Essay 2 Flashcards
Meiosis is a cause of genetic diversity in organisms,
because it is a form of reduction division; the cell produces 4 genetically distinct haploid daughter cells. One way in which mitosis causes genetic variation is in prophase I, where the homologous chromosomes coil and condense, joining at multiple points to form a bivalent. The non-sister chromatids join at the chiasmata, and this is the site of crossing over (where the chromatids exchange segments of genetic information). This causes production of new combinations of alleles on the chromatids; genetic recombination, which causes genetic variation. In the next stage, metaphase I, independent assortment also causes genetic variation. The order of chromosomes lined up at the equator of the cell is random, therefore so too is the combination of maternal and paternal chromosomes that each daughter cell receives, increasing the genetic variation by a factor of 2.
[Meiosis] is important because
each gamete is genetically varied, and since fertilisation requires the random fusion of two gametes, the probability of genetic variation is doubled. This maintains genetic diversity in a population, so it is less at risk to extinction, because some of the population is likely to carry alleles that allow the organisms to adapt to environmental catastrophe.
Further genetic variation is inadvertently caused
due to mutations that can occur in the DNA replication of the S phase of interphase, before meiosis. Such mutations could be point mutations (substitution with a different nucleotide or inversion of a portion of the nucleotide sequence; missense) or frameshift (through addition, deletion or duplication of a nucleotide in the sequence), that changes how the rest of the downsttream sequence is coded into amino acids. An example of a mutation that causes genetic variation is the substitution of a nucleotide found on the haemogloin-beta gene on chromosome 11, that results in sickle-cell anaemia.
[Mutations are] important because they allow
the process of natural selection by creating a directional selection pressure. In populations with low malarial-stress, the HbSHbS genotype has nearly been eliminated due to the reduction in oxygen transport and respiratory issues that follow. However, in areas of high malarial stress, Apicomplexa Plasmodium protist merozoites are prevented from invading sickled erythrocytes, preventing malarial infection. As such, the allele becomes advantageous and increases likelihood of survival to reproduction to be passed on, therefore retaining a stable allelic frequency in the gene pool. As such, mutations result in the genetic variation that facilitates evolution – it is extremely important in organisms.
Allelic frequency is also dependent on the inheritance
of different alleles, and inheritance is also a cause of variation in organisms. Since there are only two homologous chromosomes (maternal and paternal), there are only two loci available for an allele, so only two can present in an organism at any one time. This means that characteristics controlled by multiple alleles, such as human blood types, can exhibit increased variation in populations. Human blood types are controlled by multiple alleles of the immunoglobin gene (IA, IB, IO) that code for antigens A, B and neither on the cell-surface membrane of erythrocytes, respectively. Alles IA and IB are codominant, and IO is recessive to both. This causes genetic variation because there are several possible genotypes: IAIA, IAIB, IAIO, IBIO, IBIB, IOIO.
[Inheritance] is important in organisms
because the genotype increases pathogenic susceptibility – those with the genotype IAIA are more susceptible to Variola major infection and IOIO are more susceptible to Yersinia pestis, thus creating directional selection pressures for Asian countries experiences epidemics, and further facilitating evolution.
To survive these epidemics,
a comprehensive immune response is essential. This is achieved through the variability of antibodies, unique globular proteins produced by B cells that react with specific antigens that are complementary to the 3D tertiary of their variable regions. Antibodies are composed of 4 polypeptide chains, 2 heavy and 2 light, held together by disulphide bridges that form the hinge region, making a quaternary structure. Antibody variation is caused by their being proteinous, due to the infinite possibilities for primary structure combinations caused by the 20 naturally occurring amino acids. This results in there being different amino acid sequences in each of their variable regions (as opposed to the sequences that are the same in the constant region).
[Antibody variety] is important because
the specificity of the variable region allows for the formation of antigen-antibody complexes when the B cell encounters a pathogen. This allows the antibodies to induce opsonisation (stimulating phagocytes by making the antigen more recognisable), agglutination (binding the pathogens together for more efficient phagocytic recognition) and neutralisation (binding to pathogenic toxins to neutralise them). As such, pathogenic infection can be overcome.; it is very important for organisms.
While immunity is essential in the survival of an organism,
colonisation, and the following stages of succession, are essential in the survival of a dynamic ecosystem. The stages of succession are also an inherent cause of increased biodiversity in ecosystem. Initially, a xerophytic, nitrogenase-positive, photosynthetic pioneer species will colonise the inhospitable niche, forming a pioneer community. Notably, pioneer species’ tend to reproduce via asexual reproduction, with very little genetic variation resulting in their offspring, because the abiotic conditions are stable when there is low biodiversity, and this will increase likelihood of survival by passing on the advantageous allele that make the pioneer species adapted to its niche. This initial colonisation causes variety in abiotic conditions, making the niche more hospitable to other species better adapted than the pioneer species, which is therefore out-competed, creating a new community. This process repeats in what is known as the stages of succession, which eventually plateau into the climax community of high biodiversity.
[Succession] is important because
the population fluctuations create a proliferation of habitat and niche number, more complex food webs and a higher overall biomass that help the ecosystem to survive, and thrive, further reducing the chances of extinction.
