Cell Cycle and Chromosomes

The period after DNA synthesis has occurred but prior to the start of prophase, the cell synthesizes proteins and continues to increase in size. In a gamete, DNA is n. But in other cells, it is 2n. DNA gets double after G2 phase, so it will become 4n. So, 4 x 5 pg = 20 pg. 

In the eight-spored N. crassa, the linearly ordered ascospore pairs reflect the underlying genetic events during meiosis, and provide the clearest visual demonstration that crossing over occurs at the four-strand stage during meiotic prophase. When a gene marker is close to the centromere (no crossing over between the gene and centromere), the two alleles are likely to segregate from one another at the first division of meiosis and result in a 4:4 ascospore pattern for the segregating alleles. In contrast, when a gene marker is far from the centromere (cross over likely) the two alleles often segregate at the second division of meiosis show a 2:2:2:2 or 2:4:2 ascosporepattern for the segregating alleles.

Cytokinesis, during which the cell cytoplasm divides to form two daughter cells, begins during telophase of mitosis. It occurs differently in plant and animal cells. In animal cells, cytokinesis is accomplished by cleavage, the division or splitting of the cell cytoplasm. Cleavage begins with the formation of a cleavage furrow in the region of the metaphase plate.

During mitosis, the nuclear envelope merges with the endoplasmic reticulum (ER), and nuclear pore complexes are disassembled. In a current model for reassembly after mitosis, the nuclear envelope forms by a reshaping of ER tubules.

Chromosomal crossover (or crossing over) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis(pachytene) in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

Cell division involves the distribution of identical genetic material, DNA, to two daughters cells. The sequence of events during cell division is: DNA replication, Karyokinesis, Cytokinesis.

In mitosis: The centrioles are at opposite poles of the cell. The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate. Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).

Interphase occurs when the cell is not undergoing mitosis. It engages in metabolic activity. DNA duplicates during the synthesis phase of interphase to prepare for mitosis. 

The process in cell division in sexually reproducing organisms that reduces the number of chromosomes from diploid to haploid (half the original number). Meiosis involves two consecutive divisions of the nucleus and leads to the production of reproductive cells (gametes) in animals and to the formation of spores in plants, fungi, and most algae (the haploid spores grow into organisms that produce gametes by mitosis).

Meiosis is the process by which replicated chromosomes undergo two nuclear divisions to produce four haploid cells, also called meiocytes (sperms and eggs). Diploid (2n) organisms rely on meiosis to produce meiocytes, which have half the ploidy of the parents, for sexual reproduction. Halving the ploidy in meiocytes is essential for restoring the genetic content of the zygote to that of the parents. Meiosis uses similar mechanisms as those employed during mitosis to accomplish the separation and redistribution of chromosomes. However, several features, namely pairing and genetic recombination between homologous chromosomes, are unique to meiosis.

The kinetochore is the protein structure on chromatids, where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore forms in eukaryotes, assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis.

A cell with 2n = 16 undergoes meiosis. Each of the four daughter nuclei formed at the end of the division will have single stranded 8 chromosomes.

At diakinesis, a meiocyte has ten pairs of chromosomes. The number of chromosomes expected in each nucleus is, therefore ten at the end of I division ant ten at the end of second division. 

Prophase I has so many processes happening that it is usually separated into five stages. They are listed, in order, below with their explanations. Leptonema During this stage, the chromosomes begin to condense and become visible. Zygonema The chromosomes continue to become denser. The homologous pairs have also found each other and begin to initially align with one another, referred to as 'rough pairing'. Pachynema Coiling and shortening continues as the chromosomes become more condense. A synapsis forms between the pairs, forming a tetrad. Diplonema The sister chromatids begin to separate slightly, revealing points of the chiasma. This is where genetic exchange occurs between two non-sister chromatids, a process known as crossing over. Diakinesis The chromosomes continue to pull apart, but non-sister chromatids are still loosely associated via the chiasma. The chiasma begin to move toward the ends of the tetrad as separation continues. This process is known as terminalization. Also during diakinesis, the nuclear envelope breaks down and the spindle fibers begin to interact with the tetrad.

During fertilization, two cells fuse but the blastomeres formed from the division of the zygote contain the same number of chromosomes as the body cells because meiosis occurs during gametogenesis.