The ''pachytene'' stage (
/ˈpækɪtiːn/ ''PAK-i-teen''; from Greek words meaning "thick threads".
), also known as ''pachynema'', is the third stage of
prophase I during meiosis, the specialized cell division that reduces
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
number by half to produce
haploid gametes. It follows the
zygotene stage and is followed by the stage
Diplotene
Synapsed chromosomes
During pachytene, the homologous chromosomes are fully synapsed along their lengths by the completed synaptonemal complex protein structure formed in the previous stages. This holds the homologous closely paired, allowing intimate DNA interactions.
Chromosome condensation
The chromosomes reach their highest level of condensation during pachytene. Each chromosome consists of two closely associated sister chromatids along their entire length. The chromosomes appear as distinct, well-defined threadlike structures under the microscope.
Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology called the
pseudoautosomal region
The pseudoautosomal regions or PARs are Homology (biology), homologous sequences of Nucleotide, nucleotides found within the Sex chromosome, sex chromosomes of species with an XY sex-determination system, XY or ZW sex-determination system, ZW mech ...
.
Recombination nodules
Multiple recombination nodules are distinctly visible along the paired homologous chromosomes. These proteinaceous structures mark the sites of genetic crossover events between the non-sister chromatids that were initiated during zygotene.
Proteins like MLH1 and MLH3 stabilize the crossover events, ensuring at least one obligatory crossover per chromosome arm. This gives each chromosome a minimum of two crossover sites. Additional crossovers are also possible but regulated.
DNA repair
During pachytene, any unresolved DNA double-strand breaks from previous recombination events are repaired. Mismatch repair proteins help correct any errors in base pairing between the homologs.
Treatment of male mice during
meiosis
Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
with gamma
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:
* ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
causes
DNA damage
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is constantly modified ...
.
Homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
is the principal mechanism of
DNA repair
DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...
acting during meiosis. From the
leptotene to early pachytene stages of meiosis exogenous damage triggered the massive presence of gamma
H2AX (which forms when DNA double-strand breaks appear), H2AX was present throughout the nucleus, and this was associated with DNA repair mediated by homologous recombination components
DMC1 and
RAD51
DNA repair protein RAD51 homolog 1 is a protein encoded by the gene ''RAD51''. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to t ...
proteins.
[
]
The meiotic sex checkpoint
Pachytene is also a stage where a critical checkpoint operates to monitor proper chromosome synapsis and recombination. Errors detected at this stage can arrest the meiotic cell cycle and trigger apoptosis
Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
(programmed cell death) of the defective cell.
Transition to diplotene
Once crossover events are stabilized, the synaptonemal complex disassembles and chromosomes begin to gradually desynapse as the cell transitions into the diplotene stage.
Importance
The pachytene stage is essential for the extensive genetic recombination and accurate chromosome segregation in meiosis. Defects at this stage can lead to aneuploidy and nondisjunction.
References
{{Reflist
Meiosis
Cellular processes