In humans, somatic cells (body cells) have:
23 pairs of homologous chromosomes
one member of each pair from each parent.
The human sex chromosomes (Gonosomes)
X and Y differ in size and genetic
The other 22 pairs of chromosomes are
autosomes with the same size and genetic
Homologous chromosomes are
centromere position, and
gene locations (locus).
A locus (plural, loci) is the position of a
Different versions (alleles) of a gene may
be found at the same locus on maternal
and paternal chromosomes.
Humans and most animals and plants have
diploid body cells.
That means they have two sets of
(homologous chromosome pair) one from
Diploid is written 2n.
It refers to the total number of
chromosomes a cell can have.
Meiosis is a process that converts
diploid nuclei to haploid nuclei.
Diploid cells have 2 sets of
Haploid cells have 1 set of
Meiosis occurs in the sex organs,
producing gametes—sperm and
Fertilization is the fusion of a sperm
and egg cell.
The zygote has a diploid chromosome
Haploid gametes (n 23)
A life cycle
adults (2n 46)
Haploid stage (n)
Diploid stage (2n)
All sexual life cycles include an
a diploid stage and
a haploid stage.
Why is meiosis so important? It
produces haploid gametes which
prevents the chromosome number from
doubling in every generation. Produce
gametes for fertilization.
Meiosis is a type of cell division that
produces haploid gametes from diploid
Two haploid gametes combine in
fertilization to restore the diploid state in
MEIOSIS I consisting of 5 phases:
Interphase I, Prophase I, Metaphase I,
Anaphase I, Telophase I.
MEIOSIS II consisting of 4 phases
Prophase II, Metaphase II, Anaphase
II, Telophase II.
Cell build up energy
DNA Replication (to
Cell doesn’t change
Events occurring in the nucleus:
Chromosomes coil and become individual
chromo-somes, nucleolus and nuclear
Homologous chromosomes come together as
pairs by synapsis forming a tetrad (Each pair,
with four chromatids)
Non-sister chromatids exchange genetic materi
through the process of crossing over to ensure
Centrioli move to opposite poles with spindle
fibers between them.
Genetic recombination is the production
of new combinations of genes due to
Crossing over is an exchange of
genesbetween separate (non-sister)
chromatids on homologous chromosomes.
Non-sister chromatids join at a chiasma
(plural, chiasmata), the site of
Genetic material are exchanged between
maternal and paternal (nonsister)
reached the poles.
A nuclear envelope
and nucleolus re-forms
Each nucleus now has
the haploid number of
forming a cleavage
furrow, which extends to
for 2 separate haploid
Follows meiosis I without chromosome
Each of the two haploid products enters
Chromosomes coil and
become compact (if
uncoiled after telophase
Nuclear envelope and
nucleolus, if re-formed,
Centrioli move to
opposite poles, forming
spindle fibers between
chromosomes align on
One chromosome per
spindle fiber attached by
means of kinetochore of
Centrioli has reached the
Spindle fibers contract.
chromosomes split in
dividing in 2)
chromosomes move to
Daughter chromosomes has
reached the poles.
Two cells invaginate and
form 4 daughter haploid
They uncoil and form
Nuclear envelope and
nucleolus for around
Centrioli for centrosome.
Mitosis and meiosis both
begin with diploid parent cells that
have chromosomes duplicated during
the previous interphase.
However the end products differ.
Mitosis produces two genetically
identical diploid somatic daughter
Meiosis produces four genetically
unique haploid gametes.
An extra copy of chromosome 21 causes
Down syndrome or also known as
A. Trisomy 21
involves the inheritance of three
copies of chromosome 21 and
is the most common human
Trisomy 21 produces a characteristic set of
symptoms, which include:
characteristic facial features,
susceptibility to respiratory infections,
leukemia, and Alzheimer’s disease, and
shortened life span.
The incidence increases with the age of the
Nondisjunction is the failure of chromosomes
or chromatids to separate normally during
meiosis. This can happen during:
meiosis I, if both members of a homologous
pair go to one pole or
meiosis II if both sister chromatids go to one
Fertilization after nondisjunction yields
zygotes with altered numbers of chromosomes.
Sex chromosome abnormalities
tend to be less severe, perhaps
the small size of the Y
a single Y chromosome is enough to
produce “maleness,” even in
combination with several X
the absence of a Y chromosome yields
Chromosome breakage can lead to
rearrangements that can produce:
genetic disorders or,
if changes occur in somatic cells,
a deletion, the loss of a chromosome
a duplication, the repeat of a
an inversion, the reversal of a
chromosome segment, or
a translocation, the attachment of a
segment to a nonhomologous
chromosome that can be reciprocal.