AP Biology: Unit Four

what is mitosis?

the process where one parent cell divides to create two genetically identical "daughter" cells

what are homologous chromosomes (in mitosis)?

one from each parent, carrying the same genes replicate to form sister chromatids. they align individually at the metaphase plate, and their sister chromatids separate, ensuring each new daughter cell gets an identical complete set of chromosomes, maintaining the diploid state for growth and repair. 

why is it important that homologous chromosomes are homologous?

genetic inheritance - they ensure that offspring receive a full set of genetic instructions, one from each parent. 

genetic diversity - the variation in alleles between homologous pairs (and crossing over) creates unique individuals, driving evolution. 

what are alleles?

different versions or variants of the same gene, like different "flavors" of instructions for a trait, such as brown versus blue eyes or different blood types. You inherit one allele for each gene from each parent, making a pair (like 'A' and 'a' or 'B' and 'b')

what are diploids?

cells have two sets of chromosomes (one from each parent, like 46 in humans) and make up the body (somatic cells), divides via mitosis

what are haploids?

cells have one set of chromosomes (half the diploid number, like 23 in human sperm/egg) and are for sexual reproduction, divides via meiosis

chromosome structure: what are p & q arms?

the p arm is the short arm, derived from the French word "petite", while the q arm is the long arm, named because 'q' follows 'p' in the alphabet

function of p & q arms?

gene location - geneticists use 'p' and 'q' to pinpoint genes, e.g., 14q21 means band 21 on the long arm (q) of chromosome 14.

mapping - each arm is further divided into regions (p1, p2, q1, q2, etc.) and bands (e.g., 14q21.3) for precise mapping.

clinical significance - deletions or extra copies on a specific arm (e.g., 14q deletion) can cause genetic disorders. 

chromosome structure: what is the chromatid?

(“one side of x”) one of the two identical, duplicated strands of a single chromosome, joined at the centromere, which form before cell division to ensure each new cell gets a complete set of DNA

chromosome structure: what is the centromere?

(buttoning a shirt analogy) is a region on a chromosome that holds sister chromatids together and serves as the attachment point for spindle fibers during cell division, ensuring each new daughter cell receives the correct number of chromosomes

chromosome structure: what is the telomere?

(shoelace analogy) is a protective, repetitive DNA cap at the end of each chromosome, preventing them from fraying, sticking together, or being mistaken for broken DNA, essentially acting as a "molecular clock" that shortens with each cell division, limiting cell lifespan

what are the three forms of DNA in mitosis?

(memorize everything listed)

1. chromaTIN: during interphase, the period before mitosis, DNA is in a loose, semi-condensed form called chromatin.

2. chromosomes: as a cell enters mitosis (specifically prophase), the chromatin supercoils and condenses into highly compact, visible structures called chromosomes. this compact packaging is essential to prevent the long DNA strands from becoming a tangled mess and allows them to be easily separated into daughter cells.

3. sister chromaTIDS: after DNA replication in the S phase, each chromosome consists of two identical copies of DNA, called sister chromatids, joined at a region called the centromere.

what is the order of mitosis?

1. interphase

2. prophase

3. anaphase

4. telophase

5. daughter cells formed

6. repeat ( 2 → 4 → 8, etc.)

mitosis phase explanation: interphase

before mitosis, the cell grows, duplicates its DNA (S phase), and prepares for division

mitosis phase explanation: prophase

chromosomes coil tightly into visible X-shaped structures (sister chromatids joined at the centromere), the nucleolus disappears, and the mitotic spindle starts forming from the centrosomes moving to opposite poles.

mitosis phase explanation: metaphase

spindle fibers pull the chromosomes, lining them up perfectly along the cell's equator, the metaphase plate.

what happens after prophase but before metaphase?

the nucleus's protective wall disappears, letting the cell's spindle fibers float in and grab onto special docking stations (kinetochores) on the chromosomes, getting them ready to be pulled apart for cell division.

mitosis phase explanation: anaphase

sister chromatids separate at the centromere, pulled by the shortening spindle fibers toward opposite ends (poles) of the cell, becoming individual chromosomes.

mitosis phase explanation: telophase

chromosomes arrive at the poles and begin to uncoil, new nuclear envelopes form around each set, creating two nucleus reappearing, cytokinesis occurs, and the spindle fibers break down.

what is cytokinesis?

the final step in cell division where the cytoplasm of a parent cell divides to form two distinct daughter cells

what is G1, S, and G2 in interphase of mitosis?

(memorize everything listed)

G1 phase (growth 1)

• what happens:

  the cell grows physically, makes new proteins and organelles (like mitochondria, ribosomes). 

• purpose:

  to build up resources (energy, building blocks) and prepare for the S phase (DNA synthesis). 

• checkpoint:

  a key G1 checkpoint ensures the cell is big enough, has enough nutrients, and no DNA damage before committing to DNA replication. 

S phase (synthesis)

• what happens: the cell's DNA is replicated, creating two identical sister chromatids. 

G2 phase (growth 2)

• what happens:

  the cell continues growing, produces more proteins and organelles needed for the actual division (mitosis).

• purpose:

  to ensure everything is ready and the duplicated DNA is intact before entering mitosis. 

• checkpoint:

  a G2 checkpoint checks for any errors or damage from the S phase and halts the cycle if problems are found. 

what creates spindle fibers in mitosis and what are their purpose?

centrioles; so chromosomes separate properly