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Structure of dna
Structure of dna
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Dna structure

  1. 1. DNA & Gene Structure Elham lasemi
  2. 2. DNA is the genetic material of : Bacteria (in 1928-Frederick Griffith showed this subject by transformation) Viruses (in 1952,result of an experiment by Alfred Hershey and Martha Chase) Eukaryotic cell In some viruses, the genetic material is RNA.
  3. 3. DNA-is a molecule that all living organisms carry in every cell in their body. DNA- Contains information needed to caryy out cell activities, it is the genetic material. →every person’s DNA is unique result DNA can serve as an individual identifier.
  4. 4. In the1900-1950:a series of experiments revealed two important features of DNA. → DNA are passed down from parent to offspring → the instructions on how to create a body and control its growth and development are encoded in the DNA molecule.
  5. 5. Chargaff's rules: Edwin Chargaff at Columbia university had measured the base composition of nucleic acid . The amount of adenine nearly always equalled the amount of thymine . The amount of cytosine nearly always equalled the amount of guanine .
  6. 6. In 1951- X-ray diffraction studies Maurice Wilkins showed a diffraction pattern of DNA at a scientific meeting in Napales In 1952- Rosalin Franklin produced X-ray pictures of DNA that were critical to decoding its shape.
  7. 7. In 1953:Francis Crick and James Watson,deduce the exact structure of DNA
  8. 8. DNA-Deoxyribonucleic Acid Nucleic acids are made from repeating units of nucleotides 1) A phosphate group Two major forms of nucleic acid polymers DNA and RNA Nucleotide contain:
  9. 9. 2) A sugar 3)A nitrogenous base
  10. 10. The nucleotide structure Glycoside bond
  11. 11. The physical structure of DNA is frequently described as a “double helix.”
  12. 12. What exactly is a double helix? imagine long ladder twisted around like a spiral staircase the backbone of DNA molecule- each is made from alternating molecules : a sugar, then a phosphate, then a sugar, then a phosphate, and so on.
  13. 13. the backbone of DNA molecule- nucleotides are connected to each other to from a long chain phosohodiester bond: Fromed between the phosphate group of one nucleotide and 3´- OH of the next nucleotide.
  14. 14. The rungs of the ladder- Attached to each sugar, and protruding like half of a rung on the ladder, is one of the nitrogen- containing bases.
  15. 15. There are four types of DNA nucleotides, each differing in their nitrogen base only 1) Adenine (double ring= purine) 2) Thymine (singlering=prymidine) 3)Guanine (double ring=purine) 4)Cytosine(single ring=pyrimidine)
  16. 16. These nitrogen bases are held together by : Hydrogen bonds
  17. 17. The two strands of nucleotides are anti-parallel to each other .one is oriented 5 to 3 , the other 3 to 5 the two strand warp around each other to create the helical of the molecule
  18. 18. the three helical forms of DNA ( RNA)
  19. 19. In the cell, the most commonly seen from of DNA double helix is called the B form or the Watson- Crick helix The double helix can also exist in an A form which is shorter and wider than the B form with the bases at an angle rather than perpendicular to the helix axis The A form is seen in RNA double helices and in RNA-DNA hybrid helix structures observed in transcription and RNA processing   
  20. 20. structural variation in DNA reflects three things : The different possible conformation of the deoxyribose Rotation about the contiguous bonds that make up the phospho-deoxyribose backbone Free rotation about the C-1´- N glycosyl bond -The Watson-Crick structure is also referred to as B form DNA or B-DNA. -The B form is the most stable structure and standard point of reference in DNA study. -A and Z forms have been characterized.(Z form is very unusal)
  21. 21. The major and minor grooves are lined by sequence- specific hydrogen bonding group
  22. 22. - DNA in B form has a major groove and minor groove - The presence of the grooves allows access to the hydrogen-bonding capabilities of the exposed bases. - The hydrogen-bonding capabilities provides a mean of sequence specific interactions between DNA and the molecules is most interact with in the process of replication and transcription
  23. 23. Major and minor groove
  24. 24. Supercoiling-The double helix can also wind around itself to change the overall conformation or topology of the DNA in space. supercoiling creates tension in the DNA , and thus can only occur in the DNA has no free ends
  25. 25. Two kind of supercoiling
  26. 26. ∆L=∆W+∆T L (Linking number) W= writhing number T=Twisting number
  27. 27. DNA replication
  28. 28. Mutation change the sequence of DNA Spontaneous mutations-all organism suffer a certain number of mutations as the result of normal cellular operations or random interaction with the environment. Induced mutation-the occurrence of mutations can be increased by treatment with certain compounds ,these are called mutagens.
  29. 29. A point mutation-changes only a single base pair and can be caused by either of two types of event :  chemical modification of DNA directly changes one base into a different base.  An error during the replication of DNA cause by the wrong base to be inserted into a polynucleotide A point mutation Transition Transversion
  30. 30. Hot spots - some sites gain far more than the number of mutations ,expected from a random distribution thay may have ×10 or even × 100 more mutation than predicted by random hits.
  31. 31. Difference between DNA and RNA
  32. 32. Genome-The full set of DNA present in an individual organism The genome incloudes: - Choromosal DNA - DNA in plasmids - Organellar DNA as found in mitochondria and chloroplasts ( in eukaryotes)
  33. 33. In prokaryotest the information contained within circular pieces of DNA ( such as all bacteria)
  34. 34. In eukaryotes this information is laid out in long linear strands of DNA in the nucleus.(such as humans)
  35. 35. The genome in viruses DNA RNA dsDNA SSDNA dsRNA SSRNA
  36. 36. Chromosomes- Rather than being one super-long DNA strand, eukaryotic DNA exists as many smaller, more manageable pieces.
  37. 37. Gene - a sequence of bases in a DNA molecule that carries the information necessary for producing a functional product, usually a protein or RNA molecule.
  38. 38. Alleles-These alternative versions of a gene that code for the same feature
  39. 39. Is the size of an organism’s genome related to its complexity? - Comparing the amount of DNA present in various species, in terms of both numbers of chromosomes and numbers of base pairs, reveals a paradox: there does not seem to be any relationship between the size of an organism’s genome and the organism’s complexity.
  40. 40. Junk DNA-a huge proportion of base sequences in DNA do not code for anything and have no obvious purpose In what types of organisms do we find the most “junk DNA”? - Bacteria and viruses tend to have very little noncoding DNA; genes make up 90% or more of their DNA. It is in the eukaryotes (with the exception of yeasts) that we see an explosion in the amount of non-coding DNA, about 25% of which occurs within genes and 75% between genes
  41. 41. Non-coding regions of DNA sometimes: - take the form of short sequences that are repeated thousands of times - slightly longer repeated sequences - consists of gene fragments, duplicate versions of genes,and pseudogenes (sequences very similar to actual genes butwith a few slight alterations that make them lose their proteincoding ability) - occur both within genes in which case they are called introns and between genes
  42. 42. All the extra DNA may serve some purpose. Perhaps it is a reservoir of potentially useful sequences. Or it may have some function in regulating when genes are turned on or off.
  43. 43. thanks for your attention

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