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					                CH. 7 How Cells Reproduce

   Cells reproduce by duplicating their chromosomes.
   Single-celled eukaryotes divide by mitosis which maintains
    the parental chromosome number.
   Sexual reproduction starts with the formation of gametes by
    way of meiosis.
       o Each gamete gets half the parental chromosome
          number.
       o Later fertilization between two gametes restores the
          parental chromosome number.

Mitosis, Meiosis and the Prokaryotes

   Mitosis and meiosis are the nuclear divisions a cell
    undergoes.
   Mitosis helps organisms grow, replace worn out or dead
    cells, and repair tissue.
   Asexual reproduction (in plants and some animals) is where
    exact copies of themselves are made through mitosis.
   Meiosis gives rise to gametes, such as eggs and sperm.

Chromosome Structure
       Together the DNA and its proteins are one
        chromosome.
       Before a cell enters nuclear division it duplicates its
        chromosomes.
       Each copied chromosome stay attached to one another
        and are known as sister chromatids until late in the
        division process.
       Early in mitosis and meiosis the proteins and the DNA
        coil back on itself repeatedly into a condensed form
        known as a centromere.
         Structures in this region are docking sites for certain
          microtubules during nuclear division.

Cell Cycle
         Series of events from one cell division to the next.
         Starts with a daughter cell formed by mitosis and
           cytoplasmic division, and ends when the cell finishes
           its own division.
         During interphase a cell increases in mass, doubles the
           # of cytoplasmic components, and duplicates DNA.
         For most types of cells interphase is the longest part of
           the cell cycle.
         Thus is also known as the resting time for the cell,
           because the cell is not actively dividing.
         Once interphase is over the cell has replicated every
           one of its 46 chromosomes.
         Interphase is broken down into 3 steps: G1, S, and G2
               o G1: interval gap of cell growth and functioning
                 before the onset of DNA replication.
               o S: time of synthesis (DNA replication).
               o G2: second interval gap after DNA replication,
                 when the cell prepares for division.

Mitosis Maintains Chromosome Number

         Each species has a characteristic chromosome number,
          (the sum of all chromosomes in cells of a given type)
          humans = 46
         Our cells have a diploid number (2n)of chromosomes,
          (2 of each type)
             o Think of it like 2 volumes of books numbered 1-
                23 (1 from dad and 1from mom)
             o Except for one sex chromosome pair,
                chromosome pairs are the same length and shape
                  and carry the same hereditary instructions for the
                  same traits.
           Mitosis occurs in 4 stages: prophase, metaphase,
           anaphase, and telophase.
          In all phases the chromosomes move into new positions
           with the help of spindles, which are composed of 2 sets
           of microtubules extending from its 2 poles.
          A bipolar spindle established where all the
           chromosomes will end up.
          Chromosomes of eukaryote cells are duplicated before
           mitosis starts.
          A microtubule spindle move sister chromatids of each
           duplicated chromosome apart during mitosis.

Prophase: Mitosis begins

        The centrioles move away from each other to opposite
         sides of the cell.
        They forma bunch of fibers called mitotic spindle
         fibers.
        The spindle fibers attach to the chromosomes at the
         centromeres to move them during mitosis.
        The chromosomes condense/coil up.
        The nuclear membrane starts to break up.

Metaphase:

        The chromosomes line up, pushed and pulled by the
         spindle fibers at the equator of the cell.
        The equator is known as the metaphase plate.

Anaphase:
        The centromere that joins each pair of chromatids splits
         in two so that each chromatid separated from its
         partner.
        Now each chromatid is now called a chromosome.
        The newly separated chromosomes move toward
         opposite poles of the cell with the help of spindle fibers.
        Also the cell physically begins splitting in two.
        The area where it pinches is called the cleavage furrow,
         (cell plate formation in plants).

Telophase:
         A nuclear membrane forms in each new cell and two
           daughter cells result.
         Each of which has 46 chromosomes.
         The cytoplasm then divides during a process known as
           cytokinesis.
         Then once again the two new daughter cells enter
           interphase and the process starts over again.


7.5 Meiosis and Sexual Reproduction

      Asexual vs. sexual:
         o Asexual; one parent alone produces offspring by
           way of mitotic cell division, where the offspring
           inherits the same number and kinds of genes as the
           parent.
         o Genes are stretches of DNA, genetic info.
         o Sexual reproduction requires both a sperm and an
           egg that fuse to form a zygote.
         o Cells of new individuals contain pairs of genes on
           pairs of homologous chromosomes.
         o Each molecular form of the same gene is known as
           an allele.
Homologues

       Homologous chromosomes: chromosomes of the same
        length, shape, and size.
       46 (23+23) homologous chromosomes in human germ
        cells.
       After meiosis 23 end up in each gamete.
       Gametes have a haploid number, (half of the parental
        chromosome number), known as (n).

Two divisions not one:

         In meiosis a germ cell duplicates its DNA in interphase.
         The two DNA molecules and their proteins stay
          attached at a centromere, and are known as sister
          chromatids as long as they are attached.
         Chromosomes (in meiosis) go through 2 consecutive
          divisions that end with the formation of four haploid
          nuclei.
         No interphase between the 2 divisions which are called
          meiosis I and meiosis II.

Meiosis I:

         Consists of four phases; prophase I, metaphase I,
          anaphase I, and telophase I.
         The chromosomes have already replicated and are
          found as two chromatids held together at the
          centromere.
         The biggest difference between these 4 phases and that
          of mitosis is in the very beginning. The homologous
          chromosomes pair up in a process known as synapsis.

Prophase I:
         Synapsis occurs here.
         All chromosomes have to find their homologous partner
          and pair up.
         Prophase I is the longest phase of meiosis I.
         Instead of finding 46 replicated chromosomes floating
          around, we find 23 pairs of replicated chromosomes.
         Because each pair consists of four chromatids (two
          chromatids per replicated chromosome, and two
          replicated chromosomes), this pair is known as a tetrad
          (tetra=4).
         All the other normal events occur in prophase; the
          spindle is formed, the chromosomes condense, and the
          nuclear membrane disintegrates.
         After synapsis occurs an event called crossing over
          takes place.
             o Where like segments on homologous
                chromosomes are exchanged

Metaphase I:

         The chromosomes line upon the equator of the cells,
          however they stay in their homologous pairs.
         So, instead of 46 individual chromosomes lining up,
          there are 23 pairs of chromosomes.

Anaphase I:

         The centromeres DO NOT DIVIDE.
         Instead the homologous pairs separate with one entire
          replicated chromosome (a pair of chromatids and a
          centromere) moving to each of the opposite poles of the
          cell.

Telophase I:
         The two cells finish dividing their cytoplasm
          (cytokinesis), and nuclear membranes reform around
          the chromosomes.
         The two new cells DO NOT have 23 homologous pairs
          of chromosomes (46 total chromosomes) they only
          have 23 replicated chromosomes (each chromosome is
          made of two identical chromatids).
         Because there are no homologous pairs, the cells are
          considered haploid at this point.

Meiosis II:
         Since we are starting with the two cells formed from
            meiosis I, they have only half the number of
            chromosomes that a cell would have when undergoing
            mitosis.
         During prophase Ii the spindle forms, the nuclear
            membrane disintegrates, and the DNA condenses.
         During metaphase II the chromosomes line up
            individually along the equator.
         During anaphase II the centromere splits and the
            chromatids divide.
         Then the chromatids are called chromosomes again.
         During telophase II a nuclear membrane forms around
            the newly split chromosomes and we are left with four
            haploid cells.

Spermatogensis:

         Because the sperm and ova are the gametes, the
          formation of sperm and ova is called gametogensis.
         We start with a diploid cell called a spermatogonium.
         It undergoes meiosis and produces four sperm cells,
          which are haploid.
         It replicates all of its chromosomes during interphase.
          (now has 46 chromosomes, 2 chromatids joined at a
          centromere)
         The cell undergoes prophase I, homologous
          chromosomes pair up (synapsis) and crossing over
          occurs.
         The cell undergoes metaphase in which the paired
          chromosomes line upon spindles at the equator. 2
          centromeres on each spindle fiber.
         The cell undergoes anaphase I, but the centromeres
          don’t divide, instead the homologous chromosome airs
          separate.
         The cell finishes dividing during telophase I, and we
          now have two cells.
         Each cell has 23 chromosomes, and each chromosome
          is made up of two chromatids, still joined by a
          centromere. These cells are considered haploid.
         Each of these two cells then goes through prophase,
          metaphase, anaphase, and telophase II.
         The second set of divisions DOES resemble mitosis.
         Spermatogenesis occurs on a daily basis, beginning at
          puberty and lasting the entire lifetime of a male.
         Millions of sperm are produced on a daily basis.

Oogenesis:

        The formation of female egg cells, also known as ova.
        Oogenesis is very much like spermatogenesis: a diploid
         cell forms from a haploid cell through meiosis.
        There are some important differences:
            o The initial cell is called a primary oocyte, not a
               spermatogonium.
            o Primary oocytes live in ovaries in the female
               reproductive system.
            o The final cell is called an ovum.
             o Oogenesis results in the production of a single
               ovum from a single primary oocyte.
             o Two of the daughter cells resulting from meiotic
               division simply disintegrate.
             o These cells are called polar bodies.
             o This is very different from spermatogenesis in
               which four mature sperm are produced from a
               single spermatogonium.
             o Oogenesis occurs on a monthly basis (not daily),
               beginning at puberty and ending at menopause.
             o A single ovum is produced per month.

7.8 Cell Cycle and Cancer

         Cell cycle has built in check points where proteins
          monitor chromosomes structure and other components
          of a cell.
         Moles and other tumors are known as neoplasms.
         Cancers are abnormally growing and dividing cell of a
          malignant neoplasm.
         All cancer cells display 4 characteristics:
             o Grow and divide abnormally
             o Cells have significantly altered cytoplasm and
                plasma membrane; the membrane is leaky with
                abnormal or lost proteins.
             o Have a weakened capacity for adhesion.
                Recognition proteins are lost or altered.
                Abnormal cell migration and tissue invasion is
                known as metastasis.
             o Cancer cells usually have lethal effects

				
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