 Figure 2: Examples of polytene chromosomes Show Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes. Although he did not know it, Walther Flemming actually observed spermatozoa undergoing meiosis in 1882, but he mistook this process for mitosis. Nonetheless, Flemming did notice that, unlike during regular cell division, chromosomes occurred in pairs during spermatozoan development. This observation, followed in 1902 by Sutton's meticulous measurement of chromosomes in grasshopper sperm cell development, provided definitive clues that cell division in gametes was not just regular mitosis. Sutton demonstrated that the number of chromosomes was reduced in spermatozoan cell division, a process referred to as reductive division. As a result of this process, each gamete that Sutton observed had one-half the genetic information of the original cell. A few years later, researchers J. B. Farmer and J. E. S. Moore reported that this process—otherwise known as meiosis—is the fundamental means by which animals and plants produce gametes (Farmer & Moore, 1905).The greatest impact of Sutton's work has far more to do with providing evidence for Mendel's principle of independent assortment than anything else. Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells. To illustrate this concept, consider the variety derived from just three hypothetical chromosome pairs, as shown in the following example (Hirsch, 1963). Each pair consists of two homologues: one maternal and one paternal. Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:
When these chromosome pairs are reshuffled through independent assortment, they can produce eight possible combinations in the resulting gametes:
A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete. Thus, in the previous example of three chromosome pairs, the calculation is 23, which equals 8. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is (2n)2, which results in some fairly large numbers. But what about chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that one person could produce 223 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is (223)2 possible combinations. However, some of these combinations produce the same genotype (for example, several gametes can produce a heterozygous individual). As a result, the chances that two siblings will have the same combination of chromosomes (assuming no recombination) is about (3/8)23, or one in 6.27 billion. Of course, there are more than 23 segregating units (Hirsch, 2004). While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis. In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination. Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2n calculation. In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage. Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation. While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction, genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes. Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species.
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Down Syndrome karyotype (Dr_Microbe, iStockphoto)
Errors during meiosis can alter the number of chromosomes in cells and lead to genetic disorders.
People are always pointing out differences between cultures, ethnicities and nationalities. But we all belong to the same species. DNA evidence shows that all humans are more than 99% genetically identical. The remaining 1% percent variance in genetic makeup is due in part to mutations that occur occasionally during cell division. Let’s take a closer look at how this happens. What is DNA?DNA is short for deoxyribonucleic acid. You’ve probably heard that it’s what makes you unique. DNA is a long chain of molecules that looks like a twisted ladder. The rails are made of sugar and phosphate molecules. These are also called the “backbone” of a DNA molecule. Strands of DNA contain genes and chromosomes. Genes are like genetic paragraphs written using nucleotides. And chromosomes are like genetic books that contain large number of genes. The human body has 23 different chromosomes. Each cell has two copies of each chromosome, one from your mother and the other from your father. This makes for a grand total of 46 chromosomes. Almost every single cell in your body contains the exact same chromosomes.
The human body has 22 pairs of non-sex chromosomes, called autosomes. That leaves just one pair of sex chromosomes. These determine your biological sex. If you are biologically female, you have two X chromosomes. If you are biologically male, you have one X chromosome and one Y chromosome. What happens to chromosomes during reproduction?Reproduction depends on specialized sex cells, called gametes. These cells only contain 23 chromosomes, or half the genetic material of other cells. Gametes are commonly known as sperm cells in males, and egg cells in females. How does our body make new cells?Your cells create copies of themselves through cell division. In your non-sex cells, this is called mitosis. This is the process your cells use when you grow or heal. Another type of cell division is called meiosis. Meiosis is cell division that produces gametes. It results in four cells which each contain 23 chromosomes. These new cells are each genetically different from one another. Meiosis is a two-step process that occurs in the reproductive organs. The steps are called meiosis I and meiosis II. In meiosis I, the chromosomes line up in pairs at the centre of a cell. Then, they are pulled apart into two new cells. In meiosis II, a similar process occurs, except that each new cell contains only 23 chromosomes. The chromosomes are pulled apart by tiny molecular machines called spindles. Spindles are made up largely of microtubules and are organized by the centrosomes. Sometimes, the spindle does not do its job properly. That means new cells end up with too few or too many chromosomes. This is called nondisjunction. What is nondisjunction?Nondisjunction happens when a chromosome pair doesn’t separate during meiosis. As a result, one of the gametes contains extra chromosomes, while others have too few. Why are some people born with too few or too many chromosomes?If a sex cell that has suffered a nondisjunction event is combined with a sex cell from the opposite sex, the resulting zygote will have more or less than 46 chromosomes. This means that when that baby is born, it will have more or less than 46 chromosomes. This person will also have either too many or too few genes. Biologists call this aneuploidy. Without 46 chromosomes, the human body will make the wrong number of proteins. Proteins play many roles. For example, they help build and repair tissues, like hair and nails. They also create and regulate hormones. Without the right number of proteins, the processes in your body may not work properly.
What happens if you have too many chromosomes?If one of the original cells had an extra chromosome, the person will have trisomy. This is a type of aneuploidy. People with trisomies have three copies of a particular chromosome (instead of two). This means these individuals have a total of 47 chromosomes (n+1). Trisomies are named after the chromosome pair that gets the extra chromosome. Trisomy 21 is a fairly common aneuploidy that involves an extra chromosome 21. This is also called Down syndrome. It affects about one in every 750 babies born in Canada. Children with Trisomy 21 may experience delays when learning to crawl, walk and speak. As they get older, they may have trouble with reasoning and understanding. Two other examples are Trisomy 13 (Patau syndrome) and Trisomy 18 (Edward’s syndrome). They can both cause serious brain, heart and spinal cord defects. Many babies born with these syndromes only live a few days. What problems do extra X and Y chromosomes cause?In a female fetus, an extra X chromosome causes Triple X syndrome. It is associated with learning disabilities and organ abnormalities. In a male fetus, Klinefelter syndrome is the result of an extra X chromosome (XXY). Males with this condition have smaller testicles and are infertile. Finally, males born with an extra Y chromosome (XYY) have Jacob’s syndrome. The symptoms include language difficulties, problems with sitting and walking, and behavioural-emotional issues. People with an extra Y chromosome may also have mild autism and weak muscle tone (hypotonia). The human body is a complex machine and sometimes, it makes mistakes. By learning more about human genetics, we can better understand genetic disorders. This will help us find ways to help people who have them. Scientists are now looking for ways to edit DNA so they can reduce the effects of genetic disorders. This technique is called genome editing. You can make a difference in your community by volunteering to help people with disabilities.
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During meiosis, two chromosomes did not separate correctly. what may result from this error?
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