AP Biology - Unit 4: Cell Communication and Cell Cycle

Homeostasis

Homeostasis

Cells need to communicate in order to maintain a stable internal environment

Feedback Loops

process by which a change triggers an alarm to produce a certain result

Positive Feedback

moves a system farther away from the target of homeostasis by amplifying effects of a product/event

Negative Feedback

brings system closer to target of homeostasis when the product of reaction leads to decrease in that reaction

cell signaling

In multicellular organisms, cell-to-cell communication allows the cells of the body to coordinate their activities to maintain homeostasis… involves the plasma membrane (most of time)

Local Signaling - No Distance

Some animal + plant cells have junctions (channels) that directly connect the cytoplasm of adjacent cells (ex. cardiac muscle)

• gap junctions (animal cells)

• plasmodesmata (plant cells)

LS - Short Distance

synaptic signaling consists of electrical signal moving along nerve cell that triggers secretion of neurotransmitter molecules, triggering a response in the target cell

Long Distance Signaling

In endocrine signaling in animals, specialized cells release hormone molecules that travel via the circulatory system.

Three states of Cell Signaling

1) reception

2) transduction

3) response

reception

bind between signal molecule (ligand) and receptor

transduction

conversion of signal to cellular response via signal transduction pathway

response

a cellular activity in response to the signal

membrane receptors

g protein coupled receptors (GPCR) and ligand-gated ion channels

ligand binding

shape change in receptor

g protein-coupled receptors (GPCRs)

• plasma membrane receptors work with help of G protein

• G proteins bind to the energy-rich molecule GTP (guanosine triphosphate)

• GPCR pathways are extremely diverse in function

GDP/GTP - binding cycle

A signal induces protein to exchange its GDP for GTP, activating it. The protein then inactivates itself by hydrolyzing its bound GTP to GDP

Ligand-gated Ion Chanel Receptors

• receptor acts as a “gate” for ions when the receptor changes shape

• When ligand binds to receptor, the gate opens and allows specific ions through, such as Na+ or Ca2+

Intracellular Receptors

• found in cytoplasm or nucleus of target cells

• activated by small or hydrophobic chemical messengers

• ex. steroid and thyroid hormones

Transduction

• multi-step pathways can amplify a signal so that a few molecules can produce a large cellular response…

• at each step of signal transduction, signal is transduced/converted into a different form, via protein shape changes

transduction via protein phosphorylation

• protein kinases transfer phosphates from ATP to protein in phosphorylation

• When many of the relay molecules in signal transduction pathway are kinases, acting on other kinases, they are said to form a phosphorylation cascade

Protein dephosphorylation

• protein phosphates remove the phosphates from proteins in dephosphorylation

• Phosphateses provide a mechanism for turning off the signal transduction pathway

Transduction via second messengers

• the extracellular signal molecule (ligand that binds to the receptor is a pathway’s “first messenger”

• Second messengers are small, non-protein, water-soluble molecules or ions that spread throughout a cell by diffusion, like cyclic AMP and calcium ions (Ca2+)

Response: regulation of transcription activities

• many pathways regulate synthesis of enzymes by turning genes on or off in nucleus

Response: regulation of cytoplasmic activities

• other pathways regulate the activity of enzymes (rather than their synthesis), such as opening of an ion channel or change in cell metabolism

Protein structures

1) primary

2) secondary

3) tertiary

4) quaternary

Primary structure

Amino acid sequence of its polypeptide chain

Secondary structure

Local spatial arrangement of a polypeptide’s backbone (main chain) atoms

Tertiary structure

3-dimensional structure of entire polypeptide chain

Quaternary structure

3-dimensional arrangement of subunits in a multi subunit protein

Cell division

• the ability of organisms to produce more of their own kind best distinguishes living things from non living matter

• continuity of life is based on reproduction of cells

Cell cycle

• cell division enables multicellular eukaryotes to develop from a single cell and eventually repair or replace cells as needed

• A life of cell from its formation to its own division

Organizing DNA for cell division

• DNA is “packed” by winding around Justine’s (proteins) to form nucleosomes

• Fibers of wound DNA are called chromatin, which are tightly packed into chromosomes

Chromosome organization

• two arms are connected at a centromere

• Duplicated chromosomes consist of two sister chromatids

• All the DNA in a cell makes up the cell’s genome

• A protein

Somatic (diploid) cells

Non-reproductive cells— humans have 46 chromosomes (2 sets of 23) in diploid cells

Gamete (haploid) cells

Reproductive cells (sperm and eggs) have half as many chromosomes as somatic cells, human gametes have 23 chromosomes

Phases of Cell Growth

1) G1 phase

2) S phase

3) G2 phase

4) M phase

Interphase (90% of cell cycle) - G1 phase

Normal cell activities and growth

Interphase - S phase

Synthesis/replication (making more) of DNA

Interphase - G2 phase (second gap)

More cell activities and growth, prep for division

M Phase - Mitosis

Division of nucleus (genetic material)

M phase - Cytokinesis

Division of cytoplasm

Phases of Mitosis

1) prophase

2) prometaphase

3) metaphase

4) anaphase

5) telaphase (overlaps w/ cytokinesis)

Prophase

Chromatin condenses into chromosomes, nuclear envelope begins to break down, spindle formation begins from asters

Prometaphase

Nuclear membrane gone, chromosomes form kinetochores at the centromeres

Spindle formation

Spindle is made of microtubules and associated proteins and controls chromosome movement… starts at centrosome (complexes of centrioles) and forms aster (array of short microtubules)

Spindle

Centrosomes, microtubules and asters

Kinetochores (part of spindle formation)

Protein complexes that assemble at centromeres… some microtubules attach to kinetochores to move the chromosomes

Metaphase

Chromosomes align at the cell’s equator

Anaphase

Duplicated chromosomes are separated

Telophase and cytokinesis

Effects of prophase and prometaphase are reversed, cytoplasm divides into two daughter cells

Chromosome separation

• microtubules shorten by depolymerizing at kineto chore ends

• Chromosomes also “reeled in” by motor proteins at spindle poles, using ATP to move along filaments

• Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell

• Ring of actin filaments interact with myosin motor proteins to pinch the cell apart to form daughter cells

Binary fission

• prokaryotes (bacteria and archaea) reproduce via binary fission

• Single loop chromosome replicates, beginning at origin of replication

• Two daughter chromosomes actively move apart while cell elongates

• Plasma membrane pinches inward, dividing the cell into two

Cell cycle control system

Directs sequential events of cycle, regulated both internally and externally, including specific checkpoints where the cycle pauses until a go-ahead signal is received

G1 checkpoint

Cell size, nutrients, growth factors, DNA damage (resting state: G0)

G2 checkpoint

Cell size and DNA replication

Spindle assembly (mitosis)

Chromosome attachment to spindle

Cell cycle regulation

(Regulatory proteins) cyclins and cyclin-dependent kinases (CDK) are used

Loss of cell cycle controls

• growth factors that trigger mitosis

• Contact inhibition between cadherins (cell adhesion proteins)

• Cells that can divide indefinitely are said to have undergone transformation

Cancerous growths

Forms tumors:

• benign= abnormal cells remain only at original site

• Malignant= abnormal cells invade surrounding tissues, exporting cancer cells to other parts of body= metastasis