OnC OF PROKARYOTIC GENOME Flashcards
What is the bacterial chromosome?
Most bacterial species contain a large, circular chromosome.
It is highly compacted and found within the nucleoid, not membrane bound.
A typical chromosome is a double stranded DNA that is associated with DNA binding proteins.
Most bacterial species contain a single chromosome type in multiple copies.
Several thousand genes are interspersed throughout the chromosome. Structural genes account for the majority.
How many origins of replication does a prok chromosome have?
One.
OOR is a sequence that functions as an initiation site for the assembly of several proteins that are required for DNA replication.
What are plasmids?
Small circular pieces of DNA that exist independently of the bacterial chromosome.
They are self replicating
Not necessary for bacterial survival, however they confer advantages to bacterial survival in stressful environments.
What are the 5 cats of plasmids?
Fertility Plasmids, (F factors) allow bacteria to mate with each other, facilitating genetic recombination, which may be advantageous in a changing environment.
Resistance Plasmids (R factors), contains gene that confer antibiotic and toxin resistance.
Degradative Plasmids, carry genes that enable the bacterium to digest and utilise an unusual substance.
What is the structure of the bacterial chromosome?
To fit within the bacterial cell, chromosomal DNA must be compacted severalfold. It involves the formation of loop domains, which bases are anchored by DNA binding proteins.
Supercoiling leads to further compaction of the looped bacterial chromosome, which is negatively supercoiled. This promotes DNA strand separation in small regions, enhancing replication and transcription.
DNA gyrase and topoisomerase 1 control the degree of supercoiling.
Describe the process of binary fission.
Before the bacterial cell divides, semi-conservative replication of parental DNA begins at the origin of rep to give rise to two origins of rep.
As the chromosome continues to rep, each origin moves rapidly toward the opposite end of the cell, adhering to the CSM.
When the chromosome is replicating, the cell elongates , separating the two identical chromosome copies.
When replication is complete, the CSM invaginates and deposits new cellwall materials. Daughter cells are genetically identical to the parent cell.
What is transformation?
Transformation is the alteration of bacterial cell genotype by the uptake of naked, foreign DNA from the surrounding environment.
This DNA is released into the environment when another bacterium has died.
Many bacteria possess cell surface proteins that recognise and transports DNA from closely related species into the cell. Foreign DNA can then be incorporated into the genome, either by integration or recombination via crossing over at a homologous region.
What is transduction?
Transduction occurs when a phage infects a bacterial cell and then transfers some of the bacterial DNA to another bacterial cell.
It results due to an error in the phage reproductive cycle. A phage that carries bacterial DNA may not be able to replicate because it lacks some or all of its own genetic material. However, the phage is still able to attach and inject bacterial DNA acquired from the first cell.
Cell becomes recombinant if the DNA is incorporated into recipient cell chromosome by homologous recombination.
What is generalised transduction?
It results from an error in a phage lytic cycle.
Bacterial genes are randomly transferred from one bacterial cell to one another.
During the synthesis of phage DNA and proteins, the bacterial chromosome is degraded into small pieces.
During generalised transduction, a small piece of the host cell’s degraded DNA could be accidentally packaged within a phage capsid in place of the phage genome during assembly.
As the phages are released from its host cell via the lytic cycle, this mistake creates a phage carrying bacterial chromosomal DNA. This virus is defective as it lacks its own genetic material.
The phage attaches to another bacterium, injecting the piece of bacterial DNA from the first cell.
If crossing over takes place between homologous regions of the chromosome, recombinants are formed.
What is specialised tranduction?
Specialised tranduction results from an error in phage lysogenic cycle.
Only bacterial genes adjacent to the prophage site are efficiently transferred to another bacterium.
Prophage is incorrectly excised from bacterial chromosome and the phage DNA incorporated some bacterial genes adjacent to the prophage.
This phage carries bacterial chromosomal DNA.
Some of this DNA can replace the homologous region of the recipient cell’s DNA by homologous recombination.
What is bacterial conjugation?
A direct physical interaction between two bacterial cells and the transfer of genetic material from a donor bacterium to a recipient bacterium.
The DNA transfer is one-way, the donor uses appendages called sex pili to attach to the recipient.
The sex pilus retracts to draw the donor and recipient cells closer together upon contact.
A temporary cytoplasnic mating bridge then forms between two cells, providing an avenue for DNA transfer.
During conjugation, genes within the F factor encode proteins that promote the transfer of one strand of F factor DNA.
The DNA strand is cut at the origin of transfer and then the strand travels through the cytoplasmic mating bridge into the recipient cell.
Other strand remains in the host cell, and is restored to its OG 2-stranded condition via replication.
The F factor DNA in the host cell also becomes double stranded.
What are the regulatory sequences found in the lac operon?
- Promoter
- Found upstream of structural genes.
- Is the site where RNA polymerase binds to DNA prior to transcription of the structural genes. - Terminator
- Found downstream of structural genes
- Is the site which signals the end of transduction - CAP site is a DNA sequence recognised by an activator protein
- Operator
- Is situated between the promoter and structural genes.
- Is a binding site for the repressor.
What does the genes of the Lac Operon code for?
They code for enzymes and lie adjacent to one another.
When the RNA polymerase moves from one structural gene to the next, the genes are transcribed into a single mRNA.
The mRNA that is transcribed is described as a polycistronic mRNA as it contains the coding sequences of two or more structural genes. Introns are absent.
What are the three structural genes of the lac operon?
- lacZ
- Codes for beta-galactosidase
- beta-galactosidase hydrolyses lactose into glucose and galactose.
- A side reaction of this enzyme is to convert a small percentage of lactose into allolactose, a structurally similar lactose analog. - lacY gene
- Lactose Permease.
- Membrane protein required for the transport of lactose into the cell - lacA gene
- Codes for galactosidase transacetylase.
What is the lacI regulatory gene?
Codes for the lacI repressor, important for the regulation of the lac operon.
Is constitutively expressed at fairly low levels.
lacI gene has its own promoter called the i promoter, considered to be a regulatory gene because its sole function is to regulate the function of structural genes.
The lac repressor is synthesised in an active form and binds to the operator. When the lac repressor binds to the operator, the promoter is blocked from the RNA polymerase, and transcription of the structural gene is prevented.
The ability of the repressor to bind to the operator and inhibit transcription depends on the proteins conformation, which is allosterically regulated by an inducer.
