L27:'The Central Dogma' of molecular biology Flashcards
Provide evidence that genetic traits can be transferred between different bacterial straits
Griffiths carried out an experiment using 2 different strains of Streptococcus pneumonia:
- S (smooth) strain, trait: cells HAVE protective capsule -> protected from animal immune system -> pathogenic
- R (rough) strain, trait: cells DON’T HAVE protective capsule-> nonpathogenic
Mice were injected with the straints as follows:
Controls:
injected S strain -> mouse dies
injected R strain -> mouse healthy
injected heat-killed S -> mouse healthy
injected heat-killed S + R -> mouse dies
Explanation:
The living R cells inherit a molecule of genetic material from the dead S cells, the inherited information enables the R cells to form protective capsules, thus the inherited genetic material has caused the R cells to develop the pathogenic trait. Transformation of the R cells into S cells has occurred.
furthermore all descendants of the transformed R cells possessed the pathogenic trait, therefore pathogenicity is a heritable trait.
Define: Transformation
A change in genotype and phenotype due yo the assimilation of external DNA by a cell
Give evidence DNA is the molecule which carries the genetic material necessary for transformation
DNA molecule
Evidence 1) Griffith’s work furthered
Protein destroyed, transformation still occurs:
-Heat-killed S cells treated with protease enzymes + living R cells -> transformation still occurs
DNA destroyed, No transformation occurs:
Heat-killed S cells treated with deoxyribonuclease enzymes (DNA’se) + living R cells -> no transformation
Virus
Viruses consist of DNA (or RNA) enclosed in a simple protective coat (often a protein) and in order for more viruses to be produced a virus must infect a cell and take over the cell’s metabolic machinery.
Bacteriophages (phages)
Bacteriophages (phages) are viruses that infect and destroy bacteria.
Viruses consist of DNA (or RNA) enclosed in a simple protective coat (often a protein) and in order for more viruses to be produced a virus must infect a cell and take over the cell’s metabolic machinery
DNA is hereditary material, give experimental evidence for this in context to Viral DNAs ability to program cells.
Experiment:
T2 is a virus that infects bacteria, it’s a phage. The T2 phage attaches to a host cell via it’s tail fibres and uses it’s ‘tail sheath’ to inject genetic material through the plasma membrane and into the host cell
Both DNA and Protein are each labelled with a different radioactive source, therefore two batches of phages required:
Batch 1) T2 phages are labelled with radioactive sulphur, this labels the protein (since protein contain sulphur and DNA doesn’t ).
Batch 2) T2 phages are labelled with radioactive phosphorus, this labels the DNA (since DNA contains phosphorus and protein doesn’t ).
Samples of bacterial cells are then infected with the labelled phages (two samples = one for each batch)
Theses samples must then be tested to see which molecule -Protein or DNA- had entered the bacteria cells, i.e. which molecule would be capable of reprogramming the bacteria cells.
Infected bacterial cells are mixed in a blender, the agitation is required to ‘knock’ phage parts, outside of the bacteria, from the cells.
The mixture is centrifuged causing bacteria cells to form a dark pellet (sinks to bottom of test tube) as well as free phages and phage parts -lighter in colour- to remain suspended in liquid.
Radioactivity in the pellet and the liquid is measured to determine which contains the DNA and which contains protein.
Results show that:
- Radioactive phages DNA found in pellet
- Radioactive phages Protein found in liquid
Therefore concluded that DNA molecules entered the host cell and Protein did not.
The bacteria was returned to the medium, the infection ran it’s course and the bacteria released phages that contained radioactive phosphorous; evidently the DNA, injected into the bacteria, had a role during the infection process and thus it was concluded that the DNA contained genetic information that was responsible for the new viral DNA and proteins produced by the bacteria cells.
This experiment provides substantial evidence that DNA molecules, rather than proteins, are hereditary materials.
Evidence DNA is a genetic material, in context to Chargaff’s rules
Chargaff’s rules
1)The base composition of DNA varies between species
It is therefore credible that DNA could be responsible for genetic diversity
2)In any species the number of A and T bases is equal and the number of G and C bases is equal
Describe DNA structure founded by Franklin, Watson and Crick
Franklin used X-ray crystallography to produce a picture of the DNA molecule; this allowed Watson and Crick to deduce that DNA has a double helix structure. The image also enabled them to deduce the width of the helix and the spacing of the nitrogenous bases.
DNA has a double helix structure composed of two anti-parallel sugar-phosphate backbone strands (strands run in opposite directions), this results in a 3’ end and 5’ at a phosphate group.
Both A and G are larger bases which pair to T and C which are smaller bases, to form purines (A-T) and pyrimidines (G-C), since purines and pyrimidines are formed from a composition of similar sized bases they have a similar shape; the similarity in shape of purifies and pyramids allows for constant spacing between the two strands.
Difference between Right handed double helix and left handed double helix
Imagine holding a DNA double helix in your hand, for a right handed double helix your finger would curl up around the structure (vice versa for left handed DNA double helix).
Explain why Chargaff’s ruel is true for DNA
A pairs to T
G pairs to C
Therefore the number of A and T bases is equal and the number of G and C bases is equal
Explain why the structure of DNA provides evidence for DNA being a hereditary molecule
In order for DNA to be hereditary it must be possible to copy a DNA molecule so the genetic information can be passed from one generation to the next.
The specific paring of bases allows for replication to take place as follows:
DNA molecule unwinds and weak hydrogen bonds between base pairs break unzipping the molecule and exposing the bases of both DNA strands.
A primer attaches to one end of each exposed DNA template strand. DNA polymerase adds free complementary DNA nucleotides to the 3’ end of the growing strand.
Hydrogen bonds form between bases and chemical bonds form between phosphate and deoxyribose sugar of ajcent nucleotides.
Ligase bonds fragments into the lagging strand to form a complete strand
Each replicated DNA molecule is made of one orginal template strand and a newly synthesised strand.
