Fundamentals of Biology

(BIO301E)
Sexual Life Cycles and
Meiosis
Chapter XIII
 The family members shown in this photo have some
similar features. Offspring resemble their parents more
than they do unrelated individuals
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Offspring acquire genes from parents by
inheriting chromosomes
• The transmission of traits from one generation to
the next is called inheritance, or heredity
• Sons and daughters are not identical copies of
either parent or of their siblings
• Along with inherited similarity, there is variation
• The study of heredity and inherited variation is
called genetics

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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 somatic 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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 Human somatic cells have 23
pairs of chromosomes

• 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 have the same length, centromere
position, and staining pattern
• They also carry genes controlling the same inherited characters
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 • Human females have a homologous pair of X chromosomes
(XX)
• Human males have one X and one Y chromosome
• The remaining 22 pairs of chromosomes are called
autosomes

• 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
• For humans, the diploid number is 46 (2n = 46)
• A gamete (sperm or egg) contains a single set of
chromosomes and is thus a haploid cell (n)

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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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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
• The behavior of chromosomes is related to the
human lifecycle and other types of sexual life
cycles

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Behavior of Chromosome Sets in the Human
Life Cycle
• Fertilization is the union
of gametes (the sperm and
the egg)
• The fertilized egg is called
a zygote and has one set
of chromosomes from
each parent
• The zygote produces
somatic cells by mitosis
and develops into an adult
• Gametes are the only type
of human cells produced
by meiosis, rather than by
mitosis
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The Variety of Meiosis and Fertilization
• 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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 • Depending on the type of life cycle, either haploid
or diploid cells can divide by mitosis
• However, only diploid cells can undergo meiosis
• In all three life cycles, the halving and doubling
of chromosomes contribute to genetic variation
in offspring

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Meiosis reduces the number of chromosome
sets from diploid to haploid
• Like mitosis, meiosis is
preceded by the replication
of chromosomes
• 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
• Chromosomes duplicate before meiosis
• The resulting sister chromatids are closely
associated along their lengths
• This is called sister chromatid cohesion
• The chromatids are sorted into four haploid
daughter cells

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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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 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
• Nuclear envelope breakdown, spindle formation,
centrosome movement occur
• Later in prophase I, microtubule from one pole to
another pole attach to kinetochers, one at the
centromere of each homolog.

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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 duplicated chromosomes
• Each chromosome still consists of two sister
chromatids
• Cytokinesis usually occurs simultaneously, forming
two haploid daughter cells
• 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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 • 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 by
microtubules 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

• 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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BioFlix® Animation: Meiosis

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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 matching points
• A zipper-like structure called the synaptonemal complex holds the
homologs together tightly
• During synapsis, DNA breaks are repaired, joining DNA from one
nonsister chromatid to the corresponding segment of another

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A Comparison of Mitosis and Meiosis
• Mitosis conserves the number of chromosome sets,
producing two cells that are genetically identical
to the parent cell
• Meiosis reduces the number of chromosomes sets
from two (diploid) to one (haploid), producing
four cells that differ genetically from each other
and from the parent cell

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 • Three events are unique to meiosis, and all three
occur in meiosis I
– 1. Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
– 2. Alignment of homologous pairs at the
metaphase plate
– 3. Separation of homologs during anaphase I

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 Genetic variation contribution to evolution

• Mutations (changes in an organism’s DNA) are the

original source of genetic diversity
• Mutations create different versions of genes called
alleles
• 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
– Random fertilization

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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

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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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Crossing Over
• 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 occur per
chromosome
• Crossing over adds even
more variation
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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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Combining Maternal and Paternal DNA for
Comprehensive Ancestry Research
• Recent studies are integrating both mtDNA and Y-
DNA to construct more complete human
genealogies. These dual lineage studies provide a
broader view of how both maternal and paternal
ancestors contribute to modern populations' genetic
makeup
Females
Males
Y DNA Yes No
Mitochondria
Yes Yes
l DNA
Autosomal
Yes Yes
DNA
X Yes, their Yes, from
BIO Web of Conferences 115, 07003 Chromosome mother’s only both parents
(2024)
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Mutations in mtDNA are linked to several
inherited diseases

• Research is focusing on better understanding these

diseases and developing gene therapies to correct
mtDNA mutations
• Research is delving into the role of mitochondrial
DNA damage in the aging process, particularly its
contribution to neurodegenerative diseases like
Alzheimer's and Parkinson's disease

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mtDNA is increasingly studied as a biomarker
for chronic diseases
• Since mtDNA is inherited exclusively from the
mother, its mutations and role in cancer
development can provide unique insights into
how certain cancers may develop, progress, and
how patients respond to treatments

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The Evolutionary Significance of Genetic
Variation Within Populations
• Natural selection results in the accumulation of
genetic variations favored by the environment
• Mutations are the original source of different
alleles
• Sexual reproduction is almost universal among
animals
• Asexually reproducing organisms like the bdelloid
rotifer increase genetic diversity by incorporating
foreign DNA from the environment

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A bdelloid rotifer, an animal that reproduces
only asexually

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https://youtu.be/Rn-c1aB9uUA?si=qrUIzvJTzqsCN9rL

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Cryptobiosis

• Cryptobiosis is a survival state in which organisms

almost completely stop their metabolic activities in
order to survive extremely adverse environmental
conditions.
• During this process, the organism can withstand
extreme conditions such as dehydration, freezing,
lack of oxygen, or high salinity. However, when
conditions return to normal, the organism can
become active again and continue its life where it
left off.

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Prophase I to Anaphase

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Homologs aligned at the
metaphase plate

Dublicated chromosome Homologous chromosomes

Non dublicated chromosome Chiasmata
Kinetochore F allele
Sister chromotide Gene locus
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Sister chromotide cohesion
Cavendish banana

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 • Commercial bananas are usually triploid (3n)
hybrids and do not produce seeds.

• Therefore, they cannot naturally create genetic

diversity and develop resistance to diseases.

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https://youtu.be/_HoyzhoDJjE?si=EZiGZdg
M6ptl51dO

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