Lecture 1, September 3, 1997

DEFINE:

1. mitosis, meiosis: mitosis is the process of nuclear division in a eukaryote which produces two daughter nuclei without a change in chromosome number; meiosis is the process of nuclear division in a diploid eukaryote which produces a halving of chromosome number, with one of each homologous pair of chromosomes sorted into the daughter nuclei. (hook: These processes have been described as a series of "snapshots" that represent stages observed with the light microscope; learn the processes, and overlay the stages for ease of communication. You are responsible for the names and status of the processes in the stages, between stages, and which produce phenotypes that give rise to the names of stages; see p.56-61 (Griffiths et al.). Notice how these processes are referred to and examined throughout the semester.)
2. stages of the cell cycle: In the mitotic cycle, the cell cycle is represented by "stages" (or phases) G₁, G₂, and S phases in interphase, and by mitosis. ("G₁ phase" is the "gap" between the end of the previous division and the "synthesis" of DNA, the "S phase," followed by "G₂ phase," the gap before the beginning of mitosis.) In the meiotic cycle, there are two rounds of nuclear division, with phases G₁, S, G₂, MI, G*, MII. (MI is the first meiotic nuclear division, "G*" is the gap phase in interphase between the two divisions, and MII is the second nuclear division.) (hook: Know the fate -- amount and distribution -- of DNA throughout these cycles; also know the various ADDITIONAL processes that are taking place, sometimes continuing over several of these phases, and sometimes initiated by events in previous phases. These processes are not in your book; many will be in your previous biology courses as part of metabolism, more will be added in class.)
3. chromosome theory of inheritance: It was noticed that the chromosomes behave in a parallel fashion to the "hypothetical "genes in gametogenesis. Also, inheritance patterns are consistent with the behavior of hypothetical "factors of inheritance" and with the organelles called chromosomes. Today we see that DNA has a parallel behavior with inheritance patterns, and it has been shown to be localized in the chromosomes. As the correlations (i.e., parallels in different patterns) increase, the model (theory) grows stronger that DNA contains inherited information, and that the chromosomes organize the DNA. Nevertheless, this model, a scientific model which is refutable in principle, will be extended as simpler models are revised from new observations to be obtained that do not fit within the existing model's framework.
4. chromosome: in eukaryotes, the nuclear organelles that consist of DNA and protein, and contain most of the recognized genes of the cell. The chromosomes form compact bundles in preparation for nuclear division, and in many species they are readily visible with a light microscope at this stage. Since there are many alleles for many loci in a species, the chromosomes in any given gamete or individual contain only a fraction of the genetic information of a species.
5. chromatid: when single stranded chromosomes replicate in the S-stage of interphase two strands are formed for each chromosome. The strands are called chromatids, and each chromosome then consists of two chromatids. Each strand contains the DNA coding for the centromere, along with all other parts of the chromosome. The centromere is the site for formation of the kinetochore (see below), and together the kinetochore and the centromeres of the two chromatids function as a single unit until they separate at anaphase (mitosis) or anaphase II (meiosis). At the time they separate in anaphase, the chromatids become independent and each one is called a chromosome, now having only one chromatid. (hook: connect this behavior with the DNA content discussed later in the course)
6. centromere, kinetochore, telomere: Centromeres are the regions coded in the DNA upon which proteins assemble to form the kinetochore, which is not DNA. These specialized proteins of the kinetochore which then connect to spindle fibers when the chromosomes are moved, and hold the chromatids together until they separate; telomeres are regions coded in the DNA that keep the ends of the chromosomes from interacting, or connecting, with other chromosomal material. These terminal regions of the chromosome function in the logistics of organizing and moving the genes contained in the chromosomes, and have been recognized as part of the "biological clock" that determines the life span of a cell, that is, when the telomere clock runs out, the cell dies. (hook: The chemical nature and physical structure will be discussed later in the course.)
7. synaptinemal complex: the protein "glue" that holds homologous regions of chromosomes together during meiotic pairing; it consists of specialized proteins having a characteristic structure. The SC seems to be essential for crossing over to occur. (hook: A synaptinemal complex has not been observed in all examined organisms, but they also do not appear to have crossing over (e.g. male Drosophila); think about the significance of this as the course develops. Without a synaptinemal complex, how can homologous pairs form in order to achieve first meiotic division?)
8. haploid, diploid: "...ploid" refers to a complete set of chromosomes (approximately a single genome); haploid means one set and diploid means two sets. In sexually reproducing organisms, a diploid zygote is formed by the union of two haploid gametes.
9. chiasma: The shaped configuration in meiotic prophase (pachytene or diplotene) which results from physical exchange between chromatids attached to homologous centromeres (i.e. non-sister homologous chromatids) (hook: relate this to chemical processes in the breaking and rejoining of chromatids discussed later in the course).
10. non-disjunction: the failure, either of homologous centromeres to separate at anaphase I, or the failure of sister chromatids to separate at anaphase II or in mitosis. This occurs when the spindle fibers and centromeres fail to function normally, and results in unequal chromosome numbers in the daughter nuclei formed from the division. One of the cell cycle "checkpoints" senses when the spindle has all the chromosomes properly attached.
11. sporophyte, gametophyte: In sexually reproducing plants the diploid phase (after fertilization and before meiosis) is called the "sporophyte" while the haploid phase (after meiosis and before syngamy - fusion of two haploid gametes) is called the "gametophyte."
12. eukaryote, prokaryote: eukaryotes have chromosomes - organelles - that organize the genes contained in them for nuclear division and functional regulation in the nucleus. The vast amount of DNA must be coiled and folded to move it in an orderly fashion during division. Prokaryotes do not have a nucleus and have DNA circles with little coiling. The term "chromosome" is applied in the sense that the genes are strung together on the DNA strand. The complexity of the "gene string" is much greater in eukaryotic organisms.
13. UT-Austin EXERCISES: Chapter 3 (Griffiths et al.): Problems 1, 2, 4, 5, 6, 7 (and describe processes of DNA change, microtubule change, membrane changes).
14. TAMU exercises:

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