Lecture Outline for February 25, 2005



Topics: meiosis

Chapter 9 section 9.1, 9.3

Figures used in lecture: 9.6, 9.7, 9.8, 9.9, 9.10

Additional resources

Meiosis Animation shown in class on Monday Feb 28



Sexual Reproduction

Diversity is fostered by sexual reproduction which combines genetic information from 2 different cells.

Define/explain:  homologous chromosomes (homologous pair of chromosomes), haploid, diploid, gamete, fertilization, zygote

Understand the advantages and disadvantages of sexual reproduction and asexual reproduction.



Define/explain: meiosis, crossing over

You should know all the stages of meiosis and be able to explain in words or diagrams what occurs in each stage of meiosis. 

What are the functions of meiosis and how does meiosis accomplish these functions?

Understand the differences between meiosis I and meiosis II. 

What is the purpose of crossing over.  Be able to describe in words or diagrams the process of crossing over.

You should have a clear understanding of how meiosis increases genetic diversity among offspring.


Meiosis leads to genetic diversity

The products of meiosis I are genetically diverse for two reasons.  First, crossing over during prophase I allows the maternal chromosome to interact with the paternal one.  After crossing over, the recombinant chromatids contain some genetic material from each chromosome.  Second, which member of a pair of chromosomes goes to which daughter cell at anaphase I is a matter of pure chance…since the way in which homologous pairs line up at metaphase I is random.


Today’s Lecture (2-25-05)


Sexual Reproduction


Meiosis reduces the chromosome number by half

Diploid  à Haploid

Meiosis of diploid cells produces haploid daughter cells, which may function as gametes.


Sexual Reproduction



haploid egg and sperm produced by meiosis




            - reduces chromosome number from diploid to haploid

            - allows for genetic diversity among the gametes



1 DNA replication; 2 divisions:


Meiosis I - reduction division (diploid to haploid)

Replicated, homologous chromosomes undergo synapsis; crossing over occurs.

Homologous pairs align at the cell center; are pulled into two different daughter cells.

Each chromosome of the haploid daughter cells consists of two sister chromatids.


Prophase I



crossing over: 



Metaphase I



Meiosis II

replicated chromosomes line up in single-file at the equatorial plate during metaphase II

sister chromatids seaparate at anaphase II


Genetic Diversity

Meiosis and fertilization introduce genetic variation in
three ways:

1.Independent assortment of homologous pairs at metaphase I:


2. Crossing over between homologous chromosomes at prophase I.


3. Random chance fertilization.


Mitosis vs. Meiosis