Transcript video slide - Green River Community College

Chapter 13
Meiosis and Sexual
Life Cycles
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Hereditary Similarity and Variation
• Living organisms
– Are distinguished by their ability to reproduce
their own kind
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• 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
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• Genetics
– Is the scientific study of heredity and hereditary
variation
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• Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
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Inheritance of Genes
• Genes
– Are the units of heredity
– Are segments of DNA
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• Each gene in an organism’s DNA
– Has a specific locus on a certain chromosome
• We inherit
– One set of chromosomes from our mother and
one set from our father
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Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical
offspring by mitosis
Parent
Bud
Figure 13.2
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0.5 mm
• In sexual reproduction
– Two parents give rise to offspring that have
unique combinations of genes inherited from
the two parents
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• 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
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Sets of Chromosomes in Human Cells
• In humans
– Each somatic cell has 46 chromosomes, made
up of two sets
– One set of chromosomes comes from each
parent
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• A karyotype
– Is an ordered, visual representation of the
chromosomes in a cell
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Figure 13.3
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5 µm
Figure 13.3 Preparing a Karyotype
APPLICATION A karyotype is a display of
condensed chromosomes arranged in pairs.
Karyotyping can be used to screen for
abnormal numbers of chromosomes or
defective chromosomes associated with
certain congenital disorders, such as Down
syndrome.
TECHNIQUE Karyotypes are prepared from
isolated somatic cells, which are treated with
a drug to stimulate mitosis and then grown in
culture for several days. A slide of cells
arrested in metaphase is stained and then viewed
with a microscope equipped with a digital camera.
A digital photograph of the chromosomes is entered
into a computer, and the chromosomes are
electronically rearranged into pairs according to
size and shape.
RESULTS This karyotype shows the chromosomes
from a normal human male. The patterns of stained
bands help identify specific chromosomes and parts
of chromosomes. Although difficult to discern in the
karyotype, each metaphase chromosome consists of
two, closely attached sister chromatids (see diagram).
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Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
Figure 13.3 Preparation of a human karyotype (Layer 2)
Figure 13.3 Preparation of a human karyotype (Layer 3)
Figure 13.3 Preparation of a human karyotype (Layer 4)
Figure 13.x2 Human female chromosomes shown by bright field G-banding
Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes
Figure 13.x4 Human male chromosomes shown by bright field G-banding
Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes
• Homologous chromosomes
– Are the two chromosomes composing a pair
– Have the same characteristics
– May also be called autosomes
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• Sex chromosomes
– Are distinct from each other in their
characteristics
– Are represented as X and Y
– Determine the sex of the individual, XX being
female, XY being male
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• A diploid cell
– Has two sets of each of its chromosomes
– In a human has 46 chromosomes (2n = 46)
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• In a cell in which DNA synthesis has occurred
– All the chromosomes are duplicated and thus
each consists of two 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
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Pair of homologous
chromosomes
(one from each set)
• Unlike somatic cells
– Gametes, sperm and egg cells are haploid
cells, containing only one set of chromosomes
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Behavior of Chromosome Sets in the Human Life Cycle
• At sexual maturity
– The ovaries and testes produce haploid
gametes by meiosis
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• During fertilization
– These gametes, sperm and ovum, fuse,
forming a diploid zygote
• The zygote
– Develops into an adult organism
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• 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)
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Diploid
zygote
(2n = 46)
The Variety of Sexual Life Cycles
• The three main types of sexual life cycles
– Differ in the timing of meiosis and fertilization
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• In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Key
Haploid
Diploid
n
n
Gametes
n
MEIOSIS
FERTILIZATION
Zygote
2n
Figure 13.6 A
Diploid
multicellular
organism
2n
Mitosis
(a) Animals
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• Plants and some algae
– Exhibit an alternation of generations
– The life cycle includes both diploid and 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
2n
(b) Plants and some algae
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FERTILIZATION
2n
Mitosis
Zygote
• In most fungi and some protists
– Meiosis produces haploid cells that give rise to
a haploid multicellular adult organism
– The haploid adult carries out mitosis,
producing cells that will become gametes
Haploid multicellular
organism
Mitosis
n
Mitosis
n
n
n
Gametes
MEIOSIS
FERTILIZATION
2n
Figure 13.6 C
Zygote
(c) Most fungi and some protists
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n
• Concept 13.3: Meiosis reduces the number of
chromosome sets from diploid to haploid
• Meiosis
– Takes place in two sets of divisions, meiosis I
and meiosis II
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The Stages of Meiosis
• An overview of meiosis
Interphase
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
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Haploid cells with unreplicated chromosomes
• Meiosis I
– Reduces the number of chromosomes from
diploid to haploid
• Meiosis II
– Produces four haploid daughter cells
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• Interphase and meiosis I
MEIOSIS I: Separates homologous chromosomes
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Centrosomes
(with centriole pairs)
Sister
chromatids
Chiasmata
Spindle
Nuclear
envelope
Metaphase
plate
Homologous
Microtubule
chromosomes
Tetrad
attached to
Chromatin
separate
kinetochore
Pairs of homologous
Chromosomes duplicate
Tertads line up
chromosomes split up
Homologous chromosomes
(red and blue) pair and exchange
Figure 13.8
segments; 2n = 6 in this example
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• Telophase I, cytokinesis, and meiosis II
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
Cleavage
furrow
Figure 13.8
Two haploid cells
form; chromosomes
are still double
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
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A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from
mitosis
– By three events in Meiosis l
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• Synapsis and crossing over
– Homologous chromosomes physically connect
and exchange genetic information
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• Tetrads on the metaphase plate
– At metaphase I of meiosis, paired homologous
chromosomes (tetrads) are positioned on the
metaphase plates
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• Separation of homologues
– At anaphase I of meiosis, homologous pairs
move toward opposite poles of the cell
– In anaphase II of meiosis, the sister
chromatids separate
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• A comparison of 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
Metaphase
Chromosomes
positioned at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
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
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Sister chromatids separate during anaphase II
n
• Concept 13.4: Genetic variation produced in
sexual life cycles contributes to evolution
• Reshuffling of genetic material in meiosis
– Produces genetic variation
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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
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Independent Assortment of Chromosomes
• Homologous pairs of chromosomes
– Orient randomly at metaphase I of meiosis
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• In independent assortment
–
Each pair of chromosomes sorts its maternal and paternal
homologues into daughter cells independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Figure 13.10
Combination 1
Combination 2
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Combination 3
Combination 4
Crossing Over
• Crossing over
–
Produces recombinant chromosomes that carry genes derived
from two different parents
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Figure 13.11
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Recombinant
chromosomes
Random Fertilization
• The fusion of gametes
– Will produce a zygote with any of about 64
trillion diploid combinations
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Evolutionary Significance of Genetic Variation
Within Populations
• Genetic variation
– Is the raw material for evolution by natural
selection
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• Mutations
– Are the original source of genetic variation
• Sexual reproduction
– Produces new combinations of variant genes,
adding more genetic diversity
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1. How do cells at the completion of meiosis compare
with cells that have replicated their DNA and are just
about to begin meiosis?
1) They have twice the amount of cytoplasm and half the
amount of DNA.
2) They have half the number of chromosomes and half
the amount of DNA.
3) They have the same number of chromosomes and
half the amount of DNA.
4) They have half the number of chromosomes and onefourth the amount of DNA.
5) They have half the amount of cytoplasm and twice the
amount of DNA.
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2. Which number represents G2? *
1) I
2) II
3) III
4) IV
5) V
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3. Which number represents the DNA content of
a sperm cell?
1) I
2) II
3) III
4) IV
5) V
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4. The DNA content of a diploid cell in the G1
phase of the cell cycle is measured. If the
DNA content is x, then the DNA content of the
same cell at metaphase of meiosis I would be
1) 0.25x
2) 0.5x
3) x.
4) 2x.
5) 4x.
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5. The DNA content of a diploid cell in the G1
phase of the cell cycle is measured. If the
DNA content is x, then the DNA content at
metaphase of meiosis II would be
1) 0.25x.
2) 0.5x.
3) x.
4) 2x.
5) 4x.
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6. The DNA content of a cell is measured in the
G2 phase. After meiosis I, the DNA content of
one of the two cells produced would be
1) equal to that of the G2 cell.
2) twice that of the G2 cell.
3) one-half that of the G2 cell.
4) one-fourth that of the G2 cell.
5) impossible to estimate due to independent
assortment of homologous chromosomes.
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7. Which of the following would not be
considered a haploid cell?
1) daughter cell after meiosis II
2) gamete
3) daughter cell after mitosis in gametophyte
generation of a plant
4) cell in prophase I
5) cell in prophase II
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8. A cell in G2 before meiosis compared with one of the
four cells produced by that meiotic division has
1) twice as much DNA and twice as many
chromosomes.
2) four times as much DNA and twice as many
chromosomes.
3) four times as much DNA and four times as many
chromosomes.
4) half as much DNA but the same number of
chromosomes.
5) half as much DNA and half as many chromosomes.
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Children with Down Syndrome
Down Syndrome—As a function of mother’s age



1 in 1000 births in U.S.
1 in 12 births at age 50
Most frequent genetic
cause of mental
retardation

I.Q. = 20-50
Down Syndrome—As a function of mother’s age
Human Somatic Cells
have 46 Chromosomes


How many
chromosomes in
human gametes?
How do you
know?
Down Syndrome




How many
chromosomes are in
the egg?
Sperm?
How do you know?
D.S. is due to an
error in Meiosis
Down Syndrome
Karyotype




Down syndrome due to an
error in meiosis
 What’s meiosis?
What is wrong with the
Karyotype?
Why are most trisomies
fatal?
Trisomies involving the sex
chromo’s sex chromosomes
are not fatal.
Human Life Cycle
1.
2.
Role of mitosis?
Meiosis



3.
Sperm (23 C)
Fertilization
produces gametes
A reductive
division (46  23
chromosomes)
Don’t confuse
meiosis with mitosis
What if gametes
were made by
mitosis?
Egg (23 C)
Fertilization
Zygote (46 C)
Mitosis
Meiosis
Adult Human
(Somatic cells: 46 C)
Meiosis
Normal Meiosis followed by Fertilization
Meiosis I
Meiosis II
Fertilization
Normal sperm
Chromosomes
Normal diploid
zygote
Both daughter cells
have one copy of
each chromosome
Product of Meiosis:
Haploid Egg Cell
Genetic Basis of Down Syndrome
Nondisjunction of chromosome pair #21 during meiosis leads to Down Syndrome
No copy of
chromosome 21
Diploid minus one copy
of chromosome 21:
Zygote dies
Normal
sperm
Chromosome
pair #21
Are misaligned
Two copies of
chromosome 21
Diploid plus one extra copy
of chromosome #21:
Down syndrome
Down Syndrome—a function of mom’s age
• Why is the
incidence of
Down
Syndrome a
function of
mom’s age
and not that
of the dad?
Egg formation in humans
1.
All pre-egg cells present before
girls are born
Girls are born with about a
1000 pre-egg cells
2.
–
–
3.
At birth all Pre-egg cells are stuck
in metaphase I
Homologous chromosomes are
held in the middle of pre-egg cell
by spindle fibers
After Reaching Puberty, each
month one pre-egg cell finishes
meiosis I & II
–
–
Meiosis produces one egg
The other 3 cells are called polar
bodies and die
Mom’s Pre-egg Cells form Prenatally
1. At about 12 years, women start ovulating one egg
each month for the next 40 years.
2. A 50 yr old woman has had her eggs sitting with
chromosomes aligned in metaphase I for over 50
years!
3. Egg spindle fibers degenerate with age—Causes....
–
–
Chromosomes move to one side
Results in nondisjunction
4. Nondisjunction is rare with the larger
chromosomes,
#’s 1 – 20—Why??
–
Usually lethal because too much genetic imbalance.
Why doesn’t the age of the father influence the
incidence of Down syndrome?
1. Sex cell formation in Males
–
–
Sperm formation starts at puberty and continues daily
for life
Each pre-sperm cell divides twice to produce 4 sperm
• 200-300 million sperm produced per day!
2. Sperm forming cells do not stop in meiosis I of
metaphase
–
–
Sperm cells don't get old
Therefore, no nondisjunction
Screening for Down Syndrome: Amniocentesis
1.
Fetus located using
ultrasound, needle
inserted to remove
amniotic fluid
•
2.
3.
4.
Not performed until
16th week of pregnancy
Fluid contains fetal
biochemicals and fetal
cells from skin,
respiratory tract,
urinary tract
Culture cells for 1-2
weeks
Make Karyotype to
detect abnormal
chromosome numbers
Amniocentesis
Screening for Down Syndrome: Chorionic Villi Sampling (CVS)
1.
2.
3.
4.
Catheter inserted vaginally
and chorionic tissue
removed
Perform 9-11 weeks after
conception
Make Karyotype
CVS—
•
Done earlier in pregnancy
 Less chance of complications if
end pregnancy
•
•
5.
Slightly riskier for fetus
Greater chance of infection
Amniocentesis—have
results...
•
•
Done later in pregnancy
Slightly safer than CVS
Mitosis
1.
2.
3.
4.
5.
6.
No cross-over
Produces 2 genetically
identical somatic cells
Involves only 1 division
Chromosomes align single
file in the middle of the
cell during metaphase
Sister chromatids separate
during anaphase
Daughter cells have the
same number of
chromosomes as the
parent cell
vs.
Meiosis
Cross-over during prophase I
Produces 4 genetically different
gametes
Involves 2 divisions
Homologous pairs align in pairs
during metaphase I
Homologous pairs separate
during anaphase 1
1.
2.
3.
4.
5.
•
6.
Sister chromatids separate
during anaphase 2
Daughter cells have half the
number of chromosomes as the
parent cell
Why Meiosis Causes Genetic Variation
Independent Assortment
1.
Homologous pairs of chromosomes align
independently of one another during metaphase I
•
•
2.
Maternal and paternal chromosomes are shuffled during
meiosis
223 or 8,388,608 different combinations for each parent
Fertilization gives 70 trillion possible genetic
combinations
One Cause of Genetic Variation: Independent Assortment
Possibility 1
Two possible
arrangements of
chromosomes
during metaphase I
Possibility 2
Cross-over—the 2nd Reason
for Genetic Variation
•
•
Homologous
chromosomes exchange
parts during Prophase I
Cross over results in
thousands of genetically
different gametes
Cross-over results in genetically different gametes
1. Cross-over
2. Regions exchanged
Meiosis I
Paternal
copy
Crossing over
3. Products of meiosis
Meiosis II
Parental
Recombinant
Homologous
chromosomes
Recombinant
Maternal
copy
Parental
Homologous
chromosomes
Parent cell
Sister
chromatids
Review of Mitosis
Product of
mitosis:
Two genetically
identical diploid
daughter cells
DNA Replication
Metaphase:
Chromosomes
align single file
Anaphase Separates
sister chromatids
1. How do chromo’s align at metaphase?
2. What separates at anaphase?
3. How do the daughter cells compare genetically?
Review of Meiosis
Parent cell
1.
2.
3.
4.
Crossing over
•Product of
•meiosis:
•Four
genetically
•different
•haploid
•daughter
cells
DNA Replication
Metaphase I
(homologous
Chromosomes are
paired)
Anaphase I
separates
homologous
chromosomes
Alignment
without
replication
How do the chromo’s align at metaphase I?
What separates at anaphase I
How do the chromo’s align at metaphase II?
What separates at anaphase II?
Anaphase II
separates sister
chromatids