2. Cell growth

Question 1

What are the limits to cell growth?

Answer 1

• transport - diffusion (more trouble removing waste / absorbing nutrients - diffusion slow and inefficient over large areas)
• communication (communication from nucleus to organelles must be fast - large cell - large distance - slow)
• mRNA synthesis (larger cell - more DNA - would take too long to construct needed proteins to support large cell)

Question 2

What do cells use to make bigger cells?

Answer 2

• vacuole / adipocites - not a part of metabolism but take up space (ex: plant cells)
• syncytia - many nuclei in one cell (fusion of multiple cells) (ex: skeletal muscle)
• polytene chromosomes - parallel copies of DNA stuck together - more than one mRNA possible for synthesis at the same time (ex: fruitfly salivary glands)
• helper cells - part of the cell but not active itself - act as support (ex: granulosa cells in ovarian follicle - produce hormones)

Question 3

Explain what are polytene chromosomes

Answer 3

Polytene chromosomes - parallel **copies of DNA stuck **together - more than one mRNA possible for synthesis at the same time - high level of function

Question 4

Do cells enlarge when organisms grow?

Answer 4

No, cells remain small but their numbers increases in numerous cell divisions

Question 5

Explain the cell cycle

Answer 5

I. Interphase (G1, G2, G3)
II. Mitotic phase (P, M, A, T)

Question 6

How does the cell cycle progress in early embryo cells?

Answer 6

Embryos decouple growth and cell divisions - cells divide but they get smaller to occupy the same space - CLEAVAGE - because of limiting resources at that stage

Question 7

What molecules are used to control the cell cycle?

Answer 7

Cyclins - allow the cell to progress to the next stage

Question 8

Explain how do cyclins work

Answer 8

• cyclins work by controlling cyclin-dependent kinases (cdks) - enzymes which activate (by phosphorylation) progression to next stage of cell cycle - cyclins bind to cdks
• there are distinct restriction points (checkpoints) in the cycle - held at those points until required

Question 9

What are the key requirements for the cell to progress to the next phase in the cell cycle?

Answer 9

• enough resources (ATP, nucleotides, am. a. for synthesis)
• no signal ‘don’t progress’ - inhibitors
• signal ‘do progress’ - growth factors

Question 10

What is anisotropic growth?

Answer 10

Anisotropic growth - differential growth - when growth rates are not equal in all directions

Question 11

Is environment important for control of cell growth? How does it influence growth?

Answer 11

• environmental control can directly drive morphogenesis
• nutrient availability (can determine colony shape), agar stiffness (how easy can spread)

Question 12

What is directional cell division?

Answer 12

Directional cell division determines the tissue structure as cells divide in a specific direction to form a needed tissue structure - important for multicellular organisms (especially for non moving cells)

Question 13

What are the possible types of division based on the division plane?

Answer 13

• periclinal (inreases radius)
• anticlinal (increases circumference)
• transverse (increases length)

Question 14

How is an extra layer of tissue formed in mutant plant due to division plane change?

Answer 14

Periclinal divisions -> two cell types switch to anticlinal, one remains ant periclinal -> extra layer

Question 15

Explain Hertwig’s rule

Answer 15

Hertwig’s rule: cells orientate their division planes in the direction that will reduce the existing mechanical stress in tissues (ex: heavy earings -> earlobes stretch by directional cell division to relieve the mechanical stress; pregnant people -> skin growth to relieve the stretch)

Question 16

Does cell crowding influence cell assembly?

Answer 16

Yes, crowding of cells orientates expansion: uncrowded - random; crowded - orientated domains

Question 17

Example: how does mesentry influence gut folding?

Answer 17

• mesentry - sheet of cells - connects gut to the body
• when mesentry was removed in chicken embryo - gut didn’t fold properly
• mechanism: mesentry doesn’t expand - only gut - folding occurs because from two connected tissues only one expands (rubber band experiment)

Question 18

Explain planar cell polarity vs apico-basal polarity

Answer 18

• planar polarity: x-y plane
• apico-basal polarity: z axis

Question 19

Explain what are astral microtubules

Answer 19

Astral microtubules - in metaphase microtubules assemble metaphase plate - astral microtubules outside the organising centres towards cell membrane

Question 20

What is the function of astral microtubules?

Answer 20

To orientate the poles of cell division (planar cell polarity) - acts in trapping - attach to localised cortical microtubule binding proteins

Question 21

How do cells know which polarity to choose for oriented mitosis?

Answer 21

Cell polarity is communicated between adjacent cells - astral microtubules pulled towards specific protein complexes

Question 22

Give an example of orientated mitosis

Answer 22

• in neural tube closure - oriented mitosis to fuse the edges of the fold
• don’t fuse if insufficient folic acid - spina bifida

Question 23

Why are people generally getting taller over centuries?

Answer 23

Improved healthcare and nutrition - affects overall cell division

Question 24

Explain feotal transfusion syndrome, why it happens?

Answer 24

Monozygotic twins share the placenta - one might have more cords - get disproportionately more nutrients - twins of diff. development / size

Question 25

What has the biggest influence on height / size of animal?

Answer 25

Genetics - can vary within same species

Question 26

What is sexual dimorphism

Answer 26

- within same species **sexes show differences** in ex: size - usually **females>males **because have to produce eggs / bear offspring but also **males>females when men in competition**

Question 27

What does the vitruvian man depict?

Answer 27

Vitruvian man depicts that the human body has **specific proportions** that are more or less followed - growth guidance

Question 28

What controls organism's growth to its full size?

Answer 28

Growth hormone

Question 29

What happens when the growth hormone disfunctions?

Answer 29

An organism grows into a **functional body **but not full size - Laron syndrome

Question 30

What are the two main causes of growth hormone disfunction?

Answer 30

- no growth **hormone** / insufficient - no growth hormone **receptor**

Question 31

What organ produces growth hormone in mammals?

Answer 31

Pituitary gland

Question 32

Explain how does the body control equal limb length

Answer 32

In limbs - growth plates - when bones grow the **distance between cells and growth plate** increases - distance makes signalling less efficient - cells stop dividing <= feedback loop

Question 33

Explain rabit leg experiment used to investigate how equal limbs are grown

Answer 33

- growth of one leg of young rabit inhibited by cutting off circulation - the other leg grows normally but the inhibited leg doesn't - **not enough resources** - circulation released - inhibited leg catches up => growth plates **signal by distance** for cells to keep dividing

Question 34

What is achondroplasia?

Answer 34

Achondroplasia - most common form of **skeletal dysplasia** - impairs the growth of bone in the limbs - does not affect the trunk - skin grows as much as bones

Question 35

Give examples that applying mechanical force to human skin drives skin growth

Answer 35

- pregnancy - obesity - cultural practices

Question 36

How can the angle at which the force is applied afect the deformation / tension division?

Answer 36

Physics: corner / angle at which the force is applied **affects the tension spread** -> ex: square plane windows would crack - round windows more evenly spread tension, so don't crack

Question 37

What pathway is responsible for cell division under mechanical stress?

Answer 37

- **Piezo1** - **mechanosensitive** ion channel protein - responds to stretch activated currents - drives cell division in needed direction of mechanical stress

Question 38

How is it determined if cells should divide or die to accomodate mechanical stress?

Answer 38

Regulation of cell death/division occurs corresponding to a **ratio of cells** in a particular place - if too crowded cell death - if too sparse cells divide

Question 39

Why organs don't experience mechanical stretch when organisms move?

Answer 39

Organs are **mechanically isolated** - no stretch

Question 40

Explain spleen experiment regarding regulation of the amount of organ tissue in an organism

Answer 40

Spleen tissue regulates its amount in an organism by **cell communication** => total mass of spleen = one normal spleen

Question 41

What organ exhibits the opposite regulation of amount of tissue in an organism than spleen?

Answer 41

Thymus - when **two feotal thymus grafted** - **two separate normal size thymus formed** -> cell don't communicate how much thymus tissue exists in an organism

Question 42

Explain what is quorum sensing

Answer 42

Quorum sensing - type of **cell-cell communication** which allows to share info about **cell density and adjust gene expression** to it (autocrine signalling)

Question 43

Explain how quorum sensing operates, give example

Answer 43

Cell-cell communication is achieved by certain **thresholds of needed signalling molecules** (the more cells - the more signalling molecules - the stronger signal - response only when threshold reached -> needed signal induced (ex: Wnt4 expression in kidney cells to differentiate)

Question 44

Experiment for evidence for quorum sensing

Answer 44

- Wnt4-/- knockout - normal levels of Wnt4 - Wnt4 signal faked by Li+ ions -> small aggregates formed needed structures -> small structures as the signalling induced not when enough cells were present

Question 45

Explain trophic theory in cell growth

Answer 45

Trophic theory : cells need signals from other cells to inhibit cell death (**survival factors secreted by other cells** - detected - elective cell death stopped)

Question 46

Trophic theory: explain the experiment of chick limb innervation

Answer 46

- Trophic theory - cells need signals from other cells to inhibit cell death - neurons grow into limbs - when limb buds cut off - little motor neurons (cells need signals (neurotrophin) from other cells to inhibit death) - when two limb buds grafted in - extra motor neurons formed - the **signal (neurotrophin) intensity for neurons increased with extra muscle tissue** => proves trophic theory (when less adjacent diff tissues - less tissue in same location formed)

Question 47

Explain how the number of motor neurons is regulated in response to neurotrophin (quorum sensing)

Answer 47

1) **Mitosis** occurs despite presence / lack of neurotrophin - **neurotrophin independent** - fixed max point of cells reached 2) Depending on levels of neurotrophin **cell death is regulated** - **neurotrophin dependen**t