1. DNA Structure & Replication
Chapter 13
DNA Structure & Replication

2. The History of DNA
Fredrick Griffith (1928) working with Streptococcus pneumoniae conducted transformation experiments of virulent & nonvirulent bacterial strains
Oswald Avery (1949) showed that DNA, not protein or RNA, was responsible for the transformation seen in Griffith’s experiments
Hershey & Chase (1952) used bacteriophages (viruses) to show that DNA, not protein, was the cell’s hereditary material
Rosalind Franklin (1951) used x-rays to photograph DNA crystals
Erwin Chargaff (1950) determined that the amount of A=T and amount of C=G in DNA; called Chargaff’s Rule
Watson & Crick (1953) discovered double helix shape of DNA (A pairs with T; C pairs with G) & built the 1st model

3. Griffith’s Experiments
In 1928, Fredrick Griffith was studying the bacteria (S. pneumoniae) that cause pneumonia. - Smooth strain (virulent)  Mouse dies - Rough strain (non-virulent)  Mouse lives - Heat-killed smooth strain  Mouse lives - Heath-killed smooth + rough strains  Mouse dies

4. Griffith’s Experiments
Heat-killed, disease-causing bacteria (smooth colonies)
Harmless bacteria (rough colonies)
Control (no growth)
Harmless bacteria (rough colonies)
Heat-killed, disease-causing bacteria (smooth colonies)
Disease-causing bacteria (smooth colonies)
Dies of pneumonia
Dies of pneumonia
Lives
Lives
Live, disease-causing bacteria (smooth colonies)

5. Griffith’s Experiments
Griffith called this process transformation: one type of bacteria turned into another
ex. Rough strain turned into smooth strain when the two were mixed
Through additional experiments, Oswald Avery concluded that DNA, not RNA or protein, is responsible for transformation in bacteria
Only bacteria with undestroyed DNA could transform other bacteria
Destroyed RNA and protein didn’t make a difference

6. Hershey-Chase Experiments
Hershey and Chase confirmed that DNA, not protein, is the hereditary material by conducting experiments with bacteriophages

7. Watson & Crick
Watson and Crick created a model of DNA by using Rosalind Franklin’s DNA diffraction x-rays
Watson and Crick are associated with the discovery of DNA as a double-helix (two strands wound around each other like a spiral staircase)

8. James Watson
(1928-?)
89-years old
Francis Crick
( )
Rosalind Franklin
( )

9. DNA Structure

10. DNA Structure A DNA nucleotide is made of three components:
5-carbon deoxyribose sugar
Phosphate group
One of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), or thymine (T).

11. DNA Structure
The two categories of nitrogenous bases are purines and pyrimidines Purines: Double-ring structure - Adenine (A) and Guanine (G) Pyrimidines: Single-ring structure - Cytosine (C) and Thymine (T)

12. DNA Structure Bonds Hold DNA Together
Nucleotides along each DNA strand are linked by covalent bonds
Complementary nitrogenous bases are bonded by hydrogen bonds
Hydrogen bonding between the complementary base pairs, G-C and A-T, holds the two strands of a DNA molecule together

13. DNA Structure: Antiparallel Strands
One strand of DNA goes from 5’ to 3’
The other strand is opposite in direction, going 3’ to 5’
This will be important to remember when it comes to DNA replication

14. Chargaff’s Rule Base-pairing rules of DNA are known as Chargaff’s Rule
Adenine (A) always pairs with thymine (T)
Guanine (G) always pairs with cytosine (C)
Therefore, in a DNA strand, the % of adenine = % of thymine; % of cytosine = % of guanine
Example: If a particular DNA molecule is 30% adenine, then it is 30% thymine, which equals 60%. This means that the remaining 40% of the molecule is 20% cytosine and 20% guanine.

15. DNA Replication Steps of DNA Replication
DNA has to be copied before a cell divides
DNA is copied during the S or synthesis phase of interphase
New cells will need identical DNA strands
Steps of DNA Replication
Replication begins with the separation of the DNA strands by helicases
To begin process, RNA primase must first add a small primer (sequence of RNA)
Then, DNA polymerases form new strands by adding complementary nucleotides to each of the original strands.

16. DNA Replication Begins at origin of replication
DNA strands are opened (“unzipped”) by helicases
Two strands open, forming replication forks (Y-shaped region)
New strands grow at the forks
DNA polymerase adds complementary nucleotides to new strands
Replication
Fork
Parental DNA Molecule 3’ 5’

17. DNA Replication

18.
DNA Replication
DNA polymerase can only add nucleotides to the 3’ end of the DNA
This causes the NEW strand to be built in a 5’ to 3’ direction
Leading strand (built toward replication fork) completed in one piece
Lagging strand (built moving away from the replication fork) is made in sections called Okazaki fragments
RNA primers are removed and replaced with DNA sequences
DNA ligase joins segments together

19. DNA Replication

20. Errors in DNA Replication
Changes in DNA are called mutations
DNA proofreading and repair prevent many replication errors
Unrepaired mutations that affect genes that control cell division can cause diseases such as cancer

22. DNA Nucleotide Mutations: Overview

23. DNA Nucleotide Mutations: Substitution
C base was changed….C was substituted with a T
Results in a change (if mRNA codon is different) in a single amino acid
Sickle-Cell Anemia; Tay-Sachs

24. DNA Nucleotide Mutations: Insertion
Example: Fragile-X Syndrome
Inserted an additional base…C
Causes a frameshift mutation…pushes each base to the right one place
Now, the remainder of the gene’s codons are all changed, and so are the amino acids

25. DNA Nucleotide Mutations: Deletion
Example: Niemann-Pick Disease
Deleted a base…A
Causes a frameshift mutation…pushes each base to the left one place
Now, the remainder of the gene’s codons are all changed, and so are the amino acids