Noneukaryotic Genetic Information DNA, and in some cases RNA, is the primary source of heritable information Noneukaryotic Genetic Information

Viruses Viruses consist of the following structures: 1. Nucleic acid - either DNA or RNA contains the heredity information of the virus. 2. Protein coat - which encloses the nucleic acid. 3. An envelope made of phospholipds and proteins from the host cell surround the protein coat (capsid) of some viruses.

Viral Replication

Lytic Cycle In the lytic cycle, a virus penetrates the cell membrane of the host and uses the enzymes of the host to produce viral nucleic acids and viral proteins. The viral components are assembled into new viruses which subsequently burst from the host cell, destroying the host cell in the process. Retroviruses are special case of the lytic cycle. Retroviruses use an enzyme called reverse transcriptase to make a DNA complement of their RNA. The DNA complement can then begin the lytic cycle or the lysogenic cycle. HIV is a retrovirus.

Lysogenic Cycle In the lysogenic cycle, a virus penetrates the cell membrane of the host but rather than take over the cell the nucleic acid becomes incorporated into the host cell DNA. A virus in this dormant state is called a provirus (prophage). The virus remains inactive (being copied every time the cell divides), until some trigger causes the virus to begin the lytic cycle.

Bacterial DNA Two types of DNA in bacteria. The main form of genetic material in bacteria is a single circular chromosome made of DNA. The chromosome replicates via binary fission. In binary fission, the chromosomes replicates and the cell divides into two cells, with each cell gets an identical copy of the chromosome. Bacteria also contain plasmids, small, circular DNA molecules outside the chromosome. Plasmids replicate independently of the chromosome. Plasmids are not always necessary to the survival of the bacteria but can be beneficial to the survival of the bacteria.

Binary Fission

Genetic Variation in Bacteria Conjugation is a process of DNA exchange between bacteria. Transduction occurs when DNA is introduced into the genome of a bacterium by a virus. Transformation occurs when bacteria absorb DNA from their surroundings and incorporate it into their genome, Mutation occurs when there is a random change in the DNA

Conjugation

Transduction

Transformation

Summary of Sources of Genetic Variation Mutations also results in genetic variation

Regulation of Gene Expression Gene expression in bacteria is controlled by the operon model. An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control. Natural selection has favored bacteria that produce only the products needed by that cell. A bacteria cell can regulate the production of enzymes by feedback inhibition or gene regulation via an operon.

Parts of an Operon promoter - region is a sequence of DNA to which the RNA polymerase attaches to begin transcription operator - region can block the action of the RNA polymerase if the region is occupied by a repressor protein structural genes - contain DNA sequences that code for several related enzymes that direct the production of some particular end product regulatory genes - produces proteins that either (1) bind to the operator and block transcription(repressor proteins) or (2) bind to the repressor which causes it to release the operator and allow transcription to take place (activator proteins).

Repressible Operons – always ON Repressible operons are always turned on - meaning they produce their protein product until they are turned off. The trp operon is an example. By default the trp operon is on and the genes for tryptophan synthesis are transcribed. When tryptophan is present, it binds to the trp repressor protein, which turns the operon off. The repressor is active only in the presence of its corepressor - tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high. Repressible enzymes usually function anabolic pathways; their synthesis is repressed by high levels of the end product

Inducible Operons – always OFF Inducible operons are usually turned off - meaning they don't produce the protein until a molecule called an inducer inactivates the repressor and turns on transcription. The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose. By itself, the lac repressor is active and switches the lac operon off. A molecule called an inducer inactivates the repressor to turn the lac operon on. When there is no lactose present - there is not need for the enzymes that break it down. Inducible enzymes usually function in catabolic pathways; like digestion; their syntesis is induced by a chemical signal (the item that is meant to be digested).

Operon Assignment Due 1/16/13 Explain the concept of an operon and the function of the operator, repressor, and corepressor. Be sure to state the adaptive advantage of grouping bacterial genes into an operon. Discuss how repressible and inducible operons differ and how those differences reflect differences in the pathways they control.