1. DNA Structure & Function
Chapter 8

2. Discovery 1869: Johannes Miescher 1928: Frederick Griffith
Determined the substance we now call DNA was not a protein, but could not find function
1928: Frederick Griffith
Found that one strain of bacteria could be transformed into another
1940: Oswald Avery, Colin Macleod and Maclyn McCarty
Identified that the “transforming principle” was a nucleic acid

3. Discovery Late 1940s: Alfred Hershey and Martha Chase
Established that DNA transmits a full complement of hereditary information
1948: André Boivin and Roger Vendrely
Established that body cells of any individual of a species contain precisely the same amount of DNA
Daniel Mazia’s laboratory discovered that DNA content does not change over time

4. Discovery
Evidence showed DNA met all the essential properties of hereditary material
A full complement of hereditary information must be transmitted with the molecule from one generation to the next
An equal amount of hereditary material must be found in each cell of a given species
Hereditary material must not change in order to function as a genetic bridge between generations
Must be capable of encoding the enormous amount of information required to build a new individual

5. Structure DNA: Deoxyribonucleic Acid
Nucleic acid – genetic material! Blueprint for living organisms! Passed from generation to generation!
DNA is a polymer made of nucleotides, each with --
five-carbon sugar
deoxyribose
three phosphate groups
one of four nitrogen-containing bases
Adenine, guanine, thymine, cytosine

6. DNA Nitrogen bases are either - Purines: double ringed bases
Guanine (G) and Adenine (A)
Pyrimadines: single ring bases
Cytosine (C) and Thymine (T)
1950: Erwin Chargaff
1st Rule: within a species the amount of guanine nearly equals the amount of cytosine, and the amount of adenine nearly equals the amount of thymine (A = T and G = C)

7. Structure
1950s: James Watson and Francis Crick suspect that DNA is a helix
Rosalind Franklin made the first clear image of DNA as it occurs in cells using X-ray diffraction
She calculated that DNA is very long and identified a repeating pattern
Wilkins, Watson and Crick later stole this work and took credit for themselves, winning the Nobel Peace Prize

8. Structure
The structure of DNA is called a double helix – twisted ladder
The phosphate of one nucleotide bonds to the sugar of another – creating a backbone on the outside of the molecule [sides of the ladder]
The nitrogen bases pair with each other on the inside [rungs of the ladder]
The whole molecule is held together by hydrogen bonds

10. DNA
A purine base always bonds to a pyrimidine, ensuring equal distance between the rails of the ladder
Complementary base pairing  each base will only bond with 1 other specific base
A bonds to T [double hydrogen bond]
Remember AT2
C bonds to G [triple hydrogen bond]
Remember CG3
Order of bases in one strand determines the order of the bases in the other

11. DNA
Chargaff’s 2nd Rule: The order of nucleotides in a strand of DNA, DNA sequence, varies tremendously among species
Variations in nucleotide sequence are the foundation of life’s diversity; defines species and distinguishes individuals

12. DNA
The two sides of DNA are antiparallel – meaning they are parallel to each other but are oriented in the opposite direction of each other
Think of a 2 way street – parallel, but traffic goes in opposite directions

13. DNA
The last carbon atom on one end is the 5′ [5 prime] carbon of a sugar and the last carbon atom on the other end is the 3′ carbon of a sugar
The direction of one strand is 5’ to 3’ and the other is 3’ to 5’

14. Chromosomes
Chromosome: structure that consists of DNA and associated proteins
Carries part or all of a cell’s genetic information
DNA is arranged in segments called genes
Histone: protein that structurally organizes eukaryotic chromosomes
Chromatin: Relaxed form of DNA in the nucleus

15. Chromosomes
During most of a cell’s life, each chromosome consists of one DNA molecule
When the cell prepares to divide, it duplicates its chromosomes by DNA replication
After replication, each chromosome consists of two DNA molecules (identical sister chromatids) that attach at a centromere region

16. Chromosomes
Each species has a characteristic chromosome # (chromosomes in its cells)
Human body cells have two sets of 23 chromosomes—two of each type
Karyotype: an image of an individual’s diploid set of chromosomes

17. Chromosomes Autosome: chromosome that is the same in males and females
Two autosomes of a pair have the same length, shape, and centromere location, and hold information about the same trait
Sex chromosomes differ between males (XY) and females (XX)

18. DNA Replication
For each cell to have a copy of DNA, the cell copies its chromosomes into two sets before dividing
Before DNA replication, a chromosome has one molecule of DNA
Semiconservative replication produces two copies of DNA: one strand of each copy is new, and the other is parental

19. Semiconservative Replication
DNA helicase: enzyme responsible for unwinding and unzipping the double helix.
Hydrogen bonds are broken creating two single strands
Free floating nucleotides (in the nucleus) match up to the parent strands
Each nucleotide provides energy for its own attachment to the parent strand of DNA
Two of three phosphate groups are removed when a nucleotide is added

20. Semiconservative Replication
DNA polymerase: attaches new nucleotides to each strand and proofreads
Reads in 3′ to 5’ direction, attaches in 5’ to 3’ direction
Both strands of the parent molecule are copied at the same time
As each new DNA strand lengthens, it winds up with its template strand into a double helix

21. Semiconservative Replication
The leading strand replicates continuously [5’  3’]
The lagging strand replicates away from the fork [3’ 5’]
It is synthesized discontinuously in small segments
Okazaki Fragments: about nucleotides long
DNA Ligase: enzyme that seals gaps, so the new DNA strands are continuous

23. Semiconservative Replication
In prokaryotic cells, DNA is a loop (circular) in the cytoplasm. The DNA strand is opened at one origin of replication
Eukaryotic DNA unwinds in multiple areas during replication

24. Mutations Mistakes can and do occur during DNA replication
The wrong base is added to a growing DNA strand, a nucleotide gets lost, or an extra one slips in
Most replication errors occur because DNA polymerases work very fast, luckily, most proofread their work
They can correct a mismatch by reversing the synthesis reaction

25. Mutations
However, DNA polymerases do not copy damaged DNA very well
If proofreading and repair mechanisms fail, an error becomes a mutation
Mutation: a permanent change in the DNA sequence of a cell’s chromosome

26. Mutations Mutations can form in any type of cell
Those that occur in sex cells can be passed to offspring
Some alter DNA instructions and may have a harmful or lethal outcome – cancer
Not all mutations are dangerous
Some give rise to variation in traits; basis for evolution

27. DNA Damage
Ionizing radiation from X-rays, most UV light, and gamma rays may cause DNA damage
Breaks DNA
Causes covalent bonds to form between bases on opposite strands
Fatally alters nucleotide bases

28. Cloning Cloning: making an identical copy of something
Reproductive Cloning: technology that produces genetically identical individuals
Example: artificial embryo splitting
Animal breeders using cloning because sometimes they want an exact copy of a specific individual