4-10 Flashcards

Question

What strategies do cells use to grow large?

Answer 1

Vacuoles: Reduce active cytoplasm Syncytia: Multiple nuclei in one cytoplasm (e.g., muscle) Polytene chromosomes: Many chromosome copies (e.g., fly salivary glands) Helper cells: Transfer cytoplasm (e.g., oocyte support) Increased gene copies: Maintains DNA:cytoplasm ratio

Answer 2

They remain small but increase in number via cell division

Answer 3

G1: Cell growth S: DNA synthesis G2: Growth + mitosis preparation M: Mitosis (cell division)

Answer 4

Cyclins bind and activate Cdks, forming active enzymes that phosphorylate target proteins. Each phase’s progression depends on the previous one’s completion.

Answer 5

Cyclin D: G1 Cyclin E: S phase entry Cyclin A: S & G2 Cyclin B: Mitosis

Answer 6

Adequate resources Enough space Presence of growth signals No “don’t divide” or DNA damage signals

Answer 7

Rb (Retinoblastoma protein) blocks S-phase gene expression until phosphorylated by Cdks. Low ATP inhibits Cdk activity, preventing replication.

Answer 8

Growth factors activate Cyclin D–Cdk4/6. TGF-β and cell crowding activate p21, which inhibits Cdks. DNA damage induces p21 → Cdk inhibition → halted S-phase entry.

Answer 9

Normal cells: Rely on paracrine (cell-to-cell) signaling. Cancer cells: Often become autocrine, making their own growth factors or activating receptors abnormally.

Answer 10

Cyclin–Cdk complexes drive progression → checkpoints ensure readiness → inhibitors (p21, Rb) halt cycle under stress → deregulation leads to uncontrolled division (cancer).

Answer 11

Cells stay in place; outer cells receive more nutrients and divide faster. Growth on stiff agar is slower because movement is limited.

Answer 12

Periclinal: Increases tissue thickness (adds layers). Anticlinal: Increases circumference (surface area). Transverse: Increases length.

Answer 13

Mutants undergo extra periclinal divisions, producing additional cell layers and extra girth before differentiating.

Answer 14

Crowding: Inhibits division or causes cell death. Stretching: Activates Piezo1 Ca²⁺ channels, promoting division to relieve tension. Cells maintain homeostasis between crowding and stretch.

Answer 15

Gut: High proliferation. Mesentery: Low proliferation but provides structural support. The gut loops because it grows faster than the mesentery allows.

Answer 16

Cells divide along their longest axis to reduce mechanical stress; division orientation adapts to substrate patterns and crowding cues.

Answer 17

Apico-basal polarity: Vertical (z-axis), from basement membrane to apical surface. Planar cell polarity: Within the tissue plane (x–y axis), aligning cells across the sheet.

Answer 18

Frizzled (Fz): Localizes to the “North” side. Dishevelled (Dsh): Works with Fz. Stbm (Strabismus/Van Gogh) and Pk (Prickle): Localize to the “South” side. Together, they coordinate directional orientation across the tissue.

Answer 19

Astral microtubules connect to cortical microtubule-binding proteins that “trap” or pull spindle poles, aligning the division axis.

Answer 20

Divisions are oriented toward the tube’s edges, pushing the neural folds together for closure — failure of this can lead to neural tube defects.

Answer 21

Nutrition and disease levels strongly influence height; better nutrition and fewer diseases have increased average height over time.

Answer 22

Twins share a placenta; one twin receives more nutrient-rich blood, growing larger, while the other is undernourished and smaller.

Answer 23

Species-specific genetic control — body size depends on which growth-regulating genes are active.

Answer 24

Females often need greater nutrient reserves for reproduction; in mammals, hormonal regulation changes this pattern.

Answer 25

Body parts (e.g., arm span and height, or internal organs) grow in predictable proportions — represented by the Vitruvian “normal proportion” model.

Answer 26

GH is secreted by the pituitary gland; it stimulates growth directly in muscles and indirectly in other tissues via IGF-I and IGF-II.

Answer 27

Too much: Gigantism (overgrowth, as in pituitary tumours). Too little: Dwarfism or stunted growth.

Answer 28

GH acts on local tissues to induce IGF-I/II, which stimulate nearby cells to grow and divide — mediating overall body size.

Answer 29

Growth occurs at growth plates containing proliferating chondrocytes at the distal end; these mature, die, and calcify into bone

Answer 30

Top: Proliferating chondrocytes (dividing; stimulated by PTHrP). Middle: Pre-hypertrophic cells (Indian Hedgehog triggers PTHrP). Bottom: Hypertrophic chondrocytes (mature; promoted by CNP and IGF-I).

Answer 31

Indian Hedgehog: Stimulates PTHrP → promotes proliferation. PTHrP: Maintains chondrocyte division and delays maturation

Answer 32

IGF-I: Promotes bone formation and growth (from liver and local tissues). CNP: Encourages chondrocyte maturation (“mature!” signal).

Answer 33

Activating mutation in FGFR3 — overactive FGF signalling inhibits chondrocyte proliferation and differentiation → premature growth plate closure → short limbs.

Answer 34

Growth plates and body parts respond independently to signals. Some body regions continue growing even when others stop — demonstrating localized control of growth and proportion.

Answer 35

Skin grows in response to mechanical tension — stretching stimulates cell division, ensuring enough skin to cover the body (as seen in pregnancy or obesity).

Answer 36

Cells grown with corners under high tension divided more rapidly, showing that mechanical stress promotes growth.

Answer 37

Organs are mechanically isolated — they grow and maintain size independently while the body can move without disturbing internal organ structures.

Answer 38

Multiple foetal spleens implanted in one mouse grew to fractions of normal size, together totaling one normal spleen’s mass → indicates systemic feedback control of total organ size.

Answer 39

Multiple thymus grafts each grow to normal size, showing no feedback inhibition → indicates organ-specific control mechanisms.

Answer 40

Mechanical stress is concentrated at sharp 90° corners and reduced in rounded corners — demonstrates how tissue geometry influences local cell division.

Answer 41

Cells secrete signalling molecules (factor X) that accumulate with population density; when concentration exceeds a threshold, it triggers coordinated responses like growth arrest or differentiation.

Answer 42

Cells both secrete and detect factor X via their own receptors, allowing them to self-regulate once a collective threshold is reached.

Answer 43

Weak Wnt4: mesenchymal cells remain loose. Strong Wnt4: cells aggregate and form epithelium. Wnt⁻/⁻ mutants can’t sense density → fail mesenchymal–epithelial transition. Li⁺ mimics Wnt, even without Wnt present

Answer 44

Positive feedback amplifies Wnt expression — once cells reach a critical density, Wnt reinforces its own production to drive differentiation.

Answer 45

Once the threshold of factor X is reached, feedback strengthens the signal, making the response irreversible — cells stop dividing and differentiate.

Answer 46

Developing neurons depend on target-derived survival factors (neurotrophins); excess neurons die if they fail to receive enough trophic support.

Answer 47

Removed limb: fewer neurons survive. Added extra limb: more neurons survive. → Neuron number is proportional to target tissue size

Answer 48

NGF (Nerve Growth Factor) BDNF (Brain-Derived Neurotrophic Factor) NT-3 (Neurotrophin-3) GDNF, CNTF, HGF

Answer 49

Cells have a constitutive “death wish”. Survival factors (contact- or secretion-based) inhibit apoptosis. When deprived, cells die → maintains balance within and between tissues.

Answer 50

The process by which unspecialised cells become specialised, both in development and throughout life.

Answer 51

Organs consist of multiple specialised cell types that perform different roles, creating a division of labour that allows the organ to function efficiently

Answer 52

Chromaffin cells in the adrenal gland synthesise the hormone epinephrine (adrenaline) using specialised enzymes unique to that cell type.

Answer 53

Housekeeping proteins: Found in most cells; essential for shared functions (e.g., metabolism, DNA repair). Cell-type-specific proteins: Present only in particular cells, enabling unique functions (e.g., epinephrine synthesis in chromaffin cells).

Answer 54

All cells share about 8,000 housekeeping proteins needed for basic functions, but each cell type also makes its own special proteins — about 3,500 in muscle cells and 4,000 in neurons — which give them their unique roles.

Answer 55

Cells begin identical but diverge through differentiation, guided by: Gene expression changes, and Cell signalling, which coordinates development across the embryo.

Answer 56

Cell fate determination: The final cell identity. Cell lineage: The cell’s developmental origin. Lineage restriction: Limits on what a cell can become. Developmental potency: The range of fates a cell can adopt.

Answer 57

Germ layers (ectoderm, mesoderm, endoderm) are key intermediate stages formed during gastrulation. Each layer gives rise to specific tissues and organs (e.g., ectoderm → nervous system, mesoderm → muscle, endoderm → gut).

Answer 58

Mesoderm cells: Express specific proteins and exhibit distinct behaviours (e.g., shape changes causing invagination). Have restricted potential, meaning they can form only certain tissues like muscle, bone, or kidney.

Answer 59

Transcription factors are proteins that regulate gene transcription by binding to specific DNA sequences and switching genes on or off.

Answer 60

RNA polymerase II is guided to the promoter region by general transcription factors like TFIID, which binds to the TATA box — this sets the start point, not the timing, of transcription

Answer 61

1- The DNA binding sites in its enhancer regions. 2- Whether the matching transcription factors (TFs) are present in that cell.

Answer 62

Enhancers are regulatory DNA sequences that bind specific transcription factors to activate or repress RNA polymerase, controlling when and where a gene is express

Answer 63

Genes require combinations of transcription factors — for example, some TFs activate housekeeping genes, while others (like MyoD) activate cell-type-specific genes such as muscle proteins.

Answer 64

Histone acetyl transferase (HAT): loosens DNA–histone interaction for easier access. Chromatin-remodelling complexes: reposition nucleosomes to allow RNA polymerase binding.

Answer 65

All cells have the same genes, but different transcription factors determine which are active. Example: Red blood cells: TF1 + TF2 → β-globin gene ON. Muscle cells: TF1 + TF3 → myosin II gene ON.

Answer 66

Mesodermal cells divide and lose growth factors. They exit the cell cycle, fuse together, and express muscle-specific proteins such as myosin II.

Answer 67

MyoD is a muscle-specific transcription factor that binds to DNA at an E-box sequence (5’-CATATG-3’) as a dimer, turning on genes needed for muscle formation.

Answer 68

MyoD activates hundreds of target genes (e.g. myosin II, troponin, tropomyosin, creatine kinase), ensuring the coordinated expression of all proteins required for functional muscle cells.

Answer 69

Cells can still form muscle-like structures, but they will be non-functional, showing that MyoD’s targets are essential for muscle function, not initial differentiation.

Answer 70

By removing growth factors from the culture media, myoblasts stop dividing, fuse, and express muscle-specific proteins (myosin II), mimicking in vivo muscle development.

4-10 Flashcards

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