Transcript Understanding Our Environment

Sexual Reproduction and Meiosis
Chapter 7b
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Outline
•
•
•
•
•
•
Reduction Division
Unique Features of Meiosis
Prophase I
Metaphase I
Completing Meiosis
 Second Meiotic Division
Sexual Reproduction
 Origin and Maintenance
 Evolutionary Consequences
Reduction Division
•
In sexual reproduction, gametes fuse
(fertilization) to produce a zygote.
 Gamete formation involves a mechanism
(meiosis) that reduces the number of
chromosomes to half that found in other
body cells.
- Adult body cells are diploid.
- Gamete cells are hapolid.
 Alteration of generations.
Unique Features of Meiosis
•
•
•
Synapsis
 Homologues pair along their length.
Homologous Recombination
 Genetic exchange (crossing over) occurs
between homologous chromosomes.
Reduction Division
 Chromosomes do not replicate between
the two nuclear divisions.
Reduction Division
Copyright © McGraw-Hill Companies
Permission required for reproduction or display
Prophase I
•
Five Sequential Stages
 Leptotene - Chromosomes condense
 Zygotene - Synaptonemal complex
 Pachytene - Crossing over
 Diplotene - Cell growth
 Diakinesis - Chromosomes recondense
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Crossing Over
•
Presence of chiasmata dictates crossing
over has occurred.
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Metaphase I
•
•
Terminal chiasmata holds homologous pair
together.
 Spindle microtubules attach to kinetochore
proteins only on the outside of each
centromere.
Each joined pair of homologues lines up on
metaphase plate.
 Orientation of each pair is random.
Chiasmata and Alignment
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Completing Meiosis
•
Anaphase I
 Spindle fibers begin to shorten and pull
whole centromeres towards poles.
- Each pole receives a member of each
homologous pair.
 Complete set of haploid
chromosomes.
 Due to random orientation, meiosis
results in independent assortment.
Completing Meiosis
•
Telophase I
 Chromosomes segregated into two clusters.
- Nuclear membrane re-forms around each
daughter cell.
 Sister chromatids are no longer
identical.
Second Meiotic Division
•
•
Meiosis II resembles normal mitotic division.
 Prophase II - Nuclear envelope breaks
down and second meiotic division begins.
 Metaphase II - Spindle fibers bind to both
sides of centromere.
 Anaphase II - Spindle fibers contract and
sister chromatids move to opposite poles.
 Telophase II - Nuclear envelope re-forms.
Final Result - Four haploid cells.
Sex
•
•
Asexual Reproduction - Individual inherits all
its chromosomes from a single parent.
 Parthenogenesis - Development of adult
from unfertilized egg.
Sexual Reproduction - Produces genetic
variability.
 Segregation of chromosomes tends to
disrupt advantageous combinations.
- Only some progeny maintain advantages.
Origin and Maintenance Of Sex
•
Theories
 DNA Repair Hypothesis
- Only diploid cells can effectively repair
certain kinds of chromosome damage.
 Contagion Hypothesis
- Secondary consequence of the infection
of eukaryotes by mobile genetic
elements.
Origin and Maintenance Of Sex
•
•
Red Queen Hypothesis
 Current recessive alleles can be stored in
reserve for potential future use.
Miller’s Ratchet
 Sexual reproduction may be a method of
keeping mutational load low.
Evolutionary Consequences of Sex
•
Evolutionary Process is revolutionary and
conservative.
 Pace of evolutionary change is accelerated
by genetic recombination.
 Evolutionary change not always favored by
selection.
- May act to preserve existing gene
combinations.
Copyright © McGraw-Hill Companies Permission required for reproduction or display