Text: Meiosis
Guiding Question: What is the purpose of meiosis and how is it different from mitosis?
Text
(1): Every student in our biology class has characteristics passed
onto them by each of their parents. The instructions for each trait
are located on chromosomes and arranged in segments called
genes. The human body has 46 chromosomes: 23 from the mother
and 23 from the father. The chromosomes that make up each pair,
one chromosome from each parent, are called homologous
chromosomes. In the process of meiosis, one cell turns into four
cells and it produces sex cells. Unlike mitosis, which creates body
cells and diploid cells (46 chromosomes), meiosis creates haploid
cells (23 chromosomes). Through fertilization, the haploid cells (23
chromosomes) from mom and the haploid cells (23 chromosomes)
from dad come together to form a diploid cell (46 chromosomes
total) or what is known as a zygote.
Vocabulary
Genes:
segments of
DNA that
control the
production of
proteins
Homologous
Chromosome
s: one
chromosome
from mom
and one
chromosome
from dad are
paired
together
Gamete: sex
cells that have
23
chromosomes
Haploid: 46
chromosomes
Text Dependent Questions (IN COMPLETE SENTENCES)
1. The human body has __________ chromosomes.
2. When chromosomes pair up, they are called
_____________________ chromosomes.
3. The purpose of mitosis is to turn one cell into
___________ cells.
4. What is the one difference between mitosis and
meiosis?
5. What is the difference between haploid and diploid
cells?
6. What is a zygote?
(2): Meiosis occurs only in sex cells and therefore only occurs in
7. Where does meiosis occur?
sexually reproducing organisms. Meiosis involves TWO cell divisions
8. How many divisions in meiosis are there? How many
are min mitosis?
called meiosis I and meiosis II. Meiosis I undergoes interphase,
prophase I, metaphase I, anaphase I, and telophase I, and
Mitosis:
Meiosis:
cytokinesis while meiosis II undergoes prophase II, metaphase II,
9. During this prophase I, chromsomes pair up to become
what?
anaphase II, telophase II, and cytokinesis
(3): Just like in mitosis, interphase must also occur before the cell
10. What is crossing over?
undergoes meiosis where the DNA in cell replicates.
(4): Once the cell has prepared itself, the cell will undergo the FIRST
stage of meiosis I, which is prophase I. This is the stage where
chromatin tightens and becomes chromosomes. Chromosomes
then pair up in a process called synapsis and become homologous
chromosomes. Then a process called crossing over occurs whereby
segments of the chromosomes, and therefore DNA, is exchanged.
(5): During the second phase of meiosis I called metaphase I,
Crossing over:
a process
where
segments of
chromosomes
are mixed ,
DNA is
exchanged
between
homologous
chromosomes
11. Do you think that chromosomes are the same after
crossing over as they were before crossing over?
12. Summarize what happens in prophase I:
homologous chromosomes line up as pairs in the middle of the cell.
Spindle fibers are attached.
(6): During the third phase of meiosis I, called anaphase I, the pair
of homologous chromosomes are pulled apart as the spindle fibers
13. Summarize what happens in anaphase I. How many
chromosomes are on either side of the cell after this
phase?
begin to shorten. At this point, the chromosome number has been
reduced to half (23 chromosomes go to one side and the other 23
chromosomes go to the other side of the cell).
(7): During the fourth and final phase of meiosis I called telophase I,
14. After cytokinesis, is the DNA in each cell the same as
when the cell division started?
homologous chromosomes, each consisting of two sister
chromatids reach opposite ends of the cell. Two new membranes
begin to form around each set of sister chromatids. Cytokinesis
then occurs and the two new cells physically break apart from one
another. In each cell, there is now a unique set of DNA that is
different than the DNA that we started with because of the crossing
over that occurred in prophase I.
15. Why is the DNA different in each cell? At what point in
the cell division process did this happen?
(8): Unlike mitosis, the process doesn’t stop there though. In
16. What is different about the second cell division in
meiosis when compared to mitosis?
meiosis, there is a second set of cell division that must take place.
However, what is different in this cell division is that each cell is
dividing a haploid set of chromosomes (23 chromosomes) as
17. Summarize what happens in anaphase II:
opposed to a diploid set (46 chromosomes) like mitosis. In the first
phase of meiosis II, called prophase II, chromatin condenses back
into chromosomes. In the second phase of meiosis II, called
metaphase II, chromosomes line up in the middle and spindle fibers
attach. In the third phase of meiosis II, called anaphase II, the sister
chromatids are pulled apart and move to opposite ends of the cell.
18. How is what happens in anaphase II different than
what happened in anaphase I?
In the fourth phase of meiosis II, called telophase II, two
membranes form around the sister chromatids. Finally, at the end
of meiosis II, cytokinesis occurs and the cell physically breaks apart.
(9): To summarize: at the beginning of meiosis, a cell will start with
a diploid number chromosomes (46 chromosomes). After meiosis I,
there are two cells which each have a haploid number of
19. How many chromosomes are present at the start of
meiosis?
chromosomes. (23 chromosomes in one cell and 23 chromosomes
20. How many chromosomes are present at the end of
meiosis?
in another cell) Due to the act of crossing over in prophase I, each
of these cells is unique from the cell it began with. After meiosis II,
21. At the end of meiosis, are the cells the same as when
they began?
each of these two cells splits again, thereby forming 4 cells total
with each cell consisting of a haploid number of chromosomes (23
22. Why are the cells different than when they started?
chromosomes).
23. By the end of meiosis II, how many cells exist in total?
24. In each cell, how many chromosomes are there?