Meiosis in Life Cycles: Tutorial

Telophase 1 & cytokinesis

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Summary

All organisms that reproduce sexually divide by a unique process called meiosis. During meiosis, the number of chromosomes in the daughter cells is half of that of the parent cell. At the time of fertilization, the total number of chromosomes is restored. Gametogenesis is the process by which gametes are produced. In males, this process is spermatogenesis, and it results in four equally sized haploid cells or gametes, called sperm. In females, this process is oogenesis, and it results in one large ovum and three polar bodies. Sexual reproduction results in genetic variation due to three mechanisms: independent assortment of chromosomes, crossing over, and random fertilization. Together, these three mechanisms not only make you different from your parents, but also from your siblings. Indeed, when you are born, you are unique!

Interphase 1`

CHromosomes Duplicate

Interphase

Cells dividing by mitosis or meiosis begin by duplicating their chromosomes. A cell with four chromosomes duplicates so that each chromosome now has an identical copy called a sister chromatid.

Chromatin

Chromatin is a mass of genetic material composed of DNA and proteins that condenses to form chromosomes. During interphase, the chromatin replicates but does not condense. Each replicated chromosome has two genetically identical strands, called sister chromatids, which are attached at the centromere.

Cleavage Furrow

Cytokinesis involves a division of cytoplasm, resulting in the separation of two haploid daughter cells. In animal cells, the cell membrane constricts to form a cleavage furrow, which moves inward dividing the cell in two. The process is different in plant and animal cells. In plant cells, a new cell wall forms between the two new cells, which ultimately separates the two cells.

Sister chromatids

During anaphase I, the kinetochore microtubules (spindle fibers) begin to shorten. As a result, the attached sister chromatids—no longer identical (due to crossing over), but still joined at the centromere—are pulled toward the poles as a single unit.

Metaphase Plate

During metaphase I, homologous chromosomes, now in the form of tetrads, arrange themselves on an imaginary plane, which is at right angles to the spindle. It is called the metaphase plate, where one chromosome of each pair faces either of the two poles.

Independent Assortment

During metaphase I, tetrads align themselves along the metaphase plate. Each tetrad has one chromosome inherited from the mother, called the maternal chromosome, and the other from the father, called the paternal chromosome. In each tetrad, this orientation of the maternal and paternal chromosome toward either of the poles is completely random. As tetrads arrange themselves along the plane independent of other pairs, a daughter cell can receive any combination of maternal and paternal chromosomes. This is known as independent assortment. The number of possible combinations in a daughter cell will increase with the number of chromosomes. For example, in a cell with two pairs of homologous chromosomes, the number of possible combinations is four. For humans, who have 23 pairs, the number of combinations would be 223, or about eight million! Each gamete you produced had one of eight million probable combinations of chromosomes inherited from your parents!

Microtubes

During metaphase I, the spindle fibers attach to each sister chromatid at the kinetochore. The tension caused by the spindle fibers aligns the tetrads at the metaphase plate.

Chiasmata

During prophase I, chromatin condenses to form chromosomes. Homologous chromosomes, or homologues, pair up so that genes on one homologue line up with corresponding genes on the other homologue. This pairing is known as synapsis. Then, the nonsister chromatids connect and trade sections of genes in a process called crossing over. This process is vital to ensure genetic variation in daughter cells. The points at which the four chromatids connect are known as chiasmata.

Tetrad

During the end of prophase I, the homologous chromosomes align in pairs. Each chromosome is composed of two sister chromatids, so the pair of chromosomes is a four-part structure known as a tetrad.

Random Fertilization

Fertilization, or the union of gametes, also adds to the genetic variation. For fertilization to occur, an ovum can fuse with any of the millions of sperm present at the time of mating. Since every ovum and sperm already have a unique combination of traits, fertilization increases this uniqueness further! For example, in humans each male and female gamete can have one out of eight million chromosomal combinations possible due to independent assortment. A fusion of the two gametes will produce a zygote that can have any one out of the 64 trillion diploid combinations possible! If you consider the variations due to crossing over, the estimated variation becomes enormous. This random nature of sexual reproduction explains why siblings can be so different from each other. See how truly unique you are!

Gametogenesis

Gamete in Greek means to marry and genesis means to produce, so gametogenesis literally means marry to produce. In organisms that reproduce sexually, gametogenesis is a process in which germinal cells produce mature gametes. This process differs in males and females. In males, it's called spermatogenesis, where one germinal cell produces four equal-sized haploid sperm. In females, it's called oogenesis, where one haploid cell receives most of the cytoplasm and matures to be an ovum. The three other cells receive very little cytoplasm and mature to become polar bodies. This uneven division has a useful purpose. Upon fertilization, the ovum would require more cytoplasm for the development of the zygote. The three polar bodies usually do not participate in reproduction. Note that the male and female gametes do not undergo any further division until fertilization or the union of the gametes occurs.

Gametes

Gametes—sex cells—form by meiosis, which divides the number of chromosomes in the gametes in half. These chromosomes are represented as n and are called haploid. The chromosomes in the gametes have unique genetic information from each parent. Separation and distribution of the members of these pairs of chromosomes (along with fertilization) contribute to genetic diversity. Remember that a human nerve cell (a somatic cell) is diploid with 46 chromosomes. A gamete such as a sperm cell is haploid with 23 chromosomes. When fertilization occurs and two gametes join to become one, the diploid number of chromosomes is restored. Some terms you should know while studying meiosis are crossing over and tetrad formation. Crossing over is a process wherein sections of the pairing homologous chromosomes interchange with each other. When two pairs of sister chromatids line up on the equatorial plane during meiosis, it's called a tetrad. Explore the process of meiosis in a cell with a single pair of chromosomes.

Propphase 1

Homologous chromosomes pair and exchange chromosomes

Anaphase 1

Homologous chromosomes split up

Meiosis

Most of the genetic variation in your family and in the world is the result of what happens during this process of cellular division. Remember, the types of cells found in an organism that reproduces sexually are gametes (or germ cells), which are the sex cells, and somatic cells, which are the nonsex cells. These cells divide by different processes. Gametes and somatic cells also differ in the number of chromosomes found in their nucleus. Somatic cells are produced through the process of mitosis. During mitosis, a single parent cell produces two daughter cells genetically identical to the parent. Because they have two pairs of chromosomes, the cells are called diploid. The number of chromosomes in a diploid cell is represented as 2n. One example of a somatic cell created by mitosis is the human neuron or nerve cell. This is a diploid cell that has 46 chromosomes.

Genetic Variation

One of the key features of sexual reproduction is that the offspring, even after receiving characteristics from their parents, are not identical to either parent. This variation is a result of the behavior of chromosomes during meiosis and fertilization. Three mechanisms contribute to the reshuffling of genes in sexual reproduction: independent assortment of chromosomes crossing over random fertilization

An organism has 12 chromosomes in its germinal cells. During spermatogenesis, which of the following cells is the first to have 6 chromosomes?

Secondary spermatocytes are haploid and they give rise to spermatids.

Metaphase 1

Tetrads line up

Centrosomes

The centrosome contains a pair of centrioles. During interphase, it replicates along with its centriole pair, forming two centrosomes. Later, these two centrosomes move to opposite ends of the nucleus and form a spindle network, which is responsible for separating replicated chromosomes into the two daughter cells.

Daughter Cell

The cytokinesis and telophase I stages generally occur simultaneously. In the beginning of telophase I, each half of the cell has a complete haploid set of chromosomes, though each still has a pair of sister chromatids. During cytokinesis, the cytoplasm separates and two haploid daughter cells form.

Crossing Over

The process of crossing over takes place in prophase I of meiosis. During this phase, the homologous chromosomes lie side by side so that genes from one homologue face their corresponding genes on the other homologue. Then the sister chromatids exchange genetic material with each other to produce a new combination. This event is called crossing over. Check the illustration to see how it happens. The chromatids with the recombined genetic material are called recombinant chromatids. During metaphase II, the sister chromatids, which are no longer identical, are separated, adding to the variation. Together with independent assortment, crossing over increases the genetic variation in daughter cells. It ensures that a particular chromosome in a gamete is not identical to the maternal or paternal chromosome, but a combination of the two. In humans, each chromosome pair undergoes one to three crossover events. This varies depending upon the size of the chromosome and the position of its centromere.

Spermatogenesis

The spermatogenesis process is specific to males. Spermatozoa, or sperm, are produced in the testes, one of the male reproductive organs. In mammals, sperm production begins with sexual maturity. As the illustration on this screen shows, spermatogenesis has three phases: *Phase 1*: is a mitotic phase. The spermatogonia, or germinal cells, undergo mitosis to form diploid cells called primary spermatocytes. *Phase 2*: is a meiotic phase. With the onset of puberty, primary spermatocytes enter meiosis I to form two secondary spermatocytes, which are haploid. Secondary spermatocytes enter meiosis II giving four haploid cells called spermatids. *Phase 3*: The spermatids mature into sperm that can move spontaneously. During a successful mating, this ability enables sperm to move toward the ovum and fertilize it.

Spindle

The spindle apparatus consists of the centrosome with centrioles, spindle fibers, and related proteins. A spindle fiber, also known as a spindle microtubule, is made up of the protein tubulin. During prophase I, the two centrosomes move toward the opposite poles of the cells. By the end of this stage, the two centrosomes are parked at the opposite ends of the cell with a spindle network extended between them. The spindle tapers toward the poles and is spread out in the center.

Meiosis I

This phase includes metaphase I and anaphase I. During metaphase I, the duplicated homologous pairs of chromosomes form a structure called a tetrad. During anaphase I, the duplicated (and identical) pairs of chromosomes (tetrads) separate and segregate into two cells. After meiosis I, the cells have half the number of chromosomes as the parent cell. The two cells are genetically different.

Meiosis II

This phase separates these identical pairs into two cells producing cells that are true haploids, represented as n. Think of meiosis as reduction division. Meiosis II is nearly the same as mitosis.

Oogenesis

This process is specific to females. The female germinal cells develop into mature ova, or eggs, in ovaries, one of the female reproductive organs. Oogenesis usually begins in a female body while in the womb. The germinal cells divide mitotically to produce a large number of diploid cells. Oogenesis then enters meiosis I. The process is arrested at prophase I. The diploid cells in this stage are called primary oocytes. The primary oocytes remain inactive until puberty. At the beginning of puberty, specific hormones stimulate oocytes to complete meiosis I and enter meiosis II. Meiosis is again halted at metaphase II. The resulting haploid secondary oocyte is released from the ovary by the process called ovulation. The remaining stages of meiosis II take place only after fusion with a sperm. In oogenesis, cytokinesis is unequal because there is a single mature ovum and three smaller cells called polar bodies.