Chapter 6. Chapter 6: Chromosomes and Cell Division

Rebuilding telomeres can result in cells becoming cancerous. Therefore, individuals would have a greater chance of dying from cancer by undertaking therapy of this kind.

Prokaryotes typically have circular chromosomes attached to the cell membrane while eukaryotes have linear, free-floating chromosomes. Eukaryotic cells have more genetic material than prokaryotic cells. Additionally, eukaryotic cells typically have two or more copies of chromosomes that have been contributed by maternal and paternal gametes. Since prokaryotic cells do not take part in sexual reproduction, they do not typically have multiple copies of chromosomes.

The main phases of the eukaryotic cell cycle are interphase, during which the cell grows and prepares for division, and the mitotic phase (or M phase) during which the nucleus and genetic material divide, followed by the division of the rest of the cellular contents. Within interphase, there are several sub-phases. During Gap 1, the cell grows and performs normal functions, such as protein synthesis and waste removal. Cells spend most of their time in Gap 1. Some cells may enter a resting phase (sometimes for days or even years) called G₀,where no cell division occurs. Next, cells destined for mitosis enter S-phase (DNA synthesis), where replication of chromosomes occurs, resulting in each chromosome having an exact copy. In Gap 2, cells exhibit a high rate of growth and protein synthesis in preparation for division. The mitotic phase begins with mitosis, in which the parent cell’s nucleus (including its chromosomes) divides. This generally is followed by cytokinesis, a process that leads to the division of cytoplasm between two daughter cells.

Making a copy of the DNA ensures that daughter cells have the same genetic information as the parent cell that produced them. Cells lacking a complete copy of the genetic code will be unable to perform normal activities, perform maintenance, and reproduce.

Mitosis makes it possible for existing cells to generate new, genetically identical cells, allowing organisms to grow and to replace damaged or dead cells.

The chromosomes replicate, becoming two, identical strands of linear DNA (sister chromatids) held together at the centromere. Next, the sister chromatids condense, coiling tightly and becoming compact.

During the S phase (also referred to as DNA synthesis) of interphase, the cell replicates its DNA, thereby creating a duplicate copy of each chromosome (sister chromatids). Once this process is complete, the cell is ready to divide into two, identical daughter cells.

Cancer cells are unlike normal cells in that they lack contact inhibition, can divide indefinitely, and have reduced stickiness. Often, death occurs when cancer cells crowd out normal healthy cells, in turn causing organs and organ systems to fail.

During meiosis, two cell divisions occur. The first division separates the homologous chromosomes equally between two daughter cells. The second division separates the sister chromatids equally into the four daughter cells. These four daughter cells, called gametes, are haploid, meaning they contain a single copy of each chromosome. When haploid games fuse at fertilization (the union of two gametes), they return to the diploid state—in other words, they will have two copies of each chromosome. If gametes were not haploid, a greater number of chromosomes would result from fertilization.

Homologous chromosomes separate as a result of Meiosis I, resulting in two haploid cells with chromosomes made up of sister chromatids. Sister chromatids separate during Meiosis II, producing four haploid cells.

The larger female gamete contains larger amounts of cytoplasm with nutrients needed for development of the organism after fertilization occurs. The sperm only contributes genetic material as it enters the cytoplasm of the egg.

Sexual reproduction produces genetic variation for the following reasons: 1) alleles from two different parents are contributed during fertilization, producing offspring with a genetic make-up different from either parent, 2) crossing over during Prophase I of Meiosis results in a mixture of maternal and paternal genetic information for the sister chromatids, and 3) homologous chromosomes are randomly distributed during Meiosis I to daughter cells, resulting in a mixing of maternal and paternal chromosomes.

During crossing over, sister chromatids swap different segments of equal size between the homologous pairs of the maternal and paternal chromosomes, thereby producing sister chromatids that are no longer genetically identical. This results in unique genetic combinations and helps produce offspring that are genetically different from their parents.

As we learned in Chapter 3, when fertilization occurs, the egg contributes chromosomes as well as mitochondria, which contain their own DNA. The sperm, however, contribute only DNA and no cytoplasm and, hence, no mitochondria. Therefore, the female gamete always will contribute more genetic material than the male gamete.

Non-sex chromosomes contain genetic information regarding traits other than gender specific characteristics. The sex chromosomes contain specific genetic information that instructs the body to develop into one sex or the other.

The gender with two different sex chromosomes (i.e. the male) will determine the sex of the offspring. During meiosis in males, half of the sperm produced inherit the X chromosome while half inherit the Y chromosome. So when an egg is fertilized, it is fertilized by a sperm bearing either an X chromosome or a Y chromosome.

Unlike the X chromosome, the Y chromosome does not contain essential genetic information needed for normal development. Information contained on the much smaller Y chromosome is limited to directing the development of male gonads.