Unit 4: Cell Communication and Cell Cycle

Unicellular organisms detect and respond to environmental signals.
Taxis is the movement of an organism in response to a stimulus and can be positive (toward the stimulus) or negative (away from the stimulus).
Taxes are innate behavioral responses, or instincts. Chemotaxis is movement in response to chemicals.

The cells of multi-celled organisms must communicate with one another to coordinate the activities of the organism as a whole.
Cells communicate through cell-to-cell contact or through cell signaling. Signaling can be short-range (affecting only nearby cells) or long-range (affecting cells throughout the organism).
It can be done by cell junctions or signalling molecules called ligands that bind to receptors and trigger a response by changing the shape of the receptor protein.
Signal transduction is the process by which an external signal is transmitted to the inside of a cell. It usually involves the following three steps:
1. a signaling molecule binding to a specific receptor
2. activation of a signal transduction pathway
3. production of a cellular response
For signaling molecules that cannot enter the cell, a plasma membrane receptor is required.
Plasma membrane receptors form an important class of integral membrane proteins that transmit signals from the extracellular space into the cytoplasm. Each receptor binds a particular molecule in a highly specific way.

Ligand-gated ion channels in the plasma membrane open or close an ion channel upon binding a particular ligand. This channel opens in response to acetylcholine, and a massive influx of sodium depolarises the muscle cell and causes it to contract.
Catalytic (enzyme-linked) receptors have an enzymatic active site on the cytoplasmic side of the membrane. Enzyme activity is initiated by ligand binding at the extracellular surface.
A G-protein-linked receptor does not act as an enzyme, but instead will bind a different version of a G-protein (often GTP or GDP) on the intracellular side when a ligand is bound extracellularly. This causes activation of secondary messengers within the cell. One important second messenger is cyclic AMP (cAMP).

The set of conditions under which living things can successfully survive is called homeostasis.
Your blood glucose levels are regulated by insulin and glucagon, two hormones released from your pancreas.
Many of these responses are controlled by negative or positive feedback pathways.
A negative feedback pathway (also called feedback inhibition) works by turning itself off using the end product of the pathway. The end product inhibits the process from beginning, thus shutting down the pathway.

A positive feedback pathway also involves an end product playing a role, but instead of inhibiting the pathway, it further stimulates it.

Every cell has a life cycle—the period from the beginning of one division to the beginning of the next.
The cell’s life cycle is known as the cell cycle.
The cell cycle is divided into two periods: interphase and mitosis.

Interphase is the time span from one cell division to another.
The Three Stages of Interphase Interphase can be divided into three stages: G1, S, G2.
The most important phase is the S phase. That’s when the cell replicates its genetic material.
During interphase, every single chromosome in the nucleus is duplicated.
These identical strands of DNA are now called sister chromatids.
The chromatids are held together by a structure called the centromere.
You can think of each chromatid as a chromosome, but because they remain attached, they are called chromatids instead.
To be called a chromosome, each needs to have its own centromere.
Once the chromatids separate, they will be full-fledged chromosomes.

G1 and G2- During these stages, the cell performs metabolic reactions and produces organelles, proteins, and enzymes.
G stands for “gap,” but we can also associate it with “growth.”
These three phases are highly regulated by checkpoints and special proteins called cyclins and cyclin-dependent kinases (CDKs).
Cell cycle checkpoints are control mechanisms that make sure cell division is happening properly in eukaryotic cells.
In eukaryotes, checkpoint pathways function mainly at phase boundaries (such as the G1/S transition and the G2/M transition).
When damaged DNA is found, checkpoints are activated and cell cycle progression stops. The cell uses the extra time to repair damage in DNA. If the DNA damage is so extensive that it cannot be repaired, the cell can undergo apoptosis, or programmed cell death.
Cell cycle checkpoints control cell cycle progression by regulating two families of proteins:
- cyclin-dependent kinases (CDKs)
- cyclins.
To induce cell cycle progression, an inactive CDK binds a regulatory cyclin. Once together, the complex is activated, can affect many proteins in the cell, and causes the cell cycle to continue.
To inhibit cell cycle progression, CDKs and cyclins are kept separate. CDKs and cyclins were first studied in yeast, unicellular eukaryotic fungi.

Cancer occurs when normal cells start behaving and growing very abnormally and spread to other parts of the body.
Mutated genes that induce cancer are called oncogenes.
They are genes that can convert normal cells into cancerous cell healthy version is called a proto-oncogene.
Tumour suppressor genes produce proteins that prevent the conversion of normal cells into cancer cells. They can detect damage to the cell and work with CDK/cyclin complexes to stop cell growth until the damage can be repaired.
They can also trigger apoptosis if the damage is too severe to be repaired.

Mitosis, or cellular division, occurs in four stages:
prophase, metaphase, anaphase, and telophase.
During prophase, the nuclear envelope disappears and chromosomes condense.
Next is metaphase, when chromosomes align at the metaphase plate and mitotic spindles attach to kinetochores.
In anaphase, chromosomes are pulled away from the center. Telophase terminates mitosis, and the two new nuclei form.
The process of cytokinesis, which occurs during telophase, ends mitosis, as the cytoplasm and plasma membranes pinch to form two distinct, identical daughter cells.
Interphase Once daughter cells are produced, they reenter the initial phase—interphase —and the whole process starts over. The cell goes back to its original state. Once again, the chromosomes decondense and become invisible, and the genetic material is called chromatin again.

Mitosis achieves two things:
The production of daughter cells that are identical copies of the parent cell maintaining the proper number of chromosomes from generation to generation
The impetus to divide occurs because an organism needs to grow, a tissue needs repair, or asexual reproduction must take place.

Note