AP Biology: Unit Four

what is mitosis? (REPAIR AND REPLACE)

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. DNA finds the corresponding strand to make chromosomes

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? (2N → 2N)

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? (2N → N)

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.

******checkpoint 4: (protein: Mad1)

ensures chromosomes are properly attached to spindle fibers

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 in interphase of mitosis?

G1 phase (growth 1) (CELL GROWTH)

• 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 1: (protein: rascyclin)

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

• ******checkpoint 2: (protein: P53) analyzes every chromosome to make sure it is the right length, detects any error

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

centrioles; so chromosomes separate properly

given early and late interphase, how can you tell by looking which comes first?

early interphase only depicts one centriole

animal cells vs plant cells in mitosis

differences in the telophase with cytokinesis (cell division)

• animal cells pinch inward with a cleavage furrow

• plant cells build a new cell plate from the center out due to their rigid cell wall

• animal cells use centrioles for spindle formation, which plants lack

• circular vs rectangular

what is the discrepancy with interphase as a stage in mitosis?

interphase technically isn’t a phase, but a stage, so if prompted about the first phase, it would be prophase

what is the longest phase of the entire cell cycle? why?

interphase; requiring significant time for growth and duplication of cellular components (G1, S, G2)

why is mitosis important?

(think about homologous chromosomes too)

exact same cell and DNA replication; repair and replace

what are the only cells in the human body that don’t go through mitosis?

sex cells

put the mitosis phases in order of which phases take the longest to shortest amount of time

Interphase, Prophase, Telophase, Metaphase, Anaphase

what is S interphase of mitosis?

S phase (synthesis) (DNA REPLICATION)

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

• ******checkpoint 3: (protein: Nibrin)

• plays into DNA repair after replication

what is G2 in interphase of mitosis?

G2 phase (growth 2) (PREPARATION FOR MITOSIS)

• what happens:

  the cell continues growing, produces more proteins and organelles needed (CENTRIOLES MAKE COPIES AND MICROTUBULES ARE ADDED) for mitosis.

• purpose:

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

why are G1, S, and G2 no longer called resting phase?

they’re not resting, they are preparing for mitosis

what is meiosis? (REPRODUCTION AND GENETIC DIVERSITY)

a specialized cell division process for sexual reproduction, where one diploid cell (with two sets of chromosomes) divides twice to produce four genetically unique haploid cells (sperm or egg cells, with one set of chromosomes)

what is crossing over in meiosis and when does it occur?

the exchange of genetic material between homologous chromosomes, creating new gene combinations, occurs during Prophase I (SHUFFLES ALLELES)

why is crossing over important in meiosis?

genetic diversity: creating unique allele combinations in gametes (sperm/egg) (ex. why siblings look different, take more after a different parent, some genetic diseases only pass to one relative, etc)

what is independent assortment in meiosis and when does it occur?

the random alignment and separation of homologous chromosome pairs during Metaphase I, creating unique combinations of maternal and paternal chromosomes in each new gamete, which generates significant genetic diversity (SHUFFLES ENTIRE CHROMOSOMES)

how are “crossing over” and independent assortment different?

independent assortment is about which whole chromosome goes where (Metaphase I), while crossing over physically exchanges segments (Prophase I) before assortment happens, creating unique chromosome blueprints to then shuffle.  

what happens in meiosis 1 (after interphase)?

meiosis I

1. prophase I:

   chromosomes condense, homologous pairs find each other, and genetic material is exchanged (crossing over), nuclear envelope breaks down, spindles form. 

2. metaphase I:

   homologous pairs line up at the cell's equator (metaphase plate). 

3. anaphase I:

   homologous chromosomes are pulled to opposite poles, separating the pairs. 

4. telophase I & cytokinesis:

   chromosomes arrive at poles, new nuclear envelopes form, and the cell divides into two haploid cells, each with duplicated

what happens in meiosis 2?

meiosis II

1. prophase II:

   chromosomes condense again in 2 new cells; nuclear envelopes break down, spindles form. 

2. metaphase II:

   chromosomes align individually at the metaphase plate in each cell. 

3. anaphase II:

   sister chromatids separate and move to opposite poles as individual chromosomes. 

4. telophase II & cytokinesis:

   chromosomes arrive at poles, nuclear envelopes reform, and cytokinesis results in four genetically unique haploid daughter cells (gametes). 

key differences in mitosis vs meiosis

• Diploid/Haploid:

  • Mitosis: Produces diploid (2n) daughter cells (same number of chromosomes as the parent cell).

  • Meiosis: Produces haploid (n) daughter cells (half the number of chromosomes as the parent cell).

• Genetic Makeup (Identical/Different):

  • Mitosis: Daughter cells are genetically identical to the parent cell and to each other.

  • Meiosis: Daughter cells are genetically different from the parent cell and from each other due to crossing over and random assortment of chromosomes.

• Type of Cells That Undergo Each Process:

  • Mitosis: Occurs in somatic (body) cells.

  • Meiosis: Occurs in germ cells (cells that produce gametes, i.e., sperm and eggs).

• Type of Cells Produced:

  • Mitosis: Produces somatic cells for growth and repair.

  • Meiosis: Produces gametes (sex cells: sperm and eggs).

• Behavior of Homologous Chromosomes During Prophase:

  • Mitosis (Prophase): Homologous chromosomes do not pair up.

  • Meiosis (Prophase I): Homologous chromosomes pair up, and cross over (exchange of genetic material) occurs.

• Behavior of Homologous Chromosomes During Metaphase:

  • Mitosis (Metaphase): Individual replicated chromosomes (each with two sister chromatids) line up along the metaphase plate (equator).

  • Meiosis (Metaphase I): Homologous pairs (tetrads) line up together along the metaphase plate.

• Number of Nuclear Divisions:

  • Mitosis: One nuclear division (prophase, metaphase, anaphase, telophase).

  • Meiosis: Two nuclear divisions (Meiosis I and Meiosis II, each with prophase, metaphase, anaphase, and telophase stages).

• Number of Daughter Cells at the End:

  • Mitosis: Two daughter cells are produced.

  • Meiosis: Four daughter cells are produced. 

ex: 2N=30. how many chromosomes will be in each daughter cell for mitosis and meiosis?

mitosis: 30

meiosis: 15

why is it crucial that gametes reduce their chromosomes by half?

so an offspring recieves the correct number of chromosomes and they don’t double with each generation

why do homologous chromosomes seperate during meiosis?

ensures each gamete gets one complete set of chromosome (no missing/extra)

where can problems in meiosis arise?

if chromosomes don’t seperate properly, gametes can end up with too many/few chromosomes