Premeiotic cells have two copies of each chromosome (2n),
one derived from the paternal parent and one from the maternal
parent. For simplicity, the paternal and maternal homologs of
only one chromosome arediagrammed.
All chromosomes are replicated during the S phase before the
first meiotic division, giving a 4n chromosomal complement.
Cohesin complexes (not shown) link the sister chromatids
composing each replicated chromosome along their full lengths.
As chromosomes condense during the first meiotic prophase,
replicated homologs become paired as the result of at least one
crossover event between a paternal and a maternal chromatid.
This pairing of replicated homologous chromosomes is called
synapsis. At metaphase, shown here, both chromatids of one
chromosome associate with microtubules emanating from one
spindle pole, but each member of a homologous chromosome pair
associates with microtubules emanating from opposite poles.
During anaphase of meiosis I, the homologous chromosomes,
each consisting of two chromatids, are pulled to opposite
Cytokinesis yields the two daughter cells (now 2n), which
enter meiosis II without undergoing DNA replication. At
metaphase of meiosis II, shown here, the chromatids composing
each replicated chromosome associate with spindle microtubules
from opposite spindle poles, as they do in mitosis.
Segregation of chromatids to opposite spindle poles during
the second meiotic anaphase followed by cytokinesis generates
haploid germ cells (1n) containing one copy of each chromosome
(referred to as chromatids earlier).