II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

Reproduction is one of the main attributes of living organisms and their constituent cells. There is an intricate mechanism by which the genetic material in the nucleus is first copied and then partitioned so that each of the two daughter nuclei gets one complete copy of the genetic information. This mechanism is called mitosis in eukaryotes. The diversity of organisms is possible partly due to another mechanism for nuclear division, referred to as meiosis. Meiosis produces four daughter nuclei, each with only half the genetic information contained in the original cell, and each differing from the others with respect to the exact information contained.

Mitosis is vital for growth; for repair and replacement of damaged or worn out cells; and for asexual reproduction. The life cycle of eukaryotic cells is a continuous process typically divided into the following phases: interphaseand mitosis,which includeskaryokinesisandcytokinesis.Interphase includes three stages, referred to as G1, S and G2. In G1, a newly formed cell synthesizes materials needed for cell growth. In the S stage, deoxyribonucleic acid (DNA) is replicated. At this stage, DNA consists of long, thin strands called chromatin. When the S stage is complete, the cell enters a brief stage known as G2, when specialized enzymes correct any errors in the newly synthesized DNA, and proteins involved with the next phase, mitosis, are synthesized.

Karyokinesis occurs in four steps. In prophase the replicated, linked DNA strands slowly wrap around proteins that in turn coil and condense into two short, thick, rodlike structures called chromatids, attached by the centromere. Two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite poles of the cell. As the centrioles move apart, they begin to radiate thin, hollow, proteins called microtubules. The microtubules arrange themselves in the shape of a spindle. As the spindle forms, the nuclear membrane breaks down into tiny sacs or vesicles that are dispersed in the cytoplasm. Final disintegration of the membrane marks the beginning of metaphase.

In metaphase, the spindle fibers attach to the chromatids near the centromeres, and tug and push the chromatids so that they line up in the equatorial plane of the cell halfway between the poles. One chromatid faces one pole of the cell, and its linked partner faces the opposite pole. Anaphase begins when the centromeres split, separating the identical chromatids into single chromosomes, which then move along the spindle fibers to opposite poles of the cell. As these two identical groups of single chromosomes gather at opposite poles of the cell, telophase begins. A new nuclear membrane forms around each new group of chromosomes. The spindle fibers break down and the newly formed chromosomes begin to unwind. If viewed under a light microscope, the chromosomes appear to fade away. They exist, however, in the form of chromatin, the extended, thin strands of DNA too fine to be seen except with electron microscopes. Mitosis accomplishes replication and division of the nucleus, but the cell has yet to divide.

The final phase of the cell cycle is known as cytokinesis. It can begin in anaphase and finish in telophase; or it can follow telophase. In cytokinesis, the cell’s cytoplasm separates in half, with each half containing one nucleus. Animals and plants accomplish cytokinesis in slightly different ways. In animals, the cell membrane pinches in, creating a cleavage furrow, until the mother cell is pinched in half. In plants, cellulose and other materials that make up the cell wall are transported to the midline of the cell and a new cell wall is constructed. The new cells enter interphase, and the cell cycle begins again.

Meiosis is a process of cell division in which the cell’s genetic information, contained in chromosomes, is recombined and divided into sex cells with half the normal number of chromosomes, known as the haploid number. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance. Meiosis involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division.

To illustrate the steps of meiosis, consider a corn plant cell with 10 pairs of chromosomes, so the diploid number of chromosomes is 20. In order for the diploid corn cell to reproduce, it must undergo meiosis to produce cells with half the normal number of chromosomes. Each haploid corn cell contains only 10 chromosomes.

Each of the two consecutive cell divisions consists of four stages: prophase, metaphase, anaphase, and telophase. In prophase I each long DNA strand forms a chromosome. Since the DNA was copied during interphase, each chromosome condenses to form two identical chromatids, joined at a centromere. A corn cell has 20 chromosomes at this stage, each with two identical chromatids, making a total of 40 chromatids.

Chromosomes exist in two pairs. These pairs of chromatids gather together in groups of four called tetrads. Each corn cell contains 10 tetrads. While grouped together in tetrads, sections of the chromatids from different chromosomes exchange, or cross over. Called genetic recombination, this process is the first of two ways that meiosis mixes genetic information during sexual reproduction. Also in prophase I, two structures called centrioles separate and move toward opposite sides of the cell and the membrane around the nucleus of the cell breaks down. During metaphase I, the spindle fibers move the tetrads so that they line up in a plane halfway between two centrioles. Anaphase I begins when the spindle fibers pull the tetrads apart, pulling the chromosomes from each pair toward opposite sides of the cell. The first meiotic division concludes with telophase I, when the two new groups of chromosomes reach opposite sides of the cell. A nuclear membrane may form around the two new groups of chromosomes and a division of cell cytoplasm forms two new daughter cells.

Each daughter corn cell receives 10 chromosomes made up of a random mixture of maternal and paternal chromosomes. This second mixing of genetic information is called independent assortment. Genetic recombination and independent assortment make it possible for parents to have many offspring who are all different from each other.

In the second meiotic division the cell moves directly into prophase II, skipping the interphase replication of DNA. Each corn cell begins the second division with 10 chromosomes. Once again the centrioles radiate spindle fibers as they move to opposite sides of the cell. During metaphase II, the chromosomes line up along the plane in the center of the cell, and in anaphase II the pairs of chromatids are pulled apart, each moving toward opposite ends of the cell. Telophase II completes meiosis.

The original diploid corn cell with 20 chromosomes has undergone meiosis to form four haploid daughter cells, each containing 10 chromatids. It is now possible for two haploid sex cells to join during fertilization to form one egg cell with the normal diploid number of chromatids. After fusion and DNA replication, two haploid corn cells will yield one diploid zygote with 10 pairs of chromosomes.

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