DNA: The Molecule of Life Ch. 12-1 and 12-2

1. DNA: The Molecule of Life Ch. 12-1 and 12-2

2. What is DNA? • DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria.

3. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism.

4. I. Discovery of DNA The work of many scientists paved the way for the exploration of DNA. Way back in 1868, almost a century before the Nobel Prize was awarded to Watson, Crick and Wilkins, a young Swiss physician named Friedrich Miescher, isolated something no one had ever seen before from the nuclei of cells. He called the compound "nuclein." This is today called nucleic acid, the "NA" in DNA (deoxyribo-nucleic-acid) and RNA (ribo-nucleic-acid).

5. Erwin Chargaff discovered that in any sample of DNA the percentages of Guanine and Cytosine were almost equal. He found the same to be true of Adenine and Thymine. A = T and C = G 1949 This observation came to be known an as “Chargaff’s Rules of Base Pairing”

6. Rosalind Franklin studied the DNA molecule using a technique called “X-Ray Diffraction” 1952 Watson and Crick used this photograph by Franklin to determine the structure of DNA

7. 1953 James Watson and Francis Crick developed the “Double Helix” model of the structure of DNA

8. II. DNA Structure • DNA is a “polymer”. It is long molecule made up a series of “monomers” called NUCLEOTIDES • Each NUCLEOTIDE is made of three basic components: Deoxyribose Sugar Phosphate Group Nitrogenous Base

9. “Polymer” Nucleotide “monomer”

10. There are four different types of nucleotides found in DNA, differing only in the nitrogenous base The other two bases are called “PYRIMIDINES” Two of the bases belong to a group of compounds called “PURINES”

11. The four nucleotides are given one letter abbreviations as shorthand for the four bases

12. III. DNA Shape • The shape of DNA, discovered by Francis CrickandJames Watson, is a double helix (or “twisted ladder”)

13. The backbone of the DNA chain is formed by sugar and phosphate groups of each nucleotide

14. The rungs of the ladder are made of the four nitrogenous bases

15. IV. Complementary Base Pairing The rules of base pairing (or nucleotide pairing) are: A with T: the purineadenine (A) always pairs with the pyrimidinethymine (T)

16. G with C: the purineguanine (G) always pairs with the pyrimidinecytosine (C)

17. A pairs with T C pairs with G

18. Chargaff’s Rules The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same. Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same. A =T G =C

19. V. DNA and CHROMOSOMES Eukaryotic DNA is generally located in the cell nucleus in the form of a number of chromosomes.

20. VI. DNA Replication Before a cell divides , it duplicates its DNA in a copying process called “replication”

21. During DNA Replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. Each strand of the “Double Helix” of DNA serves as a template, or model for the new strand

22. In a eukaryotic cell, replication occurs at hundreds of places along the DNA molecule. Replication proceeds in both directions until each chromosome is completely replicated. The point at which the two strands of DNA are separated to allow replication of each strand is called the“replication fork”.

23. Steps of DNA Replication (DNADNA) Step 1: "Unzipping" of parent DNA • DNA replication begins with a parent DNA molecule unwinding and unzipping between the nitrogenous bases of the two strands.

24. The hydrogen bonds holding A-T and G-C together are relatively weak and are broken by an enzyme called DNA helicase to expose the nucleotides of both strands.

25. Step 2: Complementary base pairing of nucleotides When the parent DNA has been unzipped the nitrogenous bases that were separated are available for the attachment of free-floating nucleotides. Nucleotides containing an A, T, C, or G nitrogenous base are present in the nucleus of the cell. Free floating nucleotides

26. These free-floating nucleotides will form hydrogen bonds with a complementary nucleotide on either of the parent strands. Remember that A-T and G-Care the only complementary arrangements, as these pairs involve a purine and pyrimidine base. free-floating nucleotides forming bonds with a complementary nucleotide

27. Step 3: Backbone formation in daughter strands Once the free-floating nucleotides are in place an enzyme, DNA polymerase, will aid the formation of sugar-phosphate bonds attaching one nucleotide to the next and forming the backbone of the new daughter strand.

28. Each new DNA molecule contains one strand from the parent DNA that acted as a template and a second strand that is newly formed.

29. This DNA replication process is so accurate that errors are rare (1 in a billion) and when errors do occur they are often corrected by special repair enzymes. This repair allows for accurate replication as organisms grow and reproduce generation after generation.

30. DNA Replication DNADNA