Bacteria and viruses are model genetic organisms, why?
small
simple
genes can be manipulated
Viruses are aggregates of nucleic acids and proteins yet they are not considered living, why?
Viruses are not considered living because they cannot carry out essential life processes, such as metabolism and reproduction, without hijacking a host cell’s machinery.
protein shell of viral genome
Capsid
building block of capsids
Capsomere
Membrane covering capsids derived from host cells (phospholipid bilayer)
Viral Envelope
Like camouflage using your target’s skin
Virus that infects bacteria
Bacteriophage (phage)
Parts of Bacteriophage
Head
Tail Sheath
Tail Fiber
Process of Virus Replication
- Virus DNA injected into cell
- Viral DNA undergoes translation and transcription making Viral DNA and mRNA; mRNA translated into capsid proteins.
- Viral DNA plus capsid proteins reassemble into new virus particles that will eventually exit the cell.
Mechanisms of viral infection and differentiate
- Lytic cycle - ends in host cell lysis
- Lysogenic - no lysis of host cell
Steps of lytic cycle (TLDR at bottom of answer):
- T4 phage uses its tail fibers to stick to specific receptor sites on
the outer surface of an E.coli cell - Sheath of the tail contracts, thrusting a hollow core through the
wall and membrane of the cell. The phage injects its DNA into the
cell - Empty capsid of the phage is left as a ghost outside the cell. The
cell’s DNA is hydrolyzed - The cell’s metabolic machinery, directed by phage DNA, produces
phage proteins, and nucleotides from the cell’s degraded DNA are
used to make copies of the phage genome. The phage parts come
together. THree separate sets of proteins assemble to form phage
heads, tails, and tail fibers - Phage then directs production of lysozyme, an enzyme that
digests the bacterial cell wall. With a damaged wall, osmosis
causes the cell to swell and lyse, releasing 100 to 200 phage
particles
TLDR: Phage injects DNA into cell, phage controls all production methods, makes copies of phage genome, phage directs production of lysozyme causing burst of virus copies
Lysogenic cycle steps:
- Phage attaches to host cell and injects DNA
- Phage DNA circulates
- Phage DNA integrates into bacterial chromosome, becoming a prophage
- Bacterium produces normally, copying the prophage and
transmitting it to daughter cells - Many cell divisions produce a colony of bacteria infected with
prophage - Occasionally, a prophage exits the bacterial chromosome,
initiating a lytic cycle
TLDR: Phage injects dna into host, dna circulates and integrates into host chromosome becoming PROPHAGE, copies itself repeatedly, many cell divisions later produce infected colony, sometimes prophage exits, sometimes cell lyses.
Phages that use both cycles are called _______
Temperate phages
Viruses that contain reverse transcriptase enzyme
Retroviruses
Probable origins of viruses
Probably evolved after first cells; originated from fragments of cellular nucleic acids that could move from one cell to another
Small, circular DNA in bacteria and yeast
Replicate independently
Plasmid
DNA segments that move from one location to another
within a cell’s genome
Transposon
Naked circular RNAs that infect plants
Do not code for proteins
Disrupt plant growth and other metabolic processes
Viroids
Replicating infectious PROTEINS
May cause neurological disorders
Prions
How prions infect
Zombie-like, prion attaches to a normal protein converting it into a new prion
Incurable degenerative neurological disorder
leading to spongiform encephalopathy and caused
by a prion found in humans
Transmitted among members of the Fore Tribe of Papua New Guinea via cannibalism
Kuru Disease
Characteristics of Bacteria
Short generation time
Adaptable
Characteristic of Bacterial Genome
One double-stranded circular DNA molecule
Dense region of the DNA bounded by a membrane
Nucleoid
Bacteria divide by _______; preceeded by replication of the bacterial chromosome from a _______ origin of replication
binary fission; single
