Genetics of Viruses & Bacteria. Chapter 18

Maat: px

Weergave met pagina beginnen:

Download "Genetics of Viruses & Bacteria. Chapter 18"

Adriaan de Backer
4 jaren geleden
Aantal bezoeken:

1 Genetics of Viruses & Bacteria Chapter 18

2 Structure of Virus NOT CELLS! infectious particle consisting of a nucleic acid enclosed in a protein coat Viral Genomes Classified as DNA or RNA virus Dbl. stranded DNA,single stranded DNA, dbl. stranded RNA or single stranded RNA Genome organized as linear or circular

3 Capsids protein shell encloses the genome Rod shaped, polyhedron or complex Composed of protein subunits called capsomeres Some virus also have viral envelopes surrounding the capsid

5 General Features of Viral Reproductive Cycles Obligate intracellular parasites only reproduce within a host Lack metabolic enzymes & ribosomes Limited range of host cells they can infect ; identify their hosts by a lock-and-key approach; must be a fit between proteins on the outside of virus & specific receptors on host Wide range West Nile Narrow range measles, polio, cold, AIDS

6 Lytic Cycle Bacteriophage / phage virus that infects bacteria Results in death of the host Virulent phage one that reproduces through the lytic cycle Why don t phages exterminate bacteria? Natural selection favors bacterial mutants; changes receptor site so it isn t recognized by phage Phage DNA recognized as foreign and cut up by restriction enzymes Phages can coexist with host in the lysogenic cycle

8 Lysogenic Cycle Replicates the phage genome without destroying the host Temperate phage one capable of both lytic & lysogenic cycles ; phage is capable of giving rise to an active phage; triggered by environmental signal (radiation, chemicals) Lambda common temperate phage

9 Phage binds to surface of cell & injects DNA Lambda DNA forms a circle Either: 1. Lytic Cycle viral genes immediately cause host to make more viruses & lyse the host cell 2. Lysogenic Cycle lambda DNA incorporated by genetic recombination into host chromosome Viral DNA now known as a prophage Each time host cell prepares to divided it replicates the phage DNA & passes it to its daughter cells

12 Bacterial Genome Genome is double stranded circular DNA with proteins bacterial chromomes Densely coiled into the nucleoid region of cell Plasmids smaller circular loops of DNA ; only a few genes Reproduce through binary fission which is preceded by DNA replication

14 Bacterial Transformation Changes the bacterial cell s phenotype and genotype by uptaking DNA from the surrounding environment Since cell now has DNA from a new source it is recombinant (DNA from 2 different cells) Bacteria possess cell-surface proteins that recognize & transport DNA from closely related species into the cell

15 Bacterial Transduction Phages carry bacterial genes from one host cell to another Small piece of host DNA is trapped in phage capsid

17 Bacterial Conjugation Direct transfer of DNA between two bacterial cells DNA transfer is one way From donor / male to recipient / female Formation of sex pili depends on presence of F factor specialized plasmid Plasmid small circular self replicating DNA that is separate from bacterial chromosome ; only contains small number of genes that are not required for survival of bacteria Episome genetic element that can replicate either as part of the bacteria or idenpendently

18 Cells containing F plasmid designated as the donor ; F+ Cells lacking F plasmid designated as the recepient ; F- During conjugation an F+ cell converts an F- cell to F+

20 Operons Genes of the bacterial are switched on & off by metabolic pathways Operon model (Francois Jacob) describes the basic mechanism for control of gene expression in bacteria Genes for a related function are grouped into the same transcription unit Operator switch ; segment of DNA within the promotor region or between it and the genes that will code for an enzyme Operator controls the access of RNA polymerase to the genes Operon = Operator + Promotor + Genes Operons normally turned on Repressor proteins that binds to a specific operator, blocking attachment of RNA polymerase & turns operator off Regulatory genes code for repressor proteins

Vergelijkbare documenten

Chapter 4 Understanding Families. In this chapter, you will learn

Chapter 4 Understanding Families In this chapter, you will learn Topic 4-1 What Is a Family? In this topic, you will learn about the factors that make the family such an important unit, as well as Roles