MCDB6: Meiois/Mitosis

G1 checkpoint

A quality-control point late in G1 where the cell checks for sufficient size, nutrients, and DNA integrity before committing to DNA replication and cell division.

G2 checkpoint

A quality-control point after DNA replication (S phase) where the cell verifies that DNA was copied correctly and checks for damage before entering mitosis.

M checkpoint (Spindle Assembly Checkpoint)

A quality-control point during mitosis (specifically metaphase) that ensures all chromosomes are properly attached to spindle fibers before anaphase begins; prevents premature sister chromatid separation.

S checkpoint

A checkpoint during S phase that monitors DNA replication fidelity and responds to replication stress or DNA damage.

G0 phase

A resting or quiescent state where cells are not actively preparing to divide; cells exit the cell cycle and perform specialized functions (e.g., neurons, muscle cells).

G1 phase

The first gap phase of interphase; the cell grows, carries out normal functions, synthesizes proteins, and prepares for DNA replication.

G2 phase

The second gap phase of interphase; the cell continues to grow, produces proteins needed for mitosis, and assembles the mitotic spindle components.

S phase

The synthesis phase of interphase; DNA replication occurs, duplicating the entire genome so each chromosome consists of two identical sister chromatids.

M phase

The mitotic phase of the cell cycle; includes both mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Interphase

The combined G1, S, and G2 phases of the cell cycle; the period between cell divisions during which the cell grows and replicates its DNA.

Cell cycle

The ordered series of events a cell undergoes from its formation to its division into two daughter cells; consists of interphase and the M phase.

Mitosis

A type of cell division in which one diploid parent cell produces two genetically identical diploid daughter cells; used for growth, repair, and asexual reproduction.

Meiosis

A type of cell division that produces four genetically unique haploid daughter cells (gametes) from one diploid parent cell; involves two rounds of division (meiosis I and meiosis II).

Meiosis I

The first division of meiosis; homologous chromosomes (tetrads) are separated, reducing the chromosome number from diploid (2n) to haploid (n).

Meiosis II

The second division of meiosis; sister chromatids separate, similar to mitosis; produces four haploid cells from the two haploid cells resulting from meiosis I; no DNA replication precedes this division.

Prophase

The first stage of mitosis; chromatin condenses into visible chromosomes, the mitotic spindle begins to form, and the nuclear envelope breaks down.

Prometaphase

The stage of mitosis following prophase; the nuclear envelope fully disintegrates, spindle fibers attach to kinetochores on chromosomes, and chromosomes begin moving toward the cell's equator.

Metaphase

The stage of mitosis in which chromosomes align along the cell's equatorial plane (metaphase plate); the spindle assembly checkpoint is active here.

Anaphase

The stage of mitosis in which sister chromatids are pulled apart by spindle fibers to opposite poles of the cell.

Telophase

The final stage of mitosis; chromosomes arrive at opposite poles, decondense back into chromatin, and nuclear envelopes reform around each set of chromosomes.

Cytokinesis

The physical division of the cytoplasm following mitosis or meiosis, producing two separate daughter cells; occurs differently in animal cells (cleavage furrow) versus plant cells (cell plate).

Cleavage furrow

A pinching inward of the plasma membrane during cytokinesis in animal cells, formed by a ring of actin and myosin filaments that contracts to split the cell in two.

Cell plate

A structure formed during cytokinesis in plant cells; vesicles from the Golgi fuse at the cell's equator to form a membrane partition that develops into the new cell wall dividing the two daughter cells.

Spindle fibers (spindle apparatus)

Protein filaments (microtubules) that extend from the centrosomes/centrioles to the chromosomes; responsible for moving and separating chromosomes during cell division.

Centrosome

An organelle that organizes the mitotic spindle; in animal cells it contains a pair of centrioles; duplicated during S phase so each pole of the spindle has one.

Centrioles

Cylindrical structures made of microtubules found in animal cell centrosomes; help organize the spindle apparatus during cell division.

Kinetochore

A protein complex on the centromere of a chromosome where spindle fibers attach to pull chromosomes apart during mitosis/meiosis.

Centromere

The region of a chromosome where the two sister chromatids are joined together and where the kinetochore forms.

Sister chromatids

Two identical copies of a chromosome that are joined at the centromere after DNA replication; they separate during anaphase of mitosis and anaphase II of meiosis.

Chromosome

A condensed, coiled structure of DNA and proteins (chromatin) that carries genetic information; humans have 46 chromosomes (23 pairs).

Chromatin

The complex of DNA and histone proteins that makes up chromosomes in a less condensed form during interphase.

Homologous chromosomes (homologs)

A pair of chromosomes (one from each parent) that carry the same genes at the same loci but may have different alleles; they pair up during meiosis I.

Tetrad (bivalent)

The structure formed when two homologous chromosomes pair up (synapse) during prophase I of meiosis, consisting of four chromatids total.

Synapsis

The pairing of homologous chromosomes during prophase I of meiosis, held together by the synaptonemal complex.

Synaptonemal complex

protein scaffold between paired homologous chromosomes in prophase i. holds homologs together and allows crossing over.

Crossing over (crossover)

The exchange of genetic segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis; a major source of genetic variation.

Homologous recombination

The molecular process by which homologous chromosomes exchange DNA segments during crossing over; creates new combinations of alleles on a chromosome.

Independent assortment

The random orientation of homologous chromosome pairs at the metaphase I plate; each pair lines up independently of other pairs, creating variation in which chromosomes end up in each gamete.

Genetic variation

Differences in DNA sequences among individuals in a population; in meiosis, arises primarily through crossing over and independent assortment.

Diploid (2n)

A cell containing two complete sets of chromosomes (one from each parent); the normal state of most human somatic cells (2n = 46).

Haploid (n)

A cell containing only one set of chromosomes; the state of gametes (eggs and sperm) after meiosis (n = 23 in humans).

Gametes

Haploid reproductive cells (sperm and egg) produced by meiosis; they fuse during fertilization to restore the diploid chromosome number.

Somatic cells

All body cells other than gametes; they are diploid and divide by mitosis.

Point mutation

A change in a single nucleotide base pair in the DNA sequence; can be a substitution, insertion, or deletion of one base.

Silent mutation

A point mutation that changes a codon but does NOT change the amino acid produced (due to the redundancy of the genetic code); has no effect on the protein.

Missense mutation

A point mutation that changes a codon so that it codes for a different amino acid, potentially altering the protein's structure and function.

Nonsense mutation

A point mutation that converts a codon for an amino acid into a stop codon, causing premature termination of translation and usually a nonfunctional protein.

Frameshift mutation

A mutation caused by the insertion or deletion of one or more nucleotides (not in multiples of three) that shifts the reading frame of the genetic message, altering all downstream codons and typically producing a nonfunctional protein.

Reading frame

The way nucleotides are divided into sequential three-nucleotide codons during translation; determined by the start codon and can be disrupted by insertions or deletions.

Mutation

Any heritable change in the DNA sequence of an organism; can be beneficial, harmful, or neutral depending on the context and environment.

DNA replication

The process by which a cell copies its DNA before division; occurs during S phase; each original strand serves as a template for a new complementary strand.

Nuclear envelope

The double membrane surrounding the nucleus; breaks down during prophase of mitosis/meiosis and reforms during telophase.

Nucleolus

A structure within the nucleus where ribosomal RNA (rRNA) is synthesized; disappears during prophase when chromatin condenses.