Genetics Flashcards
Describe an unplanned or chance event that led to a major discovery in the history of early DNA research.
Griffith discovering that the combination of two non-lethal strains of bacteria could produce a fatal case of pneumonia in the mice
James Watson being allowed to see Rosalind Franklin’s unpublished X-ray crystallography results, enabling him to realize which of his own models was probably right
DNA consists of
wo long strands made up of repeating sequences of simple units called nucleotides (or bases). Each nucleotide is attached to a sugar and a phosphate group. The sugar and phosphate groups join together in adjacent nucleotides to form a backbone for the DNA molecule. The nucleotide sequences in the two strands run in opposite directions to each other, which is why we describe the molecule as anti-parallel.
Nucleotides consist of a nitrogen base, a five-carbon sugar (deoxyribose), and a phosphate group.
pyrimidine-to-pyrimidine pairings are energetically unfavourable why
molecules are too far apart for hydrogen bonding to be established. The opposite is true for purine-to-purine pairings, which are energetically unfavourable because the molecules are too close, leading to overlap repulsion.
AC and GT pairings are impossible because the positions of hydrogen donors and acceptors are mismatched.
The 5 and 3 carbon atoms play a role in identifying the start and end,
how is DNA joined
the deoxyribose and phosphate molecules are joined together by a covalent bond called a phosphodiester bond, which forms between the third and fifth carbon atoms of adjacent sugar rings. It is the sugar-to-phosphate linkage that forms the backbone of the DNA molecule.
The 5 and 3 carbon atoms play a role in identifying the start and end, Because the phosphate always joins at the 3’ end of the sugar as nucleotides are added, this means that the DNA strand has a direction.
how do you know where DNA starts
first nucleotide on the DNA strand has a phosphate group attached to the 5’ carbon in the sugar, so it is called the 5’ end. The last nucleotide on the strand has a hydroxyl group (OH) in the 3’ position of the sugar, so this is called the 3’ end.
why is dna twisted
bond angles between the phosphate and sugar, as well as between the nucleotides. By twisting into this shape, the molecule achieves its most stable configuration.
twist is about 3.4 nm long
steps of DNA extraction
First physically broken up to make it easier to get at the DNA. To break up the cell walls of the tissues to help release the DNA solution is added, called a lysis solution sometimes also called a buffer. The lysis mixture chemically broke up the cell and nuclear membranes to release the DNA into solution.
DNA is physically separated from the other chemicals in the solution through a filtering process.
DNA can be further purified by repeated washing with water and filtering. To wash the DNA out of the filter paper, since DNA is soluble in water The result will be a clear fluid containing water and dissolved DNA,
ideal fruit from which to isolate DNA, since they are octoploid, eight copies of their DNA
Used x-ray diffraction to show the helical nature of DNA
rosalind
chargaff
Identified that there is a one-to-one ratio between Adenine-Thymine and Cytosine-Guanine through experimentatio
hershey and chase
Used bacteriophages which are composed of DNA and protein, to show that DNA enters host bacteria after infected by bacteriophage
avery
Experiments showed that DNA not protein was the transforming agent in Pneumonococcus Bacteria and the hereditary molecule
griffth
Experiments using Pneumonococcus Bacteria showed bacteria can transfer genetic material through a process called transformation
levene
Proved that Nucleotides were composed of a phosphate-sugar-base complex and proposed the “polynucleotide” model for how DNA molecules were put together
miescher
Isolated nuclein and first to identify DNA as a distinct molecule
conservative model,
the parent, or original DNA molecule, produces an exact copy of itself but remains intact. The result is two daughter DNA molecules— one made entirely of new nucleotides, and the other composed of the same original nucleotides as the parent. However, because they are
identical copies, the nucleotide sequence is the same in each.
semi-conservative model,
arent DNA molecule splits apart, with each strand acting as a template for the newly synthesized strand. This produces two new DNA molecules, with each one containing one strand from the original parent DNA, and the other strand made up of new DNA. As in the conservative model, the nucleotide sequence is the same in each daughter DNA molecule. THIS IS IT
dispersive model,
parental DNA molecule is broken into fragments and that each daughter DNA is made up of a random mix of parental and new DNA, again with the same nucleotide sequence as the parent.
Meselson and Stahl experiments
use isotopes of nitrogen to mark new nucleotides, thus distinguishing them from the ones in the original DNA.
nitrogen as their indicator atom, because it needs to be absorbed in large quantities in order for DNA replication to occur. and ecoli reproduced quickly in culture and its biology was well understood. n14 and n15 as they both have different weights - separated out by density using a centrifuge.
replication in prokaryotes is much faster, why
Prokaryotes have circular strands of DNA and contain about a thousand times fewer nucleotides than the long, twisted strands of DNA found in eukaryotes
DNA replication in both eukaryotes and prokaryotes can be broken down into three main phases
Initiation: DNA is unwound and separated to expose each strand in the pair at multiple sites of replication along the DNA molecule.
Elongation: Enzymes attach complementary nucleotides onto the 3’ end of each exposed strand in a linear sequence and check for errors.
Termination: Nucleotide addition stops, enzymes are removed, and the newly formed strands of DNA coil back into the double helix shape.
Origins of Replication
Initiation
Multiple sites along DNA where replication begins
Helicase
Initiation
At each origin of replication point, the double-stranded DNA is unwound and separated by Helicase using ATP as energy.
“Unzips and unwinds” DNA
Replication Fork
Initiation
When the two DNA strands split apart with the help of Helicase, a two-pronged form that resembles a fork is created and is known as the replication fork.
Replication Bubbles
Initiation
When helicase opens up the DNA molecule somewhere in the middle, a bubble containing two replication forks is formed in the DNA, one on each side of the bubble and these are known as replication bubbles.
Topoisomerase
Initiation
In order to eliminate this coiling stress, topoisomerase, moves along the DNA, just ahead of the helicase, cutting the DNA and relaxing the coils.
Prevents super-coiling of a DNA molecule during replication
Single-stranded DNA binding proteins (SSBs)
Initiation
The SSBs help prevent the two template parent DNA strands from re-establishing hydrogen bonds and protect the exposed bases until they are ready to bond with their complementary nucleotides.
DNA Polymerase I
Elongation
DNA polymerase I is active mostly near the end of DNA replication, where it removes the RNA primer.
DNA Polymerase III
Elongation
DNA polymerase III is responsible for joining the nucleotides together and checking for errors.
Catalyzes the formation of the daughter strands in DNA replication
DNA Polymerase replaces the incorrect base with the correct one.
Deoxyribonucleoside triphosphates
dNTP
Elongation
dNTP’s are the nucleotides adenine, thymine, guanine and cytosine with two extra end phosphate groups on each nucleotide that form dATP, dTTP, dGTP and dCTP respectively that provide energy required for the formation of DNA.
Primase
Elongation
Helps to form RNA primer
RNA Primer
Elongation
A starting sequence of nucleotides for DNA polymerase III to attach to formed from RNA with the help of primase
Leading Strand
Elongation
DNA polymerase III continuously adds nucleotides, starting from the initial RNA primer in a 5’ to 3’ direction
Lagging Strand
Elongation
The lagging strand is synthesized discontinuously in short fragments in the opposite direction to the leading strand (that is, away from the fork) with the use of many RNA primers and the formation of Okazaki fragments and DNA ligase.
