Molecular Genetics chapter 12
Mystery of the Hereditary Material:
Experiments in the 1950s showed that DNA is the hereditary material
Scientists raced to determine the structure of DNA
1953 - Watson and Crick proposed that DNA is a double helix
Griffith Discovers Transformation (1928) Attempting to develop a vaccine
Isolated two strains of Streptococcus pneumoniae
Rough strain was harmless smooth strain was pathogenic
Griffith’s “Transforming Factor”
Hershey & Chase’s Experiments
Created labeled bacteriophages , Radioactive sulfur , Radioactive phosphorus
Allowed labeled viruses to infect bacteria
Asked: Where are the radioactive labels after infection?
DNA Deoxyribonucleic acid
Nucleic acids are polymers that are the blueprints for proteins.
Nucleotides: The monomers that make up nucleic acids which consist of:
5 carbon sugar
Phosphate group
nitrogenous base
NITROGENOUS BASES
DNA
Adenine (A)
Cytosine (C)
Thymine (T)
Guanine (G)
RNA
In RNA uracil replaces thymine.
Composition of DNA
Chargaff showed:
Amount of adenine relative to guanine differs among species
Amount of adenine always equals amount of thymine and amount of guanine always equals amount of
cytosine
A=T and G=C
Rosalind Franklin’s Work: Was an expert in X-ray crystallography
Used this technique to examine DNA fibers; Concluded that DNA was some sort of helix
Watson-Crick Model
DNA consists of two nucleotide strands; Strands run in opposite directions; Strands are held together by
hydrogen bonds between bases
A binds with T and C with G
Molecule is a double helix
DNA Structure Helps
Explain How It Duplicates
DNA is two nucleotide strands held together by hydrogen bonds
Hydrogen bonds between two strands are easily broken
Each single strand then serves as template for new strand
DNA Replication
Each parent strand remains intact
Every DNA molecule is half “old” and half “new”
Enzymes in Replication
Enzymes unwind the two strands
DNA polymerase attaches complementary nucleotides
DNA ligase fills in gaps
Enzymes wind two strands together
DNA – RNA – Protein
Transcription: RNA copies the code of the DNA in the nucleus and leaves the nucleus (mRNA)
Translation: Converts the mRNA message into proteins using tRNA. Occurs in the ribosomes
Codon: A 3 nucleotide code for a specific amino acid
Editing the RNA
Introns: Noncoding region of RNA
Exons: Coded regions of RNA
RNA splicing: The removing of introns and rejoining exons
Three Classes of RNA
Messenger RNA
Carries protein-building instruction
Ribosomal RNA
Major component of ribosomes
Transfer RNA
Delivers amino acids to ribosomes
Genetic Code
Set of 64 base triplets
Codons
61 specify amino acids
3 stop translation
Translation
Anticodon: A three nucleotide group on the tRNA that is complimentary to the codon of the mRNA
A specific amino acid is attached to the other end of the tRNA
Gene Mutations
Missense
Nonsense
Base-Pair Substitutions
Insertions
Deletions
Frameshift Mutations
Insertion
Extra base added into gene region
Deletion
Base removed from gene region
Both shift the reading frame
Result in many wrong amino acids