Mieosis

Diploid

A cell with two complete sets of chromosomes (2n), one from each parent.

Haploid

A cell with a single set of chromosomes (n); gametes like sperm or egg.

Homologous chromosomes

A matching pair of chromosomes carrying the same genes but possibly different alleles.

Sister chromatids

Identical copies of a chromosome connected by a centromere.

Tetrad

The four-chromatid structure formed when homologous chromosomes pair during Prophase I.

Synapsis

The pairing of homologous chromosomes during Prophase I to form tetrads.

Crossing over

Exchange of DNA between nonsister chromatids at chiasmata during Prophase I; creates new allele combinations.

Chiasma

The visible point where crossing over occurs between chromatids.

Independent assortment

Random alignment of homologous pairs during Metaphase I that produces unique gametes.

Nondisjunction

Failure of homologous chromosomes (Anaphase I) or sister chromatids (Anaphase II) to separate properly.

Aneuploidy

Abnormal chromosome number (e.g., trisomy 21).

Gamete

A haploid reproductive cell (sperm or egg) formed by meiosis.

Zygote

A diploid cell formed when two haploid gametes fuse.

Before meiosis begins

DNA replication occurs during interphase, producing duplicated chromosomes.

Prophase I (describe)

Chromosomes condense; homologous chromosomes pair to form tetrads; crossing over occurs.

Metaphase I (describe)

Tetrads align randomly on the metaphase plate; independent assortment occurs.

Anaphase I (describe)

Homologous chromosomes separate to opposite poles; sister chromatids remain attached.

Telophase I (describe)

Two haploid cells form; each chromosome still consists of sister chromatids.

Prophase II (describe)

Spindle fibers reform in each haploid cell; chromosomes re-condense.

Metaphase II (describe)

Individual chromosomes line up at the equator in each haploid cell.

Anaphase II (describe)

Sister chromatids separate and move to opposite poles.

Telophase II (describe)

Nuclei reform; cytokinesis yields four genetically unique haploid gametes.

Crossing over occurs in

Prophase I.

Independent assortment occurs in

Metaphase I.

Homologous chromosomes separate in

Anaphase I.

Sister chromatids separate in

Anaphase II.

End product of Meiosis I

Two haploid cells with duplicated chromosomes.

End product of Meiosis II

Four genetically distinct haploid gametes.

Why Meiosis I is reduction division

It reduces the chromosome number from diploid (2n) to haploid (n).

Why Meiosis II is equational division

It separates sister chromatids without changing chromosome number.

Number of DNA replications in meiosis

One, before Meiosis I.

Main sources of variation in meiosis

Crossing over (Prophase I) and independent assortment (Metaphase I).

Effect of nondisjunction in Anaphase I

One daughter cell receives both homologs and another receives none → aneuploid gametes.

Effect of nondisjunction in Anaphase II

Some gametes have an extra chromatid and others are missing one.

How many daughter cells result from meiosis?

Four.

Are meiosis daughter cells identical?

No; each is genetically unique.

Starting cell ploidy

Diploid (2n).

Resulting gamete ploidy

Haploid (n).

Which phases in meiosis resemble mitosis most?

Meiosis II stages.