The Inheritance of Abo Blood Type - Essay Example

?The inheritance of ABO blood type ABO blood type characterizes red blood cell membrane glycoproteins. ABO blood typing is important in cases of transfusion, as incompatibility may cause fatal immune reactions. Phenotypes can either be A, B, AB, or O. It is determined by the combination of multiple alleles, IA, IB and i, which produces six combinations of genotypes: IAIA and IAi, which produces phenotype A, IBIB and IBi, which produces phenotype B, IAIB, which produces phenotype AB, and ii, which produces phenotype O. As can be inferred, its inheritance is ruled by Mendelian codominance, wherein IA and IB are codominant alleles, while i is the recessive allele (Campbell and Reece, 2002). Thus, if a parent with a blood type A or B has an I-i, then it is possible that his or her child can have blood type O, especially when the other parent has an I-i or ii genotype. Production of gametes through meiosis A basic tenet of embryology is the fusion of a male and a female gamete in the process called fertilization. The gametes are haploid cells that are produced in the testes and ovaries of the father and mother, respectively. They are produced in a cell division process called meiosis. This process is special because each of the resulting daughter cells contains just half of the chromosomes of the parent cell. In effect, a child receives 50% of each of the parent’s genetic material. Briefly, it involves two major processes, Meiosis I and then II, each composed of the usual processes of (1) prophase that prepares for (2) metaphase, in which the chromosomes meet in the middle, (3) anaphase, whereby the chromosomes separate and go to two opposite ends of the dividing cell, and (4) telophase, in which the cell membranes separate to ultimately produce the daughter cells. Initially, the gonium is a diploid cell (chromosome number (n) = 46) with replicated chromosomes in the form of sister chromatids. After Meiosis I, the daughter cells are already haploid (n = 23), because what is separated during Anaphase I are the pairings of homologous chromosomes. Each chromosome carries genes for specific traits, and each chromosome of a homologous pair thus carries an allele per trait. In contrast, during Anaphase II, the sister chromatids separate, producing haploid cells with unreplicated chromosomes. Ideally, 4 daughter cells are produced per meiosis of a –gonium, and this is what happens in the production of sperm cells. However, in the case of female gamete formation, 2 daughter cells (1 from meiosis I and 1 from meiosis II), only 1 oocyte is produced from a cycle of meiosis (Campbell and Reece, 2002). DNA replication occurs in preparation for cell division How does DNA replicate? A part of the double-stranded DNA (dsDNA) unwinds, allowing DNA polymerase and DNA ligase to get into what is known as the replication bubble. The DNA polymerase adds the complement of each nucleotide in the parent strand, completing the whole length of the strands to produce two semi-conservative dsDNA, each composed of a parent strand and a daughter strand. Eventually, these two identical dsDNA, takes the form of sister chromatids, which are separated during mitosis, the somatic cell replication. The resulting daughter cells thus get identical copies of DNA, which is an exact match to the parent cell (Campbell and Reece, 2002). DNA is the genetic code that directs all cellular function Why is it necessary for each cell to bear DNA? The DNA, housed in the nucleus, is the template to produce messenger RNA (mRNA) through a process called transcription. In this process, a transcription factor recognizes the TATA sequence 25 nucleotides upstream from the transcriptional start point. This initiates the binding of RNA polymerase II to the DNA, and binding of additional transcriptional factors, opening up the double strand to produce the pre-RNA strand from 5’ to 3’. The pre-mRNA then peels off from the DNA template, and it complete detaches hundreds of nucleotides after reaching the terminating AAUAAA sequence. This undergoes further processing, which adds a guanosine triphosphate cap on the 5’ end and a poly(A) tail on the 3’ end. These help prevent mRNA degradation, and the poly(A) specifically promotes the transport of mRNA from the nucleus. Upon reaching the cytoplasm, it attaches with ribosome. Briefly, a ribosome has an mRNA binding site and three binding sites for the amino acid-delivering, transport RNAs (tRNA). Briefly, the A site, or aminoacyl-tRNA binding site, initiates the interaction between the amino acid containing-tRNA and the corresponding codon, a three nucleotide sequence in the mRNA. The ribosome then initiates the formation of a peptide bond between the amino acid in the A site and the carboxyl end of a growing polypeptide still attached to tRNA in the (2) peptidyl-tRNA binding site (P site). Translocation then ensues, with the now empty tRNA from the P site to the exit site (E site), and the polypeptide-tRNA complex from the A site to the P site. The cycle continues, until a stop codon, UAG, UAA or UGA reaches the A site, and a release factor initiates the release of the polypeptide by hydrolyzing the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. Upon proper folding, and, possibly, association with other polypeptides, this polypeptide chain becomes a protein. In turn, proteins can play various roles for the cell. It can be catalyzing enzymes, it can be a receptor, an antibody, a structural components, transport molecules. It can also be a characterizing feature of a cell. In the case of RBCs, the specific ABO protein it contains, as encoded by the corresponding alleles in the specific chromosome (Campbell and Reece, 2002). With these enormous effects of DNA on the cell, any form of disintegration has serious repercussions. Deletions, insertions and thymine dimerization are just some of the insults DNA can incur. There are DNA repair mechanisms to prevent such occurrences. However, in the event that the DNA damage is great enough for these mechanisms to repair, it can cause the absence of the proteins which the damaged DNA codes for. As well, the damaged protein from a damaged DNA can have an entirely different function (Campbell and Reece, 2002). The Control of Gene Expression However, humans have different types of cell, despite all of them having the same set of genetic material. Why is this so? Although a cell contains millions of genes through its DNA, only a small amount of it is expressed. First, differential chromatin modifications, for example DNA unpacking of only certain parts of the chromatin, control the genes that will be open to transcription. In contrast, DNA methylation inhibits the genes from being expressed. The abovementioned factors that control transcription limits the expression of genetic material to be limited. As well, posttranscriptional processes can also inhibit the production of protein from mRNA (Campbell and Reece, 2002). References Campbell, N. A. and Reece, J. B., 2002. Biology. 6th ed. San Francisco: Pearson. Read More