Chromosomes in Meiosis
CHROMOSOMES IN MEIOSIS
During the study of the cell, you learned that new cells are produced in two ways. The first way is mitosis, which provides two exact copies of the parent cell. Mitosis is important for growth, replacement, and repair. The second type of cell division is for the production of gametes or sex cells. Meiosis takes place in the gonads (sex organs) of the organism.
Here are your goals for this lesson:
- Identify and describe the stages of meiosis
- Distinguish the differences between spermatogenesis and oogenesis
- Explain how crossing-over occurs
Vocabulary
/ Anaphase I / The stage on the first meiotic division when the homologous chromosomes move to opposite poles but the sister chromatids remain together./ Anaphase II / The stage in the second meiotic division where sister chromatids of a chromosome split and migrate to opposite poles.
/ centromere / A structure on the chromosome that holds a pair of chromatids together during replication.
/ chromatid / A double-stranded chromosome following replication attached by a centromere.
/ crossing over / A condition where non-sister chromatid of homologous chromosomes exchange genes.
/ Metaphase I / The stage in the first meiotic division where the homologous chromosomes line up as a pair, forming a tetrad of chromatids, at the equator of the cell.
/ Metaphase II / The stage in the second meiotic division where the chromatid pair lines up at the equator of the cell..
/ oogenesis / Process of development of haploid female gametes.
/ polar body / One of three small cells produced during oogenesis, each contain the haploid number of chromosomes.
/ Prophase I / The stage in the first meiotic division where the doubled chromosomes become visible and nuclear membrane disappears.
/ spermatogenesis / The production of haploid male gametes by the male parent.
/ spindle fibers / A number of threadlike filaments formed between poles of the cell during cell division to which the centromeres of chromosomes attach; used to pull chromosomes apart and segregate them during anaphase.
/ Telophase I / The stage of the first meiotic division where the cell divides into two cells each containing one member of each pair of homologous chromosomes.
/ Telophase II / The stage of the second meiotic division when the cell divides into two cells with each being haploid; these cells become the gametes.
/ tetrad / During metaphase I the two pairs of chromatids of the homologous pair of chromosomes comes together; crossing-over can occur at this time.
Meiotic process. The process of meiosis involves two nuclear divisions. Preceding the first division, the DNA in the nucleus doubles by producing copies of itself. As the cell begins division, the chromatin (genetic material) condenses and twists into visible chromosomes. The DNA, which is now duplicated appears as a doubled chromosome attached in the midsection. Each part of the pair is called a chromatid and the structure that attaches the two chromatids is called a centromere. The period of time in which the chromosome is becoming visible is called Prophase I.
The two members of a pair of chromosomes (each having two chromatids) next come together to form a tetrad. Each of the pairs of chromosomes forms tetrads in the midline of the cell with the centromeres being attached to spindle fibers. This phase is called Metaphase I.
During Anaphase I the homologous chromosomes separate and migrate to opposite sides of the cell. The centromere is still holding the chromatids together. The migration appears to be by means of the spindle fibers contracting and pulling on the attached centromere and chromatids. Each of the groups of tetrads separates in this manner. During Telophase I of Meiosis I, the cell divides in half forming two cells. Both cells have a member of each pair of chromosomes still being doubled as two chromatids.
Meiosis continues rapidly and proceeds directly into Metaphase II without the duplication of DNA as at the beginning of Meiosis I. During metaphase the chromatids line up in the midplane of the cell and attach to spindle fibers.
Anaphase II is initiated by the pulling of the spindle fibers and separation of the chromatids by splitting the centromere. The chromatids move to opposite sides of the cell.
Telophase II pinches the cell into two cells, each with half the number of chromosomes as the parent cell. The result of meiosis is that one cell produces four cells, each with half of the amount of genetic material in the original parent cell.
Spermatogenesis. The basic pattern of reducing the chromosome number occurs in both plant and animal cells and in both male and female. The major difference is the timing and exact process. During spermatogenesis, male gamete (sperm) formation, four haploid (one member of each pair of chromosomes) sperm cells of equal size are produced from the original cell that begins the meiotic process.
Oogenesis. In some organisms the female, during ovum production (oogenesis) at Telophase I, has the meiotic division occur at the end of the cell rather than the middle. This unequal division results in an unequal distribution of the cytoplasm of the cell. One cell is large and the other is small; however, both cells have equal numbers of chromosomes. The small cell is called the polar body. In some species meiosis does not continue until after fertilization. However, in other species both divisions precede fertilization. Meiosis II continues in both of the cells resulting in three polar bodies and a single large haploid ovum. In the event of fertilization, the polar bodies will not produce offspring because of the limited amounts of nutrients stored in the small amount of cytoplasm. The ovum contains the major portion of cytoplasm and stored nutrients that will provide the nourishment for the new offspring during the beginning stages of development.
Crossing-over. Remember that the separation of a pair of homologous chromosomes is independent of any other pair of homologous chromosomes. When pairs of chromosomes line up on the spindle during Metaphase I, they pay no attention to any other chromosome pair. They line up purely by chance. During Metaphase I when the two pairs of chromatids are together in the tetrad, another vastly important event can occur. The chromatid strands do not remain as quiet, straight threads; but they wrap around each other and twist actively. Occasionally, during this wrapping process the chromatids break and fuse on the opposite chromatid. This rejoining process may have the head of one chromatid fused with the tail end of the other chromatid. In effect, chromatids have exchanged parts. This exchange results in new combinations of genes present on the chromosome. The process where chromatids exchange parts is called crossing over.
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