Chapter 8 Bacterial Genetics

How do eukaryotes generate genetic diversity?

sexual reproduction, diploid vs. haploid, combination of genetic material between members of a species

How is genetic diversity accomplished in prokaryotes?

reproduction by binary fission, spontaneous mutations, gene transfer

mutation

any change in a nucleotide or nucleotide sequence

wild-type

normal non mutated sequence

genotype

collection of genes present in an organism

phenotype

visible expression of genotype, observable characteristics

silent mutation

some change in genetic sequence with no visible effect

auxotroph

nutritionally deficient bacteria

prototroph

wild type with respect to nutritional requirements

spontaneous mutation

mutations that occur naturally in the absence of mutagens

induced mutation

mutation that results from exposure to chemicals or environmental agents

frameshift mutation

mutation that shifts the "reading" frame of the genetic message by inserting or deleting a nucleotide

transposable element

move between organisms and within a single chromosome, jumping genes

point mutation

single base change in DNA sequence

reading frame

Reading mRNA nucleotides in the correct groupings

homologous recombination

Exchange of genetic information between homologous DNA molecules.

How do spontaneous mutations occur

errors during replication and transposable elements

What types of mutations occur spontaneously?

base substitution, addition/removal of nucleotides, insertion mutation

What are the three types of mutations that can occur as a result of a point mutation that results in a base substitution?

silent mutation, missense mutation, nonsense mutation

silent mutations

does not result in a change in the amino acid sequence

missense mutations

results in a codon coding for a different amino acid

Consequences for missense mutation

ex: sickle cell anemia

nonsense mutations

results in a codon for an amino acid change to a stop codon

Consequences for nonsense mutation

truncated protein that is shorter than normal, won't fold or function properly

What is the effect of adding or subtracting 1 or 2 nucleotides during DNA replication?

frameshift mutation

What effect does frameshift mutation have on the gene product?

shift reading frame

What is the effect of adding or subtracting 3 contiguous nucleotides during DNA replication?

add or removes 1 codon, protein has fewer or more amino acids changing function

What is the effect of adding or subtracting 3 non-contiguous nucleotides?

shifts back to original reading frame, amino acid and protein wild type sequence

What type(s) of mutation(s) are caused by transposable elements?

Knockout mutations, insertion mutation

What are the effects of insertion mutations on the gene product?

disrupts coding region of gene, no way to make functional gene product

Why can mutations caused by transposons be classified as either spontaneous or induced?

both natural, unpredictable cellular events and from deliberate exposure to mutagenic agents

Base analogs (AZT)

structural analogs that look like nucleotide be inserted into nucleic acid, not same function

Chemical modification of bases

many carcinogens chemically modify bases

Intercalating agents (ethidium bromide)

inserts between base pairs; addition or subtraction of nucleotides doesn't base pair properly

Transposons

segments of DNA that can move from one region of DNA to another, create gene knockouts

UV radiation

cause thymine dimers which cannot be read properly by DNA poly or RNA poly

X-rays

higher in energy than UV, single and double stranded breaks in DNA result in lethal deletion mutations

Proofreading by DNA polymerase (error in nucleotide incorporation)

DNA poly removes mispaired nucleotide and replaces with correct one

Mismatch repair (error in nucleotide incorporation)

protein cuts unmethylated strand it gets degraded, DNA poly synthesizes replacement

What type of mutation is repaired by dark repair?

Thymine dimer

What human disease is a result in defects in dark repair?

Xeroderma pigmentosum: unable to fix distorting thymine dimer damage get skin cancer

Light repair (photoreactivation, thymine dimer)

light is used to break covalent bond between the thymine nucleobases, restoring original

What enzyme is associated with light repair and how does it become activated and how does it function?

photolyase identifies thymine dimers breaking covalent bonds

What types of organisms can perform light repair?

all organisms but placental mammals, humans only do excision repair

What is the difference between horizontal and vertical transmission of genes in bacteria?

horizontal one cell to another, vertical parent to daughter

What are the mechanisms by which horizontal transmission can occur?

DNA-mediated transformation, transduction, conjugation

Describe the mechanism by which DNA mediated transformation occurs.

the uptake of exogenous DNA (naked DNA)

What are the requirements for DNA mediated transformation?

Cell must be competent in order to undergo transformation, natural and artificial

In what types of bacteria can DNA mediated transformation occur?

many gram positive and gram negative

What are the two types of transduction and what is used to transfer DNA during transduction?

generalized and specialized, bacteriophage used

What types of genes can be transferred during both types of transduction?

generalized is any gene, specialized is only specific genes

Be able to describe the mechanism by which generalized transduction occurs

Bacteriophage binds and injects DNA bacteriophage replicate, cells die chromosomes chopped up and put into bacteriophage make transducing particle

What is conjugation

transfer of DNA from one bacterial cell to another

what are the requirements for conjugation to take place?

requires direct cell to cell contact

In what types of bacteria can conjugation take place?

can occur in both gram positive and gram negative bacteria

In what types of bacteria can conjugation using the F pilus occur?

In gram negative bacteria

Describe the mechanism by which conjugation occurs between an F+ cell and an F- cell.

donor F+ has plasmid and pilus that attaches to recipient F- cell pulls them together forming cytoplasmic bridge single strand of the F plasmid is transferred from the F+ to the F-

What is the difference between an F+ cell and an Hfr cell?

an F+ cell has an F plasmid as a separate, circular DNA molecule, while an Hfr cell has the F plasmid integrated into its bacterial chromosome

What is the difference between the types of genes that can be transferred by F+ cells and Hfr cells?

F+ cells transfer the F plasmid itself, Hfr cells transfer the F plasmid along with large segments of the host bacterial chromosome

Be able to describe the mechanism by which Hfr conjugation occurs.

same as F+ and F-

Be able to list and describe the characteristics of plasmids

circular double stranded DNA, replicate independently of chromosomal DNA, carry a few to several hundred genes, transferred through conjugation and in the lab

What types of genes can be carried on plasmids?

non-essential genes

How are plasmids transferred from cell to cell?

conjugation or transformation in lab

What types of genes are found on R plasmids?

antibiotic resistant genes

Be able to describe the features and the functions of the features of a transposon (transposable element)

2 insertion sequences and DNA forming mobile element

How can transposons move both within a cell and between cells?

"cut-and-paste" or "copy-and-paste" mechanisms, using transposase to move their DNA from one location to another in the genome

Be able to describe the importance of gene transfer in bacterial perspective

genetic variation, adaption to changing environment

Be able to describe the importance of gene transfer in human perspective.

spread of antibiotic resistance genes, transfer of virulence factors

Be able to describe how the CRISPR system functions

degrade proteins into chucks take pieces and array in crispr rna, combine with cas nuclease degrading phage DNA

What is the function of the CRISPR system in bacteria?

bacterial defense mechanism against viruses/bacteriophage, bacteria immune system

How can CRISPR potentially be used to treat diseases in humans?

correcting gene defects that cause genetic disorders, engineering immune cells to fight cancer