Meiosis
Meiosis is the basis of sexual
reproduction
Outline
1.
Key concepts
2.
Heredity
3.
Asexual and sexual reproduction
4.
The roles of meiosis in life cycle
5.
Meiosis
6.
Origins of genetic variation
7.
Key terms
8.
Conclusions
Key Concepts:
1. Heredity is the transmission of traits from one
generation to the next
2. Clone – a group of genetically identical
individuals produced by asexual reproduction
3. The fusion of gametes called fertilization or
syngame
Key Concepts:
4. Meiosis is the basis of sexual reproduction
5. Sexual reproduction dominates the life cycle of
nearly all animals
6. Crossing over, random segregation, and
random fertilization contribute to variation in
traits among offspring
Heredity
Asexual
reproduction
Figure 13.1 The asexual
reproduction of a hydra
Sexual reproduction
The roles of
meiosis in life
cycle
SEM of sea urchin
sperm fertilizing egg
Human Life Cycle
Figure 13.3 The
human life
cycle
The process of meiosis
DNA replication occurs in interphase prior to meiosis
Meiosis I:
1. Prophase (90%) - chromosomes condense, homologous
chromosomes pairing (synapsis) to form tetrads
Crossing-over (chiasmata) occurs between homologues
2. Metaphase - chromosomes line up at equator of cell (randomly)
3. Anaphase - homologues separate, sister chromatids remain
together (Random Segregation/Independent Assortment)
4. Telophase - chromosomes remain condensed
No DNA replication between meiosis I and meiosisII
Meiosis II: (very similar to mitosis)
Figure 13.6a The stages of
meiotic cell division
Figure 13.6b The stages of
meiotic cell division
Meiosis I
Prophase I and Metaphase I
Meiosis I
Anaphase I and Telophase I
Meiosis II
Prophase II and Metaphase II
Meiosis II
Anaphase II and Telophase II
Each daughter
cell now has a
haploid
chromosome
number
A Closer Look at Meiosis I
Prophase I
– Chromosomes
as homologues
– Chromatids are
aligned
– Pairing
Diagram of Homologous
Chromosomes and Crossing Over
Figure 13.5 Overview
of meiosis: how
meiosis reduces
chromosome number
Figure 13.8 The results of alternative arrangements of two homologous chromosome
chromosome
pairs on the metaphase (plate in meiosis I)
Possible Outcomes of Random
Alignment
Metaphase I
Origins of genetic variation
Figure 13.7a A comparison of
mitosis and meiosis
Figure 13.7b A comparison
of mitosis and meiosis
Autosomes
Haploid cells
Heredity
Diploid cells
Variation
Fertilization
Genetics
Homologous chromosomes
Gene
Zygote
Asexual reproduction Synapsis
Clone
Tetrad
Sexual reproduction Chiasma
Life cycle
Chiasmata
Somatic cells
Crossing over
Syngamy
Random segregation
In Conclusion
1. A parent cell provides each daughter cell with
hereditary instructions
2. Eukaryotes divide by mitosis or meiosis and
Prokaryotes divide by binary fission
3. Each chromosome is one DNA molecule with
proteins attached
4. Cells with a diploid number (2n) contain two of
each kind of chromosome
In Conclusion
5. Meiosis consists of two consecutive
divisions
6. In meiosis II, sister chromatids are
separated and four haploid cells are the
result
7. Crossing over, random segregation, and
random fertilization contribute to variation
in traits among offspring