Scientific Farm Animal Production
Quiz 12 :
Each species contain the characteristic number of the chromosomes occurring within the nucleus. These chromosomes carry the genetic code. Half of the genetic material from each parent is contributed to the offspring. Various species have different number of chromosomes for example, Turkeys contain 41 pairs, chickens contain 39 pairs, horses have 32 pairs, cattle and goats have 30 pairs, sheep contain 27 pairs and swine contains 19 pairs. Humans possess 23 pairs of chromosomes. Half of the chromosome from each parent passed on to the offspring and therefore, the offspring contains same number of chromosomes as that of parents.
The somatic cells replicate by a process known as mitosis in which, each parent cell divides to produce two identical daughter cells each carrying full chromosome number. Mitosis is accomplished in four stages called prophase, metaphase, anaphase and telophase. Prophase: The nucleolus is disappeared and the nuclear envelop is fragmented. The spindle fibers attached to the chromosomes at the centromere. The chromosomes are pulled to the poles. Metaphase: During this phase the chromosomes are arranged at spindle equator that is the midpoint between the spindle poles. The spindle fibers extend up to opposite poles and the unattached spindle fibers overlap behind the equator. Anaphase: During this phase, the sister chromatids separate apart to become the daughter chromosomes. The attached spindle fibers disassembled and at each pole a daughter chromosome is formed. The spindle fibers move apart so that the separation of chromosomes occurs. Telophase: During this phase, the spindle fibers disappear and the nuclear envelop is formed around each daughter chromosome. Each nucleus consists of same number and type of chromosomes similar to the parent cell. The formation of gametes (spermatozoa in males and ova in female) is accomplished by cell division process known as meiosis. Meiosis produces the daughter cells carrying half number of chromosomes from each parent. Meiosis occurs in 2 phases, meiosis I and meiosis II. In the first stage (prophase I) the diploid nuclei containing chromosomes from as tetrads and are condensed. The synapsis and crossing over takes place to exchange the fragments between the chromosomes. During metaphase I the tetrads are arranged at the spindle equator facing the opposite spindle poles. During anaphase I the tetrads are separated and resulting in the movement of dyads to the opposite poles. During telophase I the haploid daughter nuclei receive two chromosomes one from each homologous pair. During the meiosis II the chromosomes are condensed, the fragmentation of nuclear envelope takes place at prophase II stage. During metaphase II the sister chromatids or dyads are arranged at the spindle equator facing the opposite spindle poles. During anaphase II separation of sister chromatids occur and the daughter nuclei move to the poles. During telophase II, 4 haploid daughter nuclei are resulted and these are genetically different from the parent nuclei because of the crossing over event.
The formation of gametes from male and female reproductive structures occurs by a process known as gametogenesis. The male reproductive organs, the testicles produce gametes called sperms. The female reproductive organs, the ovaries produce the gametes called eggs or ova. Production of sperm is known as spermatogenesis and production of ova is known as oogenesis. Meiosis takes place in the gametes. During the process of spermatogenesis the primary spermatocyte possesses two pairs of chromosomes at synapses. Each chromosome is doubled after replication and the primary spermatocyte contains two bodies or structures formed by 4 chromosomes. By two successive cell divisions 4 cells of each containing two chromosomes are produced. The cytoplasm is lost from the spermatid cells producing a tail. Thus, the process of spermatogenesis results in the formation of the sperm. Four sperms are produced from a primary spermatocyte. The primordial germ cell contains four chromosomes in two pairs but each sperm receives two chromosomes from the germ cells. Therefore, the number of the chromosomes is reduced to half from primordial germ to sperm. The process of oogenesis involves the formation of primary oocyte of female containing a tetrad. During the first maturation division a large nutrient containing cell called secondary oocyte and a small first polar body are formed. Each of these cells contains a dyad. The second maturation division takes place producing an egg or ovum and second polar body. The ovum and second polar body possess two chromosomes in each. The first polar body may also undergo division but the polar bodies die quickly and are reabsorbed. The egg is haploid containing single chromosome of each pair that was present in the primordial germ cell.