Cell Cycle Review

Interphase- Stage at which the DNA(genetic material) is doubled

Check Points - G1 and G2.  The integrity of the DNA is checked here.

Mitosis - Somatic Cell Division

2N-4N-2N

There is 1 cell division compared to 2 in Meiosis.

What is mitosis? Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle. Interphase & mitosis Interphase The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules. Prophase Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the mitotic spindle. Metaphase Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome. Anaphase The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules. Telophase Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage. Cytokinesis In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

Meiosis - Reduction Division

Packaging up half the genetic material into either sperm or eggs

There are 2 divisions compared to 1 in Mitosis

2N-4N-2N-N

Only one oocyte gets all the nutrients, therefore the others become polar bodies and die

Note:  Homologous chromosomes pair up at Metaphase

Mendel's Laws

1. Dominance vs Recessive
2. Separation of Traits  ( law of segregation: the 2 alleles from 1 gene go to different gametes)
3. Law of Independent assortment (random fashion in which the genes line up at Metaphase

Relating all this to Mendelian Genetics

When you are looking at more than one trait.

Example

T = tall
t = small

R = round
r = wrinkled

When the chromosomes line up one combination can be

T                                  R

XXXXXXX            OOOOOOO

xxxxxxxxxxx            oooooooooo

t                                   r

Duplication at Interphase of Meiosis then line up at Metaphase

T                                  R

XXXXXXX            OOOOOOO
XXXXXXX            OOOOOOO

T                                  R                                  SEPARATION OF THE TRAITS

t                                   r

xxxxxxxxxxx            oooooooooo
xxxxxxxxxxx            oooooooooo

t                                   r                      

Result in gametes is:

T and R

t and r

after 1 cell division

Note: There is no Tr and rR combination

There will be if the chromosomes line up at Metaphase like this:

T                                  r

XXXXXXX              oooooooooo
XXXXXXX              oooooooooo

xxxxxxxxxx                OOOOOOO
xxxxxxxxxx                OOOOOOO

t                                   R

Result in gametes is:

T and r

t and R

Calculation phenotypic and genotypic ratios for a dihybrid  cross

So what we have is a dihybrid cross

(Two traits are involved)

Using height and shape as above

T = tall
t = short

R= round
r= wrinkled

TtRr                 x                      TtRr

Gametes that can be passed on are:

T and R

t and r

Because of Independent assortment other possibilities are:

T and r

t and R

The possible gametes for both parents will be the same since they are both heterozygotes for both traits

Using Punnet Square     

          TR    Tr    tR    tr

TR
Tr
rR
tr

Branching Method

Or 3:1 x 3:1

    3         1

3   9         3

1   3         1

Will produce a 9:3:3:1 phenotypic ratio

9 tall and round

3 tall and wrinkled

3 small and round

1 small and wrinkled

From this we can see the genotypic ratio that is being formed.

Another way to figure the genotypic ratio for a dihybrid cross is to think of it as a monohybrid x monohybrid

If genotypic ratio for  a monohybrid cross is 1:2:1, a dihybrid cross would be 1:2:1 x 1:2:1

so:

 1AA            1DD            1AADD

                   2Dd             2AADd

                   1dd             1AAdd

2Aa             1DD            2AaDD

                   2Dd             4AaDd

                   1dd             2Aadd                  

1aa              1DD            1aaDD

                   2Dd             2aaDd         

                   1dd             1aadd

 Giving a genotypic ratio of?

Q. What is the probability of one of the F1 being double recessive?