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9.1 Genes and Alleles
Genes• Sequences of DNA that encode heritable traits
Alleles • Slightly different forms of the same gene• Each specifies a different version of gene product
Sexual and Asexual Reproduction
Asexual reproduction (1 parent)• Offspring inherit parent’s genes• Clones (identical copies of parent)
Sexual reproduction (2 parents)• Offspring differ from parents and each another• Different combinations of alleles • Different details of shared traits
Sexual Reproduction
Meiosis, gamete formation, and fertilization occur in sexual reproduction
Meiosis and fertilization shuffle parental alleles• Offspring inherit new combinations of alleles
Flowering plant
Fig. 9.3a, p.140
anther (wheresexual sporesthat give rise tosperm form)
ovules inside anovary (where sexualspores that giveto eggs form)
Key Concepts: SEXUAL VS. ASEXUAL REPRODUCTION
By asexual reproduction, one parent alone transmits its genetic information to offspring
By sexual reproduction, offspring typically inherit information from two parents that differ in their alleles
Alleles are different forms of the same gene; they specify different versions of a trait
9.2 What Meiosis Does
Meiosis • Nuclear division mechanism that precedes
gamete formation in eukaryotic cells• Halves parental chromosome number
Fertilization• Fusion of two gamete nuclei • Restores parental chromosome number• Forms zygote (first cell of new individual)
Fig. 9.12, p.150
gametes gametes
germ cell germ cell
each chromosomeduplicated during
interphase
MEIOSIS IIseparation of
sister chromatids
MEIOSIS Iseparation ofhomologues
diploid numberrestored atfertilization
zygote
2n
2n
n
Homologues
Sexual reproducers inherit pairs of chromosomes• 1 from maternal parent, 1 from paternal parent
The pairs are homologous (“the same”) • Except nonidentical sex chromosomes (X and Y)• Same length, shape, genes
All pairs interact at meiosis• One chromosome of each type sorts into gametes
9.3 Tour of Meiosis
All chromosomes are duplicated during interphase, before meiosis
Two divisions, meiosis I and II, divide the parental chromosome number by two
Each forthcoming gamete is haploid (n)
Meiosis I
The first nuclear division
Each duplicated chromosome lines up with its homologous partner
The two homologous chromosomes move apart, toward opposite spindle poles
Prophase I
Chromosomes condense and align tightly with their homologues
Each homologous pair undergoes crossing over
Microtubules form the bipolar spindle
One pair of centrioles moves to the other side of the nucleus
Prophase I (cont.)
Nuclear envelope breaks up• Microtubules growing from each spindle pole
penetrate the nuclear region
Microtubules tether one or the other chromosome of each homologous pair
Metaphase I
Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator
Anaphase I
Microtubules separate each chromosome from its homologue, moving to opposite spindle poles
Other microtubules overlap midway between spindle poles, slide past each other to push poles farther apart
As anaphase I ends, one set of duplicated chromosomes nears each spindle pole
Telophase I
Two nuclei form • Typically, the cytoplasm divides
All chromosomes are still duplicated• Each still consists of two sister chromatids
Meiosis II
The second nuclear division
Sister chromatids of each chromosome are pulled away from each other
Each is now an individual chromosome
Anaphase II and Telophase II
In anaphase II, one chromosome of each type is moved toward opposite spindle poles• Occurs in both nuclei formed in meiosis I
By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes
plasma membrane
spindle equator (midway between the two poles)
one pair of homologous chromosomes
Prophase I Metaphase I Anaphase I Telophase I
Meiosis I
Fig. 9.5a, p.142
newly formingmicrotubules ofthe spindle
breakupof nuclearenvelope
centrosome witha pair of centrioles,moving to oppositesides of nucleus
Chromosomes were duplicated earlier, ininterphase.
Prior to metaphase I, one set of microtubules had tethered one chromosome of each type to one spindle pole and another set tethered its homologue to the other spindle pole.
One of each duplicatedchromosome, maternal or paternal, moves to a spindle pole; its homologue moves to the opposite pole.
One of each typeof chromosome hasarrived at a spindlepole. In most species,the cytoplasm dividesat this time.
Prophase II Metaphase II Anaphase II Telophase II
Meiosis II
there is no DNA replication between the two divisions
Fig. 9.5b, p.142
In each cell, one of two centrioles moves to the opposite side of thecell, and a new bipolar spindle forms.
By now, microtubules fromboth spindle poles have finished a tug-of-war.
The sister chromatids of each chromosome move apart and are now individual, unduplicated
A new nuclear envelopeencloses each parcel ofchromosomes, so thereare now four nuclei.
Haploid Daughter Cells
When cytoplasm divides, four haploid cells result
One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies
Key Concepts:STAGES OF MEIOSIS
Diploid cells have a pair of each type of chromosome, one maternal and one paternal
Meiosis, a nuclear division mechanism, reduces the chromosome number
Meiosis occurs only in cells set aside for sexual reproduction
Key Concepts:STAGES OF MEIOSIS (cont.)
Meiosis sorts out a reproductive cell’s chromosomes into four haploid nuclei
Haploid nuclei are distributed to daughter cells by way of cytoplasmic division
9.4 Meiosis Introduces Variation in Traits
Two events in meiosis cause variation in traits in sexually reproducing species• Crossing over during prophase I of meiosis• Chromosome shuffling during metaphase I of
meiosis
Prophase I: Crossing Over
Nonsister chromatids of homologous chromosomes undergo crossing over• They exchange segments at the same place
along their length
Each ends up with new combinations of alleles not present in either parental chromosome
Fig. 9.6, p.144
a A maternal chromosome (purple) and paternal chromosome (blue) were duplicated earlier, duringinterphase. They become visible in microscopes early in prophase I, when hey star to condense tothreadlike form.
b Each chromosome and its homologous partner zippertogether, so all four chromatidsare tightly aligned.
mom’salleleB
mom’salleleA
mom’salleleA
mom’salleleA
mom’salleleB
dad’sallele
a
dad’salleleb
dad’salleleb
c Here is a simple way to think about crossing over. (Chromosomes are still condensed and threadlike,and each is tightly aligned with its homologous partner.)
d Their intimate contactpromotes crossing over at different places along the length of nonsister chromatids.
e At the crossover site, paternal and maternal chromatids exchangecorresponding segments.
f Crossing over mixesup maternal and paternalalleles on homologouschromosomes.
Metaphase I: Chromosome Shuffling
Homologous chromosomes align randomly during metaphase I
Microtubules can harness either a maternal or paternal chromosome of each homologous pair to either spindle pole
Either chromosome may end up in any new nucleus (gamete)
Key Concepts: CHROMOSOME RECOMBINATION AND SHUFFLING
During meiosis, each pair of maternal and paternal chromosomes swaps segments and exchanges alleles
Pairs get randomly shuffled, so forthcoming gametes end up with different mixes of maternal and paternal chromosomes
9.5 From Gametes to Offspring
Multicelled diploid and haploid bodies are typical in life cycles of plants and animals
Plants• Sporophyte: A multicelled plant body (diploid) that
makes haploid spores• Spores give rise to gametophytes (multicelled
plant bodies in which haploid gametes form)
From Gametes to Offspring
Animals• Germ cells in the reproductive organs give rise to
sperm or eggs• Fusion of a sperm and egg at fertilization results
in a zygote
meiosisDIPLOID
fertilization
zygote
gametes spores
HAPLOID
Fig. 9.8a, p.146
meiosis
meiosis
meiosis
a Plant life cycle
(2n)
(2n)
(n)
multicelledgametophyte
multicelledsporophyte
(n) (n)
Fig. 9.8b, p.146
meiosis DIPLOIDfertilization
zygote
HAPLOID
meiosis
(2n)
(2n)
(n)gametes
multicelledbody
b Animal life cycle
Introducing Variation in Offspring
Three events cause new combinations of alleles in offspring: • Crossing over during prophase I (meiosis)• Random alignment of maternal and paternal
chromosomes at metaphase I (meiosis)• Chance meeting of gametes at fertilization
All three contribute to variation in traits
a Growth b Meiosis I andcytoplasmic division
c Meiosis II and cytoplasmic division
spermatids (haploid)
secondary spermatocytes
(haploid)
primary spermatocyte
(diploid)
Fig. 9.9, p.147
diploid malegerm cell
sperm (mature,haploid male
gametes)
a Growth b Meiosis I and cytoplasmic division
c Meiosis II and cytoplasmic division
ovum (haploid)
primary oocyte (diploid)
secondary oocyte
(haploid)
first polar body
(haploid)
three polar bodies
(haploid)
Fig. 9.10a, p.147
oogonium(diploidfemale
germ cell)
Key Concepts: SEXUAL REPRODUCTION IN LIFE CYCLES
In animals, gametes form by different mechanisms in males and females
In most plants, spore formation and other events intervene between meiosis and gamete formation
9.6 Comparing Mitosis and Meiosis
Both mitosis and meiosis require bipolar spindle to move and sort duplicated chromosomes
Some mechanisms of meiosis resemble those of mitosis, and may have evolved from them• Example: DNA repair enzymes function in both
Differences in Mitosis and Meiosis
Mitosis maintains parental chromosome number• Duplicates genetic information• Occurs in body cells
Meiosis halves chromosome number• Introduces new combinations of alleles in
offspring• Occurs only in cells for sexual reproduction
Telophase IProphase I Anaphase IMetaphase I
Fig. 9.11a, p.148
In a diploid (2n) germ cell,duplicated chromosomesnow condense. The bipolarspindle forms and tethers thechromosomes. Crossoversoccur between homologues.
Each maternal chromosomeand its paternal homologueare randomly aligned midwaybetween the two spindlepoles. Either one may getattached to either pole.
Homologouspartnersseparateand moveto oppositepoles.
There are two clustersof chromosomes. Newnuclear envelopes mayform and the cytoplasmmay divide beforemeiosis II begins.
Fig. 9.11b, p.149
In a diploid (2n) body cell, the duplicated chromosomes nowcondense. Bipolar spindle formsand tethers the chromosomes.
All chromosomesaligned at thespindle equator.
Sister chromatids of each chromosomemoved to oppositespindle poles.
Two diploid (2n) nuclei form. After cytoplasmicdivision, there are two diploid body cells.
Prophase Metaphase Anaphase Telophase
Mitosis
Fig. 9.11c, p.149
no interphaseand no DNAreplicationbetween thetwo nucleardivisions
Prophase II Metaphase II Anaphase II Telophase IIAll chromosomes stillduplicated. New spindleforms in each nucleus,tethers chromosomesto spindle poles.
All chromosomesaligned at thespindle equator.
Sister chromatids ofeach chromosomemoved to oppositespindle poles.
Four haploid (n) nucleiform. After cytoplasmicdivision, haploid cellsfunction as gametesor spores.
Key Concepts: MITOSIS AND MEIOSIS COMPARED
Recent molecular evidence suggests that meiosis originated through mechanisms that already existed for mitosis and, before that, for repairing damaged DNA
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