Haploid vs. Diploid
A cell with two sets of chromosomes isdiploid, referred to as 2n,wherenis the number
of sets of chromosomes. A cell with one set of chromosomes, such as a gamete, ishaploid,
referred to asn.So when a haploid sperm and a haploid egg combine, a diploid zygote will
be formed; in essence, when a zygote is formed, half of the DNA in the diploid zygote comes
from each parent. The process of cell division that reduces the chromosome number by half
is calledmeiosis.
Meiosis
Prior to meiosis, DNA replication occurs, so each chromosome contains twosister chro-
matidsthat are identical to the original chromosome. Meiosis is divided into two nuclear
divisions: meiosis I and meiosis II. Each of these nuclear divisions shares many aspects
of mitosis and can be divided into the same phases: prophase, metaphase, anaphase, and
telophase; however, between the two divisions, DNA replication does not occur. Through
this process, one diploid cell will divide into four haploid cells.
Meiosis I
During meiosis I, the pairs of homologous chromosomes are separated from each other. Dur-
ing prophase I, the homologous chromosomes line up together. During this time,crossing-
overcan occur (Figure5.11), the exchange of DNA between homologous chromosomes.
Crossing-over increases the new allele combinations in the gametes. Without crossing-over,
the offspring would always inherit all of the many alleles on one of the homologous chromo-
somes. Because of crossing over, the alleles on the homologous chromosomes can be scram-
bled to pass on unique combinations of alleles on the chromosome. Also during prophase I,
the spindle forms and the chromosomes condense as they coil up tightly. The spindle has
the same function as in mitosis.
During metaphase I, the homologous chromosomes line up in pairs in the middle of the
cell; that is, both chromosome of a pair will line up together. The maternal chromosomes or
paternal chromosomes can each attach to either side of the spindle. The assignment of which
side is random, so all the maternal or paternal chromosomes do not end up in one gamete.
The gamete will contain some chromosomes from the mother and some chromosomes from
the father. Note this is different than during metaphase of mitosis; although chromosomes
still line up during mitosis, the sister chromatids are separated, and each cell obtains both
the maternal and paternal chromosome of each pair.
During anaphase I, the homologous chromosomes separate. In telophase I, the spindle dis-
solves, but a new nuclear envelop does not need to form. That’s because after a brief resting
stage, the nucleus will divide again. No DNA replication happens between meiosis I and