← MCAT®

Bio/Biochem 2B: The Structure, Growth, Physiology, and Genetics of Prokaryotes and Viruses

Life without a nucleus, how it grows and swaps genes sideways, and the subcellular things that hijack it to reproduce.

20 topics · 80 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

2B. Cell Theory

  1. How it worksWalk me through the three claims of cell theory and the evidence each one rests on.
  2. How it worksHow does a swan-neck flask kill spontaneous generation without ever sealing the broth off from air?
  3. Common mistakeWhy isn't a virus a counterexample that breaks cell theory outright?
  4. Going furtherWhat happens to the theory once you add that every cell comes from a pre-existing cell?
Your turneasy1 of 3
A light microscope shows a circular field of view 1.45 millimetres across. How many rod-shaped bacteria, each 4 micrometres long, would it take laid end to end to span that field?

2B. Prokaryotic Domains: Archaea and Bacteria

  1. How it worksHow do archaea and bacteria differ in cell wall, membrane lipids, and ribosome machinery?
  2. How it worksShow me the three-domain tree and where archaea land relative to us.
  3. Common mistakeWhy isn't archaea simply an old-fashioned name for bacteria that like extremes?
Your turneasy1 of 3
The small-subunit ribosomal RNA genes of an archaeon and a bacterium are aligned over 1539 positions and differ at 527 of them. What percentage of the aligned positions are identical?

2B. Structure of the Prokaryotic Cell

  1. How it worksWalk me through a bacterial cell from nucleoid to ribosome to capsule.
  2. How it worksHow does a prokaryote transcribe and translate the same message at the same moment?
  3. Common mistakeWhy isn't a bacterial ribosome interchangeable with one of ours?
  4. Going furtherWhat happens to oxidative phosphorylation in a cell that has no mitochondria to house it?
Your turneasy1 of 3
A bacterial gene encodes a protein of 718 amino acids, and a bacterial ribosome adds about 18 amino acids per second. How many seconds does it take to finish one copy of the polypeptide once translation has begun?

2B. Bacterial Shapes and the Cell Wall

  1. How it worksHow does peptidoglycan cross-link into a mesh that holds off enormous osmotic pressure?
  2. How it worksShow me why a thick peptidoglycan layer stains purple and a thin one with an outer membrane stains pink.
  3. Worked exampleTrace what happens to a rod-shaped bacterium sitting in penicillin through one round of division.
  4. Common mistakeWhy isn't a Gram-negative cell just a Gram-positive one with less wall and nothing added?
  5. Going furtherWhat happens to shape classification when cocci stay stuck together after they divide?
Your turneasy1 of 3
A Gram-positive cell wall carries a peptidoglycan layer 62 nanometres thick, while a Gram-negative cell wall carries one 6 nanometres thick beneath its outer membrane. By what factor is the Gram-positive peptidoglycan layer thicker?

2B. Flagellar Propulsion and Chemotaxis

  1. How it worksHow does a proton gradient drive the rotary motor at the base of a bacterial flagellum?
  2. How it worksWalk me through a run and a tumble, and how a cell biases them to climb a food gradient.
  3. Going furtherWhat happens to a swimming bacterium's path when every flagellum reverses rotation at once?
  4. Common mistakeWhy isn't chemotaxis a bacterium steering itself at a target the way a swimmer would?
Your turneasy1 of 3
A swimming bacterium travels at 37 micrometres per second during a run, and its runs last 2.09 seconds on average before it tumbles. How far does one average run carry it?

2B. Reproduction by Binary Fission

  1. How it worksWalk me through binary fission from the origin firing to the septum closing between two daughters.
  2. How it worksHow does a circular chromosome get pulled apart with no spindle anywhere in the cell?
  3. Common mistakeWhy isn't binary fission simply mitosis with the nucleus left out?
Your turnmedium1 of 3
A circular bacterial chromosome of 4393 kilobase pairs is copied from one origin by two replication forks moving in opposite directions, each at 789 base pairs per second. How many seconds does it take to copy the whole chromosome?

2B. Exponential Growth and the Growth Curve

  1. How it worksHow do lag, log, stationary, and death phases each come from a different limiting factor?
  2. Worked exampleFind how many cells a culture of 500 reaches after 4 hours at a 20-minute doubling time.
  3. Common mistakeWhy isn't exponential growth a straight line when you plot cell count against time?
  4. Going furtherWhat happens to the curve when you keep feeding the culture and draining the waste away?
Your turnmedium1 of 3
A culture starts with 687 cells in fresh medium and enters exponential growth with a doubling time of 46 minutes. Assuming it stays in log phase throughout, how many cells are present after 4 hours?

2B. Aerobic, Anaerobic, and Facultative Variants

  1. How it worksHow do obligate aerobes, obligate anaerobes, facultative and aerotolerant cells each handle oxygen?
  2. How it worksWalk me through why an obligate anaerobe dies of the oxygen radicals it cannot detoxify.
  3. Worked exampleTrace where each of four bacterial types settles in a broth tube with an oxygen gradient down it.
  4. Common mistakeWhy isn't a facultative anaerobe just an aerobe that puts up with low oxygen?
Your turneasy1 of 3
Catalase converts two molecules of hydrogen peroxide into two molecules of water and one molecule of oxygen gas. A catalase-positive culture is given 66 millimoles of hydrogen peroxide and breaks all of it down. How many millimoles of oxygen gas are released?

2B. Parasitic and Symbiotic Relationships

  1. How it worksHow do mutualism, commensalism, and parasitism differ in who pays and who gains?
  2. Going furtherWhat happens to your gut community and to you when a broad antibiotic wipes it out?
  3. Common mistakeWhy isn't every bacterium living on a host a pathogen waiting for its opening?
Your turneasy1 of 3
An adult is estimated to carry about 44 trillion bacterial cells and about 35 trillion human cells. What is the ratio of bacterial cells to human cells?

2B. Genetic Adaptability and Antibiotic Resistance

  1. How it worksHow does one random mutation plus selection turn a single resistant cell into the whole population?
  2. How it worksShow me the four escapes: pump the drug out, chew it up, change the target, or block the door.
  3. Worked exampleTrace a resistance plasmid sweeping a culture faster than any mutation could manage.
  4. Common mistakeWhy isn't resistance something an individual bacterium develops because it met the drug?
  5. Ties togetherHow does gene transfer between unrelated bacteria spread resistance faster than inheritance ever could?
Your turnmedium1 of 3
Spontaneous mutation confers resistance to a given antibiotic in about one cell in 73 million. A flask holds 7 billion cells and has never met the drug. How many resistant cells does it already contain?

2B. Plasmids and Extragenomic DNA

  1. How it worksHow does a plasmid replicate on its own schedule inside a cell copying its main chromosome?
  2. How it worksWalk me through what an F plasmid, an R plasmid, and a virulence plasmid each carry.
  3. Going furtherWhat happens to a plasmid in the population once the pressure that favored it disappears?
Your turnmedium1 of 3
A cell carries 43 copies of a plasmid 9 kilobase pairs long, alongside a single chromosome of 4932 kilobase pairs. What percentage of the cell's total DNA is plasmid DNA?

2B. Transformation

  1. How it worksWalk me through a competent cell hauling naked DNA through its wall and into its chromosome.
  2. Worked exampleTrace Griffith's experiment from harmless live cells plus killed virulent ones to a dead mouse.
  3. Common mistakeWhy isn't transformation the same as a bacterium eating DNA for food?
  4. Going furtherWhat happens to transformation when the incoming DNA shares no sequence with the host genome?
Your turneasy1 of 3
Competent cells are transformed with 36 nanograms of plasmid DNA and 407 colonies grow on the selective plate. What is the transformation efficiency, in transformants per microgram of DNA?

2B. Conjugation

  1. How it worksWalk me through an F-positive cell extending a pilus and pushing one DNA strand across it.
  2. How it worksHow does an Hfr cell drag chromosomal genes along behind the plasmid it is transferring?
  3. Worked exampleFind which genes have transferred when an interrupted mating is cut off at 8 minutes.
  4. Common mistakeWhy isn't conjugation a form of sexual reproduction between two bacteria?
Your turneasy1 of 3
An Hfr strain transfers its entire chromosome of 4696 kilobase pairs to a recipient over 100 minutes at a steady rate. Mating is interrupted in a blender at minute 41. How many kilobase pairs have entered the recipient?

2B. Transposons

  1. How it worksHow does a transposase cut at the inverted repeats and relocate everything between them?
  2. How it worksShow me the difference between a transposon that moves and one that copies itself first.
  3. Going furtherWhat happens to a gene when a transposon lands in the middle of its coding sequence?
  4. Ties togetherHow do the transposons in our own genome connect to resistance jumping between bacteria?
Your turneasy1 of 3
A transposon 6511 base pairs long inserts into a chromosome. Transposase cuts the target staggered, so the 9 base pair target sequence ends up duplicated, with one copy flanking each side of the insertion. By how many base pairs does the chromosome grow?

2B. Virus Structure

  1. How it worksWalk me through a capsid, an envelope, and a viral genome, and what each part is for.
  2. How it worksHow do a bacteriophage's head, tail sheath, and tail fibers work like a syringe on legs?
  3. Worked exampleTrace the size ladder from a 100 nanometer virus to a 1 micron bacterium to a 20 micron cell.
  4. Common mistakeWhy isn't an enveloped virus tougher out in the environment than a naked one?
  5. Going furtherWhat changes about a virus's strategy when its genome is RNA instead of DNA?
Your turneasy1 of 3
A virion is 218 nanometres across. How many of them, laid end to end, would span a rod-shaped bacterium 4 micrometres long?

2B. The Generalized Viral Life Cycle

  1. How it worksWalk me through attachment, penetration, replication, assembly, and release in one animal virus.
  2. How it worksHow does an enveloped virus slip out by budding while a naked one has to burst the cell?
  3. Common mistakeWhy isn't a virus's host range set by which cells it can force its way into?
  4. Going furtherWhat happens to an infection when a drug blocks attachment rather than replication?
Your turneasy1 of 3
A flask of cultured cells receives 42 million infectious virus particles and contains 13 million cells. What is the multiplicity of infection?

2B. The Lytic and Lysogenic Cycles

  1. How it worksWalk me through the lytic cycle from injection to the burst that frees a hundred new phages.
  2. How it worksHow does a prophage stitch into the host chromosome and get copied at every division?
  3. Worked exampleTrace what happens to a lysogenic colony the moment UV light damages the host DNA.
  4. Going furtherWhat happens to the phage population when nearly every available host is already lysogenized?
  5. Common mistakeWhy isn't a lysogenic phage harmless to the bacterium quietly carrying it?
Your turneasy1 of 3
A culture of 35 thousand bacterial cells is infected so that every cell takes up one phage and enters the lytic cycle, and each infected cell lyses with a burst size of 212 phage particles. How many phage particles are in the lysate afterwards?

2B. Transduction

  1. How it worksHow does a packaging mistake during phage assembly load host DNA into an empty capsid?
  2. How it worksWalk me through generalized and specialized transduction and why they move different genes.
  3. Common mistakeWhy isn't transduction just conjugation with a virus standing in for the pilus?
Your turneasy1 of 3
During a lytic infection, about one phage particle in 35 thousand is packaged with a fragment of host DNA instead of phage DNA. A lysate contains 588 million particles. How many of them are transducing particles?

2B. Retroviruses and Reverse Transcriptase

  1. How it worksWalk me through HIV from binding a CD4 receptor to a provirus parked in a human chromosome.
  2. How it worksHow does reverse transcriptase turn an RNA genome into DNA the host will copy forever?
  3. Common mistakeWhy isn't a retroviral infection clearable once the provirus is in the genome?
  4. Worked exampleTrace where a drug blocking integrase halts the cycle, and what has already happened by then.
  5. Going furtherWhat happens to viral diversity because reverse transcriptase never proofreads?
Your turneasy1 of 3
Reverse transcriptase introduces about 4 errors for every ten thousand nucleotides it copies, and it does not proofread. A retroviral genome is 11 kilobases long. How many mutations are introduced each time the genome is copied?

2B. Prions and Viroids

  1. How it worksHow does one misfolded prion force a normal copy of the same protein into its own shape?
  2. How it worksShow me a viroid, a bare circle of RNA with no coat, causing disease in a plant.
  3. Common mistakeWhy isn't boiling or bleach enough to destroy a prion the way it destroys a virus?
  4. Ties togetherHow does a prion's chain reaction compare with the exponential burst of a replicating virus?
Your turnmedium1 of 3
In a brain region the number of misfolded prion molecules doubles every 5 months as each one converts normal copies of the same protein. Starting from 657 molecules, how many are present after 23 months?

AP®, SAT® and PSAT/NMSQT® are trademarks registered by the College Board (PSAT/NMSQT® jointly with the National Merit Scholarship Corporation). ACT® is a registered trademark of ACT Education Corp. GRE® is a registered trademark of ETS. LSAT® is a registered trademark of the Law School Admission Council. MCAT® is a registered trademark of the Association of American Medical Colleges. ISEE® and SSAT® are registered trademarks of their owners. None of these organizations is affiliated with, or endorses, Jentoo.