 74 nucleotide pairs on an average.
  8. 8.  A-form DNA:- A-DNA is a right-handed double helix made up of deoxyribonucleotides.  B-form DNA:- B-form DNA is a right-handed double helix, which was discovered by Watson and Crick based on the X- ray diffraction patterns.  Z-form DNA:- Z-form DNA is a left-handed double helix.
  9. 9.  In prokaryotic cells, DNA occurs in the cytoplasm and is the only component of the chromosomes.  In eukaryotic cells, DNA is largely confined to the nucleus and is the main component of the chromosomes.  It is combined with simple proteins to form deoxyribonucleoprotein (DNA). It is highly compacted so that the chromosomes may fit within the nucleus.  About 1.8m long DNA of a human diploid nucleus is accommodated within a nucleus of only 6 micrometer diameter.  Quantities of DNA occurs in some cytoplasmic organelles such as mitochondria and chloroplasts.
  10. 10.  The DNA content is fairly constant in all somatic cells of a given species.  The amount of DNA is doubled just before the cell division.  The gametes have half the amount of DNA as they contain half the number of chromosomes.  The total amount of DNA per haploid genome of an organism is known as its C Value.  Eukaryotic cells have more DNA than the prokaryotic cells.
  11. 11.  The double helical form of DNA molecule is remarkably stable due to two sets of forces- 1. Hydrogen bonding between all the bases along the length of the molecule, and 2. Hydrophobic and electronic interactions between aromatic surfaces of bases closely stacked vertically in each DNA chain.  Although, the individual hydrogen bond is relatively weak, the very large number of these bonds along the length of DNA molecule provide the required stability. ➢ NOTE: Optical Rotation of DNA – dextrorotatory, i.e., it rotates the polarized light to the right.
  12. 12.  Each deoxyribonucleotide unit consists of – phosphate, deoxyribose sugar & nitrogenous bases.  The nitrogenous base may be a – 1. Purine, i.e., adenine (A) or guanine (G). 2. Pyrimidine, i.e., thymine (T) or cytosine (C).  Although only four types of base pairs are involved in the formation of DNA molecule, these base pairs may occur in any sequence, and there may be any number of sequences in a molecule.  This gives an infinite variety to the DNA molecule.  However, each individual has a specific sequence of base pairs in its DNA.
  13. 13.  DNA molecules are of 2 main types- 1. Linear DNA :- found in the nuclei of eukaryotic cells. It is associated with proteins. 2. Circular DNA :- found in many viruses, all prokaryotic cells, and in the mitochondria and chloroplasts (plastids) of eukaryotic cells. It is not associated with proteins.  The ciliates have separate trophic and genetic DNA located in the macronucleus and micronucleus respectively.  The molecular weight of DNA varies from 6 to 100 million.  This indicates the presence of over 20,000 nucleotides per molecule.  When fully stretched out, the DNA of E.coli is about 1mm long and that of each human cell about 1.5 metres long.  E.coli has 4x106 base pairs present in its DNA.  Generation time of E.coli is 20mins.
  14. 14.  If we heat up a tube of DNA dissolved in water, the energy of the heat can pull the two strands of DNA apart (there's a critical temperature called the T m at which this happens). This process is called 'denaturation'; when we've 'denatured' the DNA, we have heated it to separate the strands.  The denatured DNA can reformulate hydrogen bonds between complementary single strand, making it likely to reform double helix structure again.This process is called as renaturation.
  15. 15.  The outer surface of DNA double helix is highly anionic due to the presence of large numbers of phosphate groups, which are ionized at physiological pH.  As a polyanion, DNA is capable of ionic interactions with many positively charged molecules.  In solution, DNA is generally joined by small cations, but in eukaryotic cells it is tightly associated with positively charged proteins.
  16. 16.  DNA molecule absorbs light energy.The nitrogenous bases absorb most strongly in the UV region of the spectrum near 260nm.  A denatured DNA molecule absorbs more light as it bases in single strands are exposed. The increase in the absorption of light occurs even though the amount of DNA remains the same.This phenomenon is called as hyper-chromic shift. It can be used to distinguish single or double- stranded DNA in an unknown sample.  A single stranded DNA does not show the hyper-chromatic shift.  The viscosity of a solution is proportional to the resistance of the components to flow.  Solutions of DNA are highly viscous as its molecules are very long and quite stiff due to double helix conformations.  Solutions of single stranded DNA are much less viscous as its molecules are not rigid, but tend to collapse upon themselves to assume a globular form.
  17. 17.  The DNA has 5 important functions :- i. Being the genetic material, it transmits hereditary characters from our parents to the offspring. ii. It enables the cell to grow, divide and maintain itself by directing the synthesis of structural proteins. iii. It controls metabolism in the cell by directing the synthesis of necessary enzymes proteins. iv. It contributes to the evolution of organisms by undergoing mutations. v. It brings about differentiation of cells during development. Only certain genes remain functional in particular cells.This enables the cells having similar genes to assume different structure and function. vi. It brings about continuity of life by replication. Thus, DNA is the very basis of life.
  18. 18. I. DNA molecule is a double chain of deoxyribonucleotide units. II. The successive units are joined by phosphodiester bonds in each chain (strand). III. The sugar-phosphate backbones are located on the outside of the molecule. IV. The two chains are spirally coiled around a common axis to form a right-handed double helix. V. The helix has a major groove and a minor groove alternatively. VI. The helix is 20 Å wide; its one complete turn is 34 Å long, and has 10 base pairs; the successive base pairs are 3.4 Å apart. VII. The two chains are complementary to each other with respect to base sequence. VIII. The two chains are hydrogen bonded; A (Adenine) on one chain is joined to T (Thymine) on the other chain by 2 hydrogen bonds; C (Cytosine) on one chain is linked to G (Guanine) on the other chain by 3 hydrogen bonds. IX. The DNA molecule is remarkable stable due to hydrogen bonding and hydrophobic and electronic interactions.
  19. 19. x. The two chains are anti-parallel, one aligned in 5’ → 3’ direction, the other in 3’ → 5’ direction. Nucleotide always attach at 3’ end, running 5’ → 3’ end. xi. The amount of A+G= the amount of T+C; the amount of A = the amount of T; and the amount of G = the amount of C. Sugar and phosphate groups occur in equal proportions. This is known as Chargaff’s Rule. xii. The DNA molecules undergoes denaturation and renaturation easily. xiii. The denatured DNA strands are hyperchromic. xiv. The DNA molecule can replicate and repair itself, and can also transcribe RNAs. xv. The base sequence of one chain serves as the genetic code. xvi. The DNA can function in vitro. xvii. The amount of DNA per nucleus is constant in all the body cells of a given species. xviii. Solutions of DNA are highly viscous due to long stiff molecules. xix. DNA double helix has polyanionic surface. xx. DNA is dextrorotatory. xxi. Eukaryotic DNA has many repetitive sequences. xxii. Each strand in a DNA molecule has polarity with 3’ and 5’ ends.

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