Text Assignment: Chapter 2, Pages 18 - 49
Major concepts:
Sequence of presentation: It is always difficult to decide whether to begin a genetics course with current knowledge of the behaviour of homologous chromosomes in meiosis and fertilization of predominantly diploid organisms or to start with unit factors of inheritance as abstract entities, which was all of the information Mendel had to work with when he first developed his principles of inheritance. The authors of our current textbook have chosen the former approach, with their subsequent description of Mendel's findings presented at least partially in terms of what is now known about chromosomes, meiosis, and fertilization. This lecture follows their lead.
Cell Structure: The chapter begins with a brief review of the overall structural organization of typical eukaryotic and prokaryotic cells. This should all be familiar review material, but please be sure that you understand it, at least at the level of presentation in the book.
Chromosomal structure: The text presents a rather detailed discussion of chromosomal structure as it is observed at the light and electron microscope levels. Most of this should be a review of material presented in introductory biology. Although we will not spend much time on this material in lecture, it is important to be sure that you understand it.
In brief summary, each chromosome consists of one very long continuous DNA double helix, complexed with a variety of histone and non-histone proteins (See fig. 17.9 for a preview of the molecular details of chromosome structure). Multiple higher orders of coiling and folding are needed to fit all of the DNA in a typical eukaryotic cell into the interphase nucleus, with even further condensation needed to form visible mitotic or meiotic chromosomes. Thus, for example, a typical human cell contains about 1.9 meters of fully extended double helical DNA, which is all folded into a nucleus that is only a few micrometers in diameter. Even a single haploid E. coli cell contains about 1.2 mm of double helical DNA (fig. 17.3).
Centromeres: Each eukaryotic chromosome has a dense constricted area that serves as a point of attachment for spindle fibers during cell division. Prior to the development of modern banding techniques (figures 17-13 and 17-14), the position of the centromere was one of very few identifiable morphological characteristics that could be used to identify specific chromosomes (figures 2.5 and 2.4). Four classes of chromosomes are generally recognized with respect to centromere position.
Haploid/diploid life cycles: Sexual reproduction is characterized by an alternation between haploid and diploid phases of the life cycle. Mitosis provides the primary mechanism for growth, in that it can be used to increase cell number in either phase (or in both for some species). Meiosis is a process of reduction division that generates haploid cells from diploid. Fertilization unites two haploid cells to form a diploid cell. In predominantly diploid species, such as ourselves, the haploid phase occurs only transiently during reproduction, with all mitosis and growth taking place in the diploid phase. However, some multicellular species also have a substantial haploid phases in their life cycles, including ferns (fig 2.13) and mosses.
Mitotic cycle: Prior to the development of methods for determining the exact timing of DNA synthesis, classical histologists typically divided the mitotic cell cycle into two major parts, mitosis (the period during which the actual process of division could be observed microscopically) and interphase (the time that cells spent "resting" between divisions).
Mitosis: The term "mitosis" can be somewhat ambiguous. In the original histological sense, mitosis was usually understood to refer to the entire process of cell division, which includes both nuclear division (karyokinesis) and cytoplasmic division (cytokinesis). However, the term "mitosis" is also used more narrowly to refer specifically to the process of nuclear division, which, in turn, can be subdivided into four (or five) well-defined stages:
Interphase: After methods were developed for measuring when DNA is synthesized, interphase was subdivided into three distinct parts: Although the original basis for this terminology has become obsolete, the terminology itself is still widely used.
Meiosis: Three distinct processes of major genetic importance occur during meiosis: 1) a reduction in chromosome number from diploid to haploid; 2) independent assortment of chromosomes of maternal and paternal origin into the gametes (or other haploid progeny of meiosis); and 3) recombination, such that complementary portions of homologous chromosomes are joined together to generate a single recombined chromosome with genetic contributions from both parents. The independent assortment and recombination that occur during meiosis stand in sharp contrast to the rigid conservation of the genetic composition of the parental cell that occurs as its chromosomes are duplicated and distributed strictly equally to the two daughter cells in mitosis.
Meiotic prophase The extended prophase that occurs prior to the first meiotic division (meiosis I) is typically divided into five stages, identified primarily by cytologic appearance
Meiotic divisions: During the first meiotic division, the bonding between homologous chromosomes separates, but the centromeres of duplicated chromosomes stay together. For each chromosome pair, the maternal chromosome will go to one pole and the paternal to the other. Because the orientation of each pair (relative to the spindle) is random, there is independent assortment of maternal and paternal chromosomes at this division. This gives each daughter cell a haploid number of duplicated, but not yet separated chromosomes. The second division occurs without further DNA synthesis. The centromeres separate in this division, generating a haploid number of normal chromosomes.
| One cell division results in two daughter cells | Two cell divisions result in four products of meiosis |
| Chromosome number per nucleus is maintained following division | Chromosome number is halved in final products of meiosis . |
| One S phase per division | One S phase per two divisions |
| Normally, there is no pairing of homologs | Full synapsis of homologs in prophase |
| Normally, no recombination or crossovers | At least one crossover per homologous pair . |
| Centromeres divide at anaphase | Centromeres do not divide at anaphase I, but do at anaphase II . |
| Genetic composition of daughter cells is rigidly maintained | Independent assortment and crossover promote genetic variation |
Synaptonemal complex: Electron microscopy has revealed that meiotic chromosomes that are synapsed contain an additional ultrastructural element known as the synaptonemal complex in the space between them. Elements of the synaptonemal complex begin to appear during the leptotene stage and appear to be directly involved in the subsequent pairing that occurs in the zygotene stage. A number of lines of evidence suggest that the synaptonemal complex is strictly required for tight pairing of homologous chromosomes and crossing over. For example, it fails to form in male Drosophila, which are characterized by a complete lack of genetic crossing over between homologous chromosome pairs.
Nondisjunction: The textbook includes a short section describing the phenomenon of nondisjunction, in which the chromosomal products of meiosis fail to separate, either at the first or the second meiotic division. In each case, gametes that contain abnormal numbers of chromosomes are generated. If such gametes participate successfully in fertilization, nearly diploid individuals are generated who either lack one chromosome (monosomy) or have one extra chromosome (trisomy). We will examine the consequences of this type of abnormality in chromosome number in the lectures based on chapter 9.
Polytene chromosomes: Certain tissues of Drosophila and a variety of other species contain giant polytene chromosomes that are particularly well suited for detailed examination with a light microscope. In such cases, homologous somatic chromosomes become paired and then undergo a process of repeated duplication of their DNA without strand separation or cell division. These giant polytene chromosome often contain 1000 or more parallel strands of DNA (fig. 2.17). They also exhibit characteristic banding patterns as well as enlarged areas called "puffs" that reflect regions with particularly high levels of transcriptional activity. Changes in the pattern of bands allows relatively small insertions or deletions of chromosomal material to be detected quite readily.
Lampbrush chromosomes: During the extended meiotic prophase that occurs in oocytes, it is often possible to detect partially condensed chromosomes that have loops of chromatin extending out from them, giving them an appearance much like the long-bristled brushes once used to clean the chimneys of kerosene lamps. The loops are areas where transcription is continuing despite the partially condensed nature of the chromosomes in the developing egg cells.
Insights and Solutions: Be sure to read the insights and solutions section at the end of this and each subsequent chapter. For the current chapter, the insights and solutions section is designed to help clarify the genetic processes that occur during meiosis.