Bivalent (Genetics)
Encyclopedia
A bivalent, sometimes referred to as a tetrad, is a pair of associated homologous chromosome
s held together by a complex after chromosome replication.
During meiosis
(the Prophase I stage of Meiosis I), the process of synapsis
occurs in which bivalents are formed. Each replicated chromosome is composed of two chromatid
s.
Bivalents are formed as two homologous chromosomes undergo recombination. Chromosome movement to accommodate recombination places homologous chromosomes close enough to each other for a protein complex (synaptonemal complex
), consisting of a protein (ZIP1/ZYP1 depending on species), to hold them together. This protein's sequence is not well conserved, though its structure is very similar in all species (globular domains on the N and C termini with a well defined core).
In the atypical model species D. melanogaster and C. elegans recombination occurs secondarily to synapsis. Most species Human, Mouse, Arabidopsis, Maize, Yeast, Lily, and many others create bivalents as a result of DNA repair and recombination.
Recombination produces chiasmata which hold chromosomes together after the synaptonemal complex dissolves. (Prophase 1 substage Diplotene)
Homologous chromosome
Homologous chromosomes are chromosome pairs of approximately the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father...
s held together by a complex after chromosome replication.
During meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
(the Prophase I stage of Meiosis I), the process of synapsis
Synapsis
Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are...
occurs in which bivalents are formed. Each replicated chromosome is composed of two chromatid
Chromatid
A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division . They are called sister chromatids so long as they are joined by the centromeres...
s.
Bivalents are formed as two homologous chromosomes undergo recombination. Chromosome movement to accommodate recombination places homologous chromosomes close enough to each other for a protein complex (synaptonemal complex
Synaptonemal complex
The synaptonemal complex is a protein structure that forms between homologous chromosomes during meiosis and that is thought to mediate chromosome pairing, synapsis, and recombination . It is now evident that the synaptonemal complex is not required for genetic recombination...
), consisting of a protein (ZIP1/ZYP1 depending on species), to hold them together. This protein's sequence is not well conserved, though its structure is very similar in all species (globular domains on the N and C termini with a well defined core).
In the atypical model species D. melanogaster and C. elegans recombination occurs secondarily to synapsis. Most species Human, Mouse, Arabidopsis, Maize, Yeast, Lily, and many others create bivalents as a result of DNA repair and recombination.
Recombination produces chiasmata which hold chromosomes together after the synaptonemal complex dissolves. (Prophase 1 substage Diplotene)