Encyclopedia of Biology

meiosis The reductive division of diploid cells in
ovaries and testes that produce gametes (sperm and
ova). Two divisions with several stages take place that
result in the production of four daughter cells, each of
which contain half (haploid) of the original number of
chromosomes.
See alsoMITOSIS.

melanism Amedical condition usually seen in the

skin, characterized by abnormal deposits of melanin.

See alsoINDUSTRIAL MELANISM.

membrane potential The difference in electrical

charge (voltage difference) across the cell membrane due

membrane potential 215

Chromosomes appear as

single uncoiled threads

Homologous

chromosomes attract

each other, coming

together to form

bivalents

Chromosomes shorten

by coiling and individual

chromatids become

distinguishable, giving

tetrads

Homologous

chromosomes repel

each other at the

centromeres, remaining

attached only at

chiasmata

Nuclear membrane breaks

down, spindle forms,

and bivalents align

themselves along the

spindle equator

Homologous

chromosomes continue

to repel each other, the

homologues of each pair

moving to opposite ends

of the spindle

The haploid number of

chromosomes gathers at

either end of the spindle

Two spindles form at

right angles to the first

and chromatids separate

A nuclear membrane

forms around each

group of daughter

chromosomes to form

four haploid nuclei and

the cytoplasm divides

forming four gametes

Prophase

Leptotene Zygotene Pachytene

Diplotene— Diakinesis Metaphase I Anaphase I

Metaphase II — Anaphase II Telophase

During meiosis, two divisions with several stages take place that result in the production of four daughter cells, each of which contain
half (haploid) of the original number of chromosomes.