Transcript Understanding Our Environment

Sexual Reproduction and Meiosis
Chapter 7b
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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 somatic (body) cells are diploid.
- Gamete (reproductive) cells are hapolid.
Unique Features of Meiosis
(What makes it different from Mitosis)
•
•
•
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.
The most important difference
between meiosis and mitosis is that
meiosis produces _____ cells.
A)
B)
C)
D)
Four diploid
Four haploid
Two haploid
Somatic
Prophase I



Chromosomes condense\
Homologous chromosomes pair up
Crossing over may occur while chromosomes
are very close together
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Crossing Over-during Prophase I
•
Chiasmata are Xshaped structures
•
Presence of chiasmata
indicates crossing over
has occured
Metaphase I
•
Homologues line up on metaphase plate (equator).
 Orientation of each pair is random (mom’s
chromosomes vs. dad’s choromosomes may be
to the right or to the left)
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Anaphase I

Homologues separate
- Each pole receives a member of each
homologous pair.
- Due to random orientation, meiosis results in
independent assortment.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Telophase I
•
•
•
•
Chromosomes segregated into two clusters.
Nuclear membrane re-forms around each daughter
cell.
Cells are not identical
Cytokinesis occurs to form 2 haploid cells
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Meiosis II
•
•
No replication of DNA occurs between
Meiosis I and Meiosis II
Prophase II - Nuclear envelope breaks down
and second meiotic division begins.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Meiosis II
•
Metaphase II –
 Spindle fibers bind to both sides of centromere
 Chromosomes line up (single file) along equator
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Meiosis II
•
Anaphase II - Spindle fibers contract and sister
chromatids move to opposite poles.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Meiosis II
Telophase II - Nuclear envelope re-forms.
Final Result – Cytokinesis makes 4 haploid cells.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Which meiosis phase listed below is
the most different from mitosis?
A)
B)
C)
D)
Prophase II
Metaphase I
Telophase II
Cytokinesis
Types of Reproduction
•
•
Asexual Reproduction - Individual inherits all
its chromosomes from a single parent.
Sexual Reproduction - Produces genetic
variability.
 Segregation of chromosomes tends to
disrupt advantageous combinations.
- Only some progeny maintain advantages.
Parthenogenesis
Development of adult from unfertilized egg.
Mendelian Genetics
•
•
•
•
Gregor Mendel (1822-1884)
Austrian monk
Observed plants in the
monastery garden
Father of Genetics for
determining inheritance
patterns in peas
•
•
•
Punnett Squares
Tool used to predict the
next generation of offspring
P1 cross
 Cross between two
parents pure for a trait
 Produces F1 generation
F1 cross
 Cross between the first
generation offspring
 Produces F2 generation
What is the genotype that belongs in
square 1?
A)
B)
C)
D)
TT
Tt
tt
None of the above
1
Punnett Squares
•
Genotype is the letter
combination of the alleles
 GG, Gg, or gg
•
Phenotype is the
manifestation (what trait is
produced)
 Green or yellow
What is the phenotype of the yellow
peas?
A)
B)
C)
D)
Green
Yellow
GG
Gg
What is the genotype of the yellow
peas?
A)
B)
C)
D)
Green
Yellow
gg
Gg