Text Assignment: Chapter 11, Pages 321 - 343

• Haploid/diploid life cycles
• Mitotic Cell cycle
  • Interphase (G₁, S, G₂)
  • Mitosis (Prophase, Metaphase, Anaphase, Telophase)
• Meiosis
  • Meiotic prophase (leptotene, zygotene, pachytene, diplotene, diakinesis)
  • Genetic recombination, Crossover, Synaptonemal Complex
  • Lampbrush chromosomes
  • Separation of homologous chromosomes in meiotic divisions I
  • Separation of sister chromatids in meiotic divisions II
  • Nondisjunction

Haploid/diploid life cycles: The term "haploid" refers to a genome that contains one complete set of genetic information for an organism. The term "diploid" refers to having two complete copies of the genetic information (one from each parent) in an organism that undergoes a sexual cycle. Sexual reproduction is characterized by an alternation between haploid and diploid phases of the life cycle (figure 11.16). Fertilization is a fusion process a unites the genomes of two haploid cells to form a diploid cell. Meiosis is a process of reduction division that generates haploid cells from diploid. Mitosis is a process whereby a cell duplicates all of its components to generate two daughter cells that are genetically identical to the parental cell. Mitosis can be used to increase cell number in either the haploid or the diploid phase of a sexual cycle, and in some cases in both phases. Mitosis is thus the the mechanism that is ultimately responsible both for growth in physical size of organisms and increases in numbers of individuals in populations However, it should be noted that large increases in mass can sometimes be achieved without an accompanying increase in cell number (for example in a developing egg cell prior to fertilization),

Diversity of sexual cycles: 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 (figure 11.17). Flowering plants tend to have a more substantial haploid phase in their life cycles (figure 11.21). Some "lower" plants, such as ferns have even more extensive gametohphyte (haploid) phases. There are also many "primitive" species, such as molds, that have sexual cycles but spend the majority of their life cycles in the haploid state, with only a transient diploid stage (figure 11.23). The textbook describes several quite different haploid-diploid life cycles. We will not spend much time on them in lecture, but you should be fully aware of the types of alternative life cycles that may be encountered as we study patterns of inheritance and genetic recombination in a wide range of species.

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 (figure 11.4):

1. Prophase refers of the early stages of mitosis during which the interphase chromosomes become sufficiently condensed so they can be seen as distinct "structures" with the light microscope;
2. Metaphase refers to the period during which the centromeres of the condensed chromosomes become aligned at the midpoint between the poles of the mitotic spindle (many authors now refer to a separate prometaphase characterized by breakdown of the nuclear envelope and movement of chromosomes toward the metaphase position);
3. Anaphase begins with the abrupt separation of the duplicated centromeres and encompasses the period during which the chromosomes are physically separating and migrating toward the two spindle poles;
4. Telophase refers to the final aspect of nuclear division in which the nuclear membranes are reconstituted around the two daughter nuclei and the chromosomes begin to decondense.

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.

• G₁ was defined as the "gap" in time extending from the end of mitosis to the start of DNA synthesis during which it was not known what, if anything, the cell was doing. (G₁ is now known to be a busy time of growth in size of the cell and preparation for DNA synthesis);
• S was defined as the time during which DNA synthesis was actually occurring (DNA synthesis was discussed in detail in Chapter 2)
• G₂ was defined as the second "gap", which extended from the end of DNA synthesis to the start of mitosis (M). It is now known that G₂ is also a busy period in which many biochemical and regulatory processes are taking place in preparation for M.

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. The bulk of DNA synthesis has already occurred before meiotic prophase starts, but there are some regions with delayed synthesis, as noted below.

1. Leptonema (leptotene stage) (thin thread) is the earliest stage in which visible chromosome condensation can be seen. Most of the DNA synthesis has been completed by this stage, but there still may be regions with DNA synthesis in progress that hold sister chromatids tightly together. Homologous chromosomes begin to pair, but do not yet form tight synapses.
2. Zygonema (zygotene stage) (yoked thread) exhibits the beginnings of side-by-side pairing (synapsis) of homologous chromosomes, which for the most part have already replicated and thus form a four stranded structure known as a tetrad. Note, however, that some DNA synthesis still continues in this stage.
3. Pachynema (Pachytene stage) (thick thread) is characterized by further condensation of the chromosomes, and is also the period during which genetic recombination (crossing over) occurs between maternal and paternal chromosomes. This is also the stage at which the last of the delayed DNA synthesis occurs.
4. Diplonema (Diplotene stage) (double thread) is characterized by the beginning of separation of homologues, with residual areas of close contact called chiasmata (singular = chiasma), which are sites of recombination;
5. Diakinesis is characterized by shortening of the chromosomes, due to further condensation, and by migration of chiasmata toward the ends of the chromosomes.

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 become evident near the transition from leptonema to zygonema and appear to be directly involved in the tight pairing that occurs in zygonema (figure 11.13). 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. Mutant strains of yeast that lack the ability to make proteins needed for the synaptonemal complex have been shown to be unable to complete meiosis but appear to be able to complete crossing over started in abortive meiotic prophase if they revert to vegetative (mitotic) growth (boxed example 11.3).

Lampbrush chromosomes: The textbook emphasizes the fact that transcription can occur at various stages during meiosis, particularly in developing egg cells, which undergo extensive biosynthesis and grow to a large size while they are maturing. During the extended meiotic prophase that occurs in amphibian 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 (figure 11.14).

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 one complete set of duplicated, but not yet separated chromosomes, with each individual chromosome randomly derived from one of the parents, entirely independently of the parental origin of the other chromosomes. At this stage, the cells contain a haploid number of duplicated chromosomes whose sister chromatids have not yet separated. The second division then occurs without further DNA synthesis. The centromeres separate in this division, generating a haploid number of chromosomes, each of which consists of a single chromatid. The DNA content at this stage is one half that of a pre-replication diploid cell.

Nondisjunction: One of the problems that can arise during meiosis is 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 17.

Go to Review Questions for This Lecture
Go to the Notes for the Next Lecture
Go to the Notes for the Previous Lecture
Return to Index of Lecture Notes
Return to MCDB 2150 home page