Mod 5 Molecular basis of Inheritance
What did Frederick Griffith discover?
he discovered the genetic role of DNA by working on two different strains of bacteria
What was Frederick Griffith's experiment and what was concluded?
One strain, called the S strain was pathogenic while the other R strain was harmless.
Mice were unaffected when heat-killed S cells were injected into them. However, Griffith was surprised to find that when he injected a mixture of heat-killed pathogenic S cells and living non-pathogenic R cells into, some of the living non- pathogenic R cells became pathogenic and killed the mice.
Griffith concluded that some chemical component of the dead pathogenic S cells was the cause for this pathogenic change. However, the identity of this substance was unknown.
Griffith called this phenomenon transformation.
What was Hersey and Chase's work?
Their work was to determine if the material of inheritance is DNA or protein.
They studied a bacteriophage called T2, a virus that infects the bacterium E. coli.
What is referred to as a ghost?
During infection, the protein coat called a capsid of the parent virus remains attached to the cell wall of the bacterium.
What were the two hypotheses drawn from the experiment by Hersey and Chase?
1. If the genes consist of DNA, then the radioactive DNA would be found inside the infected bacterium and protein would be present only in the ghosts outside the cell
2. If genes consist of proteins, then the infected bacterium would have radioactive protein inside and DNA outside the cell.
What was the conclusion of the experiment by Hershey and Chase?
Hershey and Chase found the radioactive DNA in the cells in the pellet and the radioactive protein in the ghosts, suspended within the liquid part of the sample.
They concluded that genetic material would be found inside the DNA cell, not proteins. This landmark study provided powerful evidence that DNA is a hereditary material.
Chargaff’s Rules: What were two of his findings?
The first finding: the base composition of DNA varies between species
The second finding in any species, the number of adenine A and thymine T bases are equal, and the number of guanine G and cytosine C bases are equal.
The significance of these rules was not fully understood until the discovery of the double helix.
What did James Watson and Francis Crick determine?
They determined the correct structure of DNA in 1953.
Who was the first person who discovered the structure of DNA?
Rosalind Franklin and her colleague Maurice Wilkins used X-ray crystallography and produced an image of DNA that allowed Watson and Crick to deduce the double helical structure of DNA.
What did Watson and Crick determine about the DNA structure?
DNA strands run in an antiparallel configuration
DNA strands form a double helix
- the hydrophilic sugar-phosphate backbone faces the exterior
- nitrogenous base pairs face the interior
- they proposed that purines always pair with pyrimidines. Specifically, DNA strands form complementary base pairs where A is always paired with T, and G is always paired with C.
What are purines and pyramides in DNA?
Purines always pair with pyrimidines.
Purines: Adenine & Guanine.
Pyrimidines.: Cyostine & Thymine
The pair: Adentine and thymine have two hydrogen bonds
The pair: Guanine and cytosine have three hydrogen bonds
What is DNA's primary structure?
Two major components:
- a backbone made up of the sugar & phosphate groups of deoxyribonucleotides
- a series of nitrogen-containing bases that project from the backbone
it also has directionality. The one end of the sugar with an exposed hydroxyl group at the 3rd carbon of the sugar molecule is the three’ end and the other end with an exposed phosphate group at the 5th carbon of the sugar molecule is the five’ end.
What was Watson and Crick's conclusion about the structure of DNA?
- a DNA molecule has two strands that run in opposite directions and these antiparallel strands twist to form the double helix structure
- the structure of DNA is stabilised by complementary base pairing, where adenine A is always paired with thymine T and guanine G is always paired with cytosine C.
What is the conservative model?
-the parent strands act as a template for the synthesis of an entirely new DNA molecule
-the two parent strands reassociate after acting as a template, thus restoring the parental double helix
-after the second round of replication, the parent DNA strand is still visible, and three brand-new DNA molecules are produced
What is the semiconservative model?
- the two strands of the parental molecule separate, and each functions as a template for the synthesis of a new, complementary strand
- as a result, newly produced DNA molecules will have one old and one new strand
What is dispersive replication?
-each strand of both daughter molecules contains a mixture of old and newly synthesized DNA
Who is Meselson and Stahl?
- Meselson and Stahl are researchers who designed the experiment that distinguishes between the three hypotheses of DNA replication
What was Meselson and Stahl's experiment?
-they grew E. Coli in the presence of heavy nitrogen 15N to label the bacterial DNA called Generation Zero and an initial example was collected, samples from Generation 1 & 2 were also collected
- in generation zero the DNA was labelled with heavy nitrogen and therefore appeared at the bottom.
-generation 1 produced an intermediate band between 15N and 14N
-generation 2 produced two bands, one at 14N suggesting newly formed DNA and one intermediate band suggesting a hybrid
What was Meselson and Stahl's conclusion on their experiment?
- the result supported the hypothesis of semi-conservative replication as the mechanism by which DNA is replicated
What is DNA polymerase?
-the addition of a nucleotide in a DNA strand
- DNA polymerase enzyme is that it can only work in one direction, adding nucleotides 3' end of the growing DNA strand with the release of two phosphates
- as a result DNA polymerase synthesis always proceeds in the 5' to 3' direction
What is Helicase?
- an enzyme that unwinds the double helix at the replication fork and breaks down the hydrogen bonds between complementary bases to separate the parent strands
What is a single-strand binding protein?
- single-strand binding protein attaches to the separated strands and prevents them from re-pairing
What is a Topoisomerase?
- breaks, swivels, and rejoins the parental DNA ahead of the replication fork, relieving the strain caused by unwinding
What is the structure of DNA when DNA replicates?
- for DNA to replicate, the double-helix structure must be open
What is a primase?
- synthesizes RNA primers, using the parental DNA as a template
What is DNA polymerase III?
- adds nucleotides to the 3' end and the leaving strand or continuous strand that replicates towards the replication fork is produced
What is the lagging strand?
- the other DNA strand that is synthesized away from the replication fork
- the lagging strand starts when primase synthesis for primer DNA polymerase III adds free nucleotide bases to the 3' end of the primer
-requires multiple primers that are synthesized properly
DNA replication where does it begin?
- replication begins at special sites called origins of replication where two DNA strands are separated, opening up to a replication 'bubble'
Prokaryotes vs Eukaryotes for replication sites?
- prokaryotes which have one circular DNA molecule have a single origin of replication
-eukaryotes have hundreds or thousands of origin sites per chromosome
What are Okazaki fragments?
- short segments at the lagging strand that are synthesized from DNA polymerase away from the replication fork
What is DNA polymerase I?
-an enzyme that removes the RNA primers from the lagging strand at the start of the Okazaki fragment and fills the gap uo by adding nucleotides
What is Lipase?
- an enzyme that joins multiple Okazaki fragments to form a continuous DNA strand
How does DNA polymerase Proofread?
- if the enzyme finds a mismatch, it pauses & removes the mismatched base that was just added
-DNA polymerase can do this because it acts as an exonuclease that removes deoxyribonucleotides from DNA
What is the term mismatch repair?
- if mistakes remain after synthesis is complete, the repair enzyme can remove the defective bases & repair them
- this is termed mismatch repair and it relies upon the original strand to use as a template for repairing the mismatch
What can single-stranded DNA damage be caused by?
-exposure to harmful chemicals or physical agents such as cigarette smoke and X-rays
- spontaneous changes from cellular metabolic products such as reactive oxygen species (ROS)
What can repair single-stranded DNA damage?
- there are two processes where a nuclease cuts out & replaces damaged stretches of DNA & the gap is filled with correct nucleotides by DNA polymerase & ligase seals the gap
What is the Base excision repair?
- removing a single damaged or incorrect base
What is the Nucleotide excision repair?
- removes helix-distorting lesions such as pyrimidine dimers caused by exposure to UV rays
What is telomere replication?
- the regions at the end of Eukaryotic linear chromosomes have Telomeres which are short repeating stretches of nucleotide bases
- whenever DNA is replicated, the chromosomes tend to become shorter
- the length of a chromosome plays a role in determining how many times a cell can divide
-telomerase is an enzyme that can repair the end of lagging strands by adding more bases
What is euchromatin?
-loosely packed chromatin
What is heterochromatin?
- highly condensed chromatin where genes aren't expressed