Chromosomes, Cell Cycle, Mitosis + Meiosis

Chromosomes

composed of chromatin (loose, thread-like structures made of DNA wrapped in histone proteins and other proteins) that have become highly condensed, that carry genetic information of a eukaryotic cell. they become visible during cell division.

Structure of Chromosomes

linear, and composed of 2 arms (shorter = p, longer = q), separated in the middle by a constricted region called a centromere.

Sister Chromatids

replicated version of a chromosome, forming an “X” shape, held together at the centromere.

Genes

coding regions on a chromosome which carry specific instructions of heritable traits and functions in an organism. the position of the gene is called locus (or loci).

Homologous Pairs/Chromosomes

a pair of chromosomes that have the same genes but different alleles (allowing genetic variation), due to each pair coming from each parent.

Genome

a haploid set of chromosomes in a given cell, including non-coding + genes.

Karyotype

complete set of chromosomes visually represented by number, size and shape in a cell.

Genotype

all the gene copies of an organism which contributes to its’ phenotype.

Cell Cycle

a series of events that takes place in a cell where it grows and divides, creating 2 daughter cells identical to each other and their parent cell (equational division for having the same # of chromosomes).

Phases of Cell Cycle

interphase (consists of G₁, S, G₂ where cell grows), mitosis (prophase, prometaphase, metaphase, anaphase & telophase) & cytokinesis (division of cytoplasm at the end of mitosis OR meiosis).

Synthesis Phase

phase between G₁ and G₂, where DNA replication occurs.

Gap 1 Phase

first phase of interphase where organelles duplicate and the cell grows. in G₁ checkpoint, it’s decided if the cell is going to replicate its DNA.

Gap 2 Phase

last phase of interphase where in G₂ checkpoint decides if the cell should undergo cell division.

Prophase

first phase of mitosis where chromosomes condense, mitotic spindle forms, and nuclear envelope breaks down.

Metaphase

second phase of mitosis where chromosomes line up on the metaphase plate, which will hit the M checkpoint. if alignment done correctly, checkpoint will allow splitting of sister chromatids, guided by kinetochore (complex that connects centromere with microtubule from mitotic spindle).

Anaphase

third phase of mitosis where connections at centromere are broken, each “daughter” chromatid pulled to towards each pole by the motor proteins in the kinetochores. this separation = disjunction.

Telophase + Cytokinesis

final phase of mitosis where nuclear envelope reforms and cell furrow (animal) or plate forms before cytokinesis, where it splits.

Meiosis

generation of gametes/haploid cells with half the genetic material from parent. occurs for sexual reproduction. is a process with cell division that happens twice.

Prophase I

longest stage of meiosis I where chromosomes condense and homology-search lining up via rough pairing (forming tetrads = 4 chromosomes), which allows the zipper protein ‘synaptonemal complex’ to form, gluing the homolog pairs together = bivalent = synapsis. crossing over (DNA piece swapping) occurs between non-sister chromatids before separating back to tetrads while glued at the crossover point (chiasmata).

Different Events that Occur in Metaphase, Anaphase + Telophase I

1. chromosomes within in the tetrad are held together at the terminal chiasmata (telomeres of chromosomes due to terminalization - occurs as the tetrads “unzip”) instead of the cohesin (ring-shaped proteins within the chromosome, heavily concentrated near the centromere, holding the sister chromatids together)

2. disjunction in anaphase I = separation of homologous pairs

3. independent assortment = random from other tetrads

4. telophase = short phase → straight to meiosis II

5. type of cell division: reductional (due to chromosomes halved via separation of homologous pairs, different from parent cell)

Different Events that Occur in Metaphase, Anaphase and Telophase II

1. disjunction in anaphase II = separation of non-identical sister chromatids

2. results in 4 non-identical haploid gametes/daughter cells

3. type of cell division: equational (same # of chromosomes as parent cell’s)

Nondisjunction

the failure of one or more homologous-chromosomes/sister-chromatids to separate normally during anaphase, resulting in an abnormal distribution of chromosomes in the daughter cells.

Aneuploidy

genetic condition where cells have an abnormal number of chromosomes (missing or extra) due to nondisjunction.

Holliday Junction

4-stranded DNA structure that forms during DNA repair or homologous recombination between non-sister chromatids, which links and facilitates crossovers between homologous chromosomes or broken DNA repair. afterwards, they’re cut vertically by resolvase, allowing recombination of genetic material (non-recomb if horizontal).