How Cells Reproduce
Chapter 8
Part 2
8.6 Sexual Reproduction and Meiosis
 Two modes of reproduction: asexual and sexual
 Asexual reproduction
• Reproductive mode by which offspring arise from
one parent and inherit that parent’s genes only
• Offspring of asexual reproduction are clones
 Clone
• A genetically identical copy of an organism
Sexual Reproduction
 Offspring of sexual reproduction vary in shared
traits
 Sexual reproduction
• Reproductive mode by which offspring arise from
two parents and inherit genes from both
Inheriting Chromosome Pairs
 Offspring of most sexual reproducers inherit
pairs of chromosomes, one of each pair from the
mother and the other from the father
 Except for a pair of nonidentical sex
chromosomes, the members of a chromosome
pair have the same length, shape, and set of
genes – these are homologous chromosomes
Chromosome Pairs
Introducing Alleles
 Paired genes on homologous chromosomes
often vary slightly in DNA sequence as alleles
 Alleles
• Forms of a gene that encode slightly different
versions of the gene’s product
 Alleles are the basis of traits
Variation in Traits
 Sexual reproduction mixes up alleles from two
parents, resulting in new combinations of alleles
(and traits) in offspring
 Variations in allele combinations are introduced
during meiosis
Meiosis Halves the Chromosome Number
 Meiosis occurs in immature reproductive cells
(germ cells) of sexually reproducing eukaryotes,
forming male and female haploid gametes
 Gamete
• Mature, haploid reproductive cell
 Haploid (n)
• Having one of each type of chromosome
characteristic of the species
Meiosis Halves the Chromosome Number
 Meiosis sorts the chromosomes into new nuclei
twice (meiosis I and meiosis II)
 Duplicated chromosomes of a diploid nucleus
(2n) are distributed into four haploid nuclei (n)
Meiosis I and Meiosis II
each chromosome in the cell pairs
with its homologous partner
then the partners separate
p. 145
two chromosomes
(unduplicated)
one chromosome
(duplicated)
p. 145
Reproductive organs
of a human male
testis
(where sperm
originate)
Fig. 8-9a, p. 144
Reproductive organs
of a human female
ovary
(where eggs
develop)
Fig. 8-9b, p. 144
Restoring Diploid Number
 Diploid number is restored at fertilization, when
two haploid (n) gametes fuse to form a zygote
 Fertilization
• Fusion of a sperm nucleus and an egg nucleus,
resulting in a single-celled zygote
 Zygote
• Diploid (2n) cell formed by fusion of gametes
• First cell of a new individual, with two sets of
chromosomes, one from each parent
8.7 Meiosis
 In meiosis, two nuclear divisions halve the
parental chromosome number
• Meiosis I
• Meiosis II
 Meiosis shuffles parental combinations of
alleles, introducing variation in offspring
• Crossing over in prophase I
• Random assortment in metaphase I
Meiosis I
 In the first nuclear division, duplicated
homologous chromosomes line up and cross
over, then move apart, toward opposite spindle
poles
 Two new nuclear envelopes form around the two
clusters of still-duplicated chromosomes
Crossing Over
 Crossing over is recombination between
nonsister chromatids of homologous
chromosomes which produces new
combinations of parental alleles
 Crossing over
• Homologous chromosomes exchange
corresponding segments during prophase I of
meiosis
Crossing Over
Fig. 8-11a, p. 148
crossover
Fig. 8-11b, p. 148
Fig. 8-11c, p. 148
A) Here, we focus on only two genes.
One gene has alleles A and a; the other
has alleles B and b.
B) Close contact between the
homologous chromosomes promotes
crossing over between nonsister
chromatids, so paternal and maternal
chromatids exchange segments.
crossover
C) Crossing over mixes up
paternal and maternal alleles on
homologous chromosomes.
Stepped Art
Fig. 8-11c, p. 148
Animation: Crossing over
Meiosis II
 The second nuclear division separates sister
chromatids
 Four haploid nuclei typically form, each with one
complete set of unduplicated chromosomes
Meiosis
1 Prophase I
2 Metaphase I
spindle
plasma
membrane microtubules
3 Anaphase I
4 Telophase I
one pair of homologous
chromosomes
centrosome
nuclear envelope
breaking up
Fig. 8-10a, p. 146
Fig. 8-10b, p. 147
plasma
membrane
spindle
microtubules
one pair of homologous
chromosomes
centrosome
nuclear envelope
breaking up
There is no DNA
replication between
the two nuclear
divisions.
Stepped Art
Fig. 8-10b, p. 147
Comparing Mitosis and Meiosis
Animation: Comparing mitosis and
meiosis
8.8 From Gametes to Offspring
 Meiosis and cytoplasmic division precede the
development of haploid gametes in animals and
spores in plants
 The union of two haploid gametes at fertilization
results in a diploid zygote
Gamete Formation in Plants
 In plants, two kinds of multicelled bodies form
• Familiar plants are diploid sporophytes that make
haploid spores
 Sporophyte
• Diploid, spore-producing body of a plant
 Gametophyte
• A haploid, multicelled body in which gametes
form during the life cycle of plants
Gamete Formation in Animals
 Germ cells in the reproductive organs of animals
give rise to sperm or eggs
 Sperm
• Mature male gamete
 Egg
• Mature female gamete, or ovum
Comparing Life Cycles
of Plants and Animals
Fertilization
 The fusion of two haploid gamete nuclei during
fertilization restores the parental chromosome
number in the zygote, the first cell of the new
individual
Animation: Generalized life cycles
8.9 When Control is Lost
 The cell cycle has built-in checkpoints that allow
problems to be corrected before the cycle
advances
 Checkpoint gene products are gene expression
controls that advance, delay, or block the cell
cycle in response to internal and external
conditions
Checkpoints and Tumors
 Checkpoint genes whose products inhibit
meiosis are called tumor suppressors
 Disruption of checkpoint gene products, such as
by mutations or viruses, causes tumors that may
end up as cancer
 Failure of cell cycle checkpoints results in the
uncontrolled cell divisions that characterize
cancer
Checkpoint Genes
 BRCA genes are tumor suppressor genes
whose products normally repair broken DNA
Cancer
 Moles and other tumors are neoplasms; a
benign neoplasm is noncancerous
 A malignant neoplasm (cancer) occurs when
abnormally dividing cells disrupt body tissues,
physically and metabolically
 Malignant neoplasms can break free and invade
other tissues (metastasize)
Metastasis
 Cancer cells may metastasize – break loose and
colonize distant tissues
4
3
1 benign tumor
2 malignant tumor
Fig. 8-14, p. 150
Three Characteristics of Cancer Cells
1. Grow and divide abnormally
2. Often have an abnormal plasma membrane,
cytoskeleton, or metabolism
3. Often have weakened capacity for adhesion
because recognition proteins are altered or lost
Skin Cancer: A Checkpoint Failure
8.10 Impacts/Issues Revisited
 The HeLa cell line was established more than 50
years ago without Henrietta Lacks knowledge or
consent
 Today, consent forms are required to take tissue
samples, and it is illegal to sell one’s own organs
or tissues
Digging Into Data:
HeLa Cells Are a Genetic Mess