AP Biology Unit 4: Cell Communication and Cell Cycle

Ligands

Ligands

The survival of a living organism depends on the ability of its cell/cells to communicate by sending, receiving, and responding to chemical signals, which are these.

Autocrine Signaling

The cell secretes a ligand, which then binds to a receptor on the cell that secreted the ligand, triggering a response within that same cell. This process is a cell signaling itself to generate a response.

Juxtacrine Signaling

Depends on direct contact between the cell that is sending the ligand and the cell that is receiving and responding to it via a surface receptor.

Paracrine Signaling

The cell secretes a ligand that travels a short distance, eliciting an effect on cells in the nearby area. These ligands are sometimes referred to as local regulators since they only affect cells in the immediate vicinity of the cell that is sending the signals. Neurotransmitters are local regulators that travel the short distance across a synapse to communicate with nearby cells.

Endocrine Signaling

When ligands travel a long distance between the sending and receiving cells.

Hormones

Ligands that travel a long distance.

Signal Transduction

Determines how a cell responds internally to a signal in its environment. Processes like gene expression, cell growth and division, and the release of hormones depend on this. It begins with a ligand, which interacts with specific target cells, which respond to the presence of the ligand. Ligands may be hydrophilic or hydrophobic. Hydrophilic ones cannot cross the phospholipid bilayer of the cell membrane and enter the cell, so they interact with receptors located on the cell membrane (cell membrane receptors). The binding of the ligand to the cell membrane receptor then triggers a series of chemical reactions inside the cell. Hydrophobic ones enter the cell by sliding between the phospholipids of the cell membrane and bind to intracellular receptors in the cytosol of the cell. Once bound to the intracellular receptor, the ligand can then cross the nuclear membrane and bind to DNA in the nucleus, changing the expression of genes.

Reception

The first step of signal transduction, the ligand binds to a specific receptor on or in the target cell. The receptor may be located on the cell membrane or in the cytosol of the target cell. Receptors contain ligand-specific binding domains. If a cell does not have the receptor for a specific ligand, the cell will not respond to that ligand. Upon the binding of the ligand to the receptor, the receptor undergoes a shape change, which triggers the next step in the process on the inside of the cell.

Transduction

The second step of signal transduction, this is a series of chemical reactions triggered by the binding of the ligand to its receptor that helps the cell choose the appropriate response. Components of it include: Signal Amplification, Kinases, Phosphatases, and Enzymes.

Signal Amplification

Signaling cascades, a series of chemical reactions in which one molecule activates multiple molecules, amplifying the cell’s response to a signal.

Kinases

Can transfer phosphate groups to other molecules, which will then activate those molecules.

Phosphatases

Can remove phosphate groups from other molecules, which will inactivate those molecules.

Secondary Messengers

Produced by enzymes, they are small molecules that propagate a signal after it has been initiated by the binding of the signaling molecule to the receptor.

Response

The final step of signal transduction, it is the result generated by the ligand. Examples include the activation of genes by steroid hormones, the opening of ligand-gated ion channels, and the initiation of cell progresses, such as apoptosis.

Signal Transduction Pathways

The series of chemical reactions that mediate the sensing and processing of stimuli. Since receptors are specific to certain ligands, a mutation in a gene that is coding for a receptor protein could result in a change in shape of the receptor such that it would no longer bind to its specific ligand. Without a functional receptor for the ligand, the cell with the mutated receptor protein would no longer be able to respond to the ligand. They can also be disrupted when molecules in the environment interfere with a ligand’s ability to bind to its receptor. A disruption to any step in the signal transduction process will affect not only that step but also any subsequent steps that are dependent on the products of the previous steps.

Feedback Mechanisms

They help living organisms respond to changes in the environment while maintaining the internal environment of the cell.

Negative Feedback

Returns a system to its original condition and helps maintain homeostasis.

Positive Feedback

Magnifies cell processes.

Cell Cycle

Is important in the growth, repair, and reproduction of cells in living organisms. The phases are: interphase (G1, S, and G2), mitosis (M phase), and cytokinesis.

Interphase

The longest phase of the cell cycle; during it, the cell grows so that it has enough material to divide between two daughter cells. The cell also replicates its DNA during this phase.

G1

During this stage, the cell grows and prepares for DNA replication, and some cellular organelles are replicated.

S (Synthesis)

DNA is replicated during this stage. When it begins, each chromosome consists of one chromatid. After DNA replication is completed, each chromosome has two identical chromatids held together by one centromere. At the end of this, the cell contains twice the amount of DNA it had at the end of G1.

G2

During this stage, the cell continues to grow and prepares the materials needed for mitosis, such as proteins that will make up the spindle fibers.

Mitosis (M Phase)

The goal of it is to make sure there is an accurate transfer of a parent cell’s complete genome to each of the two resulting daughter cells. It consists of four stages: prophase, metaphase, anaphase, and telophase.

Prophase

The nuclear membrane dissolves and the chromosomes condense and become visible. Spindle fibers also begin to form.

Metaphase

The spindle fibers have fully attached to the centromeres of each chromosome. Chromosomes are then aligned along the equator of the cell in a single column. The center of the mitotic spindle is called the metaphase plate.

Anaphase

Each chromosome splits at its centromere as opposing spindle fibers begin to shorten. The identical chromatids are pulled toward opposite ends of the cell. At this point, each chromatid now has its own centromere and is considered a separate chromosome. At the end of this phase, the cell had twice the number of chromosomes that it had at the start of the cycle.

Telophase

Two new nuclear membranes form. Each of the two nuclei now contain the same number of chromosomes and the same genetic info as the parent cell.

Cytokinesis

The division of the cytoplasm, along with all of its cellular contents, between the two daughter cells. During it in animal cells, a cleavage furrow is formed, which partitions the cytosol and its contents between the two new cells. In plant cells, a cell plate is built within the dividing cell, providing new cell wall material for each daughter cell.

Nondividing Cells

Some cells may stop dividing temporarily or permanently; either when they reach their mature, fully differentiated state or when environmental conditions are not favorable for continued growth. These cells have exited the cell cycle and are in GO. Cells may enter GO at any point in the cell cycle and may reenter the cell cycle if stimulated to do so by appropriate molecular signals.

Regulation of the Cell Cycle

It is critical to appropriate growth, repair, and reproduction of cells in living organisms.

Checkpoints

Are used during the cell cycle to achieve regulation. Some of them are controlled by cyclins and cyclin-dependent kinases.

Cyclin-Dependent Kinases

Are present at constant levels throughout the cell cycle. These kinases add phosphate groups to other molecules, activating those molecules; however, they are inactive until they are bound to cyclin proteins.

Cyclin Proteins

The levels of these proteins vary during the cell cycle, reaching their max just before mitosis starts.

Mitosis-Promoting Factor (MPF)

When cyclins are bound to cyclin-dependent kinases, this complex is formed, which triggers the start of mitosis.

Somatic Body Cells

All of the cells in an organism that are not involved with sexual reproduction are referred to as this.

Density-Dependent Inhibition

The division of somatic cells can be regulated by this.

Anchorage Dependence

When somatic cells need to be attached to a surface in order to divide.

Cancer Cells

Are not regulated by density-dependent inhibition or anchorage dependence and can continue to grow and divide under conditions that would cause normally functioning somatic cells to stop dividing.

Proto-Oncogenes

Propels cell division at a specific rate; they are necessary for regulated and controlled cell growth. A mutation in a single cell of these can cause a cell to grow out of control and can cause a tumor to form. Since a mutation in a single allele can cause a cell to grow out of control, these function in a dominant way.

Oncogenes

If proto-oncogenes are mutated, they become this, which promote abnormally high rates of cell division. They can cause tumors to form when cell division occurs too quickly and too often without regard for the neighboring cells.

Tumor Suppressor Genes

Code for proteins that detect mutations in cells that may cause tumors to develop. They prevent cell division from occurring at an abnormally fast rate. If a single mutation in one of these alleles occurs, the cell will still possess one remaining unmutated allele that is functional, which helps the organism identify cells that are dividing at a rate that is too fast. If both alleles are mutated, the growth of a tumor may occur. These function in a recessive way because both alleles of these must be nonfunctional for a cell to grow out of control.

Apoptosis

Programmed cell death when a living organism’s survival depends on some cells dying and not reproducing. It may be triggered when a cell acquires a mutation that could cause cancer.