Chromosomes and Cell Reproduction - Essay Example

Chromosomes and Cell Reproduction The term karyotype describes the number, size and centromere position/arm length ratio of the chromosome in a cell.Autosomes are the chromosomes found in all individuals, irrespective of sex. Metacentric chromosomes have a centomere near the center, and roughly equal arm lengths. Submetacentric chromosomes have noticeably unequal arm lengths. Acrocentric chromosomes have one long arm and one very short arm. Telocentric chromosomes have their centromere adjacent to one telomere. Telomeres are specialized sequences and associated proteins which protect the end of DNA from degradation, and interact with the nuclear envelope and the cytoskeleton during meiosis. The number of chromosomes in a single set, as found in a gamete, is called the haploid number and is denoted by n, so diploids have 2n chromosomes. The two sexes have different number of sex chromosomes, so males and females can have different diploid numbers. Karyotypes may be very stable through evolution. Heterozygosity for chromosomal rearrangements can prevent correct segregation of genes in meiosis. Recombination (chiasmata, cross-overs) between the normal and rearranged segments tends to produce unbalanced gametes with two copies or no copies (duplications or deletions) of some chromosome segments. These reduce fertility by failing to produce viable zygotes. This make sit difficult for the new arrangements to become established, and can act as post mating isolation mechanism between species by causing hybrid sterility. When chromosome polymorphisms persist a special mechanism is usually found that reduces the deleterious effects and maintains the polymorphisms. Rearrangements require at least two breaks and one rejoin in DNA / chromosomes. Rearrangements may occur within a chromosome, or involve exchange or transfer between chromosomes. Deletions involve loss of genetic material which is more or less deleterious, depending on the number and role of the genes which are lost. Acentric chromosomes segments (that is detached from a centromere) are lost. Duplications give an extra copy of a block of genes. They are generally less harmful than deletions, and one copy of the duplicated genes can become mutated and evolve a new role. This is the origin of all new genes through out most of the evolution. Strong selection for multiple copies of a gene usually detects cells with necessary multiple duplications, often visible as a heterogenous staining region on the chromosome. Two telocentric or acrocentric chromosomes fuse at the centromere to produce a single metacentric chromosome or submetacentric. This is called as centric fusion or Robertsonian translocation. The reverse is centric fission that is splitting of the centromere in a metacentric to give two telocentrics. In this way the chromosome can change. Correct meiotic segregation in a heterozygous cell requires that the two telocentrics segregate to one pole, and the metacentric goes to another pole. Translocations involve exchange of distal regions of non-homologous (genetically different) chromosomes. It is necessary that either both rearranged or both original chromosomes occur in the same gamete to ensure genetic balance. In meiosis, normal chromosomes and chromosomes with translocations pair up alternately to form a ring or chain of several chromosomes. If two adjacent chromosomes segregate on the same pole, genetically unbalanced gametes will be produced, but if alternate chromosomes go together the gametes will contain a complete balanced genome. Some species are heterozygous for many translocations, and form long chains or rings of chromosomes in meiosis. Inversions turn part of the chromosome around, reversing its polarity. A pericentric inversion includes the centromere, A paracentric inversion is contained in one arm of the chromosome and does not move the centromere. A chromosome with an inversion can pair with a normal chromosome in meiosis in a heterozygote if they form an inversion loop. Meiotic recombination within a n inversion loop duplicates one end of each chromatid and deletes the other end in a reciprocal manner. These recombined chromatids are therefore not viable, and do not reach the next generation. Paracentric inversions do not include a centromere. Meiotic recombination in a paracentric inversion loop generates a dicentric chromatid and acentric fragment. Pericentric inversions invert the centromeric region, and the effects of meiotic recombination in the loop duplicating and deleting the ends of the chromatid cannot be avoided because all the chromatids have one centromere. Many zygotes will have unbalanced genomes. Y chromosomes have few genes and are lost in some species. Fusion of an X chromosome with an autosome causes a copy of the free autosome to become a neo-Ychromosome, only found in males which have another copy attached to their single X. Fusion of a Y-chromosome with an autosome causes the unattached autosome to become neo-X chromosome, one free copy being in males, two copies in females as before. Polymorphisms for paracentric inversions are common in dipterans where they may contain sets of alleles coadapted to control sex, or adapted to particular ecological environments. The suppression of recombination protects the cluster of genes and facilitates their coevolution. Chromosomal change is common in evolution, and can cause hybrid sterility between closely related species, forming a post mating isolation mechanism (Winter et al., 221) Cellular reproduction is a cyclic process in which daughter cells are produced through nuclear division (mitosis) and cellular division (cytokinesis). Mitosis and cytokinesis are part of growth – division cycle called the cell cycle. Mitosis, which is divided into four stages, is relatively small part of the total cell cycle; the majority of the time, a cell is in growth stage called interphase. Interphase is divided into three parts; G1, S, and G2 . The first gap phase, G1, follows mitosis and is a period of growth and metabolic activity. The S phase follows G1 and is a period of DNA synthesis, in which the DNA is replicated. Another gap phase, G2 follows DNA synthesis and preced the next mitotic division. Certain mature cell types do not continue to divide but rmain in the interphase (in Go). Mitosis is the period in the cell cycle during which nucleus divides and gives rise to two daughter cells that have chromosome numbers identical to that of the original cell. Mitosis can be divided into four distinct stages. (prophase, metaphase, anaphase, and telophase) characterized by the appearance and orientation of the homologous pairs of chromosomes. The process of meiosis involves two cell divisions that result in the daughter cells (gametes) with half the number of chromosomes. If the number of chromosomes were not reduced prior to fertilization, the chromosome number would double in each generation. Meiosis allows the exchange of genetic material on homologous chromosomes through recombination, which occurs at the end of prophase I. recombination results in new combinations of alleles of different genes on a chromosome. Meiosis, like mitosis can be divided into distinct phases. Meiosis I includes a multipart prophase I, ( during which crossing over occurs), metaphase I, anaphase I (unlike in mitosis homologous pairs of chromosomes, not sister chromatids, separate to the opposite poles of the cell), and telophase I. Meiosis II begins with prophase II, continues onto metaphase II ( the centromeres attach to the spindle apparatus), anaphase II (as in mitosis, The centromeres separate and the chromosomes begin to migrate to opposite poles), and telophase II, which completes completes meiosis, resulting in four daughter cells with half the number of the chromosomes as the parental cell (Weaver, 6) Dissertation Topic Changes in chromosomal organization of resting lymphocytes under long standing exposure to lead in workers engaged into lead production [My paper] Complex of occupational factors characteristic for lead production induces cytogenetic effect variable in intensity, assessed through chromosomal aberrations test inresting lymphocytes. Occurrence of chromosomal aberrations in lead production workers appeared to exceed spontaneous level. We propose to look for effect of lead exposure on chromosomal organization of resting lymphocytes in lead workers engaged in lead production. REFERENCES Winter PC, GI Hickey, and HL Fletcher. Instant Notes in Genetics. 2nd ed. Oxford UK: BIOS Scientific Publishers Ltd, 2003. Weaver, Robert F. Molecular Biology. 3rd Ed. McGraw Hill International, 2005. Read More