Microbial Genetics Flashcards
The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix
Conservative model of DNA.
The two strands of the parental molecule separate, and each functions as a template for synthesis of a new, complementary strand
Semiconservative model of DNA.
Each strand of both daughter molecules contains a mixture of old and newly synthesized DNA.
Dispersive model of DNA.
Showed that DNA replicates in a semi-conservative way (1958)
The Meselson-Stahl experiment
abundant, basic proteins with a positive charge that bind to DNA.
5 main types: H1, H2A, H2B, H3, H4
~equal in mass to DNA
evolutionarily conserved
Histone-charged proteins
Packing of DNA into chromosomes:
1- Winding of DNA around histone octamer to create a nucleosome structure
2- Nucleosomes connected by strands
of linker DNA like beads on a string.
3- Packaging of nucleosomes into
30-nm chromatin fiber.
4- Formation of looped domains.
Transformation
The genetic alteration of a bacterial cell resulting from the direct uptake and incorporation of exogenous (outsider) genetic material from its surroundings through the cell membrane
Plasmid
Naturally occurring circular dsDNA molecules.
Separated from the main genomic DNA of organism.
Exist in multiple copies (a few to 100’s) in bacterial cell, 1 kb to 200 kb.
Single, circular, double stranded DNA.
Function of plasmid
-Enhancement of adaptation to bacterial environment by transferring genes horizontally (i.e., without normal conjugation).
-Responsible for out break of antibiotic resistant pathogenic bacteria, due to transfer of plasmids containing antibiotic resistance genes between bacterial spp.
Enzyme activity can be used as marker for gene insertion
Disrupted gene = nonfunctional
Intact gene = functional
Media containing XGAL chromagenic substrate used (blue colonies = intact; white colonies = disrupted)
Enzyme beta galactosidase (lac Z)
Operon
A set of adjacent structural genes, plus the adjacent regulatory signals that affect transcription of the structural genes.
Bacterial conjugation
It’s the process by which one bacterium transfers genetic material to another through direct contact. During conjugation, one bacterium serves as the donor of genetic material, and the other serves as the recipient
“F” Plasmid
‘fertility factor’ confers donor characteristics through (sex pilus) to bacterial cells.
Bacteria that have F plasmid are referred to as F+ or Donor.
Those that do not have F plasmid are called F- or recipient.
Tra-region
Genes coding the F-Pilus and DNA transfer process.
OriT
The sequence which marks the starting point of conjugative transfer
Agrobacterium
a genus of bacteria that causes tumors in plants
Ti Plasmid consists of two main regions
T-DNA gene region
Virulence gene region
Encodes for plant growth hormones including auxins and cytokinins, which promotes cell division and induce tumor formation (Crown gall disease)
T-DNA gene region
The bacterial genes in operons VirA-VirE are necessary for DNA transfer to take place
Virulence gene region
-Wound sites on plants attract Agrobacterium through the release of phenolic compounds.
Chemotactic response in Agrobacterium T-DNA transfer
VirA
Transmembrane receptor, which detects the plant aceto-syringone from the wound site
VirG
“Transcription activator, in which it activates the rest of the Vir operons”.
VirC
ssDNA synthesis of the T- DNA
VirD1
Nick the T-DNA at the L and R boarders
VirD2
Will direct T-DNA integration to the plant genome
VirE
Coating the ssDNA for protection and preservation
VirB
The mechanism for translocation and membrane porethat allows ssDNA to go from bacteria into plant cells.
Capsid:
is the protein shell that encloses the nucleic acid.
It protects the nucleic acid from digestion by enzymes
Bacteriophage lambda
is a virus that infects E. coli
Transduction
It’s the process by which a virus transfers its genetic material from one bacterium to another. Viruses called bacteriophages are able to infect bacterial cells and use them as hosts to make more viral copies
Lysogenic Cycle
Also referred to as non-virulent infection, it does not kill the host cell, instead uses it as a refuge where it can exists in a dormant state.
-The prophage genome is then replicated passively along with the host genome as the host cell divides.
Transposable elements cause genetics changes and make important contributions to the evolution of genomes through:
Insert into genes.
Insert into regulatory sequences; modify gene expression.
Produce chromosomal mutations.
Transposable elements (TEs), also known as “jumping genes
DNA sequences that can move from one location on the genome to another
Transposable elements in prokaryotes, Three examples:
Insertion sequence (IS) elements.
Transposons (Tn).
Bacteriophage Mu.
Insertion elements (IS)
-Simplest type of transposable element.
-Encode only genes for mobilization and insertion.
-Range in size from 768 bp to 5 kb.
-Ends of all known IS elements show inverted terminal repeats (ITRs).
Transposons (Tn) (Amp. marker):
Similar to IS elements but are more complex structurally and carry additional genes.
2 types of transposons:
-Composite transposons.
-Noncomposite transposons.
Composite transposons (Tn):
Carry genes (e.g., a gene for antibiotic resistance) flanked on
both sides by IS elements.
-Have IS elements
Noncomposite transposons (Tn
Carry genes (e.g., a gene for antibiotic resistance) but do not terminate with IS elements.
-Has No IS element.
Yeasts?
Unicellular fungal eukaryotic organisms that can reproduce by budding or fission.
1-It has a fast growth rate, with a generation time of only 90 minutes under optimal conditions
Criteria that made yeast a model example for studying genetics?
1-Rapid growth.
2-Nonpathogenic
3- Possesses a well-defined genetic system.
4- Unlike many other microorganisms, S. cerevisiae is viable with numerous markers.
5- A cheap source for genetical studies.
budding
Daughter cell is initiated as an out growth from the mother cell, followed by nuclear division, cell-wall formation, and finally cell separation.
-Each mother cell usually forms no more than 20-30 buds, and its age can be determined by the number of bud scars left on the cell wall.
1-If 2n is in rich media it goes through:
2-If 2n is starved for nitrogen, it goes through:
1-Mitosis
2-Meiosis
MATa or MATα.
Mating type is determined at a genetic locus named MAT
Sexual Phase
-Haploid of opposite mating type can mate to form diploid.
-Mating occurs when cells of the opposite mating type become physically close to each other.
Are similar in structure to those of other fungi and eukaryotes, consisting of repeated DNA, it functions in protecting the linear DNA from degradation by nucleases
Telomere
The attachment site for mitotic
spindle fibers, “pulls” one copy of each duplicated
chromosome into each new daughter cell.
Centromere
URA3 marker
An auxotrophic marker for transformation of S. cerevisiae, a gene that encodes a protein required for pyrimidine (Uracil) biosynthesis.
TRP1 (Tryptophan) marker
Its an auxotrophic marker. Therefore, trp1 transformed cells require tryptophan to grow.
SUP4 marker
Is a gene compensates for a mutation in the yeast host cell that causes the accumulation of red pigment
Mutation
The sequence of DNA including insertions, deletions, duplications, and translocations”.
Origin of mutation are:
-Spontaneous Mutations.
-Induced Mutations.
Mutation effect on structure
-Small scale mutations affecting one or a few nucleotides.
-Large-scale mutations in chromosomal structure
A change in the nucleic acid sequence of an organism at a specific predetermined location.
Site directed mutagenesis
Mutation effect on function
Loss-of-function mutations.
Gain-of-function mutations
Base pair substitutions.
1- Transitions
Convert a purine-pyrimidine to the other purine-pyrimidine
Base pair substitutions.
2- Transversions
Convert a purine-pyrimidine to a pyrimidine-purine.
Leading to multiple copies of
chromosomal regions, increasing the
dosage of the genes located within them
Gene duplication/ Amplifications
Insertions or Deletions of large chromosomal regions.
Leading to the gain or loss of genes within those regions”.
Chromosomal inversions
Reversing the orientation of a chromosomal segment
Amorphic
When the allele has a complete loss of function (null allele)
Neo-morphic
A mutations resulted in the change of the gene product, such that it gains a new function. These mutations usually have dominant phenotype.
