Last week we did a brief overview of the asexual cell division process known as mitosis. Mitosis is the process that promotes skin and tissue growth and accounts for over 99% of the cell division processes that occur in the body. Today we will look at the cell division process known as meiosis.
Meiosis, unlike mitosis, is the process of cell division involved with sexual reproduction. Instead of producing two new identical daughter cells as in mitosis, meiosis produces four daughter cells containing half the number of chromosomes found in the parent cells. Consequently, meiosis is not a cycle process like mitosis.
Since plants and animals have different numbers of chromosomes and different modifications of sexual determination, we will use humans as our example for explaining meiosis.
Humans contain a total of 46 chromosomes that are paired up to form 23 pairs. This is known as diploid or 2N chromosomes. Meiosis separates the pairs of chromosomes resulting in four daughter cells contain only 23 unpaired chromosomes. This is known as haploid or N chromosomes.
As in mitosis, meiosis involves a pre-process known as Interphase. Interphase is further divided into 2 phases: Growth 1 (G1) and Synthesis (S). Meiosis is divided into two major phases: Meiosis 1 and Meiosis 2. Both Meiosis 1 and Meiosis 2 are further divided into four stages: Prophase 1 & 2, Metaphase 1 & 2, Anaphase 1 & 2 and Telophase1 & 2. Prophase 1 is further divided into 6 phases or steps: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis and Synchronous Processes.
Growth 1 (G1)
In this phase, the cell prepares for division by synthesizing extra proteins and enzymes necessary to carry out the division processes. Chromosomes are found as single DNA strands that are long and loose.
Chromosomes are replicated. Even though the cell now contains 2 diploid sets of chromosomes, the cell is still designated as being diploid because it contains the original number of centromeres.
Meiosis Phase 1
The product of this phase is two daughter cells each possessing only 1 set of haploid chromosomes.
The chromosomes coil and pair up. Then the nuclear membrane disintegrates. The chromosome pairs will exchange various fragments in a process known as crossing over or genetic recombination.
- Leptotene – (from Greek meaning ‘thin threads’) The chromosomes condense into strands that are visible under a strong light microscope. When looking though the microscope, each strand appears to be a single long strand as the two sister chromatids are so tightly bound together.
- Zygotene – (from Greek meaning ‘paired threads’) The chromosomes align with each other forming homologous chromosome pairs. The paired chromosomes are referred to as bivalent chromosomes.
- Pachytene – (from Greek meaning ‘thick threads’) This is the phase where the genetic crossing over occurs between non-sister chromatids resulting in a recombination of genetic information. At the point of exchange, a chiasmata forms which holds the two non-sister chromatids together.
- Diplotene – (from Greek meaning ‘two threads’) Homologous chromosomes slightly separate from each other but remain tightly bound at the chiasmata.
- Diakinesis – (from Greek meaning ‘moving through’) The chromosomes begin to condense and the four parts of the bivalent chromosomes now become visible under the microscope. The chiasmata become visible. The nucleoli disappear. The meiotic spindle starts to form as the nuclear membrane disintegrates.
- Synchronous Processes – Two centrosomes with their pair of centrioles move to the polar ends of the cell. The microtubule fibers extend from the centrosomes and attach to the chromosomes. The point of fiber attachment is known as the kinetochore. The kinetochores function as motors that pull the chromosomes towards centriole. Each bivalent has four kinetochores, but those of sister chromatids bond and act as a single unit.
The 4 chromatid fibers (2 from each chromosome) align along the equatorial plane, also known as the metaphase plate, in the center of the nuclear area. The spindle fibers attach to the centromeres. Then the chromatids are condensed.
In this stage the homologous chromosomes are separated and pulled towards the centrioles resulting in two haploid sets of chromosomes. Note that in the male, this results in the separation of the X and Y chromosomes. The cell’s shape becomes more elongated.
The chromosomes reach their respected poles and are loosened and uncoil once again becoming chromatin. The nuclear membrane reforms around the haploid chromosomes. The nucleoli reappear. The cell membrane pinches in, dividing the cell into two daughter cells. Due to the process of crossing over that took place in Prophase 1, the sister chromatids are not identical to each other.
Meiosis Phase 2
The second phase of meiosis may or may not follow immediately upon the completion of Telophase 1 as there may be a period of time before the second phase begins. Meiosis 2 closely resembles mitosis in that no further reduction of chromosomes occurs. The result of the second phase is the production of four haploid cells or gametes.
Prophase 2 – The chromatids start to condense, becoming shorter and thicker. The centrioles migrate to the polar ends of the cell and begin to arrange the spindle fibers. The nucleoli and nuclear membrane again disappear.
Metaphase 2 – The kinetochores from each centromere attach to the spindle fibers sent out from the centrosomes. The metaphase plate forms but it is aligned at a 90 degree angle compared to the metaphase plate in metaphase 1.
Anaphase 2 – The centromeres are separated, allowing the kinetochores to pull the sister chromatids towards opposite poles of the cell. Once separated, the sister chromatids are known as sister chromosomes.
Telophase 2 – The spindle fibers disappear as the nuclear membrane reforms. The chromosomes uncoil and lengthen once again becoming chromatin. The cell membrane once again pinches off and separates the daughter cells.
Before meiosis we started with a single diploid cell. After both phases of meiosis are completed, we now have four haploid daughter cells known as gametes. All four of the female gametes will contain an X chromosome while the two of male gametes will contain an X and two will contain a Y chromosome.
Please keep in mind that this is a very simplified explanation of a very complex process. However, it should be fairly obvious from this simplified explanation how complicated meiosis is. Now can you imagine that this process evolved simultaneously in two individuals? If sexual reproduction and meiosis evolved in one individual, it would have produced haploid gametes that would have had no corresponding gamete to join with to for a new individual.
The only feasible explanation for how sexual reproduction and meiosis came to exist is that it was purposely designed by an infinite Creator God who made the male and female from the very beginning. Praise be to Him!