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Chromosomal crossover (or crossing over) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes during sexual reproduction. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis.

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Mendelian inheritance is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900. These laws were initially controversial. When Mendel's theories were integrated with the Boveri-Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.

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from research organizations

An Australian research team has solved one of biology's most fundamental questions - why males produce sperm and females produce eggs. The finding is a breakthrough that could lead to improved infertility treatment, cancer therapy and pest management.

The team, led by Dr Josephine Bowles and Professor Peter Koopman from the Institute for Molecular Bioscience at The University of Queensland, has discovered that derivatives of Vitamin A trigger the beginning of egg and sperm production, a process known as meiosis.

The cells that eventually turn into either eggs or sperm - known as germ cells - are identical in male and female embryos.

"Whether a germ cell develops into an egg or a sperm depends on the time at which meiosis begins," Professor Koopman said.

"In females, meiosis begins before birth and eggs are produced, whereas in males, meiosis begins after birth and the result is sperm."

Professor Koopman and his team found that retinoic acid, a derivative of Vitamin A, causes germ cells in female embryos to begin meiosis, leading to the production of eggs.

They also discovered an enzyme present in male embryos that wipes out retinoic acid and so suppresses meiosis until after birth, resulting in sperm production.

"This is an extremely important process that nobody has been able to figure out until now," Professor Koopman said.

"It is textbook science and it should provide the basis for a number of practical applications."

Knowledge of what triggers and suppresses meiosis may allow researchers to improve fertility, for example in the case of an infertile couple wanting a baby, or suppress it, in the case of pest management.

Professor Koopman also suggested that an inappropriate retinoid signal might give the wrong instructions to germ cells, which could lead to the formation of germ cell tumours.

"Our research has suggested a possible cause for these common testicular cancers, opening up avenues of investigation which will hopefully one day lead to a cure," Professor Koopman said.

The findings of the team will be published in one of the world's top scientific journals Science and will be available in its online version, SciencExpress, from March 31.

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fertility; pregnancy and childbirth; prostate cancer; stem cells; mating and breeding; biology; developmental biology; genetics

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Why Males Produce Sperm And Females Produce Eggs

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from research organizations An Australian research team has solved one of biology's most fundamental questions - why males produce sperm and females produce eggs. The finding is a breakthrough that could lead to improved infertility treatment, cancer therapy and pest management.

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from research organizations
Credit: Image courtesy of Science/AAAS
Meiosis, the process of halving a germ cell's chromosomes in preparation for egg or sperm production, has been one of the most studied areas of cell biology. But in mammals, the field has been divided over the question of whether meiosis is triggered by a signal within a cell or by a signal coming from the cell's environment.

Now new research from the lab of Whitehead Director David Page reveals that both sets of signals are needed to initiate meiosis.

In 2006,the Page lab showed that an external signal of retinoic acid (RA), a derivative of vitamin A, starts the expression of the Stra8 gene, which is seen at the very beginning of meiosis. While this research seemed to settle the controversy, researchers still had to deal with the reality that RA has signaling functions throughout the body at various points of development. How could such a nonspecific signal have such a specific effect on meiosis in embryonic germ cells? As it turns out, other researchers in Page's lab were grappling with that very issue.

Coincidentally, more than 10 years before the work on RA, Page's lab had been studying the effects of a few genes on human male fertility, noting at the time that one such gene, known as Dazl, codes for a germ cell-specific protein that binds to RNA. But at the time, no one knew that the Dazl gene is an integral part of the RA signaling pathway in embryonic germ cells.

"Dazl is in both the mouse and human genomes, but it has counterparts that are critical in germ cell biology throughout the entire animal kingdom," says Page.

To see how the Dazl gene affects germ cells' response to RA signaling, Page graduate students Yanfeng Lin and Mark Gill compared the embryonic ovaries of control mice to those of mice lacking the DAZL protein. They stained the ovaries for Stra8 expression, a clear indicator of meiotic initiation. The ovaries without the DAZL protein failed to express Stra8 and thus did not enter meiosis. Control mice, however, which had the DAZL protein, clearly expressed Stra8 and became meiotic.

Gill explains that the Dazl gene and its protein prepare the cell to respond to RA. "We've uncovered a transition state in development in which the cells are primed for meiosis," says Gill. "This research further explains the specificity of the retinoic acid signaling pathway that had been previously described."

Although part of the meiosis initiation mystery has been elucidated, Page concedes this is only the beginning. "There's a lot of black box here, like what is going on in the cell during this transitional state. We're addressing that as well, which is why this is a very exciting time in this field."

David Page's primary affiliation is with Whitehead Institute for Biomedical Research, where his laboratory is located and all his research is conducted. He is also a Howard Hughes Medical Institute investigator and a professor of biology at Massachusetts Institute of Technology.

Journal reference: Yanfeng Lin, Mark E. Gill, Jana Koubova and David C. Page. Science, 322 (5908):1685-1687, December 12, 2008.

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human biology; stem cells; genes; medical topics; genetics; molecular biology; cell biology; biotechnology

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Meiosis: Cueing Up At The Meiotic Starting Line

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Geneticists have been debating if meiosis requires only external or internal signals. Although earlier work revealed that an external cue from retinoic acid is needed, new research shows that embryonic germ cells must first express the Dazl gene in order to process the retinoic acid signal.

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Terms and Vocabulary for Chapter 7 Meiosis Learn with flashcards, games, and more - for free.

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SC.912.L.16.16 - Describe the process of meiosis, including independent assortment and crossing over. Explain how reduction division results in the formation of haploid gametes or spores.

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In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as Meiosis I and Meiosis II.