Cell Growth And Differentiation

Question 1

What is cell growth and differentiation?

Answer 1

• Are the basic mechanisms responsible for turning a zygote into a mature multicellular organism
• Cell growth = A bigger organism, more cells
• Differentitation = Cells become complex and end to growth
• Cell growth precedes differentiation but with some overlap

Question 2

What is cell growth and differentiation in disease?

Answer 2

• Diseases related to cell growth and differentiation fall into 3 main groups:
• Developmental conditions: Can be related to cell growth or differentiation (e.g. neural tube defects)
• Neoplasia (and metaplasia): Cancers, tumours
• Other: e.g. Cardiac hypertrophic

Question 3

Describe cell growth

Answer 3

• Has 2 main forms
• Hypertrophy: Bigger cells
• Hyperplasia: More cells (Most common form)
• Cell growth is balanced by cell death

Question 4

Describe hypertrophy

Answer 4

• Hypertrophy is simply cells growing bigger
• More proteins, more membrane etc
• Elevated protein synthesis is a big driver of increased cell size
• The heart is a classic example

Question 5

Describe Hyperplasia

Answer 5

• Is caused by cell division or proliferation
• Cell cycle

Question 6

Describe cell differentiation

Answer 6

• Exit from the cell cycle
• Differentiated cells are “post-mitotic”
• A program of cell type specific gene expression
• Cell morphology and function changes

Question 7

What does cell growth and differentiation have in common?

Answer 7

• Cell growth and differentiation are governed by the integration of multiple signals
• Intra and Extracellular signals, growth and inhibitory factors, cell adhesion
• Signals converge on the promoters of key genes
• Promoters act as “co-incidence detectors”

Question 8

Give an example of Extracellular signals

Answer 8

• Ligands: Receptors - Intracellular cascade
• 3 Broad classes:
• Paracrine: Produced locally to stimulate proliferation of a different cell type that has the appropriate cell surface receptor
• Autocrine: Produced by a cell that also expresses the appropriate cell surface receptor
• Endocrine: Conventional hormones, released systemically for distant effects

Question 9

How does Extracellular signals operate in cell growth and differentiation?

Answer 9

• Proteins that Stimulate proliferation and promote survival are called mitogens
• Induce differentiation and inhibit proliferation
• Can do either e.g. Wnt ligands
• Induce apoptosis, e.g. TNFa

Question 10

How do Extracellular signals operate?

Answer 10

1. The growth factor binds to its growth factor receptor
2. Activates a signal transduction pathway via a kinase cascade
3. This activates transcription factors in the nucleus
4. These transcription factors drive the expression of downstream genes creating mRNA
5. This is then exported back to the cytoplasm where protein synthesis, translation occurs forming proteins

Question 11

Briefly describe interphase

Answer 11

• Once the cell leaves mitosis, it enters interphase
• Refers to the 3 phases of the cell cycle that are not mitosis
• Through interphase, cells continue to grow in size

Question 12

Describe the S phase of the cell cycle

Answer 12

• S phase stands for Synthesis where DNA replication occurs
• The genome is copied with every chromosome

Question 13

What lies between the M phase and the S phase?

Answer 13

• Between the M phase and S phase on the right, it’s the G1 phase
• Between the S phase and the M phase on the left, it’s the G2 phase

Question 14

What are cells that have left the cell cycle called?

Answer 14

Quiescent cells (G0)
- Can remain there indefinitely
- Can rejoin the cell cycle into G1
- Or can begin the process to differentiation

Question 15

How many chromosomes are present in the cell throughout the cell cycle?

Answer 15

• After the cell has formed a new daughter cell, it should have the correct complement of chromosomes
• The human genome is diploid so it should be diploid
• When it goes through DNA replication, it will have double so it will have a 4N genome

Question 16

How do we measure the amount of cells in the cell cycle?

Answer 16

• Cells that have been through mitosis which are G0 and G1 will have a diploid genome
• Cell that are in G2 will have a tetraploid genome
• Those that are in the S phase will have anywhere between the 2 since they’re in the process
• This can be properly measured through flow cytometry

Question 17

Describe what FACS is and the analysis of cell DNA content

Answer 17

• If a DNA stain is applied, FACS can measure the DNA content of every cell in a population
• Data is used to plot a graph
• X axis contains amount of DNA, Y axis contains number of cells
• As the amount of DNA increases, you get an increase in the number of cells for G1
• As the amount of DNA increases, you eventually get to the tetraploid cells in G2

Question 18

What other techniques can be used to look at the cell cycle?

Answer 18

• Through Fluorescence microscopy
• Blue = DNA
• Red = Y-Tubulin
• Green = CHEK2
• Yellow = Centrioles

Question 19

What does the cell cycle checkpoints involve?

Answer 19

• Contains Controls (involving specific protein kinases and phosphatases)
• Ensure the strict alternation of mitosis and DNA replication
• The purpose of these kinases and phosphatases is to ensure synthesis and mitosis is separated from eachother
• So that mitosis can only happen if the cell have the right number of chromosomes
• And that synthesis only happens once the cell has cleared mitosis

Question 20

What is the most important cell cycle checkpoint?

Answer 20

• Called the restriction point
• Checks if DNA not damaged, cell size, nutrient stores
• Located between G1 and S phase

Question 21

Where is the second most important cell cycle checkpoint located?

Answer 21

• Called the G2 M phase checkpoint
• DNA completely replicated, DNA not damaged
• Check for DNA damage
• Checks if synthesis is completely finished
• Located between G2 and the M phase

Question 22

Where is the final checkpoint located?

Answer 22

• Located in mitosis (M phase)
• Checks the physical positioning of chromosomes on the mitotic spindle
• Makes sure when the chromosomes separate, the right number of chromosomes goes to the daughter cells

Question 23

Where does the external signals growth factors act?

Answer 23

• Act in the G1 phase
• Cells responsive to growth factors
• Main site of control for cell growth

Question 24

What are the important kinases involved in regulating the cell cycle?

Answer 24

• Cyclin dependent kinases (CDKs)
• in Humans, there are 10 genes that encode for CDK proteins
• There are more than 20 genes that encode for Cyclin proteins

Question 25

What is the expression of Cyclin controlled by?

Answer 25

Controlled by mitotic signals from growth factors

Question 26

What happens when there’s sufficient amounts of Cyclin?

Answer 26

- The Cyclin will be able to form a complex with CDK forming an active Cyclin CDK complex - This can then bind to a specific substrate

Question 27

How is the Cyclin complex regulated?

Answer 27

- Cycles of synthesis (gene expression) and destruction (by proteasome) - Post translational modification by phosphorylation - May result in activation, inhibition or destruction - Dephosphorylation - Binding of Cyclin dependent kinases inhibitors

Question 28

What is the retinoblastoma protein (RB)?

Answer 28

- RB is a key subrate of G1 and G1/S Cyclin dependent kinases

Question 29

How does RB protein work?

Answer 29

- The RB protein is bound to a transcription factor called E2F - This prevents E2F from binding to the promoter of the genes and drive their transcription - However in the presence of Cyclin D CDK4 & Cyclin E CDK2, RB becomes phosphorylated - Which then dissociates from E2F, E2F is then no longer repressed so it can bind to the promoters of its target gene - Allowing replication to start - E2F also binds to the promoter Cyclin E creating more expression of Cyclin E causing a positive feed forward loop