Chapter 10
Meiosis and Sexual Reproduction
Why Sex
Fig. 10-1b, p.154
Why sex?

Asexual
 Easier, faster
 Big population
 Indentical
 Bits can make whole
indv.
 No new combos
 All inherit the same
info
 Clones
 parthogenesis

Sexual
 Changing env
 More variety
 New combos
 Involves meiosis
(gametes) and
fertilization
 allele
Cost of Sexual Reproduction
Fig. 43-2c, p.756
43.1 (p. 756)
Cost of Sexual Reproduction

Specialized cells and structures must be formed

Special courtship, and parental behaviors can be
costly

Timing of gamete formation and mating

Nurturing developing offspring, either in egg or
body, requires resources from mother
10.2 What Meiosis Does

Meiosis – nuclear division that divides
parental c-some # by half in specialized
reproductive cells

Ex: anther, ovules
anther
ovary
Homologous Chromosomes
Carry Different Alleles

Homologous c-some –
same shape, length and
assortment of genes, line
up with each other

Paternal and maternal
chromosomes can carry
different alleles
Chromosome Number

Sum total of chromosomes in a cell

Germ cells are diploid (2n)

Gametes are haploid (n)

Meiosis halves chromosome number
Meiosis: Two Divisions

Two consecutive nuclear divisions

Meiosis I – aligns with partner

Meiosis II – sister chromatids separate

DNA is not duplicated between divisions

Four haploid nuclei form
10.4 Factors Contributing to
Variation among Offspring

Crossing over during prophase I

Independent assortment

Random alignment of chromosomes
at metaphase I

Random combination of gametes at
fertilization
Crossing Over
•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

Creates new allele combinations in
offspring
Independent Assortment

Microtubules from spindle poles attach to
kinetochores of chromosomes randomly,
between Prophase I and Metaphase I
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)
Fertilization

Which two gametes unite is random

Adds to variation among offspring
Life Cycles
•
Plant
•
Animal
Plant Life Cycle
sporophyte
zygote
fertilization
diploid
meiosis
haploid
spores
gametes
gametophytes
Fig. 10-8a, p.162
Animal Life Cycle
multicelled
body
zygote
fertilization
diploid
haploid
meiosis
gametes
Fig. 10-8b, p.162
44.2
Spermatogenesis



Spermatogonium (2n) divides by mitosis
to form primary spermatocyte (2n)
Meiosis produces haploid spermatids
Spermatids mature to become sperm
movie
Figure 44.4
Page 775
Male Hormonal Control
Hypothalamus
GnRH
Inhibin
Anterior Pituitary
FSH
Leydig
Cells
LH
Testes
Sertoli
Cells
Testosterone
Formation and Development of Sperm
44.1
Oocytes Arrested in Meiosis I

Girl is born with primary oocytes already
in ovaries

Each oocyte has entered meiosis I and
stopped

Meiosis resumes, one oocyte at a time,
with the first menstrual cycle
Ovarian
Cycle

secondary oocyte
first
polar
body
Follicle grows
and matures

antrum
Ovulation
occurs

Corpus luteum
forms
Figure 44.8
Page 778
corpus
luteum
primordial
follicle
Primary oocyte, not
yet released from
meiosis I. A cell
layer is forming
around it. A follicle
consists of the cell
layer and the
oocyte.
A transparent and somewhat elastic
layer, the zona pellucida, starts forming
around the primary oocyte.
A fluid-filled cavity (antrum)
starts forming in the
follicle’s cell layer.
Mature follicle.
Meiosis I is over.
The secondary
oocyte and first
polar body are
now formed.
primordial
follicle
first polar body
secondary oocyte
The corpus
luteum breaks
down when the
woman doesn’t
get pregnant.
A corpus luteum forms from
remnants of the ruptured follicle.
Ovulation. Mature follicle ruptures and releases the
secondary oocyte and the first polar body.
Fig. 44-8b, p.778
Female Hormonal Control
Hypothalamus
Rising
estrogen
stimulates
surge in LH
Estrogen
GnRH
Anterior pituitary
LH
FSH
Ovary
follicle growth,
oocyte maturation
Corpus luteum
forms
Progesterone,
estrogens
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
An Ancestral Connection

Was sexual reproduction a giant evolutionary step from
aseuxal reproduction?

Giardia intestinalis

Chlamydomonas

Recombination mechanisms are vital for reproduction of
euk cells may have evolved from DNA repair
mechanisms in prok ancestors