The Cell: Meiosis

The Cell: Meiosis

As part of our coverage of the intricacies of the cell, we have covered the basic structure of the cell, as well as Mitosis. Today we are going one step further in our journey through the cell, by examining the process of Meiosis. The two processes have similar names and even function somewhat similarly, but result in vastly different outcomes. This article will provide a basic walkthrough of Meiosis and explain how the process fits into a discussion of origins.

While superficially similar to Mitosis, Meiosis results in four cells rather than two, and each of the new cells has only half the chromosomes of the original cell. For example, a normal human cell has twenty-three pairs of chromosomes, for a total of forty-six. This kind of cell is called a diploid cell. Meiosis takes this diploid cell and breaks it down into what is called a haploid cell. A haploid cell has half the number of chromosomes of a diploid cell. Thus a human haploid cell has twenty-three total chromosomes.  Haploid cells are sometimes also referred to as gametes because they are generally used in sexual reproduction.  Unlike mitosis, in which the cell goes through one phase and is done, meiosis is divided into two phases; Meiosis I and Meiosis II.  Each phase is further subdivided into sections.  To help the reader visualize what I discuss below, I have linked to an animation of Meiosis from start to finish.

Meiosis I is divided into six or seven distinct phases, depending on how detailed you choose to go. The first is the Interphase. Much like in mitosis, in interphase, the cell produces duplicates of each of its chromosomes. In the cytoplasm, specialized structures called centromeres form.  These centromeres will be important later. As part of interphase, tiny fibers called microtubules begin to extend from the centromeres.

Having passed through Interphase, the cell proceeds into Prophase I.  During prophase I the cell condenses the chromosomes into visible packets and the chromosomes pair up into what are called homologous pairs. Homologous pairs are the same chromosome, but with different information on each one. One half of the pair came from the organisms mother, the other from the father. These homologous chromosomes join together into an “X” like position. Each of these pairs may exchange bits of information, in a process referred to as crossing over. This creates what is called recombinant chromosomes that have information different than what they had originally. Outside the nucleus, a structure called the Meiotic spindle forms from the centromeres, and microtubules extend from it. This will be an important structure later. Having completed Prophase I, the cell moves forward into Prometaphase I.

Prometaphase I is a relatively short, straightforward part of Meiosis I.  The nuclear envelope, which generally keeps the chromosomes out of the main body of the cell, breaks down, allowing access to the cytoplasm.  This allows the Meiotic spindle to attach to each chromosome individually by means of the microtubules. Prometaphase I is followed quickly by Metaphase I.

In Metaphase I, the newly freed chromosomes line up along what is referred to as the cellular equator. Much like the earth’s equator, the cellular equator divides the cell into two equal halves. The chromosomes align randomly along this equator.  The Meiotic spindle completes its attachment to the chromosomes, attaching to one of each pair. Metaphase I sets the stage for the dramatic happenings of Anaphase I and Telophase I which follow in rapid succession.

Anaphase I begins the process of creating two new, unique cells. The Meiotic spindle contracts, pulling the chromosome pairs apart, into opposite halves of the cell.  Because of the process which took place Metaphase I called random assortment, there is a 50/50 chance that the chromosome pulled from a given pair will be either maternal or paternal. In other words, for chromosome 1 the meiotic spindle might pull the maternal chromosome, while for chromosome 2 it may pull the paternal part of the pairing.  This will result in two distinct daughter cells after Telophase I.

As part of Telophase I, the newly separated chromosomes join together. A new nuclear membrane forms around them, creating a new nucleus on each end of the parent cell. Cytokinesis then takes place, splitting the parent cell into two distinct daughter cells. The meiotic spindle breaks down as part of this process.  Sometimes the process of reforming the nucleus is completely skipped and the new cells proceed directly into Meiosis II which closely resembles Mitosis.

Meiosis II begins with Prophase II. Prophase II consists of two daughter cells but the same processes take place in each so we will consider just one of them. Bear in mind, however, that identical processes are taking place in the second daughter cell.  Each of these daughter cells has 23 pairs of chromatids or twenty-three chromosomes.  They are considered haploid because they lack paired chromosomes.   As part of Prophase II the meiotic spindle reforms. If nuclear envelopes have reformed, they dissolve again and the chromosomes reform if they had broken up. This moves the cell into Metaphase II.

In Metaphase II, the meiotic spindle attaches to the chromatids as they line up along the new cellular equator.  The spindle attaches randomly to the chromatids, just as it did to the chromosomes in Meiosis I. This complete, the cell proceeds into Anaphase II.

Anaphase II is essentially a direct copy of Anapahse I with one key difference. Instead of pulling chromosomes to opposite ends of the cell, the meiotic spindle now pulls chromatids to opposite ends of the cell.  This is why the crossing over process from Prophase I was so important. The chromatids of a given chromosome begin interphase as identical twins, sisters in biologist terms. However, crossing over happens uniquely to each chromatid, meaning that the two chromatids are no longer identical. Thus when they have pulled apart, they create a new, unique chromosome.

In Telophase II, these new chromosomes complete their movement to opposite ends of the cell and join together. A new nuclear envelope forms around each group of chromosomes. Cytokinesis then divides each daughter cell into two new, unique daughter cells, resulting in four total daughter cells, each with their own unique set of one chromatid chromosomes.  As part of sexual reproduction, two of these daughter cells, one maternal, the other paternal, will combine, creating a complete set of genetic information for their offspring.

If somewhere about Telophase I you got lost, you are not alone.  Meiosis is a complex, deep process which is much more complicated than the brief explanation I gave above.  However, Meiosis is required for sexual reproduction.  Without it, no reproductive cells could form and reproduction could not take place. This means that sexual reproduction had to have come into being at the same time Meiosis did.  If all the physical pieces were in place for sexual reproduction, along with the instinctual knowledge of how the process worked, but Meiosis was absent, the process would not work. By a similar token, Meiosis could produce billions of gametes, but without a method for the male and female gametes to meet, Meiosis would be useless.  They had to come about together.  This presents a problem for the evolutionist because his explanation for sexual reproduction requires millions of years of chance and unguided random processes. No amount of chance could have put Meiosis and sexual reproduction together simultaneously.

Further problems develop when Meiosis as a process is examined. Meiosis is an intricate, step-by-step process. Each step must be completed in the right order, much like following an instruction manual for assembling a lawn tractor, or the process will not work.  Consider our tractor example. If one tried to put on the wheels before putting on the axles, the wheels would have no place to attach and would be useless. Meiosis is the same way. Without the Meiotic spindle forming in both Prophase I and II, nothing would move the chromosomes or chromatids to opposite ends of the cell, making the process pointless. If cytokinesis took place at the end of Metaphase I and II instead of Telophase I and II two of the newly formed cells would have no genetic information. A random process could not produce the amount of ordered design apparent in Meiosis.

Creationists have no problem with Meiosis. We believe in a God powerful and wise enough to design Meiosis to function in the exact, complex, mesmerizing way that it does. While we do take this by faith, it takes far less faith to believe a complicated process such as Meiosis was designed by an all-mighty Designer, than it does to believe it just happened randomly over millions or billions of years.

 

 

 

 

 

 

Meiosis animation

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