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The Case of the Dividing Cell: Mitosis and Meiosis in the Cellular Court
Part I
1. Cell Division
a. Interphase – Cells absorb nutrients and absorb proteins and start to replicate their
internal components, including the most important part, the genes, called DNA, (located
inside an organelle, the nucleus) which encode for every process that occurs in our
body.
b. Prophase – The nucleus starts to disintegrate, or dissolve. The genes, which are typically
in string-like form, start to coil up tightly much like a spool. The original DNA and its
copy, the sister chromatid, join together in the center and form a “x” shape called a
chromosome.
c. Metaphase – The chromosomes line up near the center on the cell. The chromosomes
attached to special fibers, called spindle fibers, which pull apart the chromatids. Some
spindle fibers elongate to expand the cell.
d. Anaphase – The chromatids travel in opposite directions as the cell elongates.
e. Telophase – The chromatids all on opposite ends. The nucleus starts to appear around
the DNA, and the chromatids start to unwind. The cell starts to become narrow in the
middle, preparing for complete division.
f. Cytokinesis – The cell completely cleaves in the middle, forming sister cells. The
cytoplasm is evenly divided between the two cells. The two cells are completely
identical and are diploid (contain two sets of chromosomes, one from the mother and
one from the father). The cells head back towards interphase to divide again.
2. Cell Division in Eukaryotes and Prokaryotes
a. Eukaryotes – Mitosis/Meiosis
i. Multiple sets linear DNA
ii. Mitosis – produces 2 identical diploid (46 chromosomes) sister cells
1. Process for dividing cells
iii. Meiosis – produces 4 haploid (23 chromosomes) gametes (sex cells)
1. Produces cells for reproduction
b. Prokaryotes – Binary Fission
i. Lack membrane bound organelles
ii. Lack Nucleus
iii. Single ring of circular DNA
iv. DNA duplicates and each ring sticks to opposite ends of cell. Cell divides.
v. Not found in humans
vi. Reproduce and divide by same process.
c. Similarities
i. DNA must be replicated
ii. Result is identical sister cells
iii. Final step is division of cytoplasm
iv. Both contain cytoplasm
Part II
1. Meiosis
a. Interphase I - Cells absorb nutrients and absorb proteins and start to replicate their
internal components, including the most important part, the genes, called DNA, (located
inside an organelle, the nucleus) which encode for every process that occurs in our
body.
b. Prophase I - The nucleus starts to disintegrate, or dissolve. The genes, which are
typically in string-like form, start to coil up tightly much like a spool. The original DNA
and its copy, the sister chromatid, join together in the center and form an “x” shape
called a chromosome. Homologous pairs (one from mom, one from dad) form a tetrad
and start to exchange genetic information, a process called crossing-over which
develops uniqueness in the final products.
c. Metaphase I - The tetrads line up near the center on the cell, facing opposite poles.
d. Anaphase I – Tetrads are pulled apart. Each pair in a tetrad moves the opposite
direction.
e. Telophase I/ Cytokinesis I – The cells divide. The cells are not identical due to crossing
over, and contain only haploid set of chromosomes.
f. Prophase II – Spindle fibers form and Centrioles move to opposite directions. The
chromosomes are already coiled from previous steps. No nucleus present. Occurs with
each of the cells from Meiosis I.
g. Metaphase II – The chromosomes are lined up at the equator, ready for separation
h. Anaphase II – The sister chromatids are pulled apart towards opposite polls
i. Telophase II/ Cytokinesis II – The chromosomes unwind. Nucleus reappears and the
fibers breakdown. The cytoplasm cleaves. Each gamete is haploid, only containing 1 set
of chromosomes, and is read to be used for reproduction.
2. Mitosis vs. Meiosis
a. Mitosis
i. Produces 2 identical sister cells
ii. Final product is diploid (two sets of chromosomes, one from each parent)
iii. Asexual reproduction
iv. Produces cells for cellular growth, and body growth and repair
v. Total 46 chromosomes in each cell
b. Meiosis
i. Produces 4 sister gametes
ii. Final product is haploid (one set of chromosomes)
iii. Crossing-Over creates diversity in gametes
iv. Sexual reproduction
v. A process that involves two sets of division
vi. Produces cells for sexual reproduction
vii. Total 23 chromosomes in each cell
3. Do prokaryotes have mitosis and meiosis?
a. No, Prokaryotes do not have mitosis and meiosis. They divide and reproduce through a
process called binary fission.
4. Can crossover occur between non-homologous chromosomes?
a. Cross-Over cannot occur between non-homologous chromosomes. Homologous pairs
consist of two chromosomes, one from the mother and one from the father. Each set of
chromosomes serves a specific purpose and codes for certain genes. Information is
located on the same chromosome number in the same location for all cells. If crossingover were to occur between two non-homologous pairs, this would disrupt the gene
organization, and a gamete could miss out on a gene. Crossing-over between
homologous pairs is vital to ensure that each new gamete produced contains
information of every type of characteristic and process.
5. Was meiosis an evolutionary necessity along with reproduction?
a. Meiosis was a crucial evolutionary necessity because it produces gametes for
reproduction. The finals products of meiosis are haploid cells, each containing half a set
of chromosomes. An offspring is produced when a gamete from the father fertilizes the
gamete from the mother. Meiosis also plays a key role in creating genetic variation
between two people. Meiosis allows the child to have the genes of both the parents,
without meiosis further generations would only be copies of either one of the parents
which means that no new organisms will be formed and we would only have copies.
Lack of genetic variation could also wipe out species due to global disasters (ex.
droughts, diseases, etc). Meiosis helps us evolve and adapt with the living condition,
ensuring that withstand the environment we life.