Overview

Chapter 9 Overview

Life is heavily dependent on information that is stored in DNA. This information is duplicated and then apportioned every time a cell divides.

9.1 An Introduction to Genetics

9.2 An Introduction to Cell Division

9.3 DNA Is Packaged in Chromosomes

9.4 Mitosis and Cytokinesis

9.5 Variations in Cell Division

DNA is an information-bearing molecule that plays a critical role in the reproduction, development, and everyday functioning of living things. DNA contains the information for the prou;duction of proteins, whiulch carry out an array of tasks in living things.
The information in DNA is encoded in chemical substances called “bases,” which are laid out along the DNA double-helix in four varieties: adenine (A), thymine (T), guanine (G), and cytosine (C). One series of bases contains information for the production of one protein, while a different series of bases specifies a different protein. Each series of protein-specifying bases is known as a gene.
Protein synthesis begins with the information in a sequence of DNA bases being copied onto a molecule called messenger RNA (mRNA). This molecule moves out of the cell’s nucleus to a structure in its cytoplasm called a ribosome. There, the mRNA “tape” is brought together with the building blocks of proteins, amino acids. As the ribosome “reads” the mRNA tape, it strings together a sequence of amino acids called for by the tape. The result is a chain of amino acids that folds into a protein.
Most of the cells in an organism contain a complete copy of that organism’s genome, meaning its collection of genetic information. Before cells divide, their genome must first be copied and the resulting copies apportioned evenly into what will become two daughter cells.

Cell division takes place because old cells die and because there are many instances in which an organism needs quantities of new cells above “replacement” level. Cell division includes the duplication of DNA (replication); the apportioning of the copied DNA into two quantities in a parent cell (mitosis); and the physical splitting of this parent cell into two daughter cells (cytokinesis). In DNA replication, the two strands of the double helix unwind, after which each single strand serves as a template for construction of a second, complementary strand of DNA. The result is a doubling of the original quantity of DNA.

DNA comes packaged in units called chromosomes. These chromosomes are composed of DNA and its associated proteins—a combined chemical complex called chromatin. Chromosomes exist in an unduplicated state until such time as DNA replicates, prior to cell division. DNA replication results in chromosomes that are in duplicated state, meaning one chromosome composed of two identical sister chromatids.
Chromosomes in human beings (and many other species) come in matched pairs, with one member of each pair inherited from the mother, and the other member of each pair inherited from the father. Such homologous chromosomes have closely matched sets of genes on them, though many of these genes are not identical. A given paternal chromosome may have genes that code, for example, for different hair or skin color than the counterpart genes on the homologous maternal chromosome. Human beings have 46 chromosomes—22 matched pairs and either a matched pair of X chromosomes (in females) or an X and a Y chromosome (in males).
Cell division fits into the larger framework of the cell cycle, meaning a repeating pattern of growth, genetic replication, and cell division. The cell cycle has two main phases. The first is interphase, in which the cell carries out its work, grows, and duplicates its chromosomes in preparation for division. The second is mitotic phase, in which the duplicated chromosomes separate and the cell splits in two.

There are four stages in mitosis: prophase, metaphase, anaphase, and telophase. The essence of the process is that duplicated chromosomes line up along an equatorial plane of the parent cell, called the metaphase plate, with the sister chromatids that make up each duplicated chromosome lying on opposite sides of the plate. Attached to fibers called microtubules, the sister chromatids are then pulled apart, to opposite poles of the parent cell. Once cell division is complete, sister chromatids that once formed a single chromosome will reside in separate daughter cells, with each sister chromatid now functioning as a full-fledged chromosome.
Cytokinesis in animal cells works through a ring of protein filaments that tightens at the middle of a dividing cell. Membranes on the portions of the cell being pinched together then fuse, resulting in two daughter cells.

Because of their cell walls, plant cells must carry out cytokinesis differently from animal cells. The plant’s solution is to grow new cell walls and plasma membranes near the metaphase plate, thus dividing the parent cell into two daughter cells. Prokaryotes such as bacteria employ a process called binary fission: They double their single, circular chromosome, with the two resulting chromosomes attaching to different sites on the plasma membrane. Then an outgrowth of plasma membrane and cell wall, called a septum, begins growing from opposite sides of the cell, in between the two chromosomes. When the two septum extensions join in the middle, they divide the one cell into two.

Chapter 9: Introduction to Genetics: Mitosis and Cytokinesis