Genetics Notes
Chapter 2 - Chromosomes and Cellular Reproduction

These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; that is what class time and the textbook is for. If you were to study only these notes, you would not learn enough genetics to do well in the course.

two major groups of organisms (figure 2.1):
prokaryotes - lack nuclei
eukaryotes - has a formed nucleus, our discussion will be confined to eukaryotes
  1. stages in the cell cycle (figure 2.9)
    G1 -- GAP 1, the metabolically active stage following cell division
    S -- DNA synthesis occurs here
    G2 -- GAP 2, getting ready to divide
    cell division -- either mitosis or meiosis

  2. a chromosome consists of one or two arms, a centromere, and a pair of telomeres (figure 2.7)
    a kinetochore is the attachment point for the spindle fibers
    a centromere is the constriction in the chromosome where the kinetochores occur (these two terms are often used interchangeably, though this is not correct)

  3. Following the S phase of the cell cycle,
    - the arms and centromeres have duplicated but the centromeres are still held together by protein, thus there appears to be only one centromere
    - a chromatid refers to each set of arms. Both sets are called sister chromatids (figure 2.7).

  4. Most eukaryotic cells are diploid, at least at one point in their life cycle, that is the chromosomes occur in pairs, one inherited from the maternal side and one inherited from the paternal side.
  1. is an asexual process for increasing the number of cells (progeny)
  2. allows accurate replacement of old cells by new cells and ensures that all of the somatic cells of an organism have the same genetic composition
  3. is the only means of reproducing for asexual species It is divided into four stages, which represent a continuum from one stage to the next. The boundaries between stages are not that clear in practice.
Prophase (Figure 2.10)
  1. chromosomes begin to thicken and shorten. By mid-prophase, the morphology of the sister chromatids can be determined.
  2. nuclear membrane disintegrates
  3. the nucleolus disappears
  4. the centrioles, which are not present in higher plants, separate and migrate to opposite poles of the cell (the centriole/centrosome replicated suring G2 phase)
  5. the spindle apparatus forms from the centriole
  6. toward late prophase, the spindle fibers attach to the kinetochores (figure 2.10). There is usually only one kinetochore associated with each chromatid and only one centromere.
Prometaphase (figure 2.10)
  1. This is a recently recognized stage that is not used by all authors. It represents a time in what could be called late prophase when the nuclear membrane disappears and the spindle fibers attached to the centromeres
Metaphase (figure 2.10)
  1. the chromosomes align themselves in the equatorial plane of the spindle
Anaphase (figure 2.10)
  1. separation of the sister chromatids with division of the centromere (which had replicated in S or G2 phase)
  2. chromatids are pulled to opposite ends of the cell. There is still a diploid number of chromosomes, one from each sister chromatid, at each end of the cell (figure 2.13).
  3. the centromeres are pulled toward the poles with the arms trailing behind
Telophase (figure 2.10)
  1. the most significant event is cytokinesis, the division of the cytoplasm This requires the synthesis of a cell plate (destined to be a cell wall in plants) in plants or a pinching in of the cell membrane in animals
  2. as far as the nucleus is concerned, telophase is a reversal of prophase
    -reform nuclear membrane
    -spindle apparatus disappears
    -chromosomes uncoil
    -nucleoli reform
  1. found only in sexually reproducing species. Meiosis is the only mechanism for the reduction of the chromosomal compliment prior to fertilization and zygote formation. Meiosis allows a halving of the chromosomal compliment such that each gamete receives one member of each homologous pair. If meiosis did not occur, during sexual reproduction the chromosome number would quickly get out of control, because the chromosome number would double with each fertilization.
  2. characterized by two divisions
    1. first a reductional division that produces haploid cells
    2. second division is equational and is similar to mitosis
Meiosis I (reductional division) Prophase I (five substages) (figure 2.17)
  1. Leptonema-first stage
    1. the chromatin begins to condense
  2. Zygonema-second stage
    1. homologous chromosomes are attracted to each other and pair up or synapse. All chromosomes are paired at the end of zygonema.
    2. synaptonemal complex (figure 2.19) begins to form between the homologues and facilitates pairing
  3. Pachynema-third stage
    1. chromosomes continue to shorten and sister chromatids become obvious
    2. exchange of material between chromatids occurs (crossing-over) (figure 2.18)
    3. synaptonemal complex begins to disappear
  4. Diplonema-fourth stage
    1. sister chromatids begin to separate except at chiasmata
  5. Diakinesis-fifth stage
    1. chromosomes continue to shorten
    2. chromosomes continue to repel each other and the chiasmata move to the ends of the tetrad (terminalization)
    3. spindles attach to the centromeres
Metaphase I
  1. chromosomes align along the equatorial plane of the cell
Anaphase I (figure 2.15)
  1. homologous chromosomes are pulled apart by the spindle fibers
  2. centromeres do not divide, so that only homologous chromosomes are separated, not sister chromatids
  3. this results in a dyad (pair of sister chromatids) at each pole
Telophase I
  1. is similar to telophase in mitosis. In some species, the cell will skip this stage and go to Prophase II.
  1. some cells may enter an interphase, but no DNA synthesis will occur
Meiosis II (equational division)

Telophase I, interphase, and prophase II are highly variable in length depending on the species. In some cases, the phases may be essentially non-existent or may last for hours to months. prophase II, metaphase II, anaphase II, and telophase II are essentially like mitosis.

1) the sister chromatids separate during anaphase II

2) the genetic material is again reduced by half but there is no further reduction in chromosome number.

  1. allows the production of new combinations of alleles by....
    1. random assortment of what were originally maternal and paternal chromosomes at anaphase I (figure 2.19)
    2. exchange of genetic material between maternal and paternal chromosomes during pachynema of prophase I (figure 2.18)
  2. maintains constant amounts of genetic material between generations
The process of creating new arrangements either by....
  1. crossing over during pachynema (figure 2.18) or
  2. independent segregation in Anaphase I (figure 2.19) is called genetic recombination These processes contribute to great diversity among the offspring, which is a strong selective advantage for sexual reproduction.
If there were only 1000 genes in the genome and 2 alleles at each gene, there would 3^1000 possible genetic combinations among the individuals in a population. In actuality, there are many more genes than this, with more numbers of alleles per gene than two, so you can see that the possible number of combinations is very large.

Spermatogenesis (figure 2.24)
  1. spermatogonium (2N=46)
  2. primary spermatocyte (2N=46)
  3. 2 secondary spermatocytes (N=23)
  4. 4 spermatids
  5. 4 sperm

    All sperm have equal amonts of genetic material and equal amounts of cytoplasm.
Oogenesis (figure 2.24)
  1. oogonium (2N=46)
  2. primary oocyte (2N=46)
    In humans, meiosis I is stopped in diplonema before birth. It is not completed until ovulation.
  3. secondary oocyte (N=23) + a polar body (lost)
    In humans, meiosis II is completed after fertilization.
  4. ootid + a polar body (lost)
  5. ovum

    a) This yields 1 ovum and 2 polar bodies
    b) The first polar body does not undergo a second division and the ovum winds up with about 90% of the original cytoplasm.

Links on the Web
Mitosis -- Mitosis home page that gives an interactive explanation with detailed pictures and descriptions in plain English.

The Cell Cycle & Mitosis Tutorial -- Provides a simple description of the miotic phases with 3D pictures and an animated video.

Cell Division--Genetic Consequences -- Website from Cornell's biology department. Gives photographic review of mitosis and meiosis phases. Asks questions for review of both, as well as the major differences between the two processes. Highlights major terminology and Mendel's principles of segregation and independent assortment.

Last updated on 28 August 2005
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