Key Concepts for Week 4: Lectures 10-12
Monday, Oct. 19, Lecture 10: DNA The Molecular Basis of Inheritance
 Introduction to molecular structure of DNA
o Double helix
 Purine paired with pyrimidine
o History of DNA
 Frederick Griffith: He found that dead cells caused a heritable change allowing R cells to make
capsules.
 Hershey and Chase: DNA, not protein functions as the genetic material of phage T2. Convinced
scientists that DNA was the hereditary material.
 Structure of DNA
o Maurice Wilkins and Rosalind Franklin: x-ray crystallography to look at DNA molecule
o Watson and Crick: DNA helical structure
o Two strands are antiparallel (5’ to 3’)
o A-T and G-C are bound by hydrogen bonds. Base pairing agreed with Chargaff’s rules.
o Covalent bonds link the units of each strand.
Wednesday, Oct. 21, Lecture 11: DNA Replication and Repair
 DNA  DNA
 Happens in the nucleus
 How does DNA replicate and repair itself
o Lots of molecular machinery at play, many proteins
o Each strand serves as a template – parent strands unwind and two daughter strands are synthesized
complimentary to parent strand (semi conservative model)
 DNA replication, initial steps
o Begins at origin of replication and happens in both directions until whole DNA is replicated
o Helicases unwind the double helix at a replication fork
o Single-stranded proteins bind onto stabilize single-stranded DNA to use as a template
o Topoisomerase rejoins the DNA strands, prevents DNA from being too tightly wound
 DNA replication, synthesis of polynucleotides
o RNA primer starts nucleotide synthesis (primer created by primase)
o DNA polymerase adds nucleotides onto 3’ end (strand elongates in the 5’-3’ end) – this is the leading
strand
o Lagging strand: opposite direction of synthesis, completed by Okazaki fragments that are joined by DNA
ligase
 DNA repair
o DNA Polymerases are the proofreaders to check new nucleotides
 Telomeres
o Repeat sequence of DNA that acts as a buffer to protect organism’s gene
 Telomerase
o Enzyme that lengthens the DNA sequence and prevents it from becoming too short (multiple rounds of
DNA replication cause shortening of the DNA)
Friday, Oct. 23, Lecture 12: Transcription
 DNA  mRNA (occurs in the nucleus)
 Initial DNA strand used to create primary transcript (mRNA)
 Codon: three nucleotide sequence that codes for an amino acid (20 AAs) – triplet of nucleotides is the smallest
unit to code for all AAs
o 64 codons total with 3 being STOP codons to end translation
 Template strand: one DNA strand that is designated for transcription
 Steps of transcription:
o
o
o
Initiation
 Promoter region important for initiating transcription
 Transcription factors bring in machinery needed for starting transcription (e.g. allowing binding
of RNA polymerase)
 Transcription initiation complex
Elongation
 DNA double helix untwists as RNA polymerase moves and transcribes strand
Termination
 5’ end gets a nucleotide cap (G-cap)
 3’ end gets a poly A tail
 Modifications at ends of strand help protect strand from degradation and allows transport of
mRNA out into nucleus for translation