Genetics

(a) An organism that is heterozygous at two genetic loci on different chromosomes has alleles that are transmitted by the process of mitosis. To begin with, there are four phases that the cell must go through in order for it to grow, replicate DNA, and divide. These four phases consist of Gap 1, Synthesis, Gap 2, and mitosis. In Gap 1 it is basically resting and it is where cellular growth occurs, thus doubling organelle mass membranes and cytoplasm. Synthesis is where DNA replication or in other words the DNA is doubled. Gap 2 is where they make sure DNA is ready and is preparing for mitosis. Mitosis is the division of DNA into two masses, thus DNA and cell mass is divided into two daughter cells. Mitosis includes prophase, metaphase, anaphase, and telophase. Chromosomes must be replicated prior to mitosis. Each contains 1000's of genes which code for proteins, thus being very thin and fragile. They are packaged with proteins called histones. DNA wraps around histones though being nuclosomes. Nucleosomes coil are the chromatin which are compacted enough to fit in nucleus but can not identify and see individual ones. DNA is the most condensed in mitosis when chromatin coils and folds resulting in chromosomes; become visible when cell is about to divide, and no transcription off of chromosome it is too compacted. In addition, chromosome replication occurs in S-phase. A chromosome with one chromatid/chromosome is copied from a chromosome with two chromatids/chromosomes. Chromatids are attached at the centromere a sequence of DNA. At the end of interphase (Gap 1, Synthesis, Gap 2) DNA is in the form of chromatin and all chromosome have a pair of chromatids. Centriole replication has occurred (two centrioles or MTOC's in plants). In prophase, it follows a cell where 2N is equal to four, thus having four chromosomes in a diploid cell. In early prophase chromosomes condense and mitotic spindle begins to form which is composed of microtubules and emanate from centrioles. Then, in late prophase nuclear envelop vesicultes and bundles of microtubules called spindle fibers attach to the centromere of the chromosome at the kinetochore. In metaphase, chromosomes line up at the metaphase plate. Anaphase consists of anaphase A and anaphase B. In anaphase A, kinetochore proteins between chromatids dissolve, allowing each chromatid to go its own way; also chromosomes move toward opposite poles as the spindle shortens. In anaphase B, Spindle is elongated by the fibers sliding. One chromatid of each chromosome pair moves toward the centriole. In telophase, nuclear envelop reforms and karyokinesis is complete. Likewise, chromosomes decondense back into chromatin and spindle dissociates. Cytokenisis usually occurs which in animal cells cleavage furrow forms and in plant cells cell plate forms. Furthermore,