Ch. 11: Meiosis & Reproduction
I. Section 1 – Reproduction
A. Reproduction
1. Reproduction – process of producing offspring.
2. Occurs in 2 ways:
Asexual reproduction & Sexual reproduction
3. Which evolved first? Asexual Reproduction
B. Asexual Reproduction
1. Asexual Reproduction – DOES NOT involve sperm & egg. One
parent transfers a copy of ALL of its genetic information to its
offspring; they are genetically identical or clones of one another.
NO NEW GENES; NO GENETIC VARIATION.
2. Types of Asexual Reproduction
a. Binary Fission – carried out by prokaryotic bacteria; single
chromosome is duplicated & then the cell splits into 2
genetically identical cells. Division of cytoplasm does not have
to be equal.
b. Fragmentation – the parent organism breaks up into several
pieces, each of which develops/regenerates into a new
organism genetically identical to the parent.
Ex: sea stars (aka: starfish) & sponges reproduce by
fragmentation
c. Budding – a new individual develops off the body of the
parent. Ex: hydra, potatoes, spider plants, yeast cells
d. Parthenogenesis – female organism makes an egg that
develops into a new individual WITHOUT being fertilized by the
male. Ex: water fleas – all offspring will be female; some fish,
insects & reptiles
C. Sexual Reproduction
1. Sexual Reproduction – INVOLVES sperm & egg
(Aka: gametes, germ cells, reproductive cells, sex cells)
2. Most eukaryotic organisms (animals, fungi, plants, protists)
reproduce sexually.
3. Offspring are genetically different from their parents; have some
characteristics of both because they received half their genes
from mom & half from dad.
4. When sperm & egg unite the fertilized egg is called a zygote. The
zygote divides by mitosis to form a multicellular organism.
5. Types of Fertilization
a. External Fertilization – eggs are fertilized outside the body of
the female. Female releases the eggs & the male releases the
sperm in close proximity to the eggs & fertilization occurs.
Occurs mainly in aquatic organisms (animals that live in water)
such as fish & amphibians (frogs, salamanders, toads).
b. Internal Fertilization – eggs are fertilized inside the body of
the female & then are either laid as eggs or develop inside the
uterus of the female. Eggs are produced in the body of the
female & the male must deposit its sperm into the female’s
body. Ex: insects, some fish (like guppies), reptiles (snakes,
turtles, crocs, alligators, lizards), birds, & mammals.
D. Germs Cells & Somatic Cells
1. Germ Cells – sperm & egg cells (aka: reproductive cells, gametes,
sex cells).
2. Somatic Cells – all of the other cells in the body EXCEPT sperm &
egg. Ex: brain, skin, muscle, bone, blood cells
E. Advantages of Sexual Reproduction
1. Sexual reproduction produces offspring with new gene
combinations (genetic variation) making them better adapted to
survive changes in the environment; hopefully the entire
population won’t be wiped out - some of the individuals may be
able to survive & produce future generations.
2. Advantages of asexual reproduction are that the organism doesn’t
have to use energy to produce gametes & doesn’t have to find a
mate. Disadvantage of asexual reproduction: NO NEW GENETIC
INFO. GETS PASSED ON TO OFFSPRING; NO GENETIC
DIVERSITY!!!
F. Chromosome Number
1. Chromosomes contain thousands of genes that determine how an
organism develops & functions.
2. Every species of organism has a specific number of chromosomes.
(See chart on page 249)
3. Humans have 46 individual chromosomes or 23 pairs of
chromosomes.
G. Haploid & Diploid Cells
1. Somatic/body cells have 2 sets of chromosomes & are diploid.
Remember, half of your chromosomes come from mom & half
come from dad.
2. The diploid number is represented as 2n.
3. The human diploid number is 46, so 2n = 46.
4. Gametes/sperm & egg cells have one set of chromosomes & are
haploid.
5. The haploid number is represented as n.
6. Human sperm & egg cells haploid number is 23, so
n = 23.
7. human egg (23) + human sperm (23)  zygote (46)
n
+
n
 2n
8. The process of fertilization restores the diploid number in the
zygote.
H. Homologous Chromosomes
1. The prefix “homo” means “same or similar”.
2. Homologous chromosomes – chromosomes that are similar in
size, shape, & the kinds of genes they carry.
3. One chromosome in the homologous pair comes from the mother
& the other from the father. Your mom gave you 23 of her
chromosomes & your dad gave you 23 of his.
I. Autosomes & Sex Chromosomes
1. Autosomes – chromosomes carrying genes that DO NOT
determine the sex of the organism. Humans have 22 pairs of
autosomes or autosomal chromosomes.
2. Sex Chromosomes – carry genes that determine the sex of the
organism. Humans have one pair of sex chromosomes;
chromosome pair #23 is your sex chromosomes.
3. Human sex chromosomes are the X & Y chromosomes.
a. Human females have two X sex chromosomes (XX).
b. Human males have an X & a Y sex chromosome (XY).
c. Who determines the sex of the child in humans?
The male because he produces 2 types of sperm:
X sperm & Y sperm
II. Section 2 – Meiosis
A. Introduction
1. Meiosis in your sex cells produces four daughter cells with the n
or haploid number of chromosomes. Your body cells have the 2n
or diploid number of chromosomes.
2. Your sex cells/gametes must have half as many chromosomes as
your somatic/body cells; how does this happen? meiosis
3. Meiosis is a type of cell division that produces four daughter cells
with half as many chromosomes as the parent cell
4. In mitosis the nucleus of the cell divides once; in meiosis the
nucleus of the cell divides twice.
5. All cells in the body of a sexually reproducing organism (plants,
animals, fungi) go through interphase & therefore go through the
G1, S , & G2 phases ; the DNA of the cell is replicated.
6. The cell must divide twice in meiosis: Meiosis I and Meiosis II
B. Stages of Meiosis
1. Meiosis I
a. Prophase I – occurs after interphase (G1, S, G2).
 Chromatin condenses into chromosomes
 Nuclear membrane & nucleolus break down
 Spindle fibers/microtubules form
 Homologous chromosomes pair up into groups of 4
sister chromatids called a tetrad (NOTE: “tetra” means
“four”). This process is known as synapsis. Sister
chromatids may exchange genetic material in a process
called crossing over.
 The sister chromatids (blue & green) are now all
carrying slightly different genetic information. Causes
genetic variation/new gene combinations.
b. Metaphase I
 Homologous chromosome tetrads move as a group to
the metaphase plate/equator & line up there. Spindle
fibers help to move the tetrads.
c. Anaphase I
 Homologous chromosome pairs separate; spindle fibers
move the chromatid pairs to opposite poles. Chromatids
are still joined at the centromere.
d. Telophase I
 Two new nuclei form & cytokinesis (division of the
cytoplasm) occurs.
 At this point the 2 cells are haploid/n but the
chromatids are still paired together. Each of the cells
must divide again.
2. Meiosis II – Chromosomes are NOT copied in meiosis II
a. Prophase II
 Spindle fibers/microtubules reform
b. Metaphase II
 Paired sister chromatids attach to spindle fibers & line
up at equator of each cell
c. Anaphase II
 Sister chromatids split apart & the chromosomes are
pulled to the opposite poles in each of the 2 cells.
d. Telophase II
 Nucleus & nucleolus reform, spindle fibers disappear,
chromosomes uncoil into chromatin.
 Cytokinesis happens in each of the 2 cells; 4 haploid
cells are formed.
3. Important Facts About Meiosis
 Synapsis, tetrad formation & crossing over between
homologous chromosomes happens in Prophase I
 After telophase I/cytokinesis in Meiosis I the two cells are
haploid, but the chromatids are still paired together; the
cells’ nuclear material must divide again.
 Meiosis II produces 4 haploid cells in telophase II.
C. Gametogenesis – formation of gametes/sex cells/sperm & egg
1. Spermatogenesis – formation of sperm
 Occurs in the testes
 All 4 haploid cells develop flagellum & survive
2. Oogenesis – formation of the egg/ovum
 Occurs in the ovaries
 In meiosis I, during cytokinesis, there is unequal division of
the cytoplasm. One cell receives almost all of the
cytoplasm & the other almost none; the smaller cell is
called a polar body. The 2 cells divide again in meiosis II &
now there are 3 polar bodies & one large ovum/egg.
 The one egg cell/ovum must get most of the cytoplasm
because that’s where the nutrients are to feed the
developing zygote if the egg gets fertilized.
D. Comparing Mitosis and Meiosis
MITOSIS
MEIOSIS
Occurs in which type of
cell; SOMATIC CELLS
(body cells) or
GAMETES (sperm & egg)
SOMATIC CELLS
GAMETES
DNA Replication Occurs
in Which Phase/Stage
1 IN S PHASE OF
INTERPHASE
1 IN S PHASE OF
INTERPHASE
Number of Divisions
ONE
TWO
Synapsis of Homologous
Chromosomes (Does it
occur? When & How?)
NO
YES – PROPHASE 1.
Number of Daughter
Cells Formed
2
4
Daughter Cells
Diploid or Haploid?
DIPLOID/2N
HAPLOID/N
Role in Your Body?
REPLACES
DEAD/WORN OUT
CELLS, HEAL
WOUNDS, & HELPS
ORGANISM GROW
TO PRODUCE
HAPLOID SEX CELLS.
HOMOLOGOUS
CHROMOSOMES LINE UP
SIDE BY SIDE FORMING A
GROUP OF 4 CHROMATIDS
KNOWN AS A TETRAD
E. Genetic Variation
1. How can genetic variation help a population? Helps the
individuals adapt to changes in their environment. Not all may
be able to adapt but those that can keep the species from going
extinct & help them evolve.
2. Identify & describe conditions that cause genetic variation.
a. Crossing over between homologous chromosomes gives
those chromosomes different genes.
b. Random Fertilization – don’t know which of the sperm will
fertilize the egg. There are 223 or 8 million different gene
combinations for a male & there are 223 or 8 million different
gene combinations for a female. This means that for any
couple there are over 70 TRILLION different gene
combinations for the zygote.
c. Independent Assortment – the random distribution of
homologous chromosomes during meiosis.
Example: Individual with genes AaBb can form 4 different
types of sex cells due to independent assortment:
AB, Ab, aB, ab
d. Mutations – occasionally there are mistakes that happen
when the DNA is copied; these mutations can also lead to
genetic variation.