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DNA

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DNA

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  1. 1 DNA

  2. 2 DNA DNA stands for deoxyribose nucleic acid This chemical substance is present in the nucleus of all cells in all living organisms DNA controls all the chemical changes which take place in cells The kind of cell which is formed, (muscle, blood, nerve etc) is controlled by DNA The kind of organism which is produced (buttercup, giraffe, herring, human etc) is controlled by DNA

  3. 3 DNA molecule DNA is a very large molecule made up of a long chain of sub-units The sub-units are called nucleotides Each nucleotide is made up of a sugar called deoxyribose a phosphate group -PO4 and an organic base

  4. 4 Ribose & deoxyribose Ribose is a sugar, like glucose, but with only five carbon atoms in its molecule Deoxyribose is almost the same but lacks one oxygen atom Both molecules may be represented by the symbol

  5. 5 The bases Adenine (A) Thymine (T) Cytosine (C) (G) Guanine The most common organic bases are

  6. 6 Nucleotides The deoxyribose, the phosphate and one of the bases Combine to form a nucleotide PO4 adenine deoxyribose

  7. Joined nucleotides PO4 PO4 PO4 PO4 7 A molecule of DNA is formed by millions of nucleotides joined together in a long chain sugar-phosphate backbone + bases

  8. 8 In fact, the DNA usually consists of a double strand of nucleotides The sugar-phosphate chains are on the outside and the strands are held together by chemical bonds between the bases

  9. 2-stranded DNA PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 9

  10. 10 Bonding 1 Adenine Thymine Guanine Cytosine The bases always pair up in the same way Adenine forms a bond with Thymine and Cytosine bonds with Guanine

  11. 11 Bonding 2 PO4 PO4 thymine adenine PO4 PO4 cytosine guanine PO4 PO4 PO4 PO4

  12. 12 Pairing up PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4

  13. 13 The paired strands are coiled into a spiral called A DOUBLE HELIX

  14. 14 THE DOUBLE HELIX bases sugar-phosphate chain

  15. 15 A DIY model ofpart of a DNA molecule

  16. 16 replication Before a cell divides, the DNA strands unwind and separate Each strand makes a new partner by adding the appropriate nucleotides The result is that there are now two double-stranded DNA molecules in the nucleus So that when the cell divides, each nucleus contains identical DNA This process is called replication

  17. 17 The strands separate PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4

  18. PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 18 Each strand builds up its partner by adding the appropriate nucleotides

  19. Genetic code 1 19 The sequence of bases in DNA forms the Genetic Code A group of three bases (a triplet) controls the production of a particular amino acid in the cytoplasm of the cell The different amino acids and the order in which they are joined up determines the sort of protein being produced

  20. Genetic code 2 Ser-Cyst-Val-Gly-Ser-Cyst Ala Val Val-Cyst-Ser-Ala-Ser-Cyst-Gly Val- Cyst-Ala-Ala-Ser-Gly 20 This is a small, imaginary protein molecule showing how a sequence of 5 different amino acids could determine the shape and identity of the molecule Each amino acid (Serine, Cysteine, Valine, Glycine and Alanine) is coded for by a particular triplet of bases

  21. Coding Cytosine Adenine Thymine 21 For example Valine Codes for Cytosine (C) Alanine Codes for Guanine (G) Adenine (A)

  22. Triplet code 22 This is known as the triplet code Each triplet codes for a specific amino acid CGA - CAA - CCA - CCA - GCT - GGG - GAG - CCA - Ala Val Gly Gly Arg Pro Leu Gly The amino acids are joined together in the correct sequence to make part of a protein Ala Val Gly Gly Arg Pro Leu Gly

  23. 23 DNA and enzymes The proteins build the cell structures They also make enzymes The DNA controls which enzymes are made and the enzymes determine what reactions take place The structures and reactions in the cell determine what sort of a cell it is and what its function is So DNA exerts its control through the enzymes

  24. Genes 24 A sequence of triplets in the DNA molecule may code for a complete protein Such a sequence forms a gene There may be a thousand or more bases in one gene

  25. Question 1 Which of the following are components of nucleotides? (a) deoxyribose (b) amino acids (c) phosphate (d) enzymes (e) organic bases

  26. Question 2 Which of the following represent a correct pairing of bases? (a) adenine with thymine (b) adenine with guanine (c) thymine with adenine (d) guanine with cytosine (e) thymine with thymine

  27. Question 3 DNA molecules are formed from (a) organic bases (b) amino acids (c) deoxyribose (d) nucleotides

  28. Question 4 Which of the following are organic bases? (a) Valine (b) Guanine (c) Thymine (d) Serine

  29. Question 5 Replication of DNA occurs (a) During cell division (b) before cell division (c) at any time

  30. Question 6 A nucleotide triplet codes for (a) a protein (b) an amino acid (c) an enzyme (d) an organic base

  31. Answer CORRECT

  32. Answer INCORRECT