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Chapter 13
Meiosis and Sexual Life Cycles
Life Cycle: Span of time from one generation to the next.
This Chapter deals with how one generation produces the
next generation for animals with a sexual life cycle.
A. Somatic cells include all cells other than egg and sperm
cells. Each normal somatic cell in humans has 46
chromosomes. (The cell cycle discussed in Chapter 12 was
for somatic cells.)
- The 46 chromosomes = 23 pairs of homologous
chromosomes.
- The same genes are carried on each homologous
chromosome.
- The exceptions are the XX chromosomes in females
(homologous), and the XY chromosomes in males (partly
homologous). Y has genes that are not present on X. The X
and Y chromosomes are called sex chromosomes. The other
chromosomes are called autosomes.
B. The other human cells that are not somatic cells
are called gametes. There are two types of gametes. Males
have sperm cells and females have egg cells.
- A sperm cell has 22 autosomes plus either one Y or one X
chromosome (the sex chromosome).
- An egg cell has 22 autosomes plus one X chromosome (the
sex chromosome).
- Because gametes contain only one set of chromosomes,
they are said to be haploid; the number of chromosomes is
referred to as 1n = 23 (n = one set of chromosomes)
- Somatic cells contain duplicate sets of chromosomes, and
are said to be diploid; the number of chromosomes is
referred to as 2n = 46
- Diploidy (where 2n = 46) arises when a sperm cell fertilizes
an egg cell. A fertilized egg is called a zygote.
1n (♀) + 1n (♂) = 2n
- The zygote grows and the cell divides again and again to
create the mature adult. All somatic cells undergo mitosis for
this process (See Chapter 12).
- Eventually, the adult will form either sperm or eggs (which
are 1n); and the process starts over again.
- The process by which haploid cells are produced is called
meiosis. Note: if somatic cells did not undergo meiosis in
producing sperm and egg cells, the next generation would
contain double the number of chromosomes in a somatic cell
(4n). Thus, meiosis is a vital process in reproduction to keep
the chromosome number correct!
Please memorize Figure 13.5 (p. 241) – The human life cycle.
The process in Figure 13.5 is the human life cycle; it
depends on sexual reproduction. Why do most animals
use sexual reproduction? There are certain advantages:
 Genes from two parents are both present in the
offspring; this allows for trying out different sets of
genes to see which combination works best.
 If one gene is mutated and of no value to the offspring,
there is a second gene from the other parent that can
often be used to offset the “bad” gene.
 For this reason, sexual reproduction is common in
higher organisms where generations alternate between
1n and 2n.
In Figure 13.6 you can see how sexual reproduction occurs
for three different groups of organisms  note the alternation
of generations between 1n and 2n!
By contrast, some organisms reproduce asexually - Film
showing “Budding” by hydra.
 Note that for each life cycle, the constant theme is that
meiosis reduces the number of chromosomes in the gametes
to 1n. Fertilization then produces a 2n zygote.
C. Meiosis – Purpose is to reduce the
chromosome number from diploid (2n) to haploid (1n) in
the gametes.
In Meiosis there are three stages: Replication of
chromosomes during Interphase, then  Meiosis I
(homologous chromosomes separate), then  Meiosis II
(sister chromatids separate).
The basic outline of the process of meiosis is given
in Figure 13.7 (p. 243) – Overview of meiosis: how
meiosis reduces chromosome number.
Note where split between 2n and 1n occurs.
Note that the haploid cells are the sperm and ovum. One
somatic cell gives rise to either four sperm or four ova.
- The Sexual Life Cycle of a cell can be divided into phases:
1. Interphase (Same as in mitosis)
Meiosis I:
2. Prophase I
3. Metaphase I
4. Anaphase I
5. Telophase I and Cytokinesis
Meiosis II:
6. Prophase II
7. Metaphase II
8. Anaphase II
9. Telophase II and Cytokinesis
You need to memorize Figure 13.8 and all details associated
with the stages of meiotic cell division.
1. Interphase
- Chromosomes are replicated similar to interphase in mitosis
- Centrosomes also replicate
2. Prophase I  Main goal is to rearrange the chromosomes
by swapping segments between sister (homologous)
chromosomes. Reason? To create new combinations of
genes to create offspring that have some variability. This will
be very important in the Chapter on Mendelian genetics!
- Chromosomes condense
-Homologous chromosomes pair up – process is called
synapsis (two sister chromatids are present for each
homologous chromosome)
- Clusters of four chromatids, called tetrads, become visible
- Chromatids criss-cross at various places. These sites of
crossings are called chiasmata. So at the chiasmata,
segments of chromatids are swapped.
- Spindle begins to form; microtubules attach to kinetochore
- 90% of meiosis is spent in Prophase I
3. Metaphase I
- Homologous pairs of chromosomes align at the metaphase
plate (**Note that during mitosis, chromosomes are not
paired during alignment)
- Chromosomes are attached to centrosomes by
microtubules
4. Anaphase I
- Homologous chromosomes separate (sister chromatids
remain attached)
- Note that chromosomal rearrangement has occurred due to
criss-crossing – This is evident in Figure 13.8 but not in the
following short movie.
5. Telophase I and Cytokinesis
- Cleavage furrow becomes apparent
- Division of cytoplasm results in two (2) haploid cells (1n =
23). Note: These are haploid cells with two sister
chromatids.
Review Figure 13.8 (p. 244) – we are presently where Meiosis
1 stops.
We now need to split apart the sister chromatids and this
will happen in Meiosis II.
6. Prophase II - Spindle begins to form
7. Metaphase II
- Chromosomes align at the metaphase plate
- Kinetochores attach to microtubules leading to centrioles
8. Anaphase II
- Sister chromatids separate
- Chromosomes are pulled to opposite ends of the cell
9. Telophase II and Cytokinesis
- Nuclei form around chromosomes
- Cytoplasm divides to form two (2) cells
Then end result is: From one cell we have created four (4) haploid
(1n) daughter cells with unreplicated chromosomes
Figure 13.9 (p. 246) – A comparison of mitosis and meiosis.
Note 2n and 1n designations of the cells!
D. Outcome of Sexual Reproduction 
Genetic Variation Among Offspring that occurs in three
ways.
1. Independent assortment
- Positioning of homologous chromosomes occurs by chance
Figure 13.10 (p. 248) – The results of alternative
arrangements of two homologous chromosome pairs on the
metaphase plate in meiosis I.
2. Crossing over
- Results in the production of recombinant
chromosomes. Recombinant chromosomes form when DNA
from two parents occurs on the same chromosome. This
occurs during cross-over.
- Figure 13.11 (p. 249) – The results of crossing over during
meiosis.
3. Random fertilization
- There are 8 million possible chromosome combinations,
using 46 chromosomes, when producing egg and sperm
cells.
- Egg x Sperm = 64 Trillion combinations