Unit 10 part 1 Flashcards
Cells can repair
their DNA
who introduced the double helix
james watson and francis crick 1953
- prior identifying molecules of inheritance wasa. major problem.
molecules of inheritance
dna
Series of famous experiments showing the DNA is the genetic material
- Genetic Material: DNA or Protein?
- Frederick Griffith in 1928
- Alfred Hershey and Martha Chase in 1952
- Erwin Chargaff in 1950
Griffith’s Experiment:
- Two strains of bacteria, one pathogenic and
one harmless - Transformation: change due to the introduction of foreign DNA
- People still thought protein was genetic material – a lot of heterogeneity and specificity of function
Hershey and Chase’s Experiment
- More evidence for DNA as genetic material
- Studied viruses that infect bacteria, known as bacteriophage (phage)
- A virus is DNA or RNA enclosed by a protective capsid, often protein
- Experiment shows only one of the two components of phage (DNA or protein) enters an E. coli cell during infection
- They concluded that the injected DNA of the phage provides the genetic information
- DNA is a polymer of nucleotides:
- Consisting of:
- Nitrogenous base * Sugar
- Phosphate group
- The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C)
Chargaff’s Rules
- In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next
- Two findings became known as Chargaff’s rules:
- The base composition of DNA varies between species
- In any species, the number of A = T, and the number of G = C
- Maurice Wilkins and Rosalind Franklin
- Using X-ray crystallography to study
molecular structure - Franklin produced a picture of the DNA molecule using this technique
- Franklin’s X-ray crystallographic images of DNA allowed Watson to deduce that DNA was helical
Watson used the X-ray images to deduce:
- Helical structure
- Width of the helix
- Spacing of the nitrogenous bases
- The pattern in the photo suggested:
- DNA molecule was made up of two strands, forming a double helix
Watson built a model in which
the backbone were antiparallel
Pairing a purine (AorG) with a pyrimidine (CorT) resultedin a uniform width consistent with the X-ray data
The Watson-Crick model explains Chargaff’s rules:
in any organism the amount of A = T, and the amount of G = C
DNA Double Helix
- Right-Handed Helix
- Antiparallel Sugar Phosphate backbone * Nitrogenous bases
- Major / minor groove
- Hydrogen bonds
DNA replication and Repair
* The relationship between structure and function is evident in the double helix
- Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material
The Basic Principle: Base Pairing to a Template Strand
- The two strands of DNA are complementary
- Each strand acts as a template for a new strand in replication
- In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
Competing Models of DNA Replication
- conservative model
- dispersive model
- semiconservative model (watson and crick)
- Conservative model -
the two parent strands rejoin
( one brand new one original ) first replication
(3 brand new 1 new) second replication
dispersive model
each strand is a mix of old and new
- first replication is 50/50 new old for both
- second replication is 25/25 each
- splotchy
semiconservative model watson and crick
When a double helix replicates, each daughter molecule will have one old strand (“conserved” from the parent molecule) and one newly made strand
- 1:1 (new to old ratio) first replication
- 2 1:1(new old ratio) then 2 brand new second replication
Determining model of DNA replication
Experiments by Matthew Meselson and Franklin Stahl supported the semiconservative model
- bacteria cultured in medium,
- bacteria transfered to medium with a lighter isotop, - dna was centrifuged,
- centrufuged again where the less dense and more dense portions separated
Prokaryotic DNA Replication
- The copying of DNA is rapid and accurate * Numerous proteins and enzymes
contribute to DNA replication - Replication begins at sites called origins of replication (ori)
- Where the two DNA strands are separated, opening up a replication “bubble”
Eukaryotic DNA Replication
- A eukaryotic chromosome may have hundreds or even thousands of origins of replication
- Replication proceeds in both directions from each origin, until the entire molecule is copied
Replication fork:
at the end of each replication bubble
* Y-shaped region where new DNA strands are elongating
* Paired (one on either side of replication bubble)
Helicase:
enzyme that untwist the double helix at the replication forks
- Single-strand binding proteins:
bind to and stabilize single- stranded DNA
- Topoisomerase
relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands
cleaves the dna strand
DNA Polymerases:
- Enzymes that catalyze synthesis
of new DNA at replication fork - Require a primer (so they can add nucleotides) and a DNA template strand
