Synapsis

Synapsis Definition

Synapsis is an event that occurs during meiosis in which homologous chromosomes pair with their counterparts and remain bound due to the exchange of genetic information. During meiosis, homologous chromosomes are paired and then separated to reduce the genetic content of the resulting gamete cells. Synapsis does not occur during mitosis, because homologous chromosomes do not pair with their counterparts. Synapsis starts when proteins connecting the various chromosomes to the nuclear membrane start to migrate. The proteins move around until their counterpart is found, and the synaptonemal complex is formed. The synaptonemal complex consists of protein and RNA, and hold the homologous chromosomes together.

Function of Synapsis

The synapsis of homologous chromosomes has a couple different functions in organisms. First, it holds the homologous chromosomes together through metaphase I of meiosis I, which allows them to be aligned on the metaphase plate and separated. This is a key task during meiosis, as this is how the genetic information in each gamete is reduced. Secondly, it allows homologous chromosomes to exchange information through crossing over. Crossing over occurs when similar portions of homologous chromosomes exchange position. This can redistribute how alleles interact with each other in the resulting organism. Synapsis ensures that alleles contained in these regions can be distributed independently of their parental source.

Related Biology Terms

  • Meiosis – The process sexually-reproducing organisms use to create gametes with half the required genetic material to create a zygote.
  • Crossing-overGenetic recombination that occurs during meiosis, leading to novel forms.
  • Recombination – The mixing of genetic material from different strands of DNA.
  • Synaptonemal Complex – A protein and RNA structure that aids in forming the connections during synapsis of homologous chromosomes.

Quiz

1. If synapsis did not occur during meiosis, which of the following would be true?
A. Meiosis would be mitosis
B. No organism could exist
C. Gametes would still be created

Answer to Question #1
A is correct. Without synapsis, meiosis is essentially two divisions identical to mitosis. Synapsis binds the homologous chromosomes, which causes them to be separated during the first division of mitosis. With only half of a full genome, the cell proceeds directly into another cell division and creates gametes. Without synapsis the original division of chromosomes would be the same as in mitosis, and would create cells that were identical to the original cell. Thus, the DNA would be replicated again before the next division, just as in mitosis. Many organisms exist without meiosis, as they reproduce asexually. Without meiosis, however, gametes cannot be created for sexually reproducing organisms. Gametes require a reduction of genetic material, which requires the synapsis of homologous chromosomes.

2. During meiosis in an organisms, the process of synapsis malfunctions and does not release the homologs of a single pair of chromosomes. This extra copy gets transferred to the gamete, and eventually to the zygote. Why is this a potential problem?
A. Organisms cannot survive unless meiosis happens perfectly
B. The proteins produced by the chromosome may occur in excess
C. The extra chromosome will cause DNA damage

Answer to Question #2
B is correct. Polysomy is a condition found throughout the animal kingdom, in which an animal has more chromosomes than typical of other members of the species. This may or may not present a problem. If the proteins produced by the chromosome do not inhibit the development or functioning of the organisms, the polysomy may remain in the population. This can be seen in many varieties of plants. In humans and other vertebrates, polysomy can be developmentally abnormal and lead to conditions such as Klinefelter syndrome and Down syndrome. This is often caused by the overexpression of certain proteins or hormones which are coded for by the extra chromosome.

3. The synaptonemal complex holds homologous chromosomes together, all along their length. This is easy because homologous chromosomes are typically the same shape. In mammals, the sex chromosomes are specialized, and the female (X) chromosome is larger than the male (Y). What does this require during the synapsis of these homologous chromosomes?
A. The homologs do not undergo synapsis
B. The synaptonemal complex must only bond the homologs at the ends of the chromosomes
C. The chromosomes will undergo synapsis the same way most chromosomes do

Answer to Question #3
B is correct. The synaptonemal complex is dependent on the shape and bonding capabilities of the chromosome, which is determined by the genetic code. The male (Y) chromosome is much smaller than the female (X) because the two chromosomes code for different things. Because the shape and size are so much different, the synaptonemal complex can only connect the two homologs as the ends of each chromosome. This is different than the synaptonemal complex in most homologous chromosome synapsis events.
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