Download Meiosis I

Chapter 13
Meiosis and Sexual
Life Cycles
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: What accounts for family resemblance?
Fig. 13-1
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Overview: Variations on a Theme
•
Living organisms are distinguished by their ability to reproduce their
own kind
•
Genetics is the scientific study of heredity and variation
•
Heredity is the transmission of traits from one generation to the next
•
Variation is demonstrated by the differences in appearance that
offspring show from parents and siblings
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Q: 자손은 어떻게 부모와 닮은 형질을 획득하게
되나? 13.1: Offspring acquire genes from parents by
• Concept
inheriting chromosomes
•
In a literal sense, children do not inherit particular physical traits from
their parents
•
It is genes that are actually inherited
Inheritance of Genes
•
Genes are the units of heredity, and are made up of segments of DNA
•
Genes are passed to the next generation through reproductive cells
called gametes (sperm and eggs)
•
Each gene has a specific location called a locus (=address) on a
certain chromosome
•
Most DNA is packaged into chromosomes
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Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical
offspring by mitosis
A hydra reproduces by budding.
The bud, a localized mass of
mitotically dividing cells, develops
into a small hydra, which detaches
from the parent
Bud
Figure 13.2 (a)
An individual that reproduces asexually gives
rise to a clone, a group of genetically
Parent
identical individuals from the same parent
 Genetic differences in asexually
reproducing organisms can be arisen by
changes in the DNA called mutations (See
Chapter17)
0.5 mm
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• In sexual reproduction
– Two parents give rise to offspring that have
unique combinations of genes inherited from
the two parents
Figure 13.2(b)
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Redwoods
Q: 유성생식 생활사의 특징은?
• Concept 13.2: 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
Q: 염색체의 행동패턴은 인간 생활사와 다른
유성생식 과정과 어떤 관련성이 있나?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: 인간세포에서 발견되는 염색체 세트의
차이는?
• Human
somatic cells (any cell other than a gamete) 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 are the same length and carry genes
controlling the same inherited characters
•
The sex chromosomes (성염색체) are called X and Y
•
Human females have a homologous pair of X chromosomes (XX)
•
Human males have one X and one Y chromosome
•
The 22 pairs of chromosomes that do not determine sex are called
autosomes (상염색체=보통염색체)
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•
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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
A gamete (sperm or egg)
contains a single set of
chromosomes, and is
haploid (n)
•
For humans, the haploid
number is 23 (n = 23)
•
Each set of 23 consists of
22 autosomes and a
single sex chromosome
•
In an unfertilized egg
(ovum), the sex
chromosome is X
•
In a sperm cell, the sex
chromosome may be
either X or Y
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다음과 같이 2n=6인 세포를 가정했을 때
아래의 질문에 답하시오
(1) What is the haploid number of the following cell?
(2) Is a “set” of chromosomes haploid or diploid?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
인간 생활사에서 염색체의 행동 패턴은?
•
The human life cycle begins when a haploid sperm from the father
fuses with a haploid egg from the mother
•
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
•
At sexual maturity, the ovaries and testes produce haploid gametes
•
Gametes are the only types of human cells produced by meiosis,
rather than mitosis
•
Meiosis results in one set of chromosomes in each gamete
•
Fertilization and meiosis alternate in sexual life cycles to maintain
chromosome number
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The human life cycle
Key
Haploid gametes (n = 23)
Haploid (n)
Ovum (n)
Diploid (2n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
Ovary
Testis
Mitosis and
development
Figure 13.5
Multicellular diploid
adults (2n = 46)
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Diploid
zygote
(2n = 46)
유성생식의 다양성
• The alternation of meiosis and fertilization (key
events that contribute to genetic variation among offspring)
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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Three types of sexual life cycles: animals
•
Gametes are the only
haploid cells in
animals
•
They are produces by
meiosis and undergo
no further cell division
before fertilization
•
Gametes fuse to form
a diploid zygote that
divides by mitosis to
develop into a
multicellular organism
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Three types of sexual life cycles: Plants and algae
•
Plants and some algae
exhibit an alternation
of generations
•
This life cycle includes
both a diploid and
haploid multicellular
stage
•
The diploid organism,
called the sporophyte,
makes haploid spores
by meiosis
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Three types of sexual life cycles: fungi and protists
•
In most fungi and some
protists, the only diploid
stage is the singlecelled zygote; there is
no multicellular
diploid stage
•
The zygote produces
haploid cells by meiosis
•
Each haploid cell grows
by mitosis into a haploid
multicellular organism
•
The haploid adult
produces gametes by
mitosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
유성생식 유형간의 차이점과 유사점
• 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 contributes to genetic variation in
offspring
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
감수분열이란?
• Concept 13.3: 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 sets of 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 (see Next Figure 13.7)
•
•
After chromosomes duplicate,
two divisions follow
–
Meiosis I (reductional
division): homologs
pair up and separate,
resulting in two haploid
daughter cells with
replicated
chromosomes
–
Meiosis II (equational
division) sister
chromatids separate
The result is four haploid
daughter cells with
unreplicated chromosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Outline of Meiosis
Ref: Chapter 4 of Genetics edited by Hartwell et al.
Female (2n)
Male (2n)
Ovary :
Testis :
Germ cells (2n)
Germ cells (2n)
Meiosis
Eggs (n)
Sperms (n)
Gametes
Fertilization
Zygote (=embryo: 2n)
Mitosis
(100 times in somatic cells)
Offspring (2n)
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제1 감수분열의 특징
•
Meiosis I is preceded by interphase, when the chromosomes are
duplicated to form sister chromatids
•
The sister chromatids are genetically identical and joined at the
centromere
•
The single centrosome replicates, forming two centrosomes
제1 감수분열의 진행단계
•
Division in meiosis I occurs in four phases
–
Prophase I
–
Metaphase I
–
Anaphase I
–
Telophase I and cytokinesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 13.8a
Prophase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Telophase I and
Cytokinesis
Anaphase I
Metaphase I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Each pair of homologous
chromosomes separates.
Chromosomes line up
Duplicated homologous
chromosomes (red and blue) by homologous pairs.
pair and exchange segments;
2n  6 in this example.
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Cleavage
furrow
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
Prophase I
•
Prophase I typically occupies more than 90% of the time
required for meiosis
•
Longest (90% of meiosis), most complex phase : takes many
days, months, even years (arrested at prophase I until ovulation)
•
Chromosomes begin to condense
•
In synapsis, homologous chromosomes loosely pair up, aligned
gene by gene
•
In crossing over, nonsister chromatids exchange DNA segments
•
Each pair of chromosomes forms a tetrad, a group of four chromatids
•
Each tetrad usually has one or more chiasmata, X-shaped regions where
crossing over occurred
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Metaphase I
•
•
•
In metaphase I, tetrads 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 tetrad
Microtubules from the other pole are attached to the kinetochore of the other
chromosome
Anaphase I
•
In anaphase I, pairs of homologous chromosomes separate
•
One chromosome moves toward each pole, 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 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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Meiosis – Prophase I
(“thin” in Greek)
(“Conjugation”)
(“thick”)
(=zipper with remarkable precision)
Pachytene
Figure 4.13
 bivalent: each synapsed chromosome pair
(=tetrad for four chromatids)
 Pairing b/w X and Y chromosomes: limited pairing via a small region of homology
 Recombination nodules: a structure appeared along the synaptonemal complex; a
site of crossing-over b/w nonsister chromaitds (maternal & paternal)
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Meiosis-Prophase I-continued
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Mechanism of Crossing-over
Crossing over during prophase produces recombined chromosomes
Bivalent or tetrad
Fig. 4.14 a-c
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Mechanism of Crossing-over
Fig. 4.14 d, e
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Meiosis I – Metaphase and Anaphase
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Meiosis – Telophase I and Interkinesis
Fig. 4.13e
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Meiosis – Prophase II and Metaphase II
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제2 감수분열의 진행단계
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 13.8b
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
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Haploid daughter
cells forming
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A Comparison of Mitosis and Meiosis
•
Mitosis conserves the
number of chromosome
sets, producing cells that are
genetically identical to the
parent cell
•
Meiosis reduces the number
of chromosomes sets from
two (diploid) to one
(haploid), producing cells
that differ genetically from
each other and from the
parent cell
•
The mechanism for
separating sister chromatids
is virtually identical in
meiosis II and mitosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 13.9b
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One, including prophase, metaphase,
anaphase, and telophase
Two, each including prophase, metaphase, anaphase,
and telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many
chromosomes as the parent cell; genetically different
from the parent cell and from each other
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes
by half and introduces genetic variability among the
gametes
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•
Three events are unique to meiosis, and all
three occur in meiosis l
– Synapsis and crossing over in prophase I:
Homologous chromosomes physically
connect and exchange genetic information
– At the metaphase plate, there are paired
homologous chromosomes (tetrads),
instead of individual replicated
chromosomes
– At anaphase I, it is homologous
chromosomes, instead of sister chromatids,
that separate
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Inquiry: What prevents the separation of sister
chromatids at anaphase I of meiosis?
• Sister chromatid cohesion allows sister chromatids of a
single chromosome to stay together through meiosis I
• Protein complexes called cohesins are responsible for
this cohesion
• In mitosis, cohesins are cleaved at the end of metaphase
• In meiosis, cohesins are cleaved along the chromosome
arms in anaphase I (separation of homologs) and at the
centromeres in anaphase II (separation of sister
chromatids)
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유성생식과정에서 발생한 유전변이의
역할은?13.4: Genetic variation produced
• Concept
sexual life cycles contributes to evolution
in
•
Mutations (changes in an organism’s DNA) are the original source of
genetic diversity (유전적 다양성)
•
Mutations create different versions of genes called alleles
(대립유전자)
•
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
homologues into daughter
cells independently of the
other pairs
•
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 begins very
early in prophase I, as
homologous chromosomes
pair up gene by gene
•
In crossing over,
homologous portions of
two nonsister chromatids
trade places
•
Crossing over contributes
to genetic variation by
combining DNA from two
parents into a single
chromosome
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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(조) diploid
combinations
 No wonder brothers and sisters can be so different: You really
are unique!!!
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Evolutionary Significance of Genetic
Variation Within Populations
• Natural selection results in the accumulation of genetic
variations favored by the environment
• Sexual reproduction contributes to the genetic variation in
a population, which originates from mutations
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Privet shrubs and humans each have a
diploid number of 46 chromosomes per cell.
Why are the two species so dissimilar?
(a) Privet chromosomes undergo only mitosis.
(b) Privet chromosomes are shaped differently.
(c) Human chromosomes have genes grouped
together differently.
(d) The two species have appreciably different
genes.
(e) Privets do not have sex chromosomes.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings