- Disjunction / Non disjunction
- Mitosis – diploid. Identical. Stability.
- Meiosis – haploid. Combination. Variation.
- Sexual reproduction ensures genetic continuity and genetic variety, producing offspring's that often differ greatly from patients.
- Associated with gametogenesis.
- Counterbalances fertilization - makes sure # of chromosomes remains constant
- production of sex cells (gametes)
- 4 daughter cells with 1/2 (haploid) sets of chromosomes
- 1. Chromosomes found in homologous pairs
- identical but may have varying gene messages
- b. Humans have 46 (23 pairs)
- 22 homologous pairs (autosomes)
- X and Y (23rd pair) sex chromosomes
- c. Pairs separate during reproduction
- Offspring receives info from each parent
- Exchanging genetic material during crossing over (Prophase I)
- d. Separated during meiosis (Anaphase I)
- Homologous chromosomes form pairs in form of tetrads (4 chromatids)
- Involves 2 divisions = Meiosis I and II
- Consists of 2 identical sister chromatids attached at their centromeres
- centriole pairs also replicate into two pairs
- Condenstaion. Chromatin thickens and coils. Become visible .
- Synapsis occurs Homologous chr. pair up forming tetrads.
- Sister chromatids are attached at centromeres.
- Nonsister chromatids features crossing over resulting in chiasmata. = More than 1 can form = genetic variation)
- • spindle forms from microtubules
- • nuclear envelope/nucleoli disperse
- 5 stages: Leptonema, zygonema, pachynema, diplonema and diakenesis.
- Chromosomes have thickened.
- Each tetrad interacts with spindle fibers.
- Movement to the equitorial plate.
- Homologues are destined to separate towards opposite poles
- sister chromatids remain attached while homologues move towards the opposite pole. (Mitosis =sister chromatids are moved apart)
- One half of each tetrad ( one pair of sister chromatid) is pulled toward each pole of the dividing cell.
- No attraction of the sister chromatids – reduction in the number of chromosomes.
Telophase I + Cytokinesis
- Very short phase compared to mitosis
- Each pole now has a haploid set of chromosomes composed of two sister chromatids attached at the centromere
- cytokinesis occurs producing two daughter cells
- No interphase – No DNA replictaion. Already 2 chromatids.
2nd division (II)
- Prophase II = spindle apparatus forms
- Chromosomes move towards the metaphase II plate
- Metaphase II = chromosomes align on metaphase plate
- kinetochores of sister chromatids point towards opposite poles
- Anaphase II = centromeres of sister chromatids separate
- sister chromatids are pulled towards opposite poles
- Telophase II = I member of each homologue pair is present at each pole.
- Each chromosome is referred to as monads.
- 4 haploid gametes results.
- Haploid state has been achieved.
- If crossing over has occurred = each monad is a combination of maternal and paternal genetic information ie. Receives info from grandparents. = increases genetic variation.
- Mechanism that allows for genetic variation.
- Reduction of diploid – diploid means that each gamete will only carry one form of gene for a particular characteristics.
- Crossing over results in the exchange of genetic info from maternal to paternal chromosomes = possibility of new combination of genes.
- Random assortment = mixture of maternal and paternal chromosomes = more combination.
Nondisjunction and Translocation
- Too few or too many chromosomes in gametes
- Translocation: attachment of all or part of chromosome to another chromosome
- Nondisjunction: failure of chromosomes to separate
- 1. Translocation of chromosome #15 to #21, or
- 2. Nondisjunction with extra #21
- Nondisjunction of sex chromosomes
- 1. XO- Turner Syndrome; sterile female
- 2. XXY- Kleinfelter Syndrome; sterile male
- 3. XXX- Metafemale; limited fertility