1. DNA Structure and Function
Chapter 14
General Biology I
BSC 2010
DNA Structure and Function
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2. Human DNA There are 23 pairs of chromosomes in the cell nucleus.
one set of 23 from mother, the other set of 23 from father
each chromosome has thousands of genes
a gene is a sequence of DNA that codes for a functional product
One set of 23 human chromosomes represents the human haploid genome.
3 billion base pairs
20,000 – 25,000 genes
Mitochondrial DNA is inherited from the mother.

3. Griffith Discovered Transformation
2 strains of Streptococcus pneumoniae
R strain was not deadly to mice
S strain was deadly
Inject R  mouse lives
Inject S  mouse dies
Inject S destroyed by heat  mouse lives
Inject a mixture of destroyed S with live R  the mouse dies!
credit: modification of work by NIH
The live R bacteria were transformed to be like the deadly R bacteria. Genetic material escaped from the dead S cells and entered the live R cells.

4. DNA is responsible for the genetic transformation.
Avery, McCarty, and MacLeod Discovered the Cause of Transformation
DNA is responsible for the genetic transformation.

5. Chase and Hershey Confirmed the Findings
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6. Meanwhile… Chargaff’s Rules
Adenine A = T Thymine
Guanine G = C Cytosine
The quantities of certain DNA components always matched.
We now know this concept as
“the complementary base-pairing rules”.

7. The Building Blocks of DNA are Nucleotides
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8. 1950s: Watson and Crick Publish the Structure of DNA
credit: modification of work by Marjorie McCarty, Public Library of Science
They were aided by the work of Rosalind Franklin , Maurice Wilkins, and Linus Pauling.

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DNA Structure
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DNA has (a) a double helix structure with (b) covalent phosphodiester bonds and hydrogen bonds. The (c) major and minor grooves are binding sites for DNA binding proteins during processes such as transcription (the copying of RNA from DNA) and replication.

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DNA Sequencing
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Fred Sanger developed the dideoxy chain termination sequencing method in the 1990s. (“The Sanger Method”)

11. DNA Sequences Can Help Us Understand Our Ancestry
video link 17:01
credit: modification of work by Marjorie McCarty, Public Library of Science

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The three suggested models of DNA replication. Grey indicates the original DNA strands, and blue indicates newly synthesized DNA.
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13. Meselson and Stahl Showed DNA Replication is Semi-Conservative
Radioisotope- labeled DNA helped them distinguish new strands from old strands.
credit: modification of work by Mariana Ruiz Villareal

14. DNA Replication in Prokaryotes
The single circular chromosome is replicated bidirectionally, starting at the origin. (~1000 nucleotides per second)

15. credit: modification of work by Mariana Ruiz Villareal
A DNA Replication Fork
credit: modification of work by Mariana Ruiz Villareal

16. DNA Replication Summary
This list is found in Chapter 14, Section 4 of the text.
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17. Enzymes Involved in Prokaryotic DNA Replication
This table is found in Chapter 14, Section 4 of the text. Download for free at

18. DNA Replication in Eukaryotes
The large, multiple, linear chromosomes are replicated bidirectionally, simultaneously from many origins (~100 nucleotides per second)

19. Telomeres
Eukaryote chromosomes are linear (not circular), and therefore have ends.
These ends get shorter with each round of replication.
The sequences at the ends, known as telomeres, do not contain functional genes.

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The ends of linear chromosomes are maintained by the action of the telomerase enzyme. Telomerase is typically active in germ cells and adult stem cells. It is not typically active in adult somatic cells.
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21. credit: US Embassy Sweden
Elizabeth Blackburn, 2009 Nobel Laureate, is the scientist who discovered how telomerase works.

22. This table is found in Chapter 14, Section 5 of the text
This table is found in Chapter 14, Section 5 of the text. Download for free at

23. DNA Repair Corrects Mutations and Damage
DNA polymerase is very accurate. It can do proofreading of its own work- and fix mistakes.
If proofreading fails, mismatch repair can recognize incorrect base pairs and fix them. This can only happen very soon after DNA replication.
Nucleotide excision repair can fix problems even long after DNA replication.
Despite these mechanisms, errors and damage still accumulate over the life of a cell.
Mutations in repair genes have been known to cause cancer.

24. Proofreading
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Proofreading by DNA polymerase corrects errors during replication.
Specific repair enzymes recognize the mispaired nucleotide and excise part of the strand that contains it; the excised region is then resynthesized.

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Mismatch Repair
In mismatch repair, the incorrectly added base is detected soon after replication. The mismatch repair proteins detect this base and remove it from the newly synthesized strand by nuclease action. The gap is now filled with the correctly paired base.
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26. Nucleotide Excision Repair
Nucleotide excision can repair thymine dimers. When exposed to UV, thymines lying adjacent to each other can form thymine dimers. In normal cells, they are excised and replaced.
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27. Types of Mutations in DNA
Mutations are variations in the nucleotide sequence of DNA.
Induced mutations result from an exposure to chemicals, UV rays, x-rays, or some other environmental agent.
Spontaneous mutations occur without any exposure to any environmental agent; they are a result of natural reactions taking place within the body.
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