Download About reproduction

Chapter 9
The passage of life’s organization and
information from one generation to the
next
One way, but are there others?
How do organisms pass genetic information?
Are the contributions the same from males and females?
What kinds of mishaps occur and where do they originate?
General Life Strategies
Asexual reproduction
corms
fragmentation
bulbs
•No exchange of genetic material
•Offspring are genetically identical to parents
•No time ‘wasted” finding a mate
•No courtship
Figure 9.8
Bacterial Duplication
Some Interesting Strategies
The life cycle of aphids can involve a mix of
parthenogenetic (asexual) and sexual
reproduction. Parthenogenetic reproduction
provides the development of young from
unfertilized eggs. The young are female and
genetically identical to the parent. Eggs
typically hatch in spring and develop into
wingless females which then produce live
young. After some generations of
parthenogenesis, winged reproductive
males and females are produced which
mate and lay eggs.
Another Interesting Organism

In approximately 15 of the
Cnemidophorus species there
are no males. They reproduce by
parthenogenesis.

Parthenogenesis is rare in
vertebrates. The offspring of
parthenogenic lizards are clones,
identical to the mother.
Human Cloning
1998 Mice
1997 Dolly
2000 Monkey Business
United Nations (Nov. 20, 2001) - A
key General Assembly committee
backed a resolution calling for a
treaty to ban the cloning of human
beings, saying it was "contrary to
human dignity.“ Under the draft
resolution, a group would meet
twice next year to define what
should be negotiated in an
international convention to ban
reproductive cloning.
Box 9.3, Figure 1 But something else is happening: genetic recombination
BACTERIAL CONJUGATION AND RECOMBINATION
Hfr cell
Normal cell
Conjugation tube
1. Hfr cells contain
genes that allow them
to transfer some or all
of their chromosome
to another cell.
2. Conjugation tube
connects Hfr cell to
normal cell. Copy of Hfr
chromosome begins to
move to recipient cell.
3. Homologous sections
of chromosome synapse.
4. Cells separate. Section
of Hfr chromosome
integrates into recipient
chromosome by
crossing over.
Figure 9.9
Some comparisons between asexual and sexual reproduction
Asexual reproduction
Sexual reproduction
Generation 1
Generation 2
Generation 3
So, what good are males???
Genetic Recombination: Sexual Reproduction

What are the benefits?
• Two copies of each gene
(provides instructions)
• “Sharing” of beneficial genes
• “Infinite” number of
combinations (variation)
Genetic Recombination: Sexual Reproduction

What are the Costs?
• Courtship expenses
• Two parents investing resources
• “Complicated” process to make
gametes
• Dangerous!
Genetic Recombination: Sexual Reproduction

What are the Costs?
• Courtship expenses
• Two parents investing resources
• “Complicated” process to make
gametes
• Dangerous!
Genetic Recombination: Sexual Reproduction

What are the Costs?
• Courtship expenses
• Two parents investing resources
• “Complicated” process to make
gametes
• Dangerous!
Genetic Recombination: Sexual Reproduction

What are the Costs?
• Courtship expenses
• Two parents investing resources
• “Complicated” process to make
gametes
• Dangerous!
Life Cycle Strategies Involving Sexual Reproduction

Diploid Dominant (two copies of each chromosome)

Haploid Dominant (one copy of each chromosome)

Alteration of Generations
Figure 9.7a
MEIOSIS:
2n >> n
Diploid dominant
Haploid
gametes (n)
Diploid
adult
MITOSIS
FERTILIZATION
Diploid
zygote
2n
Figure 9.7b
Haploid dominant
MEIOSIS
MITOSIS
Haploid
cell
Diploid
cell
Haploid
adult
MITOSIS
FERTILIZATION
Haploid
gametes
Figure 9.7c, upper
Alternation of generations
MEIOSIS
MITOSIS
Haploid
cells
Haploid
gametes
Diploid plant
Diploid
cell
Haploid
plant
MITOSIS
MITOSIS
FERTILIZATIION
Figure 9.10a
Evidence for the benefits of sexual reproduction: resistance
Snails subject to parasitism by trematode worms (Lively)
Figure 9.10b
Are genetically diverse populations more resistant to parasites?
0.40
0.30
Male frequency
0.20
0.15
0.10
0.05
0.01
0.00
0.00
0.05
0.15
0.30
Frequency of infection by parasites
0.50
Meiosis is a Special Type of Cell Division that
Occurs in Sexually Reproducing Organisms

Meiosis reduces the chromosome number by half,
enabling sexual recombination to occur.
• Meiosis of diploid cells produces haploid daughter cells,
which may function as gametes. (Fig. 9.2a-c, 9.3)
Figure 9.2c
A full complement of chromosomes is restored during fertilization.
Female
gamete
Male
gamete
n = 23 in
humans
n = 23 in
humans
Fertilization
Diploid offspring
contains homologous
pair of chromosomes
Figure 9.2a
Each chromosome replicates prior to undergoing meiosis.
Paternal
chromosome
Maternal
chromosome
(n = 23 in humans)
(n = 23 in humans)
Duplication in
S phase
Sister chromatids
Centromere
Homologous pair of
premeiotic chromosomes
Figure 9.2b
Parent cell
contains
homologous
pair of
chromosomes
Homologs
separate
at meiosis I
Sister
chromatids
separate at
meiosis II
Four daughter cells contain one chromosome each.
These cells become gametes.
Daughter
cells
contain
just one
homolog
MEIOSIS II
MEIOSIS I
During meiosis, chromosome number in each cell is reduced.
Figure 9.3, left
PRIOR TO MEIOSIS
MEIOSIS I
Chromosomes
replicate, forming
sister chromatids.
Homologous chromosomes separate.
Sister chromatids
1. Chromosomes
replicate in
parent cell.
Tetrad (4 chromatids from
homologous chromosomes)
Chiasma
2. Synapsis of homologous
chromosomes. Crossing
over of non-sister chromatids.
3. Tetrads
migrate to
middle of cell.
4. Homologs
separate.
Figure 9.3, right
MEIOSIS II
Sister chromatids separate
5. Cell divides.
6. Chromosomes
begin moving to
middle of cell.
7. Chromosomes
line up at middle
of cell.
8. Sister
chromatids
separate.
9. Cell division
results in four
daughter cells.
Meiosis is a Special Type of Cell Division that
Occurs in Sexually Reproducing Organisms

Meiosis reduces the chromosome
number by half, enabling sexual
recombination to occur.
• Gametes undergo fertilization,
restoring the diploid number of
chromosomes in the zygote.
23 pairs of chromosomes in humans
But what about the difference in size
between the egg and sperm?
Can be “extrachromosomal” factors in cytoplasm of egg:
Mitochondria, chloroplasts, infectious agents, chemicals
Box 9.1 Figure 1
Figure 9.1a,b
12 types of chromosomes in the lubber grasshopper
e
b
k
a
d
j
X
i
h
f
c
g
Each type of chromosome has two homologs.
e
b
k
a
d
j
f
X
c
i
h
g
Meiosis is a Special Type of Cell Division that
Occurs in Sexually Reproducing Organisms

Meiosis and fertilization introduce genetic variation
in several ways:
Independent assortment of homologous pairs at metaphase I:
• Each homologous pair can orient in either of two ways
at the plane of cell division. (Fig. 9.5a,b)
• The total number of possible outcomes = 2n (n = number
of haploid chromosomes). (Fig. 9.6)
• Crossing over between homologous chromosomes
at prophase I.
Figure 9.5a
Hypothetical example
Eye color
Gene that
contributes
to brown
eyes
Hair color
Gene that
contributes
to blue
eyes
Gene that
contributes
to black hair
Maternal
chromosome
Paternal
chromosome
Maternal
chromosome
Gene that
contributes
to red hair
Paternal
chromosome
Figure 9.5b
During meiosis I, tetrads can line up two different ways
before the homologs separate.
OR
Brown eyes
Black hair
Blue eyes
Red hair
Brown eyes
Red hair
Blue eyes
Black hair
Figure 9.6
Crossing over
EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING
because of crossing over
1. Parent cell
with four
chromosomes.
2. Crossing
over during
meiosis I.
3. Homologs separate.
(Pairing of chromosomes 4. Gametes
depends on independent produced by
assortment.)
meiosis II.
5. Offspring
produced by selfing
(only some of the
possibilities shown.)
Box 9.2, Figure 1a,b: Crossing over involves breakage and reunion of chromatids
Shape of chromosome 9 varies in two maize strains
Knob
No knob
Long
Short
Strain 1
Strain 2
Genes on chromosome 9 also vary
Colored kernels
Colorless kernels
Waxy kernels
Starchy kernels
Strain 1
Strain 2
Box 9.2, Figure 1c
Predictions of crossing over hypothesis
If crossing over results in exchange of
genetic material between two
chromosomes, the products of
meiosis will look like this:
Products
of meiosis
Chromosome
shape:
Long
with
knob
Short
with
knob
Long
with
no knob
Short
with
no knob
Traits
contributed
to offspring:
Colored,
waxy
kernels
Colored,
starchy
kernels
Colorless,
waxy
kernels
Colorless,
starchy
kernels
Experimental results support these predictions
Figure 9.4c
Figure 9.4b
Figure 9.4d
The Consequences of Meiotic Mistakes

Nondisjunctions occur when homologous chromosomes
fail to separate at meiosis I or when chromatids fail to
separate at meiosis II.
• Fertilization can result in embryos that are 2n + 1
(a “trisomy”) or 2n - 1. (Fig. 9.11)
• Abnormal copy numbers of one or more chromosomes
is usually, but not always, fatal (Example: Down
syndrome). (Fig. 9.12)
• Human survivors: trisomics = 13, 18, 21
Figure 9.11
NONDISJUNCTION at Meiosis I: most common cause, weak meiosis I
alignment checkpoint in females???
n+1
n+1
n–1
2n = 4
n=2
1. Meiosis I starts
normally. Tetrads line
up in middle of cell.
n–1
2. Then one set of
homologs does not
separate (= nondisjunction).
3. Meiosis II
occurs normally.
4. All gametes have an abnormal
number of chromosomes--either
one too many or one too few.
Figure 9.12
Incidence of Down syndrome
per number of births
1
46
1
100
1
290
1
2300
1
1600
1
1200
1
880
20
24
28
32
Age of mother (years)
37
42
47
Other Consequences of Meiosis

Polyploidy can occur when whole
sets of chromosomes
fail to separate at meiosis I or II.
• The resulting 2n gametes, if
fertilized by normal sperm,
create 3n zygotes (triploid).
• Organisms with an odd number
of chromosome sets
cannot produce viable gametes
(Example: seedless fruits).
3n = 2X1 chromosome separation
at meiosis I = unbalanced gametes,
undeveloped seeds
So where does this take us?



How do mitosis and meiosis figure into the
passage of genetic information?
What are “patterns of inheritance”?
How do genes determine organismic
characteristics