DNA Replication and Mitosis Flashcards
Why did most scientists think that proteins were the genetic material before it was discovered that it is actually DNA?
Because they are made of 20 amino acids while DNA are made of just 4, meaning it seemed like there was more potential for complexity and variety in proteins
T. H. Morgan
Figured out that genes are located on chromosomes, but still didn’t know whether was proteins or DNA were the genetic material (the two components of chromosomes)
(early 1900s)
Fredrick Griffith
worked with two strains of baccterium:
- S cells (pathogenic for mice)
- R cells (harmless for mice)
When he mixed heat-killed remains of the S cells with living cells of the R cells, some R cells became pathogenic
(Early/Mid 1900s)
Oswald Avery, Maclyn McCcarty, and Colin MacLeod
repeated Griffith experiment, but once they killed pathogenic cells, they separated them into different molecules (DNA, etc), co-incubated separately with the living nonpathogenic cells, and only the DNA was able to make it pathogenic, and thus DNA was the transformation factor
announced that the transforming substance in bacteria was DNA
BUT
many biologists remained skeptical because there was little known about DNA
(Mid 1900s)
Bacteriophages (shortly, phages)
viruses that infect bacteria and were (and still are) widely used in molecular genetics research
What actually is a virus and how does it infect a cell
a virus is DNA (sometimes RNA) enclosed by a protective coat, often simply protein (bactoriophage)
they infect cells by injecting the DNA into the cell but not inserting the protein into it
Alfred Hershey and Martha Chase
Showed that DNA is the genetic material of a phage known as T2 through blender experiment
experiment showed that only one of the two components of T2 (DNA or Protein) enters an E. coli cell during infection
(mid 1900s)
explain the blender experiment
concluded that the injected DNA of the phage provides the genetic information:
virus injects DNA into bacteria, blended to separate bacteria from everything inside, and centrifuged cells form a pellet
DID THIS TWICE,
in 1, made Protein Radioactive (actually made Sulfur radioactive because it occurs in protein but not DNA), and radioactivity was found in liquid (phage protein)
In 2, made DNA radioactive (actually made Phosphorus radioactive because it occurs in DNA but not protein), and pellet was radioactive (phage DNA)
Erwin Chargaaff (and his rules)
Two findings became known as Chargaff’s rules
Chargaff’s rules:
1. In DNA,
- amount of A = amount of T
- amount of C = amount of G
- The base composition of DNA varies between species
(Mid 1900s)
A Piece of DNA has 30% Adenine, what percent Guanine does it have?
20% Guanine
(100 - (30*2))/2
Maurice Wilkins and Rosalind Franklin
used technique called X-ray crystallography to study molecular structure
Franklin produced a picture of the DNA molecule using this technique
Watson and Crick
The X-ray images enabled Watson to deduce that the DNA molecule was made up of two strands, forming a double helix
Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA
They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
What is the word to describe the two strands of DNA that can form a double helix?
complementary
Describe Watson and Crick’s model of DNA replication
they had a semiconservative model, meaning each daughter molecule will have one old strand conserved from the parent strand, and one new strand
What were the three competing models of DNA replication? What experiment was ran to figure out the right one (and who was behind the experiment)?
The three competing models were the…
1. conservative model
2. semiconservative model
3. dispersive model
Experiments by Matthew Meselson and Franklin Stahl supported the simconservative model:
They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope
The first replication produced a band of hybrid DNA, eliminating the conservative model
A second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semiconservative model
What are the sites DNA replication begins at called? What to they create?
origins of replication, create a replication “bubble”
What are the grey strands called?
grey strands are called daughter strands
where is a replication fork located
the ends of a replication bubble
What are leading strands and lagging strands?
The daughter strand that is made continuously is the leading strand, the one that is made in okasaki fragments is the lagging strand
What is the order of enzymes that help DNA replication?
- Helicase
- single strand binding proteins (SSBP)
- topoisomerase
- primase
- DNA polymerase III
- DNA polymerase I
- Ligase
- Telemersae (only in some cells)
What is the role of helicase in DNA replication?
To break hydrogen bonds between two strands and open up an origin of replication
What is the role of single stranded binding proteins in DNA replication?
to prevent DNA strands from binding back together after they are made a bubble (they want to because they are complementary), to stabilize single stranded DNA
What is the role of topoisomerase in DNA replication?
To break DNA strands (covalent bonds), let them untangle, and then rejoin them
What is the cell cycle?
a sequence of events that extends from the time a cell is first formed from a dividing parent cell until its own division.
What happens during G1 phase of interphase?
Cells do their job (there is one chromatid that is considered also one chromosome)
What happens during S phase of Interphase?
The Chromatid replicates and makes sister chromotids (still considered one chromosome)
DNA replicates
