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Bio/Biochem 2C: Processes of Cell Division, Differentiation, and Specialization

How one cell becomes two under tight control, how that control breaks in cancer, and how one fertilized egg becomes every tissue you have.

20 topics · 85 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.

2C. Phases of the Cell Cycle: G0, G1, S, G2, M

  1. How it worksWalk me through G1, S, G2, and M and what physically changes in the cell during each.
  2. How it worksHow does one chromosome become two identical sister chromatids over the course of S phase?
  3. Worked exampleFind the DNA content of a human cell in G1, in G2, and right after cytokinesis.
  4. Common mistakeWhy isn't a chromosome carrying two sister chromatids counted as two chromosomes?
Your turneasy1 of 3
A diploid somatic cell belongs to an organism with 22 pairs of chromosomes. How many chromatids does one such cell contain during G2, after S phase is complete?

2C. Mitosis: Prophase Through Telophase

  1. How it worksWalk me through prophase, metaphase, anaphase, and telophase as one continuous motion.
  2. How it worksHow does a cell guarantee each daughter gets exactly one copy of every chromosome?
  3. Worked exampleTrace one chromosome through all four mitotic stages in a cell where 2n equals 4.
  4. Going furtherWhat happens at cytokinesis in an animal cell compared with a plant cell?
  5. Common mistakeWhy isn't metaphase the stage where the chromosomes get pulled apart?
Your turneasy1 of 3
An organism has 22 pairs of chromosomes. Its somatic cell enters mitosis, and anaphase separates every pair of sister chromatids. At that instant, before the cell divides, how many chromosomes does the single cell contain?

2C. Mitotic Structures and Chromosome Movement

  1. How it worksHow do centrioles, asters, and spindle fibers set up two poles on opposite sides of a cell?
  2. How it worksWalk me through a kinetochore gripping a microtubule at the centromere and holding on.
  3. Going furtherWhat happens to a chromosome when its kinetochore microtubules shorten from both ends at once?
  4. Common mistakeWhy isn't the nuclear envelope simply shoved aside rather than dismantled during mitosis?
Your turneasy1 of 3
An organism has 22 pairs of chromosomes. At metaphase of mitosis each sister chromatid carries one kinetochore at the centromere region. How many kinetochores are present in one dividing cell?

2C. Control of the Cell Cycle, Checkpoints, and Growth Arrest

  1. How it worksHow do cyclins rising and falling switch their kinases on and off around the cycle?
  2. How it worksWalk me through the G1, G2, and spindle checkpoints and what each one actually inspects.
  3. Worked exampleTrace what a cell does when the spindle checkpoint finds one chromosome still unattached.
  4. Common mistakeWhy isn't a checkpoint just a timer that waits a fixed number of minutes?
  5. Going furtherWhat happens to a cell parked in G0 that never receives another growth signal?
Your turneasy1 of 3
A cell from an organism with 24 pairs of chromosomes is held at the spindle checkpoint because 4 of its chromosomes have not yet attached to microtubules from both poles. How many of its chromosomes are correctly attached?

2C. Loss of Cell Cycle Control in Cancer

  1. How it worksHow does a broken checkpoint let damaged DNA get copied into every descendant cell?
  2. How it worksShow me contact inhibition working in a normal dish and failing in a transformed one.
  3. Common mistakeWhy isn't cancer caused by cells simply dividing faster than normal cells do?
  4. Going furtherWhat happens to a solid tumor once it outgrows its own blood supply?
  5. Ties togetherHow do the cell cycle checkpoints and the DNA repair machinery report to each other?
Your turnmedium1 of 3
Oxygen diffuses only about 165 micrometres from the nearest capillary before it is used up. A spherical tumour nodule of radius 718 micrometres has grown with no blood vessels inside it. What is the radius of its necrotic core?

2C. Oncogenes and Tumor Suppressors

  1. How it worksHow does a proto-oncogene turn into an oncogene from a single activating mutation?
  2. How it worksWalk me through p53 sensing DNA damage and choosing between arrest, repair, and suicide.
  3. Worked exampleTrace a cell through the two hits that knock out both copies of one tumor suppressor.
  4. Common mistakeWhy isn't a tumor suppressor a gene whose job is to kill cancer cells?
Your turnmedium1 of 3
A child inherits one defective copy of a tumour suppressor gene in every cell. The chance that any one retinal precursor cell suffers a mutation knocking out the remaining good copy is 1 in 7 million, and the retina contains about 14 million such precursor cells. How many independent tumours are expected?

2C. Apoptosis and Programmed Cell Death

  1. How it worksWalk me through apoptosis from caspase activation to a tidy package a macrophage carries off.
  2. How it worksHow do the intrinsic and extrinsic pathways start in different places and meet at the same caspases?
  3. Common mistakeWhy isn't apoptosis the same as a cell bursting open from injury?
  4. Going furtherWhat happens to the surrounding tissue when a cell dies by necrosis instead?
Your turneasy1 of 3
An adult body of about 37 trillion cells loses roughly 76 billion of them to apoptosis each day, replacing them by division. What percentage of the body's cells is turned over daily?

2C. Gametogenesis by Meiosis

  1. How it worksWalk me through meiosis I separating homologs and meiosis II separating sister chromatids.
  2. How it worksHow do crossing over and independent assortment make every gamete genetically unique?
  3. Worked exampleFind how many genetically distinct gametes 23 chromosome pairs give before any crossing over.
  4. Common mistakeWhy isn't meiosis II just mitosis happening in a haploid cell?
Your turnmedium1 of 3
An organism has 17 pairs of chromosomes. Counting independent assortment alone and ignoring crossing over, how many genetically distinct gametes can one individual produce?

2C. Ovum and Sperm: Formation, Morphology, and Contribution

  1. How it worksHow does unequal cytoplasmic division throw away three nuclei as polar bodies?
  2. How it worksWalk me through a spermatogonium becoming four motile sperm over about 64 days.
  3. Going furtherWhat happens to an oocyte between birth and the ovulation that releases it decades later?
  4. Common mistakeWhy isn't the sperm's contribution to the zygote equal to the egg's in every respect?
Your turneasy1 of 3
In a study, 142 primary spermatocytes and 142 primary oocytes each complete meiosis to produce functional gametes. How many more functional gametes come from the spermatocytes than from the oocytes?

2C. The Reproductive Sequence: Fertilization to Birth

  1. How it worksWalk me through fertilization, implantation, development, and birth on one timeline.
  2. How it worksHow does the placenta take hormone production over from the corpus luteum around week ten?
  3. Worked exampleTrace the embryo day by day from the ampulla of the tube to the uterine wall.
  4. Going furtherWhat happens when implantation occurs in the fallopian tube instead of the uterus?
Your turneasy1 of 3
A woman has a 28-day cycle and ovulates on day 14, with fertilisation on the same day. Implantation in the uterine wall begins 8 days after fertilisation. On which day of the cycle does implantation begin?

2C. Fertilization

  1. How it worksWalk me through the acrosome reaction, membrane fusion, and the cortical reaction that follows.
  2. How it worksHow do the fast block and then the slow block keep a second sperm out?
  3. Going furtherWhat happens to a zygote when two sperm do manage to fuse with the same egg?
  4. Common mistakeWhy isn't the sperm's work finished the moment its head touches the egg surface?
Your turneasy1 of 3
An organism has a haploid number of 24 chromosomes. Two sperm fuse with one egg and all three nuclei combine. How many chromosomes does the resulting zygote contain?

2C. Cleavage and Blastula Formation

  1. How it worksHow does cleavage carve one enormous zygote into ordinary-sized cells with no growth in between?
  2. How it worksWalk me through morula to blastula and the hollow cavity that opens up inside.
  3. Worked exampleTrace an embryo from 1 cell to 16 and find what happens to its total volume along the way.
  4. Common mistakeWhy isn't determinate cleavage the same process as indeterminate cleavage under another name?
  5. Going furtherWhat happens when a single blastomere is separated from the others at the four-cell stage?
Your turnmedium1 of 3
A zygote of volume 657 picolitres undergoes 6 rounds of cleavage with no growth between divisions. What is the volume of each blastomere afterwards?

2C. Gastrulation and the Primary Germ Layers

  1. How it worksWalk me through invagination at the blastopore producing endoderm, mesoderm, and ectoderm.
  2. How it worksHow does the archenteron become the gut tube running through the finished animal?
  3. Going furtherWhat happens to the germ layers when gastrulation movements are blocked partway through?
  4. Common mistakeWhy isn't the blastopore always destined to become the mouth?
Your turneasy1 of 3
An embryo entering gastrulation has 687 cells. Of these, 25 percent are allocated to endoderm and 39 percent to mesoderm, with the remainder becoming ectoderm. How many cells become ectoderm?

2C. Major Structures Arising From the Germ Layers

  1. How it worksWalk me through ectoderm making both the skin and the entire nervous system.
  2. How it worksHow does mesoderm give rise to muscle, bone, blood, kidney, and gonad from one sheet?
  3. Worked exampleTrace the germ layer of origin for the lung lining, the heart, the spinal cord, and the pancreas.
  4. Going furtherWhat happens to the neat classification when one organ draws cells from two germ layers?
Your turnmedium1 of 3
A pathologist examines tissue from a 10 week embryo and identifies four structures: the epithelial lining of the lung, the heart, the spinal cord and the pancreas. Which germ layers did these four come from, in that order?

2C. Neurulation and the Neural Crest

  1. How it worksWalk me through notochord signalling, neural plate folding, and the neural tube zipping shut.
  2. How it worksHow do neural crest cells break loose from the folds and migrate all over the embryo?
  3. Worked exampleTrace four neural crest derivatives from the tube's edge to where they end up in the body.
  4. Common mistakeWhy isn't the neural crest just the part of the ectoderm that stayed where it started?
  5. Going furtherWhat happens to the embryo when the neural tube fails to close at one end?
Your turneasy1 of 3
Neural tube defects occur in about 15 pregnancies per 10,000 in a given region. If that region records 602 thousand births in a year, how many affected pregnancies are expected?

2C. Determination, Differentiation, and Gene Regulation in Development

  1. How it worksHow does a cell become determined before it looks any different under a microscope?
  2. How it worksWalk me through transcription factors switching on one gene set and locking another shut for good.
  3. Common mistakeWhy isn't differentiation a matter of a cell throwing away the genes it stops needing?
  4. Going furtherWhat happens to a determined cell transplanted into a completely different part of the embryo?
Your turneasy1 of 3
A human cell carries about 21 thousand protein-coding genes, and a given differentiated cell type expresses roughly 53 percent of them at any time. How many genes is that cell expressing?

2C. Induction, Cell-Cell Communication, and Migration

  1. How it worksHow does a morphogen gradient hand cells at different distances different instructions?
  2. How it worksWalk me through an organizer inducing a neural plate in tissue that was going to make skin.
  3. Going furtherWhat happens to a limb bud when a signalling region is grafted onto its opposite edge?
  4. Common mistakeWhy isn't embryonic cell migration just cells being shoved along by the tissue around them?
Your turnmedium1 of 3
A morphogen released from a localised source falls to half its concentration for every 48 micrometres travelled. What fraction of the source concentration remains at a distance of 166 micrometres?

2C. Stem Cells and Pluripotency

  1. How it worksWalk me through totipotent, pluripotent, multipotent, and unipotent as a narrowing funnel.
  2. How it worksHow does an adult stem cell divide asymmetrically to renew itself and make a specialist at once?
  3. Going furtherWhat happens to an ordinary skin cell when four transcription factors are forced into it?
  4. Ties togetherHow does the germ layer a stem cell is committed to limit which tissues it can still make?
Your turneasy1 of 3
A basal stem cell divides asymmetrically, renewing itself and producing one committed daughter. That daughter then divides 8 more times before all of its descendants differentiate. How many differentiated cells does one asymmetric division ultimately yield?

2C. Cell Death in Development and Regenerative Capacity

  1. How it worksHow does programmed death carve structures that growth on its own could never shape?
  2. How it worksWalk me through a salamander regrowing a whole limb from a blastema at the cut surface.
  3. Going furtherWhat happens to a developing hand when the death signal in the webbing never fires?
  4. Common mistakeWhy isn't human wound healing the same process as salamander limb regeneration?
Your turneasy1 of 3
About 43 thousand cells occupy the webbing between two developing digits. Programmed cell death removes 92 percent of them as the fingers separate. How many of those cells remain?

2C. Senescence and Aging

  1. How it worksHow do telomeres shortening at every replication act as a counter the cell cannot reset?
  2. How it worksWalk me through a senescent cell staying alive, staying put, and refusing to divide again.
  3. Common mistakeWhy isn't cellular senescence the same thing as the cell dying?
  4. Ties togetherHow does telomerase switching back on in a tumor cell erase the limit senescence imposes?
Your turneasy1 of 3
A cultured human fibroblast starts with telomeres 13 kilobases long and loses about 147 base pairs from them at every division. Senescence sets in when they reach 6 kilobases. How many divisions does that allow?

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