Biology 102 DNA

Lecture Outline 1.Overview: Function of DNA 2.How did we first know that DNA was the molecule of heredity? 3.Determining the structure of DNA 4.Watson and Crick’s Model: Key features of DNA

1. Function of DNA DNA codes for proteins only… DNA codes for proteins only… How then can it code for everything we are? How then can it code for everything we are?…ENZYMES!

2. How did we first know that DNA was the molecule of heredity? Griffith’s experiments on bacterial infections in mice: Griffith’s experiments on bacterial infections in mice: Avery et al. determined this “transforming molecule” was DNA

3. Determining the structure of DNA Why was the question important? Why was the question important? What did various scientists know about the structure, and when did they know it? What did various scientists know about the structure, and when did they know it? How was the structure determined? How was the structure determined? Did everyone get the credit they deserved? Did everyone get the credit they deserved?

Why was it important to know the structure of DNA? How did the structure of DNA relate to its function (i.e. how did DNA “work”?_ How did the structure of DNA relate to its function (i.e. how did DNA “work”?_ Examples: Structure vs. function… Examples: Structure vs. function… How did genetic material replicate itself? How did genetic material replicate itself? How was it possible for genetic material to code for proteins? How was it possible for genetic material to code for proteins?

What did particular scientists know, and when did they know it? The men: John Watson, Francis Crick, Maurice Wilkins, and Linus Pauling The men: John Watson, Francis Crick, Maurice Wilkins, and Linus Pauling What they knew What they knew 1.Sugar and phosphates were linked together in a backbone

What did particular scientists know, and when did they know it? (cont.) 2. The molecule contained four types of nitrogenous bases, 2 with single rings (the pyrimidines) and 2 with double rings (the purines) 3. Purines and pyrimidines existed in a 1:1 ratio (Chargaff’s rules)

Clarification: nitrogenous base vs. nucleotide Nitrogenous base is the two or one-ringed molecule: Nitrogenous base is the two or one-ringed molecule: Nucleotide is the nitrogenous base plus the sugar and phosphate Nucleotide is the nitrogenous base plus the sugar and phosphate

The woman: Rosalind Franklin What critical information did she provide? What critical information did she provide? 4. This x-ray diffraction picture suggested that DNA existed as a helix 5. The width of the helix could also be determined from this picture, and they recognized that it had a constant width. The picture was deliberately withheld from Linus Pauling. Without it, he published the wrong structure The picture was deliberately withheld from Linus Pauling. Without it, he published the wrong structure

How was the structure determined? Watson and Crick played around with three dimension models made with wires until they got it right… Watson and Crick played around with three dimension models made with wires until they got it right… Review what they knew… Review what they knew…

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig Watson and Crick’s Model: Key features 2 strands 2 strands Strands run in opposite directions Strands run in opposite directions Bases are paired, A with T, and G with C. Always three rings across Bases are paired, A with T, and G with C. Always three rings across The two strands are held together by hydrogen bonds between that link the base pairs. The two strands are held together by hydrogen bonds between that link the base pairs.

Did everyone get the credit they deserved? Watson and Crick published their findings in Nature, April 1953 Watson and Crick published their findings in Nature, April 1953 Rosalind Franklin was not acknowledged in the original paper Rosalind Franklin was not acknowledged in the original paper Watson, Crick and Wilkins shared the Nobel prize in 1962 Watson, Crick and Wilkins shared the Nobel prize in 1962 Note: Rosalind Franklin was not eligible for the Nobel Prize because she had died a few years before of cancer. Note: Rosalind Franklin was not eligible for the Nobel Prize because she had died a few years before of cancer. Watson later acknowledged Franklin’s contribution in his book, The Double Helix. Watson later acknowledged Franklin’s contribution in his book, The Double Helix.

DNA Replication The key: Complementary base pairing The key: Complementary base pairing Strand unwinds Strand unwinds New nucleotides form complementary base pairs with both strands New nucleotides form complementary base pairs with both strands The backbone is “fused”. The backbone is “fused”.

DNA Replication (cont.)

Replication is “semi-conservative”, meaning that each “daughter” DNA strand is made up of one “old” and one “new” strand. Replication is “semi-conservative”, meaning that each “daughter” DNA strand is made up of one “old” and one “new” strand.

DNA Replication (cont.) Replication occurs simultaneously at many sites along the DNA molecule Replication occurs simultaneously at many sites along the DNA molecule This allows for rapid replication. A human cell can copy its 6 billion base pairs and divide into daughter cells in only a few hours. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Details of DNA replication When does DNA replication occur? When does DNA replication occur? Prior to cell division, DNA must be replicated Prior to cell division, DNA must be replicated What is the function of DNA replication? What is the function of DNA replication? Each cell has a complete copy of the genome Each cell has a complete copy of the genome

Details of DNA replication (cont.) Step 1: The two “parent” strands unwind Step 1: The two “parent” strands unwind DNA helicase (an enzyme) separates the two strands (breaks the H-bonds) DNA helicase (an enzyme) separates the two strands (breaks the H-bonds) The replication “bubble” is formed The replication “bubble” is formed Step 2: New nucleotides are complimentary base-paired with those on the parent strand Step 2: New nucleotides are complimentary base-paired with those on the parent strand Step 3: DNA polymerase (an enzyme) joins together the sugar and phosphates to form the “backbone” of the molecule Step 3: DNA polymerase (an enzyme) joins together the sugar and phosphates to form the “backbone” of the molecule DNA polymerase   DNA polymerase DNA helicase 

Details of DNA replication (cont.) Nucleotides can only be added to the left on the upper strand, and to the right on the lower strand Nucleotides can only be added to the left on the upper strand, and to the right on the lower strand Recall that strands run in opposite directions… Recall that strands run in opposite directions… Since unwinding occurs to the left, only the upper strand can be synthesized continuously Since unwinding occurs to the left, only the upper strand can be synthesized continuously The lower strand is synthesized discontinuously The lower strand is synthesized discontinuously Fragments joined together by DNA ligase (an enzyme) Fragments joined together by DNA ligase (an enzyme) DNA polymerase   DNA polymerase DNA helicase  Fragments joined by DNA ligase

DNA Replication (cont.) Proofreading Proofreading A function of DNA polymerase A function of DNA polymerase Initial error rate: Approximately 1:10,000 base pairs Initial error rate: Approximately 1:10,000 base pairs Final error rate: Only 1:1 billion pairs Final error rate: Only 1:1 billion pairs DNA polymerase and other repair enzymes go back and correct mistakes DNA polymerase and other repair enzymes go back and correct mistakes Aging Aging Genetic errors and loss add up over time. Genetic errors and loss add up over time. Environmental factors that speed up process include Environmental factors that speed up process include UV radiation UV radiation Body temperature Body temperature What else can you think of? What else can you think of?