AP Biology
Notes:  Meiosis

Heredity

DNA:   Type of nucleic acid that is a polymer of four different kinds of nucleotides
Genes: Units of hereditary information that are made of DNA and are located on chromosomes
                    * Have specific sequences of nucleotides, the monomers of DNA
                    *Mast genes program cells to synthesize specific proteins; the action 
                      of these proteins produce an organism's  inherited traits.
Locus: Specific location on  a chromosome that contains a gene

Inheritance is possible because
    * DNA is precisely replicated producing copies of genes that can be passed along 
       from parents to offspring
    * Sperm and ova carrying each parent's genes are combined in the nucleus 
       of the fertilized egg

Human life cycle
                               

somatic cell:  Any cell other then a  sperm or egg cell
    Human somatic cells contain 46 chromosomes distinguishable by differences in size, 
    position of the centromer or staining

Karyotype:  A display or photomicrograph of an individual's 
 somatic-cell metaphase chromosomes that are arranged in a
 standard sequence

   Human Karyotype are often made with lymphocytes
   Can be used to screen chromosomal abnormalities

Homologous chromosomes :   a pair of chromosomes that have the same size, 
                                              centromer position and staining pattern.

   Homologous autosomes carry the same genetic loci; however sex chromosomes carry
   different loci even though they pair during prophase of meioses I.

Autosomes:  A chromosome that is not a sex chromosome
Sex chromosome: Dissimilar chromosomes that determine and individual's sex
        *Females  have a homologous pair of X chromosomes
        *Male have one X and Y chromosome
        *Thus humans have 22 pairs of autosomes and one pair of sex chromosomes
        *One homologue is inherited form each parent
        *Thus the 46 somatic cell chooses are actually two sets of 23 chromosomes; one a
          maternal set and the other paternal set
        * Somatic cells in human and most other animals or diploid

Diploid:  Condition in which cells contain two sets of chromosomes; abbreviated as 2n
Haploid: Condition in which cells contain one set of chromosomes; it is the chromosome 
                number of gametes and is abbreviated as n
Gamete: A haploid reproductive cell.
    Sperm cell and ova are gametes and they differ from somatic cells. In their chromosome number.  
    Gametes only have one set of chromosomes.

The diploid number is restored when two haploid gametes unit in the process of fertilization.  
Sexual intercourse allows a haploid sperm cell from the father to reach and fused with 
an ovum from the mother

Zygote: A diploid cell that results from the union of two haploid gametes
    Contains the maternal and parental haploid sets of chromosomes from
     the gametes and is diploid

Gametes
        Gametes are produced in the ovaries or testes by the process of meiosis
        Meiosis is a special type of cell division that produces haploid cell s and compesnate
        for the doubling of chromosome number that occurs at fertilization 
        Meiosis in humans produces sperm cells and ova which contain 23 chromosomes
        When fertilization occurs, the diploid condition (2n=46) is restored in the zygote.

The stages of meiotic cell division:
Meiosis I the cell division segregate the two chromosomes of each homologous pair
and reduces the chromosome number by one-half.  It includes the following four phases

        Prophase I: This is a longer and more complex process than  prophase of mitosis
            *chromosomes condense
            *Synapsis occurs.  During this process homologous chromosomes come together 
              as pairs chromosomes condense further until they are distinct structures that can
              be seen with a microscope.  Since each chromosome has two chromatids, each 
              homologous pair in synapsis appears as a complex of four chromatids or a tetrad

            * In each tetrad sister chromatids of the same chromosome are attaches at their centromeres. 
               Nonsister chromatids are linked by X-shaped chiasmata sites where homologous strand 
               exchange or crossing-over occurs

            *Centriole pairs move apart and spindle microtubules form between them 
              Nuclear envelope disperse
  
            *Chromosomes begin moving to the metaphase plate, midway between the two
             poles of the spindle apparatus Prophase I typically occupies more than 90% of 
             the time required for meiosis

        Metaphase I:  Tetrads are aligned on the metaphase plate

        Anaphase I:  Homologues separate and are moved toward the poles by 
                              the spindle apparatuses Sister chrmatids remain attahced at their 
                              centromeres and move as  aunit toward the same ple, while the homoluge
                               moves toward the opposite pole.
Telophase I and Cytokinesis.  The spindle apparatuus continues to separate homologus
                                                   chromosome pairs until the chromosomes reach the poles

        *Cytokinesis occurs simultaneously with telophase I forming tow haploid daughter cells
          Cleaves furrows form in animal cells, and cell plat form in plant cells
           In some species, nuclear membranes and nucleoli reaper and the cell enters
           a period of interkinesis before meiosis II.  In other species the daughter cells 
           immediately prepare for meioses II

Meiosis II is very similar to mitosis  enough said!!