evolution and genetics of microbes

Question 1

properties of prokaryotes

Answer 1

• unicellular
• no membrane bound organelles
• small, large surface area (unable to engulf other cells and store resources)
• DNA arranged in single circular chromosome and plasmids, single point replication occurs
• large metabolic diversity

Question 2

evolution of prokaryotes

Answer 2

• oldest type of organism
• dominated from 4,000-1,000MYA
• caused massive environmental changes; oxygenation of atmosphere, suffocation of ocean depths (anoxic bacteria producing hydrogen sulphide) and precipitation of global freezes
• changed very little over evolutionary time, remained very simple

Question 3

lifestyle of prokaryotes

Answer 3

• make use of patchy resources in space or time
• remain in stasis/ dormant when resources are low
• divide rapidly when resources available

Question 4

selection in prokaryotes

Answer 4

• best survivors selected for when resources are scarce
• fastest replicators selected for when resources are abundant
• best dispersers selected for when resources are patchy

Question 5

genome of prokaryotes

Answer 5

• much smaller than eukaryotes, increasing replication rate but reducing metabolic diversity
• can acquire genes from environment if it changes, from living and dead cells
• tend to lose genes if not immediately required, short term strategy

Question 6

phylogenetics of prokaryotes

Answer 6

• very complicated due to lateral gene transfer between organisms and species
• core genes inherited vertically, can form a phylogenetic tree showing distinct, recognisable species

Question 7

extremophiles

Answer 7

• bacteria that can live in extreme environments
• radioactive environments
• dry valleys of Antarctica
• deepsea hydrothermal vents and Earth’s crust (extreme heat and pressure)
• acidic and alkaline environments e.g. volcanic springs

Question 8

anaerobes

Answer 8

• unable to metabolise in the presence of oxygen
• dominated earth before great oxygenation event, now in anaerobic pockets such as animal guts and anoxic mud

Question 9

prokaryote cooperation

Answer 9

• form interdependent communities of multiple species
• metabolically diverse so no competition, can recycle each other’s waste products
• form mats and biofilms, attach to surfaces, replicate and send out signals for others to join
  e.g. stromatolites

Question 10

stromatolites

Answer 10

• large bacterial communities
• prokaryotes build up mound, with photosynthetic cyanobacteria on surface and anaerobes and nitrogen fixers beneath
• seen in fossil records
• only seen in specific places today with low nutrients and few grazers e.g. shark bay

Question 11

extremophile, archaea strain 121

Answer 11

• iron-reducing metabolism
• present in hydrothermal vents, metabolises at 120C

Question 12

extremophiles, sulfolobus

Answer 12

• acidophile and thermophile
• present in boiling sulphurous springs
• oxidises sulphur, producing sulphuric acid, meaning they have little competition

Question 13

anaerobes, methanogens

Answer 13

• archaea that react hydrogen with carbon dioxide to form methane
• hydrogen only freely found in anoxic environments
• only survive when oxygen and sulphate is absent, otherwise outcompeted for hydrogen by sulphate reducing bacteria
• found in bottom of freshwater lakes, stagnant marshes and in guts of herbivores/vegetarians

Question 14

prokaryotic chromosomes

Answer 14

• single circular model of DNA
• single origin of replication
• semi-conservative replication

Question 15

plasmids

Answer 15

• smaller separate circular DNA molecules
• replicates independently of chromosome, does not replicate and split uniformly during cell division

Question 16

phages

Answer 16

-viruses of bacteria
- genetic material is RNA or DNA, chromosome surrounded by protein coat
- genetic material injected into cell and reverse transcribed if RNA

Question 17

phages, lytic cycle

Answer 17

• plasmid DNA injected directly into chromosome or forms plasmid in bacteria
• turns off synthesis of bacterial proteins
• makes many copies of phage
• lysis, bacterial cell wall is broken open, new phages released to infect new bacteria

Question 18

bacterial inheritance, vertical transmission

Answer 18

• transfer of DNA from mother to daughter cell through cell division

Question 19

inheritance, horizontal transmission

Answer 19

• transfer of DNA from donor to recipient cell without cell division
• by conjugation or transformation

Question 20

horizontal transmission, conjugation of F plasmid

Answer 20

• F plasmid in donor cell causes bacteria to synthesise pili
• pili make contact with another bacterium and draws it closer
• undergoes rolling circle replication, where single stranded copy of F plasmid DNA is transferred to recipient cell from pore
• complementary strand synthesised by recipient and donor

Question 21

horizontal transmission, conjugation of Hfr strain

Answer 21

• F plasmid DNA incorporated into donor chromosome by recombination
• rolling circle replication
• recipient synthesises complementary strand and becomes diploid (merozygote)
• recipient undergoes recombination, double crossover between circular chromosome and linear fragment, forming recombinant chromosome
• rest of DNA is lost

Question 22

entry mapping, interrupted mating experiments

Answer 22

• mix Hfr and recipient strains
• separate ‘mating’ cell pairs after specific time period and kill them
• test recipient cells for presence of donor alleles
• donor alleles appear in recipient cell in a specific order after a specific time period, allows mapping of genes
• F plasmid DNA always transferred last

Question 23

formation of F’ plasmid by defective excision

Answer 23

• F’ plasmid can exit Hfr chromosomes by recombination between flanking regions
• can take genes from chromosome with it from flanking regions
• genes from chromosome transferred to new cell by conjugation, creating partial diploid
• useful for study of dominance

Question 24

R plasmid

Answer 24

• carries alleles for drug resistance
• can spread rapidly through populations by conjugation if it is also an F plasmid/ an F plasmid carrier

Question 25

horizontal transmission, transformation

Answer 25

DNA fragments in environment from lysis of another bacterial cell or living bacteria that has shed DNA enter bacterium through cell wall and plasma membrane and incorporate into chromosome through recombination

Question 26

temperate phages

Answer 26

- can switch between lytic and lysogenic cycle - enters lysogenic cycle if there is a low concentration of host cells to infect due to a lack of resources

Question 27

lysogenic cycle

Answer 27

- temperate phage occupies dormant host without killing it - either transfers DNA as a plasmid or integrated into host's DNA as a prophage - confers resistance to to other phages of the same type - can become active when cell becomes active, multiplying and causing lysis

Question 28

phages, generalised transduction

Answer 28

- phages may accidentally incorporate fragments of bacterial chromosome and inject it into new bacterial cell - genes which are close together more likely to be recombined together with phage DNA - more likely to be transduced together, allows linkage mapping

Question 29

phage crosses

Answer 29

- several phages infect the same cell - recombination can occur between the different phage DNA and the bacterial DNA taken up by the phages - allows for linkage mapping

Question 30

specialised transduction

Answer 30

- prophage breaks out of bacterial chromosome by crossing over between flanking regions - can incorporate parts of bacterial chromosome nearby and transfer to new bacterial chromosome with prophage