Bio 30: Meiosis & Gameotogenesis

Meiosis

-cell division in sexually reproducing organisms that produces sex cells (gametes) with half the parental chromosome number

→ testes & ovaries

Haploid ($n$)

-a single complete set of chromosomes ($23$ in human gametes).

Diploid ($2n$)

- two complete sets of chromosomes ($46$ in human somatic cells), one set inherited from each parent.

Mitosis vs. meiosis daughter cell ploidy

Mitosis produces daughter cells with the full diploid ($2n$) chromosome number, whereas meiosis yields cells with the haploid ($n$) chromosome number.

Fertilization

The union of a haploid ($n$) sperm and a haploid ($n$) egg that restores the diploid ($2n$) number to produce a zygote.

Zygote

The single diploid ($2n$) cell resulting from fertilization that subsequently develops into a multicellular organism via mitosis.

Homologous chromosomes

Paired chromosomes that are identical in size, shape, and gene locations, with one inherited from each biological parent.

Autosomes

The non-sex chromosomes (pairs $1$ through $22$ in humans) that contain genes for general body characteristics.

Human sex chromosome combinations

$XX$ for biological females and $XY$ for biological males (representing the $23^{\text{rd}}$ pair).

Timing of chromosome replication prior to meiosis

The $S$ phase of interphase, converting single-stranded chromosomes into double-stranded chromosomes composed of two sister chromatids.

Meiosis I

- reduction division

- homologous chromosome pairs separate into two intermediate cells, reducing ploidy from diploid ($2n$) to haploid ($n$).

Meiosis II

- division phase where sister chromatids separate, resulting in a total of four genetically unique haploid ($n$) cells.

Prophase l

-nuclear membrane dissolves

-centrioles move to poles

-spindle fibers form

-chromosomes come together in homologous pairs

Tetrad

-composed of 4 chromatids

Synpasis or Crossing Over

-chromosomes intertwine

-segements exchange

Metaphase l

-homologous chromosomes attach to spindle fibres

-line up on equatorial plate (AS PAIRS!!!)

Anaphase l

-spindle fibres elongate

-homologous chromosomes move to opposite poles→ segregation

Reduction Division

-one chromosomes from each pair found in new cells

Telophase l

-membranes forms along each nucleus

-each cell NOT identical: each contains ONE homologous chromosome

→ carry same genes, but diff variations of those genes.

-cytoplasm divides after telophase (nuclear membrane may or may not reform)

Prophase ll