chapter 9 the passage of lifes organization and information from one generation to the next one way,...
TRANSCRIPT
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 StrategiesAsexual reproduction
corms
bulbs
fragmentation
•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
1997 Dolly
1998 Mice
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.
BACTERIAL CONJUGATION AND RECOMBINATION
Hfr cell Normal cell
Conjugation tube
1. Hfr cells containgenes that allow themto transfer some or all of their chromosometo another cell.
2. Conjugation tubeconnects 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.
Box 9.3, Figure 1 But something else is happening: genetic recombination
Generation 1
Generation 2
Generation 3
Asexual reproduction Sexual reproduction
Figure 9.9
Some comparisons between asexual and sexual reproduction
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
Diploid adult
MITOSIS FERTILIZATION
MEIOSIS:
2n >> n
Haploidgametes (n)
Diploidzygote
Figure 9.7a
Diploid dominant
2n
FERTILIZATION
MITOSIS
MITOSISMEIOSIS
Diploid cell
Haploid cell
Haploid gametes
Haploid adult
Haploid dominant
Figure 9.7b
Diploid plant
Diploid cell
Haploid cells
Haploid gametes Haploid
plant
MITOSIS
MEIOSISMITOSIS
FERTILIZATIION
MITOSIS
Alternation of generations
Figure 9.7c, upper
Snails subject to parasitism by trematode worms (Lively)
Figure 9.10a
Evidence for the benefits of sexual reproduction: resistance
0.40
0.30
0.20
0.15
0.10
0.05
0.01
0.00 0.05 0.15 0.30 0.50
0.00
Mal
e fr
equ
ency
Frequency of infection by parasites
Figure 9.10b
Are genetically diverse populations more resistant to parasites?
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)
A full complement of chromosomes is restored during fertilization.
Femalegamete
n = 23 in humans
Fertilization
Diploid offspringcontains homologouspair of chromosomes
Malegamete
n = 23 in humans
Figure 9.2c
Each chromosome replicates prior to undergoing meiosis.
Maternalchromosome
Centromere
Homologous pair of premeiotic chromosomes
Duplication in
S phase
Paternal chromosome
Sister chromatids
Figure 9.2a
(n = 23 in humans) (n = 23 in humans)
During meiosis, chromosome number in each cell is reduced.
Parent cellcontainshomologouspair of chromosomes
ME
IOS
IS I
Homologs separate
at meiosis I
Sisterchromatids separate at meiosis II
Daughtercellscontainjust one homolog
Four daughter cells contain one chromosome each. These cells become gametes.
ME
IOS
IS I
I
Figure 9.2b
PRIOR TO MEIOSIS MEIOSIS I
Homologous chromosomes separate.
Sister chromatids
Tetrad (4 chromatids from homologous chromosomes)
Chiasma
1. Chromosomesreplicate inparent cell.
2. Synapsis of homologous chromosomes. Crossing over of non-sister chromatids.
3. Tetrads migrate to middle of cell.
4. Homologsseparate.
Chromosomes replicate, forming sister chromatids.
Figure 9.3, left
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.
Figure 9.3, right
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.
But what about the difference in sizebetween the egg and sperm?
Can be “extrachromosomal” factors in cytoplasm of egg:Mitochondria, chloroplasts, infectious agents, chemicals
23 pairs of chromosomes in humans
Box 9.1 Figure 1
12 types of chromosomes in the lubber grasshopper
Each type of chromosome has two homologs.
ab
c
d
X
e
fh
g
ij
c
X
ba j
e
k
kd
g
f
hi
Figure 9.1a,b
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.
Hypothetical example
Eye color
Gene thatcontributes to browneyes
Gene thatcontributes to blueeyes
Maternalchromosome
Paternalchromosome
Hair color
Gene thatcontributes to black hair
Gene thatcontributes to red hair
Maternalchromosome
Paternalchromosome
Figure 9.5a
During meiosis I, tetrads can line up two different waysbefore the homologs separate.
OR
Brown eyesBlack hair
Blue eyesRed hair
Blue eyesBlack hair
Brown eyesRed hair
Figure 9.5b
2. Crossingover duringmeiosis I.
1. Parent cellwith four chromosomes.
3. Homologs separate.(Pairing of chromosomes depends on independentassortment.)
4. Gametes produced by meiosis II.
5. Offspring produced by selfing (only some of the possibilities shown.)
EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING because of crossing over
Figure 9.6
Crossing over
Shape of chromosome 9 varies in two maize strainsKnob
Long
Strain 1 Strain 2
No knob
Short
Genes on chromosome 9 also vary
Colored kernels
Waxy kernels
Strain 1 Strain 2
Colorless kernels
Starchy kernels
Box 9.2, Figure 1a,b: Crossing over involves breakage and reunion of chromatids
Predictions of crossing over hypothesis
Products of meiosis
Chromosome shape:
Traits contributedto offspring:
Long withknob
Short withknob
Colored, waxy kernels
Longwithno knob
Shortwith no knob
Colored, starchykernels
Colorless, waxykernels
Colorless, starchykernels
If crossing over results in exchange ofgenetic material between twochromosomes, the products ofmeiosis will look like this:
Experimental results support these predictions
Box 9.2, Figure 1c
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
n + 1
n + 1
n – 1
n – 1
1. Meiosis I starts normally. Tetrads line up in middle of cell.
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.
NONDISJUNCTION at Meiosis I: most common cause, weak meiosis I
alignment checkpoint in females???
2n = 4 n = 2
Figure 9.11
Inci
den
ce
of
Do
wn
syn
dro
me
per
nu
mb
er o
f b
irth
s
20 24 28 32 37 4742
Age of mother (years)
12300
11600
11200
1100
146
1290
1880
Figure 9.12
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