AP Biology - unit 4

Cell communication and the cell cycle

What are signals in cell communication most often?

Chemicals

What is the purpose of cell communication?

• To respond to environmental stimuli

• Sense available nutrients and adjust metabolism

• Locate a suitable mate

• Inform other cells of how many cells are around

• Coordinate multicellular responses

What are the different types of cell communication?

• Local signalling

• Long-distance signalling

• Local regulators

What do cell junctions do?

Directly connect the cytoplasm of adjacent cells

• Signalling substances (chemicals) in the cytoplasm can pass freely between adjacent cells

What is paracrine signalling?

When a local regulator diffuses through extracellular fluids

What is synaptic signalling?

In the animal nervous system when a neurotransmitter is released in response to an electrical signal

What is endocrine signalling?

Hormonal signalling in animals

How does endocrine signalling work?

• Specialised cells release hormones

• Travel to target cells via the circulatory system

Why do the hormones only affect the targeted cell?

Because the ability of a cell to respond to a signal depends on whether or not it has a receptor specific to that signal.

What is an overview of signal transduction?

A signal on the cell’s surface is converted into a specific cellular response

What processes do cells receiving signals go through?

• Reception

• Transduction

• Response

Reception:

• A signalling molecule binds to a receptor protein, causing it to change shape

• Binding between a signal molecule and a receptor is highly specific

What is often the initial transduction of the signal?

A shape change in a receptor

What are most receptor proteins?

Plasma membrane proteins

What are the 3 types of plasma membrane proteins?

• G protein-coupled receptors

• Receptor tyrosine kinases

• Ion channel receptors

Transduction:

• Cascades of molecular interactions relay signals from receptors to target molecules in the cell

• Usually a multistep process

What affect does the fact that transduction is often a multistep process have?

• It can greatly amplify a signal

• Can coordinate and regulate the cellular response

How does transduction work?

• The receptor activates another protein, which activates another, and so on…

• At each step, a signal is transduced into a different form, usually a shape change in a protein.

What is cyclic Amp (cAMP)?

One of the most widely used second messengers

What is adenylyl cyclase, and what does it do?

• An enzyme in the plasma membrane

• Converts ATP to cAMP in response to an extracellular signal

Response:

Cell signalling leads to regulation of transcription or cytoplasmic activities

Where does the response occur?

Either in the nucleus or the cytoplasm

How do many signalling pathways regulate the synthesis of enzymes or other proteins?

Usually by turning genes on/off in the nucleus

Signal termination:

• Inactivation mechanisms are an essential aspect of cell signalling

• If the concentration of external signalling molecules falls, fewer receptors will be bound

• Unbound receptors revert to an inactive state

How do changes in the environment affect cellular signalliing?

• they can initiate, alter, or limit signal transduction

  • Ligand concentration

  • Inhibitor presence/absence

What is apoptosis?

Programmed cell death

Why do cells undergo apoptosis?

• Infection

• Damage

• At the end of their functional lives

How does apoptosis work?

• Components of the cell are chopped up and packaged into vesicles that are digested by scavenger cells

• Prevents enzymes from leaking out of a dying cell and damaging neighbouring cells.

What are mutations?

Changes in the sequence of DNA that codes for proteins

What can mutations alter?

• Shape

• Active site location

• Etc…

What can changes in any of the proteins/enzymes involved in signal transduction lead to?

• Inability to make proteins

• Inability to regulate cell cycle/cancer

• Apoptosis of the cell

• Alteration of the pathway

What is pathway inhibition?

Involves chemicals that block an aspect of signal transduction

What is external inhibition?

Inhibits reception - blocks reception molecule in the membrane

What is internal inhibition?

Inhibits transduction - blocks one of the intermediary molecules from forming

How can inhibition be useful?

Inhibition may make an effective treatment for certain types of cancer

What is homeostasis?

The tendency to resist change in order to maintain a stable, relatively constant internal environment.

What do negative feedback loops do?

counteract changes of various properties from their target values, known as set points

What do positive feedback loops do?

Amplify their initial stimuli - they move the system away from it’s starting state

What is the purpose of cell division in multicellular and unicellular organisms?

• Multicellular

  • Development from a fertilized cell

  • Growth

  • Repair

• Unicellular

  • Reproduction

Genome:

Complete set of genetic information

Chromosomes:

DNA molecules packaged within cell

Chromatin:

Complex of protein and DNA that condenses during cell division (in eukaryotes)

Somatic cells:

• Non-reproductive cells

• Have two sets of chromosomes

Gametes:

• Reproductive cells

• Sperm and eggs

• have half as many chromosomes as somatic cells

What does the cell cycle consist of?

• The mitotic phase

  • mitosis

  • cytokinesis

• Interphase

  • Cell growth

  • Copying chromosomes

  • Preparing for cell division

In which phase does 90% of the cell cycle take place?

Interphase

What are the subphases of interphase?

G1 - “first gap”

G0 - resting phase

S phase - “synthesis”

G2 phase - “second gap”

What are the phases of Mitosis?

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase & cytokinesis

  • Cytokinesis is well underway by late telophase

Name the phases of mitosis

1. G2 of interphase

2. Prophase

3. Prometaphase

4. Metaphase

5. Anaphase

6. Telophase & Cytokinesis

What do mitotic spindles include?

• Microtubules

• Centrosomes

• Asters

What is the function of microtubules?

Attach to the kinetochores, move chromosomes to the metaphase plate

What are the functions of centrosomes?

• Where assembly begins

• Microtubule organizing centar

• Centrosome replicates migrate to opposite ends of the cell, as spindle microtubules grow out from them

What is the purpose of asters?

• Radial array of short microtubules

• Extends from each centrosome

What happens in anaphase?

• Sister chromatids separate

• Move along the kinetochore microtubules toward opposite ends of the cell

• Microtubules shorten

  • Depolymerizing at their kinetochore ends

• Nonkinetochore microtubules

  • Overlap and push against each other, elongating the cell

What happens in Telophase?

Daughter nuclei form at opposite ends of the cell.

How does cytokinesis differ between animal and plant cells?

In animal cells:

• Process of a cleavage

• Forms a cleavage furrow

In plant cells:

• A cell plate forms

What is binary fission?

Reproduction by cell division

• In prokaryotes:

  • Bacteria and Archaea

What happens in binary fission?

• The chromosome replicates, beginning at the origin of replication

• Two daughter cells move actively apart

What two types of controls regulate the cell cycle?

• Internal

• External

How does cell cycle regulation work?

The clock has specific checkpoints where the cell cycle is halted until a go-ahead signal is received

What are regulatory proteins involved in cell cycle control?

• Cyclins

• Cyclin dependant kinases (cdks)

What are three important checkpoints in the cell cycle?

• G1

• G2

• M

What signal comes from the G1 checkpoint and what does it lead to?

The go-ahead signal, it leads to the completion of S, G2, and M phases and divide

What happens if the go-ahead signal is not recieved?

• The cell will exit the cycle

• Switch to a non-dividing state called the G0 phase

What is anchorage dependance?

for cells to divide, they must be attached to a substratum

What do density-dependent inhibition and anchorage dependence do?

Check the growth of cells at an optimal density

What peculiar thing do cancer cells not exhibit?

• neither type of regulation

• They do not respond normally to the body’s control mechanisms

Why do cancer cells divide uncontrollably?

• They do not need growth factors to grow and divide

• They make their own growth factor

• They may convey a growth factor signal without the presence of a growth factor

• They may have an abnormal cell cycle control system