In genetics, a chiasma (pl. chiasmata) is the point of contact, the physical link, between two (non-sister) chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called a chromosomal crossover, but this is much more frequent during meiosis than mitosis.[1] In meiosis, absence of a chiasma generally results in improper chromosomal segregation and aneuploidy.[2] Points of crossing over become visible as chiasma after the synaptonemal complex dissembles and the homologous chromosomes slightly apart from each other. The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 by Frans Alfons Janssens, a Professor at the University of Leuven in Belgium.[3][4] When each tetrad, which is composed of two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata. The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-overs" of genetic material are thought to occur during the previous pachytene stage. Sister chromatids also form chiasmata between each other (also known as a chi structure), but because their genetic material is identical, it does not cause any noticeable change in the resulting daughter cells. In humans, there seems to be one chiasma per chromosome arm,[5] and in mammals, the number of chromosome arms is a good predictor of the number of crossovers.[6] Yet, in humans and possibly other species, evidence shows that the number of crossovers is regulated at the level of an entire chromosome and not an arm.[2] The grasshopper Melanoplus femurrubrum was exposed to an acute dose of X-rays during each individual stage of meiosis, and chiasma frequency was measured.[7] Irradiation during the leptotene-zygotene stages of meiosis, that is, prior to the pachytene period in which crossover recombination occurs, was found to increase subsequent chiasma frequency. Similarly, in the grasshopper Chorthippus brunneus, exposure to X-irradiation during the zygotene-early pachytene stages caused a significant increase in mean cell chiasma frequency.[8] Chiasma frequency was scored at the later diplotene-diakinesis stages of meiosis. These results suggest that X-rays induce DNA damages, likely including double-strand breaks, and these damages are repaired by a crossover pathway leading to chiasma formation.
🏠 HomeSubjects ➗ Math🧪 Science🏛️ History📺 Arts & Humanities🤝 💻 Engineering & Technology💰 Business📚 OtherResources 📓 Study Guides🏆 Leaderboard💯 All Tags❓ Unanswered🔀 Random Tags🎒 Geometry In the Meiosis unit we will learn how the random alignment of the homologous pairs of chromosomes gives new combinations of the original maternal and paternal chromosomes present in the gamete mother cell. We will also see how crossing over between members of homologous pairs of chromosomes leads to the formation of chromosomes that contain new combinations of the alleles of the genes.We will learn how meiosis takes place in reproductive tissue. In male animals, meiosis takes place in the testes and in females within the ovaries. In plants, meiosis occurs in the anthers to form pollen grains and within the ovary to form ovules This unit will last 3 school days Essential idea:
Nature of science:
Understandings 10.1 U1 Chromosomes replicate in interphase before meiosis. (Oxford Biology Course Companion page 440).
During the S phase of the cell cycle, so that each chromosome has a copy of itself and consists of two sister chromatids. During meiosis I, chromosomes condense and synapse to form bivalents (homologous chromosomes are aligned next to each other).
10.1 U2 Crossing over is the exchange of DNA material between non-sister homologous chromatids. (Oxford Biology Course Companion page 440).
Crossing over involves the exchange of segments of DNA between homologous chromosomes during Prophase I of meiosis
 10.1 U3 Crossing over produces new combinations of alleles on the chromosomes of the haploid cells. (Oxford Biology Course Companion page 441).
As a result of crossing over, chromatids may consist of a combination of DNA derived from both homologues - these are called recombinants (new combinations). The formation of recombinants basically makes the possible allele combinations unlimited. Crossing over results in new combinations of alleles in haploid cells and thus increases the genetic diversity of potential offsp 10.1 U4 Chiasmata formation between non-sister chromatids can result in an exchange of alleles. (Oxford Biology Course Companion page 440).
During prophase I of meiosis, homologous chromosomes become connected in a process known as synapsis. During synapsis, prophase I, homologous chromosomes pair forming bivalents in synapsis, non-sister chromatids may break and recombine with their homologous partner (crossing over). These non-sister chromatids remain physically connected at these points of exchange – regions called chiasmata. Chiasmata are points where two homologous non-sister chromatids exchange genetic material during crossing over in meiosis. Chromosomes intertwine and break at the exact same positions in non-sister chromatids. The two chromosomes are now attached at the same corresponding position on the non-sister chromatid. Many chiasmata can form between the chromatids.Once attached the non-attached portions of the chromatids actually repel each other. Chiasmata refer to the actual break of the phosphodiester bond during crossing over. The chiasmata are separated during anaphase 1 which can result in an exchange of alleles between the non-sister chromatids from the maternal and paternal chromosomes.
10.1 U5 Homologous chromosomes separate in meiosis I. (Oxford Biology Course Companion page 443).
Prophase I
Metaphase I
Anaphase I
Telophase I
image from DNA Encyclopedia 10.1 U6 Sister chromatids separate in meiosis II. (Oxford Biology Course Companion page 444).
Prophase II
Metaphase II
Anaphase II
Telophase II
MEIOSIS = DIPLOID -> DIPLOID -> HAPLOID
image from What is Meiosis? 10.1 U7 Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I. (Oxford Biology Course Companion page 444).
When Mendel first did his experiments on pea plants, he looked at the traits that were passed on from generation to generation. He did not know how the traits were inherited in terms of meiosis. We now know that independent assortment is an essential component in explaining how chromosomes align themselves during meiosis law of independent assortment applies only to traits carried on different chromosomes, i.e.unlinked genes
Skills 10.1 S1 Drawing diagrams to show chiasmata formed by crossing over. (Guidance: Diagrams of chiasmata should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma was formed) (Oxford Biology Course Companion page 442).
Draw diagrams to show chiasmata and the resulting chromosomes formed during crossing over.
Key Terms
Classroom Material Topic 10.1 Review PowerPoint and Notes on Topic 10.1 by Chris Payne Correct use of terminology is a key skill in Biology. It is essential to use key terms correctly when communicating your understanding, particularly in assessments. Use the quizlet flashcards or other tools such as learn, scatter, space race, speller and test to help you master the vocabulary. Video Clips Amoeba Sisters as they explore the meiosis stages with vocabulary including chromosomes, centromeres, centrioles, spindle fibers, and crossing over. Hank gets down to the nitty gritty about meiosis, the special type of cell division that is necessary for sexual reproduction in eukaryotic organisms Crossing Over and Variability Time-lapse microscopy of a meiosis overlaid with animation graphic |
What is the result of one chromatid segment forming a chiasma with a homologous chromatid segment?
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