Biology 102Biology 102

DNADNA

Lecture OutlineLecture Outline

1.1. Overview: Function of DNAOverview: Function of DNA

2.2. How did we first know that DNA was the How did we first know that DNA was the molecule of heredity?molecule of heredity?

3.3. Determining the structure of DNADetermining the structure of DNA

4.4. Watson and Crick’s Model: Key features Watson and Crick’s Model: Key features of DNAof DNA

1. Function of DNA1. Function of DNA

DNA codes for DNA codes for proteinsproteins only… only…How then can it code for everything we are?How then can it code for everything we are?

……ENZYMES!ENZYMES!

2. How did we first know that DNA 2. How did we first know that DNA was the molecule of heredity?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 3. Determining the structure of DNADNA

Why was the question important?Why was the question important?

What did various scientists know about the What did various scientists know about the structure, and when did they know it?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 Why was it important to know the structure of DNA?structure of DNA?

How did the structure of DNA relate to its How did the structure of DNA relate to its function (i.e. how did DNA “work”?_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 How was it possible for genetic material to code for proteins?code for proteins?

What did particular scientists know, What did particular scientists know, and when did they know it?and when did they know it?

The men: John Watson, Francis Crick, Maurice The men: John Watson, Francis Crick, Maurice Wilkins, and Linus PaulingWilkins, and Linus Pauling

What they knewWhat they knew

1.Sugar and phosphates were linked together in a backbone

What did particular scientists know, What did particular scientists know, and when did they know it? (cont.)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. Clarification: nitrogenous base vs. nucleotidenucleotide

Nitrogenous baseNitrogenous base is the two or one-ringed is the two or one-ringed molecule:molecule:

NucleotideNucleotide is the nitrogenous base plus is the nitrogenous base plus the sugar and phosphatethe sugar and phosphate

The woman: Rosalind FranklinThe 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 helix5. 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 The picture was deliberately withheld from Linus Pauling. Without

it, he published the wrong structureit, he published the wrong structure

How was the structure determined?How was the structure determined?

Watson and Crick played around with Watson and Crick played around with three dimension models made with wires three dimension models made with wires until they got it right…until they got it right…Review what they knew…Review what they knew…

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 16.5

Watson and Crick’s Model: Watson and Crick’s Model: Key featuresKey features

2 strands2 strands Strands run in opposite Strands run in opposite

directionsdirections Bases are paired, A with Bases are paired, A with

T, and G with C. Always T, and G with C. Always three rings acrossthree rings across

The two strands are held The two strands are held together by hydrogen together by hydrogen bonds between that link bonds between that link the base pairs.the base pairs.

Did everyone get the credit they Did everyone get the credit they deserved?deserved?

Watson and Crick published their findings in Watson and Crick published their findings in Nature, April 1953Nature, April 1953 Rosalind Franklin was not acknowledged in the Rosalind Franklin was not acknowledged in the

original paperoriginal paper Watson, Crick and Wilkins shared the Nobel Watson, Crick and Wilkins shared the Nobel

prize in 1962prize in 1962 Note: Rosalind Franklin was not eligible for the Nobel Note: Rosalind Franklin was not eligible for the Nobel

Prize because she had died a few years before of Prize because she had died a few years before of cancer.cancer.

Watson later acknowledged Franklin’s Watson later acknowledged Franklin’s contribution in his book, The Double Helix.contribution in his book, The Double Helix.

DNA ReplicationDNA Replication

The key: Complementary base pairingThe key: Complementary base pairing Strand unwindsStrand unwinds New nucleotides form complementary base pairs New nucleotides form complementary base pairs

with both strandswith both strands The backbone is “fused”.The backbone is “fused”.

DNA Replication (cont.)DNA Replication (cont.)

DNA Replication (cont.)DNA Replication (cont.) Replication is “semi-conservative”Replication is “semi-conservative”, meaning that , meaning that

each “daughter” DNA strand is made up of one “old” each “daughter” DNA strand is made up of one “old” and one “new” strand.and one “new” strand.

DNA Replication (cont.)DNA Replication (cont.)

Replication occurs simultaneously at many sites Replication occurs simultaneously at many sites along the DNA moleculealong 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 replicationDetails of DNA replication

When does DNA replication occur?When does DNA replication occur?Prior to cell division, DNA must be replicatedPrior 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 genomeEach cell has a complete copy of the genome

Details of DNA replication (cont.)Details of DNA replication (cont.)

Step 1Step 1: The two “parent” strands unwind: The two “parent” strands unwind DNA helicase (an enzyme) separates the two strands (breaks DNA helicase (an enzyme) separates the two strands (breaks

the H-bonds)the H-bonds) The replication “bubble” is formedThe replication “bubble” is formed

Step 2Step 2: New nucleotides are complimentary base-paired : New nucleotides are complimentary base-paired with those on the parent strand with those on the parent strand

Step 3Step 3: DNA polymerase (an enzyme) joins together : DNA polymerase (an enzyme) joins together the sugar and phosphates to form the “backbone” of the the sugar and phosphates to form the “backbone” of the molecule molecule

DNA polymerase

DNA polymerase

DNA helicase

Details of DNA replication (cont.)Details of DNA replication (cont.) Nucleotides can only be added to the left on the Nucleotides can only be added to the left on the

upper strand, and to the right on the lower strand 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 Since unwinding occurs to the left, only the upper strand

can be synthesized continuouslycan be synthesized continuously The lower strand is synthesized discontinuouslyThe lower strand is synthesized discontinuously Fragments joined together by Fragments joined together by DNA ligaseDNA ligase (an enzyme) (an enzyme)

DNA polymerase

DNA polymerase

DNA helicase

Fragments joined by DNA ligase

DNA Replication (cont.)DNA Replication (cont.)

ProofreadingProofreading A function of DNA polymeraseA function of DNA polymerase Initial error rate: Approximately 1:10,000 base pairsInitial error rate: Approximately 1:10,000 base pairs Final error rate: Only 1:1 billion pairsFinal error rate: Only 1:1 billion pairs

DNA polymerase and other repair enzymes go back and DNA polymerase and other repair enzymes go back and correct mistakescorrect mistakes

AgingAging Genetic errors and loss add up over time.Genetic errors and loss add up over time. Environmental factors that speed up process includeEnvironmental factors that speed up process include

UV radiationUV radiation Body temperatureBody temperature What else can you think of?What else can you think of?