Meiosis and Sexual
Reproduction
Asexual
Reproduction
• Single “parent” produces
offspring
• Growth and repair
• Mitosis produces daughter
cells genetically identical to
one another and to the
parent = “clones”
Sexual
Reproduction
• “Germ cells” undergo
meiosis and cytokinesis
• Daughter cells become
gametes (sex cells)
• Gametes meet (hopefully!) at
fertilization
•
Produces genetic variation
among offspring
• This variation in traits is the
basis for evolutionary
change
Chromosome
Structure
Explore the “chromosome
viewer” at this site based on
information from the Human
Genome project (& the source
of this figure):
http://www.ornl.gov/sci/techresources/
Human_Genome/posters/chromosome
/index.shtml
Homologous Chromosomes
may carry different Alleles
• Each diploid cell has two of
each chromosome (23 pairs
in humans)
Mom
Dad
• One chromosome in each
pair from mother, other from
father
• Paternal and maternal
chromosomes may carry
different alleles
Perhaps different forms of the same gene?
Ploidy: Chromosome Number
p. 141
• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
1-5
• Gametes are haploid (n)
6-12
• Meiosis halves chromosome number,
leaving one complete set of 23 in each
13-18
gamete
Are these chromosomes replicated?
19-22
23: XX
Meiosis: Two Divisions
•
Two successive nuclear divisions
1. Meiosis I
(Without DNA replication in between)
2. Meiosis II
•
Four haploid nuclei are formed
– All (potentially) genetically different
•
Events are similar to those in mitosis
(prophase, metaphase, anaphase, telophase)
Meiosis I
p. 141
(one homologous pair of chromosomes)
What is the ploidy level of this nucleus?
Are the chromosomes replicated?
Each homologue in the
cell pairs with its partner,
then the partners
separate
What is the ploidy level of these nuclei after meiosis I?
Meiosis II
• The two sister chromatids of each
duplicated chromosome are separated
from each other
two chromosomes
(unduplicated)
one chromosome
(duplicated)
Meiosis I - Stages
Prophase I
Metaphase I Anaphase I
p. 142
Telophase I
Meiosis II - Stages
Prophase II
p. 143
Metaphase II Anaphase II Telophase II
3 main reasons for genetic
variability as a result of
sexual reproduction:
1. Crossing over
•
Prophase I
2. Independent Assortment (of chromosomes)
•
Metaphase I
3. Random Fertilization
•
random re-combination of genes in fertilized
gamete
Crossing Over
p. 144
E
E
e
e
•All four chromatids are closely aligned
•Homologous chromosomes exchange
l
l L
L
segments
•New combinations of alleles created
What would result if sister chromatids exchanged
segments?
Germ cell

“synapsis”
big eyes,
no light
Bioluminescenc and eye size in viper fish. What
traits would be linked after crossing over?
Big Eyes (E)
small eyes (e)
small eyes
light
Light (L)
no light (l)
Independent
Assortment
• random alignment of
chromosomes during
Metaphase
• number of possible
combinations of
chromosomes in a
gamete is:
2n
n = # chromosome types
p. 145