AP Bio- Cell Communication + DivisionChromatin

Chromatin

• The complex of DNA and proteins (mainly histones) that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in a dispersed form as long, thin fibers.

Chromosome

• : A threadlike structure made of DNA and proteins that carries genetic information. Chromosomes are condensed forms of chromatin, visible during cell division.

Sister Chromatid

Two identical copies of a chromosome produced by DNA replication, joined together at a centromere until they separate during cell division.

Kinetochore

• A protein complex assembled on the centromere that serves as the attachment site for spindle microtubules during cell division.

Mitotic Spindle

A structure made of microtubules that organizes and moves chromosomes during mitosis and meiosis

G1

• Cell grows, produces proteins and organelles.
  Appearance: The nucleus is intact, DNA is in the form of uncoiled chromatin.

S phase

• DNA replication occurs, producing sister chromatids.
  Appearance: Chromatin is still diffuse, but DNA content doubles.

G2

• G2 (Gap 2): Cell prepares for division, synthesizing proteins and duplicating centrosomes.
  Appearance: Nucleus intact, chromatin uncondensed, centrosomes visible near nucleus.

Prophase

• Chromatin condenses into visible chromosomes; spindle begins to form; nuclear envelope breaks down.
  Appearance: Thick, X-shaped chromosomes visible; spindle fibers radiating

Metaphase

• Chromosomes align at the metaphase plate; spindle fibers attach to kinetochores.
  Appearance: Chromosomes lined up in the center of the cell.

Anaphase

• Sister chromatids separate and move toward opposite poles.
  Appearance: Chromatids pulled apart, moving to opposite ends.

Telophase

• Nuclear envelopes reform around chromosomes; chromosomes decondense back into chromatin.
  Appearance: Two nuclei forming, chromosomes less distinct.

Metastasis

• occurs when cancer cells break away from the primary tumor, travel through the bloodstream or lymphatic system, and establish new tumors in distant organs or tissues.

• Common Sites of Spread: Bone, liver, lung, and brain

Proto-oncogenes

• are normal genes that encode proteins involved in stimulating cell growth, division, and survival.

• Function: They act like the “gas pedal” of the cell cycle, helping cells progress through division when appropriate.

• Mutation Effect: When they are mutated or abnormally activated, they become oncogenes, which can drive uncontrolled cell proliferation.

Tumor Supressor Genes

• are genes that produce proteins to regulate cell growth, repair DNA damage, and initiate apoptosis (programmed cell death).

• Function: They act like the “brakes” of the cell cycle, preventing damaged or abnormal cells from dividing.

• Mutation Effect: If thode genes are inactivated or mutated, cells lose these safeguards, increasing the risk of cancer.

Direct Contact Signals

1. Cells communicate by physically touching.

2. Example: Immune cells recognizing antigens, or plasmodesmata in plants.

Non Synaptic Signals

1. Cells release local regulators (like growth factors) that diffuse to nearby cells.

2. Acts over short distances.

3. Example: Growth factors stimulating nearby cells to divide.

Synaptic Signals

1. Specialized local signaling in neurons.

2. Neurotransmitters cross the synaptic cleft to stimulate target cells.

3. Example: Acetylcholine released at a neuromuscular junction.

Endocrine Signals

• Long-distance communication using hormones carried in the bloodstream.

• Example: Insulin secreted by the pancreas travels to muscle and liver cells.

Short Distance Signaling

• Synaptic signals are highly localized, crossing tiny gaps between neurons and target cells.

Long Distance Signaling

• signals use the circulatory system to reach distant cells.

• Example: Insulin secreted by the pancreas travels through blood to regulate glucose uptake in muscle and liver cells.
  

Components of a signal transduction pathway: Reception

• A signaling molecule (ligand) binds to a receptor protein on the cell surface or inside the cell.

Components of a signal transduction pathway: Transduction

1. The signal is relayed inside the cell through a cascade of molecular interactions.

2. Often involves second messengers (like cAMP, Ca²⁺) and phosphorylation cascades that amplify the signal.

Components of a signal transduction pathway: Reception

• The cell carries out a specific activity, such as activating a gene, producing a protein, or changing metabolism.

• Example: transcription of a gene for a growth factor receptor.
  

Quorum Sensing

• a type of cell-to-cell communication used by bacteria.

• Mechanism: Bacteria release signaling molecules (called autoinducers) into their environment. As the population grows, the concentration of these molecules increases.

• Threshold: When the concentration reaches a critical level, it triggers coordinated changes in gene expression across the bacterial community.

Tetrads

• Forms during prophase I of meiosis.

• It’s the structure created when two homologous chromosomes pair up (each chromosome has already replicated into two sister chromatids).

• So, a one of these has 4 chromatids aligned together.

Homologus Chromsomes

• A pair of chromosomes (one from each parent) that are the same size, shape, and carry genes for the same traits.

• Example: Humans have 23 pairs of these (46 total). Chromosome 1 from mom pairs with chromosome 1 from dad.

Diploid

• A cell that has two sets of chromosomes (one from each parent).

• Humans: number = 46 (23 pairs).

• Most body (somatic) cells are diploid.

Haploid

• A cell that has one set of chromosomes.

• Humans: have 23.

• Gametes (sperm and egg) are haploid, so when they fuse at fertilization, the diploid number is restored.

Synapsis

• The process during prophase I of meiosis when homologous chromosomes pair up side by side to form tetrads (4 chromatids).

• This allows crossing over (exchange of genetic material between homologous chromatids), which increases genetic diversity.

Nondisjunction

• During meiosis, chromosomes must separate properly.

• Sometimes, homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate.

• Result: Gametes end up with too many or too few chromosomes.

Aneuploid Gametes

abnormal number of chromosomes (not a whole extra set, just one or more missing/extra).

Down Syndrome

Extra chromosome 21 → developmental delays, characteristic facial features.

Triple X

Extra X chromosome in females → often mild symptoms, sometimes taller stature.

Klinefelter syndrome

Extra X in males → reduced fertility, some feminized traits.

Turner Syndrome

Missing one X in females → short stature, infertility, heart issues.