Download Holiday time test notes

Holiday time test notes. Yet another way to review as if your text, class notes, cool youtube videos, and
the Kahn Academy weren't enough. Happy Holidays!
The main concepts of the test this week will be eukaryotic cell structure (chapter 6... the subcellular
organelles), the cell cycle, mitosis, DNA replication, and meiosis.
We learn about meiosis in the context of animals (humans specifically), but later in the course when we are
focusing on different categories of organisms, you will see that the sexual life cycle can be quite different,
but meiosis does the same thing.... it reduces the chromosome number in half.
Meiosis in animals: Spermatogenesis is the production of sperm cells. Oogenesis is the production of egg
cells. Both are meiosis and they occur in testes and ovaries respectively.
Meiosis occurs in primary sex cells (germ cells) only. These primary sex cells have both maternal and
paternal chromosomes for each of the 23 pairs... 46 total. A primary spermatocyte or oocyte prepares for
meiosis just as it would for mitosis... by replicating all of its DNA. After DNA replication there are 92
DNA molecules residing on 46 chromosomes, each consisting of 2 identical sister chromatids connected at
the centromere. Meiosis starts out just like mitosis, but begins looking very, very different from mitosis in
Prophase I. As if by magic, the maternal and paternal chromosomes (homologs) somehow find one another
and become intimately entwined. This initmate coupling of the 2 pairs (4 chromatids) is called synapsis,
and it forms a "tetrad" of 4 chromatids. As if all of this were not impressive enough, while the homologous
pairs are intimately entwined, one of the maternal sister chromatids will exchange a whole section of the
chromosome with one of the paternal chromatids... producing "hybrid" chromatids. This unimaginably
complex process of enhancing genetic variation is called crossing over. Oh yes... it happens in Prophase I.
Did I mention that crossing over occurs in prophase I? Tetrads are aligned in the central plane in
metaphase I, with the homologous pairs clinging to one another as if they were deeply in love and did not
want to ever let go of one another. Then the spindle fibers begin to pull and pull until the homologs are
separated. However, because of crossing over, each one takes a bit of the other to the "other side" in
Anaphase I. Telophase one finishes Meiosis I, but the chromoses do not decondense, and the sister
chromatids are still connected at the centromere. Meiosis II starts with only one of each pair, but each
chromosome is duplicated. Meiosis II follows the path of MITOSIS. In anaphase II, sister chromatids are
finally torn apart. The resulting cells (there are 4 haploid cells resulting from the original primary sex cell)
are all genetically distinct from one another owing to that exchange that occured in prophase I called
crossing over. (Did I mention that crossing over occurs in prophase I?) Watch the videos over and over.
The musical accompaniment is lovely.
MEIOSIS I - the reduction division... diploid (2n) to haploid (n)
Prophase I - Homologous chromosomes find each other, become intimately entwined, and exchange
regions of maternal and paternal chromatids in "crossing over."
Metaphase I - Tetrads are lined up at the mid plane... still intimately entwined.
Anaphase I - The homologs are pulled apart by spindle fibers
Telophase I - No uncoiling. Sister chromotids remain attached at the centromere... the resulting nuclei are
now haploid.
MEIOSIS II - mitotic division; haploid to haploid
Prophase I - heck, the chromosomes are already condensed
Metaphase II - line up in the middle
Anaphase II - Sister chromatids finally pulled apart.
Telophase II - Haploid nuclei ready to go off to "finishing school" to become sperm or egg.
DNA Replication in Eukaryotes.
When the replication signal is released, origins of replication are found all over the place on each of the
DNA molecules. Origins of replication are called "bubbles." On either end of each bubble is a "replication
fork" so the machinery of replication is operating both two forks in each of the thousands of bubbles. At
each fork there is a leading strand and a lagging strand. As the "semiconservative" replication continues,
the bubbles will coalesce until all of the DNA molecules are replicated (semiconservative means that each
of the "daughter" molecules has one strand from the original template DNA, and one brand new strand).
Leading/Lagging; 3' - 5'; 5' - 3':
Before DNA can be replicated, the complimentary strands must be separated by an enzyme called helicase.
To prevent the strands from pairing up again in the wake of helicase, single stranded binding proteins are
attached. DNA is a polymer of nucleotides. The enzyme(s) that make new R/DNA polymers are called
polymerases. DNA polymerase is limited. First, it can only read the 3' - 5' template strand and make the
new molecule in the 5' - 3' direction. (All polymerases work in this way.) Another limitation is that DNA
polymerase cannot start a new strand... it can only add to what is already started. So the new molecules of
DNA start with short (about 200 bases) molecules of RNA, put into place by RNA Primase. The leading
strand is the one at the fork that is being read 3' - 5' and constructed continuously in the 5' - 3' direction by
DNA polymerase. There is a problem over on the other strand - the lagging strand. DNA polymerase is
working in the opposite direction of replication! Instead of making the new strand continuously, the
lagging strand must make fragments (Okasaki fragments... discovered by Okasaki), and then join the
fragments together. Remember the RNA primers? They have to be removed and replaced with DNA
nucleotides. A special DNA polymerase does this. This computer animation of the process is pretty cool.
See if you can spot the helicase, leading and lagging strands, Okasaki fragments, and DNA polymerase.
http://www.youtube.com/watch?v=4jtmOZaIvS0&feature=related
Here's another video http://www.youtube.com/watch?feature=endscreen&v=-mtLXpgjHL0&NR=1 .
Neither one of these videos is very long (2 minutes or less). They will be much more helpful if you read
the short section in the book and the notes above first.