Bellringer
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Why is genetic diversity beneficial to populations?
How does sexual reproduction increase genetic diversity?
How does meiosis increase genetic diversity?
Why don’t siblings all look almost exactly alike?
Chapter 13
Meiosis and Sexual
Life Cycles
Hereditary similarity and variation
• Living organisms are, by definition, capable of reproducing
their own kind.
• Offspring inherit genetic information from their parent(s)
• Genetic information that can be passed on is hereditary
• Different forms of the same gene (alleles) code for different
phenotypes (gene expression)
• Heredity
• Is the transmission of traits from one generation to the next
• Variation
• Shows that offspring differ somewhat in appearance from parents
and siblings
Figure 13.1
Genetics
• Scientific study of heredity and hereditary variation
Inheritance of Genes
• Offspring inherit genes, in the form of chromosomes, from
their parents
• Chromosomes
• A complex of proteins and a molecule of DNA
• 46 chromosomes in humans
• Genes
• units of heredity
• segments of DNA
Chromosomes
• Each gene in an organism’s DNA
• Has a specific locus on a certain chromosome
• We inherit
• 1 set of chromosomes from mom & 1 from dad
• Corresponding chromosomes have the same genes, although
possibly different alleles
• 2 copies of every gene (except for some genes on sex
chromosomes)
Comparison of Asexual and
Sexual Reproduction
• asexual reproduction
• 1 parent produces genetically identical
offspring by mitosis
• sexual reproduction
• 2 parents have offspring with unique
combinations of genes inherited from
both parents
Parent
Bud
Figure 13.2
0.5 mm
Sexual Life Cycles
• Fertilization & meiosis alternate in sexual life cycles life cycle
• Generation to generation sequence of stages in the
reproductive history of an organism
Sets of Chromosomes in
Human Cells
• Somatic cells – all of the cells of the body except fot the sex
cells
• 46 chromosomes in humans
• Gametes – sex cells
• 23 chromosomes in humans
Karyotype
• Visual representation of
the chromosomes in a
cell
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Figure 13.3
5 µm
LE 13-3
Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
Chromosomes
• Homologous chromosomes
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•
•
•
2 chromosomes, making a pair
Same length, centromere position, & staining pattern
also called autosomes
22 pairs in us
• Sex chromosomes
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•
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•
•
Are distinct from each other in their characteristics
X&Y
Determine sex of the individual,
XX female
XY male
Chromosomes
• Haploid
• 1 set of chromosomes
• Human = 23 (n=23)
• diploid (2n)
• 2 sets of each of its chromosomes
• human= 46 chromo’s (2n = 46)
Chromosomes
• In a cell in which DNA synthesis has occurred
• All the chromo’s are duplicated & thus each consists of 2 identical
sister chromatids
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Figure 13.4
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Chromosomes - gametes
• Haploid (only 1 set of chromosomes)
• Contains every type of autosome
• Contains only one or the other of the sex chromosomes (X or
Y)
Behavior of Chromosome Sets
in the Human Life Cycle
• At sexual maturity
• ovaries & testes make haploid gametes by meiosis
• During fertilization
• sperm & ovum fuse, forming a diploid zygote
• The zygote
• Develops into an adult organism
The Human Life Cycle
Key
Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Ovum (n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
Ovary
Testis
Mitosis and
development
Figure 13.5
Multicellular diploid
adults (2n = 46)
Diploid
zygote
(2n = 46)
The Variety of Sexual Life
Cycles
• 3 types of sexual life cycles
• Differ in timing of meiosis & fertilization
Animal Life Cycled
• Meiosis occurs during gamete formation
• Gametes are the only haploid cells
Key
Haploid
Diploid
n
n
Gametes
n
MEIOSIS
2n
Figure 13.6 A
Diploid
multicellular
organism
FERTILIZATION
Zygote
2n
Mitosis
(a) Animals
Plant Life Cycle
• Plants & some algae
• Show an alternation of generations
• life cycle has both diploid & haploid multicellular stages
Haploid multicellular
organism (gametophyte)
n
Mitosis
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
Diploid
multicellular
organism
(sporophyte)
Figure 13.6 B
FERTILIZATION
2n
(b) Plants and some algae
2n
Mitosis
Zygote
Fungi and some protists
• Meiosis makes haploid cells that make a haploid multicellular
adult organism
• haploid adult carries out mitosis, making cells that will be
gametes
Haploid multicellular
organism
n
Mitosis
Mitosis
n
n
n
Gametes
MEIOSIS
FERTILIZATION
2n
Figure 13.6 C
Zygote
(c) Most fungi and some protists
n
Sexual Life Cycles
• Depending on the type of life cycle, either haploid (n) or
diploid (2n) cells can divide by mitosis
• only diploid cells can undergo meiosis
• In all 3 life cycles, changes in the number of chromosomes
contribute to genetic variation in offspring
Meiosis
• Meiosis reduces the number of chromo sets from diploid to
haploid
• Takes place in 2 sets of divisions, meiosis I & meiosis II
The Stages of Meiosis
Interphase
• An overview of meiosis
• 2 cell divisions result in 4
daughter cells,
• rather than the two
daughter cells in mitosis
• Each daughter cell has only
½ as many chromosomes as
the parent cell
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Figure 13.7
Haploid cells with unreplicated chromosomes
The Stages of Meiosis
• 1st cell division (meiosis I), homologous chromosomes
separate
• Meiosis I results in 2 haploid daughter cells with replicated
chromosomes
• In the 2nd cell division (meiosis II), sister chromatids separate
• Meiosis II results in 4 haploid daughter cells with unreplicated
chromosomes
Summary
• Meiosis I
• Reduces # of chromo’s from diploid to haploid
• Meiosis II
• makes 4 haploid daughter cells
Interphase and meiosis I
MEIOSIS I: Separates homologous chromosomes
INTERPHASE
PROPHASE I
METAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Centrosomes
(with centriole pairs)
Sister
chromatids
Nuclear
envelope
Chromatin
Chiasmata
Spindle
Tetrad
Chromosomes duplicate
Figure 13.8
ANAPHASE I
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
Metaphase
plate
Homologous
Microtubule
chromosomes
attached to
separate
kinetochore
Tertads line up
Pairs of homologous
chromosomes split up
Telophase, cytokinesis, and meiosis II
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
Figure 13.8
Two haploid cells
form; chromosomes
are still double
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
A Comparison of Mitosis and
Meiosis
• Meiosis can be distinguished from mitosis by 3 events
• Prophase I
• Synapsis & crossing over
• Homologous chromosomes physically connect & exchange
genetic info
• Tetrads on the metaphase plate
• In metaphase I of meiosis, paired homologous chromosomes
(tetrads) are positioned on the metaphase plates
• Separation of homologues
• anaphase I of meiosis- homologous pairs move toward opp poles
of the cell
• anaphase II of meiosis- sister chromatids separate
Mitosis and Meiosis
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Sister chromatids
separate during
anaphase
Anaphase
Telophase
2n
Tetrads
positioned at the
metaphase plate
Metaphase I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
Daughter cells of meiosis II
Figure 13.9
Sister chromatids separate during anaphase II
n
Sources of genetic variation
• Genetic variation produced in sexual life cycles contributes to
evolution
• Reshuffling of genetic material in meiosis
• Produces genetic variation
Origins of Genetic Variation
Among Offspring
• In species that produce sexually
• The behavior of chromosomes during meiosis and fertilization is
responsible for most of the variation that arises each generation
Independent Assortment of
Chromosomes
• Homologous pairs of chromosomes
• Orient randomly at metaphase I of meiosis
• In independent assortment
• Each pair of chromosomes sorts its maternal & paternal
homologues into daughter cells independently of the other pairs
Crossing Over
• Crossing over
Prophase I
of meiosis
• Produces recombinant
chromosomes that carry
genes derived from 2
different parents
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Figure 13.11
Recombinant
chromosomes
Random Fertilization
• The fusion of gametes
• The number of possible complete genotypes for a gamete is
2^n
• The number of possible complete genotypes for any zygote is
2^n x 2^n
• In humans, n=23
How many possible gametes can one person produce?
How many possible zygotes could any two people produce?
Evolutionary Significance of Genetic
Variation Within Populations
• Genetic variation
• Is the raw material for evolution by natural selection
• Mutations
• Are the original source of genetic variation
• Sexual reproduction
• Produces new combinations of variant genes, adding more
genetic diversity
• Mutations
• Are the original source of genetic variation
• Sexual reproduction
• Produces new combinations of variant genes, adding more
genetic diversity