Chapter 13

Sexual Life
Cycles and
Meiosis

Lecture Presentations by
Nicole Tunbridge and
© 2018 Pearson Education Ltd.
Kathleen Fitzpatrick

Variations on a Theme

§ Offspring resemble their parents more than they do

unrelated individuals
§ Heredity is the transmission of traits from one
generation to the next
8£
§ Variation is demonstrated by the differences in
appearance that offspring show from parents and
siblings
§ Genetics is the scientific study of heredity and
variationb
Heredity 1- vari 1- ion
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Figure 13.1

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Figure 13.1a

A sperm fertilizing an egg

IN

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Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
§ In a literal sense, children do not inherit particular
physical traits from their parents
s
§ It is genes that are actually inherited

:* it

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Inheritance of Genes

§ Genes are the units of heredity and are made up of

segments of DNA
§ Genes are passed to the next generation via
reproductive cells called gametes (sperm and eggs)
§ Most DNA is packaged into chromosomes
§ Humans have 46 chromosomes in the nuclei of their
2Nsomatic cells, all cells of the body except gametes
and their precursors
§ A gene’s specific position along a chromosome is
called its locus
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 Comparison of Asexual and Sexual
Reproduction * →

§ In asexual reproduction, a single individual passes

all of its genes to its offspring without the fusion of
gametes
§ A clone is a group of genetically identical individuals
from the same parent
b-
§ In sexual reproduction, two parents give rise to
offspring that have unique combinations of genes
inherited from the two parents

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Figure 13.2

0.5 mm

Parent

Bud

n (a) Hydra (b) Redwoods

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Concept 13.2: Fertilization and meiosis
alternate in sexual life cycles
§ A life cycle is the generation-to-generation
sequence of stages in the reproductive history of an
-

organism

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Sets of Chromosomes in Human Cells

§ Human somatic cells have 23 pairs of chromosomes

-

j
§ A karyotype is an ordered display of the pairs of
chromosomes from a cell
§ The two chromosomes in each pair are called
homologous chromosomes, or homologs
§ Chromosomes in a homologous pair are the same
length and shape and carry genes controlling the
same inherited characters

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Figure 13.3

Application Technique
Pair of homologous
duplicated chromosomes

Centromeres
5 µm

Sister
chromatids
Metaphase
chromosome

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Figure 13.3a

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Figure 13.3b

5 µm

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§ The sex chromosomes, which determine the sex of

the individual, are called X and Y
§ Human females have a homologous pair of X
chromosomes (XX)
§ Human males have one X and one Y chromosome
§ The remainingo
22 pairs of chromosomes are called
autosomes
b

be {
im .

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§ Each pair of homologous chromosomes includes
-

one chromosome from each parent

§ The 46 chromosomes in a human somatic cell are
two sets of 23: one from the mother and one from
the father
§ A diploid cell (2n) has two sets of chromosomes

c.
§ For humans, the diploid number is 46 (2n = 46)

G. HIM %

be 511646 see

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§ In a cell in which DNA synthesis has occurred, each

chromosome is replicated
§ Each replicated chromosome consists of two
identical sister chromatids

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 chromosomes ?
Figure 13.4
How M qn T
'

I
•

Maternal set of chromosomes (n = 3)

0
2n = 6
Paternal set of chromosomes (n = 3)

Sister chromatids
of one duplicated 0
chromosome ① Centromere

③ ⑧
⑨
Two nonsister ⑥ Pair of homologous
]
chromatids in chromosomes
a homologous pair (one from each set)
0
? His is 2- Ñ8
→ Has is 3
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§ A gamete (sperm or egg) contains a single set of

chromosomes and is thus a haploid cell (n)
§ For humans, the haploid number is 23 (n = 23)
§ Each set of 23 consists of 22 autosomes and a
single sex chromosome
§ In an unfertilized egg (ovum), the sex chromosome
is X
§ In a sperm cell, the sex chromosome may be either
X or Y

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Behavior of Chromosome Sets in the Human
Life Cycle
←wt ③I
§ Fertilization is the union of gametes (the sperm and
the egg) 2N Cell -
-
.is
§ The fertilized egg is called a zygote and has one set
of chromosomes from each parent In 1- In = 2h
§ The zygote produces somatic cells by mitosis and
develops into an adult

→ 0%1
ein $1 .

In
growth
mitosis
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GA GI ing
§ At sexual maturity, the ovaries and testes produce
haploid gametes
§ Gametes are the only types of human cells produced
by meiosis, rather than mitosis Wsj

ingest
- -

§ Meiosis results in one set of chromosomes in each

gamete
Is
§ Fertilization and meiosis alternate in sexual life
cycles to maintain chromosome number

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Figure 13.5
Haploid gametes (n = 23)
Egg (n)

Sperm (n)

MEIOSIS FERTILIZATION

Testis
Ovary
Diploid
zygote
(2n = 46)

Mitosis and
development

Multicellular diploid Haploid (n)

adults (2n = 46) Diploid (2n)
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The Variety of Sexual Life Cycles

§ The alternation of meiosis and fertilization is

common to all organisms that reproduce sexually
§ The three main types of sexual life cycles differ in
the timing of meiosis and fertilization

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Figure 13.6
II. ( 2n ) b- ( Miosis Ilg
( mitosis/ Alin it

co
Haploid (n) Haploid multi-
Diploid (2n) cellular organism Haploid unicellular or
-

(gametophyte) multicellular organism

n Gametes n
n Mitosis n Mitosis Mitosis n Mitosis
n

0
n n n n
MEIOSIS FERTILIZATION Spores n n
Gametes Gametes n
MEIOSIS FERTILIZATION

2n Zygote 2n MEIOSIS FERTILIZATION

2n
2n Zygote
2n
Mitosis Mitosis Zygote

iÉÉ①
Diploid Diploid
multicellular multicellular
organism organism
(sporophyte)
(a) Animals (b) Plants and some algae (c) Most fungi and
some protists

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Animals life cycle

§ Gametes are the only haploid cells in animals

§ They are produced by meiosis and undergo no
further cell division before fertilization
§ Gametes fuse to form a diploid zygote that divides
by mitosis to develop into a multicellular organism

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Figure 13.6a

n Gametes n

MEIOSIS FERTILIZATION

Zygote
2n 2n
Diploid
multicellular Mitosis
organism

Haploid (n)
(a) Animals Diploid (2n)

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plants
81*46 I
peantW.oI@AoJ.W
.

§ Plants and some algae exhibit an alternation of

¥14
generations
( In)
§ This life cycle includes both a diploid and haploid
multicellular stage
§ The diploid organism, called the sporophyte, makes
haploid spores by meiosis

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§ Each spore grows by mitosis into a haploid organism
called a gametophyte → In
§ A gametophyte makes haploid gametes by mitosis
§ Fertilization of gametes results in a diploid
sporophyte
23 e is In
mitosis -

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Figure 13.6b
Haploid multi-
cellular organism
(gametophyte)

Mitosis n Mitosis
n n
n n
Spores
Gametes

MEIOSIS FERTILIZATION

2n
2n
Diploid Zygote
multicellular Mitosis
organism
(sporophyte)
Haploid (n)
(b) Plants and some algae Diploid (2n)
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 fungi 8 some protists

§ In most fungi and some protists, the only diploid

stage is the single-celled zygote; there is no
b
multicellular diploid stage -6 2h ) .

§ The zygote produces haploid cells by meiosis

§ Each haploid cell grows by mitosis into a haploid
multicellular organism
§ The haploid adult produces gametes by mitosis

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Figure 13.6c

Haploid unicellular or
multicellular organism

Mitosis n Mitosis

n
n n
Gametes n

MEIOSIS FERTILIZATION

2n

Zygote
Haploid (n)
(c) Most fungi and some protists Diploid (2n)
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§ Depending on the type of life cycle, either haploid or
diploid cells can divide by mitosis
§EHowever, only diploid cells can undergo meiosis
§ In all three life cycles, the halving and doubling
of chromosomes contributes to genetic variation
in offspring

trona
-
one for

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Concept 13.3: Meiosis reduces the number of

chromosome sets from diploid to haploid
§ Like mitosis, meiosis is preceded by the replication
of chromosomes
i.mg
§ Meiosis takes place in two consecutive cell divisions,
called meiosis I and meiosis II
§ The two cell divisions result in four daughter cells,
rather than the two daughter cells in mitosis
§ Each daughter cell has only half as many
chromosomes as the parent cell

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 The Stages of Meiosis
IF &
§ Chromosomes duplicate before meiosis
§ The resulting sister chromatids are closely
associated along their lengths
Ñ Is
§ This is called sister chromatid cohesion →
§ The chromatids are sorted into four haploid daughter
cells

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Figure 13.7
Interphase
Pair of
=
homologous
chromosomes
in diploid
parent cell
Pair of duplicated Chromosomes
homologous =
duplicate
chromosomes

Sister Diploid cell with

chromatids duplicated
chromosomes

iii.HI Meiosis I
1
Homologous
chromosomes
separate
Haploid cells with
duplicated chromosomes
Meiosis II

'Ww←
2 Sister chromatids
separate

est 2m£
Haploid cells with unduplicated chromosomes
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Figure 13.7a

Interphase

Pair of homologous
chromosomes in
diploid parent cell

Chromosomes
Pair of duplicated duplicate
homologous chromosomes

Sister
chromatids Diploid cell with
duplicated
chromosomes
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Figure 13.7b

Meiosis I

1
Homologous
chromosomes
separate

Haploid cells with

duplicated chromosomes
Meiosis II

2 Sister chromatids
separate

Haploid cells with unduplicated chromosomes

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§ Division in meiosis I occurs in four phases:
§ prophase I →
§ metaphase I
§ anaphase I
§ telophase I and cytokinesis

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is

)A&yH1
GroPhase one

a
Prophase I
§ In early prophase I, each chromosome pairs with its
homolog and crossing over occurs
§ X-shaped regions called chiasmata are sites
of crossovers

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Metaphase I
§ In metaphase I, pairs of homologs line up at the
metaphase plate, with one chromosome facing each
pole
§ Microtubules from one pole are attached to the
kinetochore of one chromosome of each pair
§ Microtubules from the other pole are attached to the
kinetochore of the other chromosome

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Anaphase I
§ In anaphase I, pairs of homologous chromosomes

=
separate
§ One chromosome of each pair moves toward
opposite poles, guided by the spindle apparatus
§ Sister chromatids remain attached at the centromere
and move as one unit toward the pole

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Telophase I and Cytokinesis
§ In the beginning of telophase I, each half of the
cell has a haploid set of chromosomes; each
chromosome still consists of two sister chromatids
§ Cytokinesis usually occurs simultaneously, forming
two haploid daughter cells

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§ In animal cells, a cleavage furrow forms; in plant

cells, a cell plate forms
§ No chromosome replication occurs between the end
of meiosis I and the beginning of meiosis II because
the chromosomes are already replicated

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 over Ssw
Grossing )A
Figure 13.8a

MEIOSIS I: Separates homologous chromosomes

Prophase I Metaphase I Anaphase I Telophase I

and Cytokinesis

Centrosome Sister
(with Kinetochore chromatids
centriole (at centromere) remain
Sister pair) attached

:
chroma-
Chiasmata
tids
Spindle Kinetochore
micro- microtubules
tubules

Cleavage
Homologous furrow
Pair of Fragments chromosomes
homo- of nuclear separate
envelope Metaphase
logous
plate
chromo-
somes Centromere

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§ Division in meiosis II also occurs in four phases:

§ prophase II
§ metaphase II
§ anaphase II
§ telophase II and cytokinesis
§ Meiosis II is very similar to mitosis

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Prophase II
§ In prophase II, a spindle apparatus forms
§ In late prophase II, chromosomes (each still
composed of two chromatids) move toward the
metaphase plate

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Metaphase II
§ In metaphase II, the sister chromatids are arranged
at the metaphase plate
§ Because of crossing over in meiosis I, the two sister
chromatids of each chromosome are no longer
genetically identical
§ The kinetochores of sister chromatids attach to
microtubules extending from opposite poles

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Anaphase II
§ In anaphase II, the sister chromatids separate
§ The sister chromatids of each chromosome now
move as two newly individual chromosomes toward
opposite poles

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Telophase II and Cytokinesis

§ In telophase II, the chromosomes arrive at opposite
poles
§ Nuclei form, and the chromosomes begin
decondensing
y

:& .

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§ Cytokinesis separates the cytoplasm
§ At the end of meiosis, there are four daughter cells,
each with a haploid set of unreplicated
chromosomes
§ Each daughter cell is genetically distinct from the
others and from the parent cell

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Figure 13.8b

MEIOSIS II: Separates sister chromatids

Prophase II Metaphase II Anaphase II Telophase II

and Cytokinesis

Sister chromatids
separate Haploid daughter
cells forming

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 Crossing Over and Synapsis During Prophase I

§ After interphase, the sister chromatids are held

together by proteins called cohesins
§ The nonsister chromatids are broken at precisely
corresponding positions é ④ I - > •
←

←
§ A zipper-like structure called the synaptonemal
complex holds the homologs together tightly
* to
§ During synapsis, DNA breaks are repaired, joining
DNA from one nonsister chromatid to the
ccorresponding segment of another
b
gtso
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Figure 13.9a

non sister Pair of homologous

(
chromosomes:

DNA Centromere DNA Paternal

breaks Cohesins breaks sister
chromatids

0 Maternal
sister
1 chromatids
Synaptonemal
complex forming

2
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Figure 13.9b

Sister chromatids Synaptonemal

Sister complex
chromatids

Crossovers
3

Chiasmata imma

0
4
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A Comparison of Mitosis and Meiosis

( 2h ) ? 'M ( 2n )
-

W I :&
.
.

I
§ Mitosis conserves the number of chromosome sets,
producing cells that are genetically identical to the
parent cell P.im/2n) 64 > &
→
§ Meiosis reduces the number of chromosomes sets
from two (diploid) to one (haploid), producing cells
that differ genetically from each other and from the
parent cell
Ib ) ¥-45
) A1 (
(meiosis

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§ Three events are unique to meiosis, and all three
occur in meiosis I
§ Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
§ Homologous pairs at the metaphase plate
§ Separation of homologs during anaphase I
b

toss

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Figure 13.10a

MITOSIS MEIOSIS
Parent cell Chiasma MEIOSIS I

Prophase Prophase I
Chromosome Chromosome
Duplicated Pair of
duplication duplication
chromosome 2n = 6 duplicated
homologs
Individual Pairs of
Metaphase homologous Metaphase I
chromosomes
line up. chromosomes
line up.
Anaphase Anaphase I
Sister chromatids Homologs
Telophase separate. Telophase I
separate.

Daughter
Sister cells of
meiosis I
chromatids MEIOSIS II
2n 2n separate.
Daughter cells n n n n
of mitosis Daughter cells of meiosis II

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Figure 13.10aa

MITOSIS MEIOSIS
Parent cell Chiasma MEIOSIS I
Prophase Prophase I
Chromosome Chromosome Pair of
Duplicated duplicated
chromosome duplication 2n = 6 duplication
homologs

Individual Pairs of
Metaphase chromosomes homologous Metaphase I
line up. chromosomes
line up.

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Figure 13.10ab

MITOSIS MEIOSIS
Anaphase Anaphase I
Sister
Telophase chromatids Homologs Telophase I
separate. separate.

Daughter
Sister cells of
chromatids meiosis I
MEIOSIS II
2n 2n separate.
n n n n
Daughter cells
of mitosis Daughter cells of meiosis II

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 .MY# 2h
Figure 13.10b
Ingi 2h ? 4
9

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§ Sister chromatid cohesion allows sister chromatids

to stay together through meiosis I
*

§ In mitosis, cohesins are cleaved at the end of

metaphase
-

§ In meiosis, cohesins are cleaved along the

chromosome arms in anaphase I (separation of
homologs) and at the centromeres in anaphase II
(separation of sister chromatids)

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Concept 13.4: Genetic variation produced in
sexual life cycles contributes to evolution
§ Mutations (changes in an organism’s DNA) are the
original source of genetic diversity
§ Mutations create different versions of genes called
b
alleles j'µl
§ Reshuffling of alleles during sexual reproduction
produces genetic variation

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Origins of Genetic Variation Among Offspring

§ The behavior of chromosomes during meiosis and

fertilization is responsible for most of the variation
that arises in each generation
§ Three mechanisms contribute to genetic variation:
§ Independent assortment of chromosomes
§ Crossing over * y
§ Random fertilization §
Aldiss
f.
i

www.t I

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Independent Assortment of Chromosomes

§ Homologous pairs of chromosomes orient randomly

at metaphase I of meiosis
§ In independent assortment, each pair of
chromosomes sorts maternal and paternal homologs
into daughter cells independently of the other pairs
b

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§ The number of combinations possible when

chromosomes assort independently into gametes is
2n, where n is the haploid number
§ For humans (n = 23), there are more than 8 million
(223) possible combinations of chromosomes

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Figure 13.11_1

Possibility 1 Possibility 2

Two equally probable

arrangements of
chromosomes at
metaphase I

Miss.SI
j
's
2. Is

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Figure 13.11_2

Possibility 1 Possibility 2

Two equally probable

arrangements of
chromosomes at
metaphase I

Metaphase II

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Figure 13.11_3

Possibility 1 Possibility 2

Two equally probable

arrangements of
chromosomes at
metaphase I

Metaphase II

Daughter
cells
Combination 1 Combination 2 Combination 3 Combination 4

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Crossing Over just in ( non sister

cronidef,

I to
§ Crossing over produces recombinant
chromosomes, which combine DNA inherited from
each parent
§ Crossing over contributes to genetic variation by
combining DNA from two parents into a single
chromosome
§ In humans, an average of one to three crossover
events occurs per chromosome

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Figure 13.12_1
Prophase I Nonsister chromatids
of meiosis held together
during synapsis
Pair of
homologs 1 Synapsis and
crossing over
Chiasma

Centromere

TEM

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Figure 13.12_2
Prophase I Nonsister chromatids
of meiosis held together
during synapsis
Pair of
homologs 1 Synapsis and
crossing over
Chiasma
2 Movement to
the metaphase I
plate
Centromere

TEM
Anaphase I

d
recombinant chromosomes,

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Figure 13.12_3
Prophase I Nonsister chromatids
of meiosis held together
during synapsis
Pair of
homologs 1 Synapsis and
crossing over
Chiasma
2 Movement to
the metaphase I
plate
Centromere
3 Breakdown of
TEM
Anaphase I proteins holding
sister chromatid
arms together

Anaphase II

© 2018 Pearson Education Ltd.

Figure 13.12_4
Prophase I Nonsister chromatids
of meiosis held together
during synapsis
Pair of
homologs 1 Synapsis and
crossing over
Chiasma
2 Movement to
the metaphase I
plate
Centromere
3 Breakdown of
TEM
Anaphase I proteins holding
sister chromatid
arms together

Anaphase II

Daughter
cells
Recombinant chromosomes
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Random Fertilization

§ Random fertilization adds to genetic variation

because any sperm can fuse with any ovum
(unfertilized egg)
§ The fusion of two gametes (each with 8.4 million
possible chromosome combinations from
independent assortment) produces a zygote with
any of about 70 trillion diploid combinations

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§ Crossing over adds even more variation

§ Each zygote has a unique genetic identity

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The Evolutionary Significance of Genetic
Variation Within Populations
this .
81 * Is I
§ Natural selection results in the accumulation of
genetic variations favored by the environment
§ Sexual reproduction contributes to the genetic
variation in a population, which originates from
mutations
§ Animals that always reproduce asexually are quite
rare *
§ Organisms like the bdelloid rotifer increase their
genetic diversity through incorporation of foreign
DNA from the environment

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