1. DNA and RNA
Chapter 12

2. How do we know DNA is the genetic material??
Scientists were not sure if protein or DNA was genetic material (both in chromosomes)
Griffith (1920s)
Experimented with mice and two strains of pneumonia bacteria
R (rough) – no disease
S (smooth) – causes disease, virulent

4. Griffith’s Experiment
Found that R strain was transformed by heat-killed S strain
What substance transformed the R strain??
Avery performed further tests in 1940s
Treated R strain + heat-killed S strain with:
Protease – enzyme that destroys protein
DNase – enzyme that destroys DNA

5. Avery’s Experiment

6. Avery’s Experiment DNA caused the transformation of R strain
Scientists still not convinced!

7. Hershey and Chase Exp.
Tested bacteriophages (viruses that infect bacteria)
DNA contains phosphorus
Protein contains sulfur
Radioactively labeled phosphorus and sulfur in two batches of phage
Infected bacteria with phage
Only radioactive phosphorus found in bacteria → DNA entered

9. DNA is the genetic material, not protein
And the point is . . .
DNA is the genetic material, not protein

10. DNA Donor
Who’s da’ mama?
Clone
Surrogate mom

11. DNA Structure Double helix Sugar-phosphate backbone 4 nitrogen bases
2 strands twisted around each other
Sugar-phosphate backbone
4 nitrogen bases
Adenine
Thymine
Guanine
Cytosine
Base Pairs

12. Nucleotide Structure Phosphate group Deoxyribose sugar – 5 carbon
Nitrogenous base

13. Four Nucleotides

14. Base Pairing What patterns do you see in this data?
Source of DNA A T G C
Streptococcus
Yeast
Herring
Human
What patterns do you see in this data?

15. Chargaff’s Rule Erwin Chargaff – 1949 Found that # of T = A and G = C
Only explanation is that T pairs with A and C pairs with G
If a DNA sample is 20% A, what % will C be?
That’s right % (20% A = 20% T, leaves 60% for G and C, 30% each)

16. Double Helix Wilkins and Franklin (1952)
Found that DNA was helix shaped through X-ray pictures

17. First DNA Model James Watson & Francis Crick (1952)
Constructed a model of DNA using others’ data
Twisted ladder with base pairs as rungs of ladder
Hydrogen bonds hold base pairs together
Nobel Prize 1958

18. DNA!

20. Chromosomes Prokaryotes – one circular chromosome in cytoplasm
Eukaryotes – many linear chromosomes in nucleus
Fruit fly: 8 Human: 46
Oak tree: Chimpanzee: 48
DNA is wrapped around histones (proteins) to fit more in small space

21. Chromosome Structure

22. DNA Replication Occurs during S phase of Interphase
Exact copies made of all DNA
Three steps:
Unwind – enzyme is helicase
Unzip – enzyme is DNA polymerase
Copy – enzyme is DNA polymerase
Each strand acts as template for new strands

23. DNA Replication DNA not copied one base at a time
Thousands of replication forks on each chromosome
Speeds up replication from weeks to minutes
Strands are copied in opposite directions
Proofreading enzyme checks for errors

24. DNA Replication

25. DNA Replication Semi-Conservative
Half of parent DNA molecule is conserved (saved) in daughter DNA molecule
Daughter DNA is half old and half new DNA

26. DNA Replication

27. RNA Made of nucleotides Single stranded Uracil instead of Thymine
Phosphate
Ribose sugar
Nitrogenous base
Single stranded
Uracil instead of Thymine
A pairs with U

28. Types of RNA Messenger RNA – mRNA Transfer RNA – tRNA
Carries genetic messages out of nucleus to ribosome b/c DNA can’t leave nucleus
Transfer RNA – tRNA
Brings amino acids to mRNA at ribosomes
Ribosomal RNA – rRNA
Makes up ribosome

29. Protein Synthesis Each chromosome has hundreds of genes
Each gene codes for one protein
2 steps:
Transcription: DNA → mRNA
In nucleus
Translation: mRNA → protein
At ribosome

30. Transcription
Translation

31. Transcription DNA unwinds, unzips
RNA polymerase (enzyme) adds RNA nucleotides to one strand of DNA
Promoters in DNA sequence tell enzymes where gene begins
Single stranded pre-mRNA leaves, DNA strands rejoin

32. Transcription

33. RNA Processing pre-mRNA contains:
Introns – nonsense sequence
Exons – expressed sequence
Introns are “cut” out and exons are spliced together before mRNA leaves nucleus

34. RNA Processing

35. Translation
mRNA message is translated into an amino acid (protein) sequence
Every 3 RNA bases = one amino acid
Called a codon (there are 64!)
mRNA attaches to ribosome in cytoplasm
tRNA matches codon with amino acid

36. tRNA Clover leaf shaped strand of RNA
Has anti-codon at one end and corresponding amino acid at other end
Anti-codon pairs with codon on mRNA

37. Translation Starts at AUG – start codon
tRNA brings in amino acids for each codon
Amino acids attached to growing polypeptide chain (protein)
Stops at UAA, UAG, or UGA – stop codons
Ribosome helps to position all molecules correctly

38. Translation

39. Translating the Code

40. Practice! DNA mRNA codon Amino Acid ATC TAC GAT CCG UAG Stop! AUG
Start –
Methionine
CUA
Leucine
GGC
Glycine

41. Mutations Gene Chromosomal Point mut. – substitute a single base
Changes one aa
Frameshift – insert or delete a single base
Changes entire aa sequence from mut. Forward
Chromosomal
Deletion, Duplication, Inversion, Translocation

42. Gene Mutations DNA: TAC GCA TCC mRNA: AUG CGU AGG AA: Met-Arg-Thr
DNA: TAC GTA TCC
mRNA: AUG CAU AGG
AA: Met-His-Thr
Substitution

43. Gene Mutations DNA: TAC GCA TGG mRNA: AUG CGU ACC AA: Met-Arg-Thr
DNA: TAT CGC ATG G
mRNA: AUA GCG UAC C
AA: Ile-Ala-Tyr
Insertion

44. Gene Mutations DNA: TAC GCA TGG mRNA: AUG CGU ACC AA: Met-Arg-Thr
DNA: TAG CAT GG
mRNA: AUC GUA CC
AA: Ile-Val-?
Deletion

45. Chromosomal Mutations

46. Causes of Mutations Internal External
Mistakes in DNA replication
External
Radiation, chemicals, high temps
Mutagens: chemicals that cause mut.
Mutations in body cells only affect that person
Mutations in sex cells can spread throughout a population

47. Gene Regulation Genes are turned on or off as needed by the cell
Repressors turn genes off by binding to DNA and preventing RNA polymerase from binding