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AP BIOLOGY:
Chapter Twelve Outline
INTRODUCTION TO MEIOSIS
Different Chromosome Numbers Are Found in One Organism
Gametes contain half as many chromosomes as somatic cells
Zygotes are produced by fusion of gametes
Each gamete contains a single complement of genetic material
The zygote contains two copies of each chromosome
Fusion of gametes is called fertilization or syngamy
Fusion of body cells would sequentially increase chromosome number
Meiosis Is an Important Event fig 12.1
Serves to stabilize the chromosomes number
Is a reduction division process
THE SEXUAL LIFE CYCLE
Fertilization and Meiosis Constitute a Cycle of Sexual Reproduction fig 12.2
Body cells of adult are diploid and possess two sets of chromosomes
Gametes are haploid and possess a single set of genetic material
An individual inherits genes from its father and its mother fig 12.3
In humans, 23 chromosomes from the mother's egg
In humans, 23 chromosomes from the father's sperm
Life Cycles Show Pattern of Alternating Chromosome Numbers
Alternate between diploid and haploid number fig 12.4
After syngamy the zygote divides by mitosis
All adult somatic cells are genetically identical to the zygote
Sexual Reproduction Varies by Kingdom
Unicellular organisms
Individual cells function directly as gametes
Zygote may divide mitotically or meiotically
Plants
Haploid cells are produced through meiosis
Cells divide mitotically to form multicellular haploid phase
Special haploid cells differentiate into eggs or sperm
Animals
Gamete-producing cells differentiate from somatic cells early
Referred to as germ line cells
Somatic cells are diploid and reproduce by mitosis
Diploid gamete-producing cells, produce haploid gametes by meiosis
THE STAGES OF MEIOSIS
Meiosis Consists of Two Rounds of Nuclear Division
Process has much in common with mitosis
Different due to two unique features
Homologous chromosomes (homologues) pair along their length
Crossing over: genetic exchange occurs between pairs
Chromosomes come together along equatorial plate
Homologues drawn to opposite poles
Clusters at poles are haploid
Each chromosome still composed of two chromatids
Chromosomes do not replicate between divisions
Second division is nearly identical to mitosis
Sister chromatids dissimilar because of crossing over
Process is continual, arbitrarily divided into stages
Two stages called meiosis I and meiosis II
Each stage divided into prophase, metaphase, anaphase and telophase fig 12.5
Meiosis prophase I more complex than mitosis prophase
The First Meiotic Division
Prophase I
Individual chromosomes become visible under light microscopy
Each chromosome replicated thus present as two sister chromatids
Ends of chromatids attach to nuclear envelope at specific sites
Membrane sites of homologues are adjacent
Members of homologous pairs brought close together
Homologues line up side by side in synapsis fig 12.6
Forms resultant synaptonemal complex
Held together by protein latticework
Each gene held in precise register with its corresponding gene
DNA duplexes unwind
Single strands of DNA pair with complementary strand from other homologue
Synapsis initiates a complex event called crossing over
DNA segments are exchanged between sister chromatids
When process is complete synaptonemal complex breaks down
Homologous chromosomes released from the nuclear membrane
Homologues do not separate completely
Sister chromatids held together by their common centromere
Paired homologues held together at points of crossing over
Points of crossing over may be visible as X-shaped chiasma fig 12.7
Chiasma indicates that two chromatids have exchanged parts
Chiasma move to ends of arms as chromosomes separate
Metaphase I
Nuclear envelope disperses, microtubules form spindle as in mitosis
Different from mitosis as homologues are paired
Terminal chiasmata: when chiasma reaches ends of chromosome
One side of centromere faces outward
One side of centromere faces other homologue fig 12.8
Spindle microtubules can only attach to outer face of centromere
Centromere of each homologue attached to only one pole
Joined homologues line up on metaphase plate
Attachment of homologue to a pole is random fig 12.9
Anaphase I
With completion of spindle attachment, microtubules shorten
Chiasma broken, centromeres pulled toward each pole
Chromosomes dragged along
Individual centromeres not pulled apart
Sister chromatids do not separate
Each pole has a complete set of haploid chromosomes
Each set contains one member of each homologous pair
Poles receive homologues randomly
Genes on different chromosomes assort independently
Telophase I
Each pole has full complement of chromosomes
Each chromosome exists as sister chromatids joined by centromere
Chromatids are not identical because of crossing over
Cytokinesis may or may not occur at this point
The Second Meiotic Division fig 12.10
Is a simple mitotic division using the products of meiosis I
Two clusters at either pole divide mitotically
Spindle apparatus binds to sides of centromeres
Centromeres divide
Chromatids drawn to opposite poles
End result is four haploid complements of chromosomes
Nuclear envelopes reform
In animals, cells develop into gametes
In plants, fungi, protists cells may proliferate via mitotic divisions
WHY SEX?
Not All Reproduction Is Sexual
Asexual reproduction
Individual inherits all chromosomes from one parent
Individual is genetically identical to parent
Bacterial cells reproduce by binary fission
Protists divide asexually unless under stress
Multicellular organisms
May reproduce by budding off localized masses of cells
Sponges reproduce asexually by fragmentation
Development from an unfertilized egg via parthenogenesis
Example: bees
Fertilized eggs become diploid females
Unfertilized eggs become haploid males
Example: vertebrates fig 12.11
Evolutionary Rationale for Sexual Reproduction
Problems associated with sexual reproduction
Advantage to species which benefit from genetic variability
Evolution occurs because of changes at level of the individual
Recombination is evolutionarily both constructive and destructive
Segregation of chromosomes disrupts beneficial gene combinations
Diverse progeny will be less well-adapted than parents
Complex adaptations are less likely to benefit from recombination
Trends for asexually-reproducing organisms
Live in harsh habitats
Premium on well-adapted, genetically uniform individuals
Benefit to sexual reproduction is yet unknown
Meiotic recombination among protists is often absent
Sex may only occur under stressful conditions
In some protists diploid is transient or only haploid phase exists
With stress haploids fuse forming diploid zygote
Resulting diploid may not persist
Sex may have evolved in protists to repair DNA damage
Particularly double-stranded breaks in DNA
Breaks induced by radiation or chemicals
Repair of such damage is necessary in longer-lived organisms
DNA repair through mechanism of synaptonemal complex
Transient diploid stage allows for such repair
Special yeast mutations
Repair system inactivated for double-strand breaks
Crossing over also prevented
THE EVOLUTIONARY CONSEQUENCES OF SEX
Principal Factors in the Evolution of the Eukaryotes
Reassortment of genetic material occurs during meiosis
Represents an enormous factor in initiation of genetic variability
In humans 23 chromosomes are from each parent
Each chromosome segregates independently of all others
Gamete possibilities equals 223 (over eight million)
Fertilization squares the number of possibilities (70 trillion)
Crossing over further adds to the variability
Evolutionary Consequences of Sex Are Profound
Genetic diversity is raw material of evolution
Pace of evolution increased with greater genetic diversity
Evolutionary Process Is Revolutionary and Conservative
Revolutionary as the pace is quickened by genetic variability
Conservative as variation is not always favored by selection
Acts to preserve existing combinations of genes
Greater in asexual organisms that are not highly mobile
Live in extremely demanding habitats
In vertebrates, the evolutionary premium is on versatility, thus sex
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