Chapter 10: Meiosis and Sexual
Reproduction (Outline)
 Reduction in Chromosome Number
 Homologous Pairs
 Genetic Recombination
 Crossing-Over
 Independent Assortment
 Fertilization
 Phases of Meiosis
 Meiosis I
 Meiosis II
 Meiosis Compared to Mitosis
 Human Life Cycle
Meiosis: Halving the Chromosome
Number
 Special type of cell division
 Used only for sexual reproduction
 Halves the chromosome number prior to
fertilization




Parents diploid (2n)
Meiosis produces haploid gametes (1n)
Gametes fuse in fertilization to form diploid zygote
Becomes the next diploid generation
 If gametes were not haploid the number of
chromosomes would double itself in each
generation
Homologous Pairs of
Chromosomes
 In diploid body cells, chromosomes occur in pairs
 Diploid cells have two of each type
 Human cells have 46 chromosomes in 23
homologous pairs
 Homologous Chromosomes
 Paired chromosomes in somatic cells
 Similar in size, shape and position of their centromeres
 Carry information about the same genetic traits (not
always the same information)

When stained, they show similar banding patterns
Homologous
Chromosomes
Homologous Pairs of
Chromosomes
 Homologous chromosomes have genes
controlling the same trait at the same position
 Each gene occurs in duplicate, why?
 The variants that exist for a gene are called
alleles
 An individual may have:
 Identical alleles for a specific gene on both homologs
(homozygous for the trait), or
 A maternal allele that differs from the corresponding
paternal allele (heterozygous for the trait)
Overview of Meiosis
 Meiosis requires 2 nuclear divisions and
produces 4 haploid daughter cells
 Cells are diploid at beginning of meiosis
 Pairs of chromosomes are called homologues
 Meiosis I
 Homologues line up side by side at equatorsynapsis
 Synapsis results in a bivalent
 When pairs separate, each daughter cell
receives one member of the pair
 Cells are now haploid
Overview of Meiosis (cont.)
 Meiosis II
 No replication of DNA occurs in this division, why?
 Centromeres divide & sister chromatids migrate to
opposite poles to become individual chromosomes
 Each of the four daughter cells produced has the
haploid chromosome number and each
chromosome is composed of one chromatid
 In plants, daughter cells are haploid spores
that germinate to haploid generation; gametes
produced by mitosis
 In animals, daughter cells are gametes (i.e.
sperm or eggs)
Meiosis Overview
Genetic Variation
 Meiosis helps ensure genetic recombination
 In a changing environment, asexual
reproduction might be disadvantageous
 Sexual reproduction might give offspring better
chance of survival
 Meiosis brings about genetic variation in two
key ways:
 Crossing-over
 Independent assortment
Crossing-Over
 Exchange of genetic material between nonsister
chromatids of a bivalent during meiosis I
 At synapsis, a nucleoprotein lattice appears
between homologues
 Holds homologues together and aligns DNA of
nonsister chromatids
 Allows crossing-over to occur
 Homologues are held together by chiasmata
 Homologues then separate and are distributed
to different daughter cells
Synapsis and crossing over
Independent Assortment
 Independent assortment:
 When homologues align at the metaphase plate:
 They separate in a random manner
 The maternal or paternal homologue may be
oriented toward either pole of mother cell
 Causes random mixing of blocks of alleles into
gametes
Fertilization
 Gametes produced by one person are
genetically different those produced by another
person
 When gametes fuse at fertilization:


Chromosomes donated by the parents are combined
In humans, (223)2 = 70,368,744,000,000
chromosomally different zygotes are possible
 If crossing-over occurs only once

(423)2, or 4,951,760,200,000,000,000,000,000,000
genetically different zygotes are possible
 Remember, crossing-over can occur several
times in each chromosome!
Significance of Genetic Variation
 Asexual reproduction produces genetically
identical clones
 Sexual reproduction produces genetic variety
 Asexual reproduction is advantageous when
environment is stable
 However, if environment changes, genetic
variability introduced by sexual reproduction
may be advantageous
Phases of Meiosis I
 Prophase I
 Each chromosome is internally duplicated
(consists of two identical sister chromatids)
 Homologous chromosomes (maternal
homologue and paternal homologue) align side
by side (synapsis)
 Synapsis results in association of four
chromatids (a tetrad)
 Paired homologous chromosomes exchange
genetic material (crossing-over)
Phases of Meiosis I
 Metaphase I
 Homologous pairs (bivalents) or tetrads
arranged onto the metaphase plate
independently
 The centrioles are at opposite poles of the cell
 Spindle fibers from one pole of the cell attach
to one duplicated chromosome of each pair
(seen as sister chromatids)
Phases of Meiosis I
 Anaphase I
 Synapsis breaks up
 Homologous chromosomes separate from one
another and move towards opposite poles
 Each pole randomly receives a maternal or
paternal chromosome from each homologous
pair
 Each is still an internally duplicate chromosome
with two chromatids
Phases of Meiosis I
 Telophase I
 Daughter cells have one internally duplicate
chromosome from each homologous pair
 One (internally duplicate) chromosome of each
type (1n, haploid)
 Nuclear envelope may reorganize, and
cytokinesis may take place
 Interkinesis
 Similar to mitotic interphase but shorter
 No replication of DNA, why?
Meiosis I
Phases of Meiosis II:
Similar to Mitosis
 Prophase II – Chromosomes condense
 Metaphase II – chromosomes align at
metaphase plate
 Anaphase II
 Centromere dissolves
 Sister chromatids separate and move to
opposite poles (daughter chromosome)
 Telophase II and Cytokinesis II
 Four haploid cells
 All genetically unique
Meiosis II
Overview of Meiosis I & II
Contrasting Mitosis and Meiosis
 Several fundamental differences between the
two processes include:
 Meiosis requires two nuclear divisions, but mitosis
requires one nuclear division
 Meiosis produces four daughter cells, but mitosis
results in two daughter cells following cytokinesis
 In meiosis, daughter cells are haploid, whereas
mitosis preserves chromosome number
 In meiosis, daughter cells are genetically different
from parent and each other, but mitosis results in
daughter cells that are genetically identical to parent
and to each other
Meiosis vs. Mitosis
 Occurrence
 Meiosis occurs only at certain times in the life cycle
of sexually reproducing organisms
 In humans, meiosis occurs in reproductive organs
and produces gametes
 Mitosis is more common since it occurs in all
tissues during growth & repair
 Process
 Meiosis I compared to Mitosis
 Meiosis II compared to Mitosis
Meiosis I Compared to Mitosis
Meiosis II Compared to Mitosis
Life Cycle Basics:
Plants
 Life cycle – reproductive events that occur from one
generation to the next similar generation
 Haploid multicellular gametophyte alternate with
diploid multicellular sporophyte
 Mosses are haploid most of
their life cycle
 In fungi and most algae, only
the zygote is diploid
 In plants, algae and fungi,
gametes are produced by
haploid individuals
Life Cycle Basics:
Animals
 In animals, somatic cells are diploid and
multiply by mitosis – the only haploid cells
produced are gametes
 Gametes develop when germ line cells undergo
meiosis
 Gametogenesis is the formation of gametes
 Spermatogenesis (male gametogenesis)
forms four haploid sperm cells for each cell
that enters meiosis
 Oogenesis (female gametogenesis) forms
one egg cell (ovum) for every cell that
enters meiosis, plus polar bodies
The Human Life Cycle
 A sperm and egg fuse at
fertilization
 Results in a zygote
 Undergoes mitosis
 Results in multicellular embryo
 As a result of mitosis, each
somatic cell in body
 Has same number of
chromosomes as zygote
 Has genetic makeup
determined when zygote was
formed
Oogenesis in Humans
 Ovaries contain oogonia that produce primary
oocytes during fetal development
 Primary oocyte continue to develop at onset of
puberty and divide through meiosis I into two cells
 One of these cells (secondary oocyte) receive most
of the cytoplasm; the other polar body may divide
or disintegrate
 Secondary oocyte begins meiosis II but stops at
metaphase II
 Then leaves the ovary and enters the oviduct
 If sperm enters, meiosis II continues & another polar
body forms
Spermatogenesis in Humans
 Spermatogenesis takes place within the testes
 Stem cells within the testes (spermatogonia)
become primary spermatocytes; undergo
spermatogenesis
 Meiosis produces haploid secondary
spermatocytes (meiosis I) and haploid
spermatids (meiosis II)
 Four spermatids are produced from the original
primary spermatocyte – each differentiates into
a mature sperm (spermatozoa)
Gametogenesis in Mammals