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Meiosis and Sexual Reproduction
Ch. 7
Biology
Ms. Haut
Introduction to Heredity
• Offspring acquire genes from parents by
inheriting chromosomes
• Inheritance is possible because:
– Sperm and ova carrying each parent’s genes are
combined in the nucleus of the fertilized egg
Actual transmission of genes depends on
the behavior of chromosomes
•Chromosomes-organizational unit of hereditary
material in the nucleus of eukaryotic organisms
•Contain hundreds of thousands of genes, each of
which is a specific region of the DNA molecule, or
locus
Meiosis
•
•
•
•
Reduces chromosome number from diploid to haploid
Increases genetic variation among offspring
Steps resemble steps in mitosis
Single replication of DNA is followed by 2
consecutive cell divisions
– Meiosis I
– Meiosis II
• Produces 4 different daughter cells which have half
the number of chromosomes as the original cell
Interphase I
• Chromosomes replicate
(still as chromatin)
• Duplicated chromosomes
consist of 2 identical sister
chromatids attached by
centromere
• Centriole pairs replicate
Meiosis I
• This cell division separates the 2
chromosomes of each homologous pair and
reduce the chromosome number by one-half
Prophase I
• Chromosomes condense
• Synapsis occurs
(homologues pair)
• Chromosomes seen as
distinct structures; each
chromosome has 2
chromatids, so each
synapsis forms a tetrad
Prophase I
• Sister chromatids held
together by
centromeres; nonsister chromatids held
together by chiasmata
where crossing-over
occurs (exchange of
DNA)
Late Prophase I
• Centriole pairs move
apart and spindle fibers
form
• Nuclear envelope
disappears and nucleoli
disperse
Prophase I
Metaphase I
• Homologous
chromosomes line up
along metaphase plate
Metaphase I
Anaphase I
• Homologous
chromosomes separate,
independently from
others
Anaphase I
Telophase I and Cytokinesis
• Each pole now has a
haploid set of chromosomes
(each with 2 sister
chromatids)
• Usually, cytokinesis occurs
simultaneously with
telophase I, forming 2
haploid daughter cells
(cleavage furrow forms in
animals; cell plate forms in
plants)
Telophase I
Meiosis II
• This cell division separates the 2 sister
chromatids of each chromosome
Prophase II
• Spindle apparatus forms
and chromosomes move
toward metaphase II
plate
Prophase II
Metaphase II
• Chromosomes align
singly on the
metaphase plate
Metaphase II
Anaphase II
• Sister chromatids of each
pair (now individual
chromosomes) separate
and move toward
opposite poles of the cell
Anaphase II
Anaphase II
Telophase II and Cytokinesis
• Nuclei form at
opposite poles of the
cell
• Cytokinesis occurs
producing 4 haploid
daughter cells (each
genetically different)
Telophase II
Telophase II
Key Differences Between Mitosis
and Meiosis
• Meiosis is a reduction division
– Mitotic cells produce clones (same xsome #)
– Meiosis produces haploid cells
• Meiosis creates genetic variation
– Mitosis produces 2 identical daughter cells
– Meiosis produces 4 genetically different daughter cells
• Meiosis is 2 successive nuclear divisions
– Mitosis has one division
Mechanisms of Genetic Variation
1. Independent assortment—each pair of
homologous chromosomes separate
independently
–
Results in gametes with different gene
combinations
2. Crossing-over—exchange of genetic
material between non-sister chromatids
–
Results in genetic recombination
3. Random fertilization—random joining of
two gametes
Importance of Genetic variation
• Essential to evolution (change over time)
• Variation can cause changes that leads to
different traits
– Some favorable
– Some unfavorable
Spermatogenesis
• Process of sperm production
• Results in 4 viable sperm
Oogenesis
• Process of egg (ova) production
• Results in 1 viable egg and 3 polar bodies
that will not survive
• Polar bodies result from an uneven division
of cytoplasm
Asexual Reproduction
• Prokaryotes—binary
fission
• Eukaryotes—2
mechanisms
– Fission—separation of a
parent into two or more
individuals of identical
size
– Budding—new
individuals split off from
existing ones
Asexual
Reproduction
Advantages
Disadvantages
Sexual
Reproduction
•Produce offspring •Genetic diversity
in short time
•Allows
•Uses no energy
adaptation to
changing
environment
•No genetic
•Uses energy
diversity
•Harder to adapt to
changing
environment
Eukaryotic Sexual Life Cycles
• Life cycle—entire span in the life of an
organism from one generation to the next
• All sexually reproducing organisms follow
a basic pattern
– Alteration between diploid and haploid
chromosome numbers
Haploid Life Cycles
n
n
Fresh water
green algae
n
Diploid Life Cycles
Alternation of Generations Life
Cycles
Parthenogenesis
• New individual develops
from an unfertilized egg
• Offspring is a clone of the mother
• Occurs in long absences of male
companionship