Download AP Bio Ch 15

Microbial Models: The Genetics of Viruses and Bacteria
Viral Genetics
virus
- nucleic acid enclosed by a protein coat
viral genomes
- may be double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded
DNA
capsid
- protein coat enclosing the viral genome
viral envelope
- membrane around some viral capsids (protect and help attachment)
- derived from host membrane
bacteriophages
- bacterial viruses
viruses
- limited number of host cells (host range)
- recognize host cells by a complementary fit between viral protein and receptor sites on host cell
viral reproduction steps (lytic cycle):
1. virus attaches to host cell
2. virus injects nucleic acid into host cell
3. viral nucleic acid takes over host metabolic machinery to make viral particles
4. viral particles assemble into new viruses
5. viral particles released by cell lysis
temperate virus
- sometimes nucleic acid incorporates into host DNA
- viral DNA replicates when host DNA replicates
- does not necessarily kill host cell (called a prophage)
- can revert to a lytic cycle (usually due to an environmental trigger such as radiation, chemicals)
lysogenic cycle
- new viral DNA made but viruses not made
viral envelopes
- helps virus enter host cell
- new viruses can leave host cell without necessarily destroying it (new viruses bud from cell surface)
retrovirus - RNA-containing virus
- contain enzyme called reverse transcriptase which changes its RNA to DNA once in host cell
example:
HIV (human immunodeficiency virus) which causes AIDS (acquired immunodeficiency syndrome)
vaccines
- major weapon for preventing viral infection
- harmless variants or derivatives of pathogenic microbes
- stimulate immune system to mount defenses against the actual pathogen
viroid
- tiny molecules of naked circular RNA that infect plants
- don’t code for proteins; can disrupt metabolism of plant and stunt its growth
prions
- infectious proteins
- cause some degenerative brain diseases such as Creutzfeld-Jakob disease (mad cow disease)
Bacterial Genetics
bacterial chromosome
plasmid
- 1 double-stranded circular molecule
- found in nucleoid region of cytoplasm
- transcription and translation can occur simultaneously
- small double-stranded ring of DNA carrying extrachromosomal genes in a bacterium
- example:
R plasmids
- carry genes that transfer antibiotic resistance
binary fission
- asexual reproduction producing offspring genetically identical to parent
- how most bacteria multiply
- DNA replication precedes it
transformation
- gene transfer by which a bacterium takes up foreign DNA from surroundings
conjugation
- bacteria form little bridges (sex pili) between them and 1 transfers genetic info to the other
F plasmid
- contains genes necessary for production of a sex pilus
transduction
- a bacteriophage carries some DNA from 1 bacterium to another
- penicillin resistance in bacterium can be passed this way
generalized transduction
- during new phage production, some of bacterial DNA is accidentally put into capsid
- after lysis, phage introduces the bacterial DNA into a new bacterium
specialized transduction
- involves a virus in lysogenic cycle
- when virus becomes lytic it may bring some host DNA with it when it pulls from it
transposons
- DNA sequences that can move from 1 location in the genome to another
- occurs in prokaryotes and eukaryotes
operon
- cluster of adjacent genes with related functions
- common in bacteria and phages
- 1 promoter region (RNA polymerase transcribes all genes in operon)
operator
- DNA segment in promoter region which controls access of RNA polymerase to genes in operon
- acts as on/off switch
- always “on”, needs to be repressed to be shut “off”
repressor
- protein that binds to operator and blocks transcription (reversible)
The Organization and Control of Eukaryotic Genomes
eukaryotic DNA
- complexed with protein to form chromatin
- condenses into chromosomes during mitosis
nucleosomes
- look like beads on a string
- formed from DNA wrapped around histones
histone
- small protein that binds to DNA
gene amplification - selective replication of certain genes
- exist as tiny DNA circles separate from chromosomes
- results in an increase expression of the gene
- occurs at certain times in development (temporary)
cell differentiation - cells become specialized during an organism’s development
- cells diverge in form and function
A typical human cell expresses only 3-5% of its genes at any given time.
carcinogen
- physical agents (X-rays, chemicals) that cause cancer by mutating DNA
oncogenes
- cancer-causing genes found in viruses
- viruses play a role in ~ 15% of human cancers (leukemia, liver cancer, cervical cancer)
proto-oncogenes
- normal genes that code for proteins that stimulate normal cell growth and division
- can mutate into oncogenes
DNA Technology
recombinant DNA - hybrid of DNA from 2 or more sources
- DNA with gene of interest in eukaryotes may be transferred into bacterial DNA
genetic engineering - technology that produces new organisms or products by transferring genes between cells
biotechnology
- manipulation of organisms and their components to perform practical tasks or provide useful
products
- ranges from wine making to manipulating DNA
restriction enzyme - cuts DNA at a specific site (restriction site)
- discovered in bacteria (1960s) where they’re used to cut up foreign DNA
- 100s have been identified
- very specific; recognize short nucleotide sequences in DNA
- cut phosphodiester bonds of both strands in a staggered way (creating sticky ends)
sticky ends - single-stranded end of a double-stranded DNA fragment
- will base pair with other DNA molecules with complementary sticky ends
DNA ligase - will seal the strands catalyzing the formation of new phosphodiester bonds
cloning vector
- plasmid used to carry foreign DNA into a cell
forming recombinant DNA:
1. a plasmid is cut by a restriction enzyme
2. DNA of interest is cut with the same restriction enzyme
3. fragments from gene of interest inserted into plasmid (they have the same sticky ends)
recombinant plasmid
- new plasmid with gene(s) of interest (AKA vector DNA)
- introduced into host (by transformation)and replicates with host
gel electrophoresis - separates DNA fragments (cut with restriction enzymes) according to size
- electrical charge sent through gel
- DNA added to a well at negative end of gel
- DNA (negatively charged) move towards positive pole of electric field
- smaller fragments move faster through gel
- restriction enzymes used to create DNA fingerprints
DNA fingerprints
- give precise patterns of DNA fragments
- major role in identifying criminals from DNA evidence
- uses restriction fragment length polymorphisms (RFLPs)
- restriction enzymes produce different RFLPs in different people (DNA is specific to each person)
Southern blot
- technique using gel electrophoresis to identify nucleotide sequences in a DNA sample
- involves using a radioactively labeled DNA probe complementary to the DNA sequence of interest
gene cloning
- used to obtain large amounts of a gene of interest (ex: insulin for diabetes treatment)
- plasmids and DNA of interest are both cut with same restriction enzyme then join together when
mixed
- recombinant plasmids are transformed into bacterial cells
- bacterial cells reproduce containing gene of interest
polymerase chain reaction - used to make large amounts of a particular sequence of DNA in a very short time
- DNA polymerase & nucleotides added to DNA with gene(s) of interest
- DNA is heated which separates strands then cools which allows primers to bind
- DNA polymerase adds nucleotides to double the DNA
- repeated over and over (each cycle doubles the amount of DNA)
- used to detect HIV presence in cells, diagnose genetic disorders & amplify trace amounts
of DNA found at crime scenes
Human Genome Project
- mapping of nucleotide sequence in human genome using PCR amplification
transgenic animals
- commercially produced animals that contain DNA from other species
genetic engineering in plants
nitrogen fixation
- used successfully in agriculture
- ex: herbicide resistance, ripening (spoilage) suppression, etc.
- conversion of atmospheric nitrogen (useless to plants) into N-containing compounds
- performed by nitrogen-fixing bacteria in soil or plant roots
- bacteria produce N-containing compounds which are taken up by plants and used to make amino
acids and nucleotides
- recombinant DNA technology may be used to increase nitrogen fixation of these bacteria
The Genetic Basis of Development
embryonic development involves:
cell division
cell differentiation
- cells become specialized in structure and function
morphogenesis
- establishment of organism’s overall shape
- involves both of the above and selective cell death
animal development
- involves movement of cells and tissues
plant development - growth not limited to embryonic and juvenile periods as with animals
- apical meristems
- root tips possessing perpetual embryonic tissues
(continually grow)
totipotency in plants
- cells retain the ability to give rise to all specialized cells of the plant
nuclear transplantation in animals - used to clone mammals
cloning Dolly:
- mammary cells taken from a sheep and grown in culture (low nutrient culture caused cell to enter G0 phase and
dedifferentiate)
- egg cell taken from another sheep and nucleus removed
- electric current used to fuse a mammary cell with enucleated egg cell
- cell grown in culture (6 days)
- embryo implanted in uterus of third sheep
- Dolly resulted