Meiosis

Meiosis

How Chromosomes are passed from parent to offspring

Each human cell (aside from red blood cells and gametes) contains a full set of 46 chromosomes. Clearly havoc would result if a sperm and egg cell each containing 46 human chromosomes were to fuse! Not only would the resulting offspring have 98 chromosomes in each cell but the number would keep on doubling with each successive generation. For this reason a process other than mitosis which produces cells with a diploid number of chromosomes is necessary to produce the sperm and egg cells.

The process by which the chromosome number is halved and chromosomes are sorted and packaged to be passed on to an organism’s offspring is called meiosis.

Each of the resulting reproductive cells, or gametes (sperm and egg), has only a single set of 22 autosomes plus a single sex chromosome, either an X or a Y. A cell with a single chromosome set is called a haploid cell.

By means of sexual intercourse, a sperm cell carrying one 23 chromosome set from the father reaches and fuses with an egg cell carrying a corresponding set of 23 chromosomes from the mother.

The resulting fertilized egg, or zygote, contains the two haploid sets of chromosomes bearing genes originating in both the maternal and paternal family lines.

As a human develops from a zygote to a sexually mature adult, the new combination of genes in the zygote are passed on with precision to all somatic cells of the body by the process of mitosis.

The process of meiosis has many similarities to the process of mitosis--chromosomes replicate before the process begins and shorten and thicken to look like the chromosomes at the beginning of mitosis. A spindle with fibers appears and the nuclear membrane dissolves.

However, as meiosis begins, each chromosome is mysteriously attracted to its special homologous partner. The two #1 chromosomes--one from the paternal set and one from the maternal set--wrap tightly about each other in a process called synapsis. Since each #1 chromosome is already doubled, a tetrad of four chromosomes is created. The same thing happens with the other chromosomes. Each homologous pair forms its own tetrad. All the tetrads arrange themselves on the spindle. Two quick divisions follow and the chromosomes are pulled apart. The four chromosomes of each tetrad are first separated into twos and then into ones.


Before meiosis begins each chromosome in the pair has already doubled. The double maternal chromosome (black) and the double paternal chromosome (white) attach to the spindle.	As the cell divides, the double maternal chromosome and the double paternal chromosome move toward opposite poles.	The pair in each daughter cell starts to pull apart in the direction of the arrows. Another cell division occurs.	The final result is four cells, each containing the haploid number of chromosomes. Two of the cells contain a chromosome from the maternal set and two contain a chromosome from the paternal set.

The diagram above shows only a single chromosome pair. Actually everything that is happening to this tetrad is happening simultaneously to the other 22 tetrads. The result is always four cells, each having a single #1 chromosome, and one #2 and one #3 and so on up to one each of 23 chromosomes. Thus each cell has one complete set of chromosomes and is ready to become either a sperm or egg cell.

The chromosomes in each sperm or egg are a random mix from the maternal set and paternal set of the original cell.

Because each of the 23 pairs of chromosomes can line up in two different ways one person can produce more than 8 million (2²³) different kinds of eggs or sperm. When fertilization occurs, 2^{23
x} 2²³or 70 trillion different zygotes are possible!

For the genealogist, this means that any one of his or her chromosomes could have come from any one of eight great-grandparents or, going back say twenty generations, from any one of over a million potential ancestors (in reality, many of these potential ancestors may be the same individuals as lines cross and intermarry over generations.) In the same light, after so many generations of shuffling and dividing, contributions from some ancestors may have been lost completely. With only one exception, it is impossible to trace the path of individual chromosomes in the nucleus from generation to generation.

Cross Over Animation

And, to complicate matters even further, the chromatids in a tetrad pair so tightly at the beginning of meiosis that non-sister chromatids from homologous chromosomes actually exchange genetic material in a process known as crossing over. This further shuffles the ancestral genes so that a single chromosome in a gamete may contain genes from both maternal and paternal ancestors. Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time. It is estimated that during meiosis in humans, there is an average of two to three crossovers for each pair of homologous chromosomes.

Differences Between Sperm and Egg Formation

What they mean to the molecular genealogist

The process of meiosis and gamete formation is fundamentally the same in males and females. However, whereas gametogenesis (formations of gametes) results in four functional sperm cells for each meiotic division in males, the same process in females gives rise to only a single functional egg capable of being fertilized and developing into an embryo.

A mature sperm has a head, which contains a nucleus with its haploid set of chromosomes, a long tail which propels the sperm through its fluid surroundings, and between these a midpiece containing several hundred mitochondria which supply the energy necessary for the long journey in search of the egg.

On the other hand, the development of the egg cell involves an unequal cell division. It produces one relatively large primary egg cell which receives all of the cell parts--including thousands of mitochondria--from the starting cell while the other three cells that result from the meiotic process form small polar bodies that, at first, attach themselves to the surface of the primary egg but eventually deteriorate.

At fertilization, the entire sperm does not enter the egg. Practically the only contribution that the sperm cell makes to the zygote is its haploid nucleus with its set of 23 chromosomes. Importantly to the genealogist, the tail and midsection of the sperm drop off outside the egg meaning that virtually no mitochondria from the male parent enter the zygote. All of the mitochondria (with its own DNA you will recall) in the developing embryo come from the mother.

The diagram below illustrates the differences between the formation of sperm and egg cells.

The cross-over animation above is a modification of an animation by
Hironao NUMABE, M.D, Tokyo Medical University, Department of Pediatrics Genetic Study Group http://www.tokyo-med.ac.jp/genet/mfi-e.htm

I have also included an exceptionally nice movie of cells undergoing meiosis which I found at http://www.fed.cuhk.edu.hk/~johnson/animations/cell_division/meiosis_movie.mov and use with permission though the webmaster was uncertain of the origin. Unfortunately, the size of this file is 11.8 MB meaning that unless you have a high speed Internet connection the download time could be as much as 18 minutes! For the fortunate with cable, DSL or satellite connections, click here

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