Chapter 10
Meiosis and Sexual Reproduction
Objectives

1. Contrast asexual and sexual types of
reproduction that occur on the cellular and
multicellular organism levels.

2. Understand the effect that meiosis has on
chromosome number.

3. Describe the events that occur in each
meiotic phase.
Objectives
 4.
Compare mitosis and meiosis; cite
similarities and differences.
 5.
Contrast meiosis in plant and animal
life cycles.
10.0 Why Sex
 Asexual
reproduction is easier and faster
 One parent alone transmits genetic
information to offspring. (all clones)
 Sexual reproduction can be an alternative
adaption in changing environments.
(survival)
 Male and female must find each other and
exchange genetic material.
Why Sex
 Sexual
reproduction has advantages when
other organisms change. (Predators and
prey, Hosts and pathogens)
 The outcome of sexual reproduction is
offspring that display novel combinations
of traits.(diversity)
10.1 Alleles and Sexual Reproduction
 Sexual
Reproduction involves

Meiosis

Gamete production

Fertilization
 Produces
offspring
genetic variation among
Introducing Alleles
– each unique molecular form of the
same gene.
 Allele
 Such
tiny differences affect thousands of
traits.
 Alleles
are one reason why individuals do
not all look alike.
 Sexual
reproduction leads to new alleles
Homologous Chromosomes
Carry Different Alleles
 Cell
has two of each chromosome
 One
chromosome in each pair from
mother, other from father
 Paternal
and maternal chromosomes carry
different alleles
Homologous
Chromosomes
Fig. 10-2, p.156
Sexual Reproduction
Shuffles Alleles
 Through
sexual reproduction, offspring
inherit new combinations of alleles, which
leads to variations in traits
 This
variation in traits is the basis for
evolutionary change
Alleles
Section 10.2: What Meiosis Does
 Meiosis
is a nuclear division process that
divides a parental chromosome number by
half in specialized reproductive cells.
 Sexual
 Unlike
reproduction will not work without it.
mitosis, meiosis sorts out
chromosomes into parcels two times.
Germ cells undergo meiosis and
cytoplasmic division
 Meiosis
involves only the sex cells.
 Cellular
descendents of germ cells
become gametes, (sperm and egg)
 Gametes
meet at fertilization
Fig. 10-3, p.156
Chromosome Number
 Sum
total of chromosomes in a cell
 Germ
cells are diploid (2n), they have a
pair of each type of chromosome. We call
them homologous chromosomes.
 Gametes
 Meiosis
number
are haploid (n)
halves parental chromosome
Meiosis: Two Divisions
 Two
consecutive nuclear divisions

Meiosis I

Meiosis II
 DNA
is not duplicated between divisions –
NO Interphase
 Four
haploid nuclei form
Meiosis I – Prophase I, Metaphase
I, Anaphase I, Telophase I
Each homologue (matching chromosome) in the
cell pairs with its partner,
then the partners
separate
p. 158
Meiosis II - Prophase II, Metaphase
II, Anaphase II, Telophase II

The two sister chromatids of each duplicated
chromosome are separated from each other
two chromosomes
(unduplicated)
one chromosome
(duplicated)
p. 158
10.3 Meiosis I -Prophase I
 Each
duplicated
chromosome pairs with
homologue
 Homologues swap
segments (crossing
over).
 Each chromosome
becomes attached to
spindle
Fig. 10-5, p. 158
Metaphase I
 Chromosomes
are
pushed and pulled
into the middle of
cell by microtubules
 The spindle is fully
formed
Fig. 10-5, p. 158
Anaphase I
 Homologous
chromosomes
separate and begin to
move toward pole.
 The
sister chromatids
remain attached
Fig. 10-5, p. 158
Telophase I
 The
chromosomes
arrive at opposite poles
 Usually followed by
cytoplasmic division.
 Now have two haploid
cells (n).
 Chromosomes are still
duplicated.
Fig. 10-5, p. 158
Prophase II

In each daughter cell
microtubules attach to
the kinetochores of
the duplicated
chromosomes.

One chromatid of
each chromosome
becomes tethered to
one spindle pole.
Fig. 10-5, p. 158
Metaphase II
 In
each daughter
cell duplicated
chromosomes line
up at the spindle
equator, midway
between the poles
Fig. 10-5, p. 158
Anaphase II
II
 In
each daughter
cell sister
chromatids
separate and move
toward opposite
poles to become
independent
chromosomes.
Fig. 10-5, p. 158
Telophase II
II
 The
chromosomes
arrive at opposite ends
of the cell
 A nuclear envelope
forms around each set
of chromosomes, each
cell divides in half.
 Four haploid (n) cells.
Fig. 10-5, p. 158
Section 10.4: How Meiosis Introduces
Variations in Traits
over – a molecular interaction
between a chromatid of one chromosome
and a chromatid of the homologous
partner.
 Crossing
 This
really is gene swapping.
Crossing Over
• During Prophase I each
chromosome becomes
zippered to its homologue
•All four chromatids are
closely aligned
•Nonsister chromosomes
exchange segments
Effect of Crossing Over
 After
crossing over, each chromosome
contains both maternal and paternal
segments
 Breaks
up old combinations of alleles and
creates new allele combinations in
offspring
Random Alignment
 During
transition between prophase I and
metaphase I, microtubules from spindle
poles attach to kinetochores of
chromosomes.
 Initial contacts between microtubules and
chromosomes are random, there is no
particular pattern to the metaphase
position of chromosomes.
Random Alignment
 Either
the maternal or paternal member of
a homologous pair can end up at either
pole. This can also lead to different traits
in each new generation.
 The
chromosomes in a gamete are a mix
of chromosomes from the two parents.
Possible Chromosome
Combinations
As a result of random alignment, the number
of possible combinations of chromosomes
in a gamete is:
2n
(n is number of chromosome types)
Possible Chromosome
Combinations
 Thus,
every time a human sperm or egg
forms, there is a total of 8,388,608 or
 223
 Possible
combinations of maternal and
paternal chromosomes.
Section 10.5: From Gametes to
Offspring
 The
life cycle of most plant species
alternates between sporophyte and
gametophyte stages.
 A sporophyte is a spore producing body
that makes spores by the process of
meiosis.
 A spore is a haploid reproductive cell that
undergoes mitosis and gives rise to a
gametophyte.
 A gametophyte gives rise to gametes,
which can then be fertilized and form the
zygote.
Plant Life Cycle
sporophyte
zygote
fertilization
diploid
meiosis
haploid
spores
gametes
gametophytes
Fig. 10-8a, p.162
Gamete formation in animals
 In
the male reproductive system, a germ
cell develops into four haploid cells, each
becoming a sperm.
 In the female reproductive system, a germ
cells develops into one haploid ovum, or
egg, and three polar bodies. The polar
bodies eventually degenerate.
 When fertilization occurs the diploid
number is restored.
Animal Life Cycle
multicelled
body
zygote
fertilization
diploid
haploid
meiosis
gametes
Fig. 10-8b, p.162
Fertilization
 Male
and female gametes unite and nuclei
fuse
 Fusion
of two haploid nuclei produces
diploid nucleus in the zygote
 Which

two gametes unite is random
Adds to variation among offspring
Factors Contributing to Variation
among Offspring
 Crossing
over during prophase I
(average of 2 or 3 in every human
chromosome)
 Random alignment of chromosomes at
metaphase I
 Random
combination of gametes at
fertilization
10.6 Mitosis & Meiosis
Compared
Mitosis

Functions



Meiosis

Asexual reproduction
Growth, repair
Occurs in somatic
cells
 Produces clones
Function

Sexual reproduction

Occurs in germ cells

Produces variable
offspring
Prophase vs. Prophase I
 Prophase

(Mitosis)
Homologous pairs do not interact with each
other
 Prophase

I (Meiosis)
Homologous pairs become zippered together
and crossing over occurs
Anaphase, Anaphase I, and
Anaphase II
 Anaphase

I (Meiosis)
Homologous chromosomes separate from
each other
 Anaphase/Anaphase

II (Mitosis/Meiosis)
Sister chromatids of a chromosome separate
from each other
Results of Mitosis and Meiosis
 Mitosis

Two diploid cells produced

Each identical to parent
 Meiosis

Four haploid cells produced

Differ from parent and one another
Repair of DNA breaks
 Checkpoint
genes code for proteins that
can recognize and repair breaks in the
double-stranded DNA molecules of
chromosomes.
 If they detect a problem, there is a pause
in the cycle until the DNA is repaired.
Review of Meiosis
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An Ancestral Connection

Was sexual reproduction a giant evolutionary step from
aseuxal reproduction?

Giardia intestinalis – single-celled parasite, does not
have a mitochondria, does not form a spindle during
mitosis, and has never been observed to reproduce
sexually.

Chlamydomonas – a single-celled alga, haploid cells
reproduce asexually by mitosis. They can also fuse and
form diploid individuals.