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Chapter 13
Meiosis
and Sexual
Life Cycles
~
 Gene – segment of DNA that code for basic
units of heredity
 Gametes – (sperm, egg) reproductive cells
that transmit genes from one generation to
the next
 Locus (loci) – location of gene on a
chromosome
Types of Reproduction
 Asexual




single parent passes all of its genes to the offspring
offspring arises from mitosis
exact copy of parent
offspring referred to as clone
 Sexual
 two parents contribute to the genes of offspring
 Results in greater genetic variation
Chromosomes
 Homologous chromosomes
 Pair of 2 chromosomes
 Carry genes that control same characteristics
 ie. If gene for eye color is at specific locus of a chromosome, it’s
homologous will have the same gene at the same locus
 Similar length
 Same centromere position
 One set (23 chromosomes) is inherited from mom and the other
(23) are inherited from dad (making 46)
Chromosomes
 The only exceptions to the homologous rule is
with sex chromosomes
 the 23rd pair of chomosomes are the sex
chromosomes and determine gender
 If XX, then female. If XY then male. (offspring
will always receive X from mom, and X or Y can
come from dad)
 Autosomes – refers to the other 22 kinds of
chromosomes that are not sex chromosomes
Karyotype
 Picture of the
homologous
chromosomes
 Shows all 46
chromosomes paired up
with the chromosome
that is like them
 notice that this is a
karyotype for a female
(XX)
Fertilization
 During fertilization, one
haploid gamete from
each parent will fuse
 The result is a zygote
(fertilized egg)
 The zygote is diploid,
arising from two
haploids
Meiosis
 Meiosis and mitosis
are similar in that both
are preceded by DNA
being replicated
 Key differences is that
mitosis involves 1 cell
division, where
meiosis involves 2 cell
divisions
Meiosis 1

The 1st cell division is referred to as meiosis 1
 Prophase I
 Chromosomes condense
 Synapsis occurs – joining of homologous chromosomes
forming tetrad
 Crossing over occurs – “mixing genes” . DNA from one homologous is
exchanged with another (results in variation…no chromosome is identical to the
other anymore)
 Chiasmata – area of crossing over
 Nucleus disappears, spindles appear and attach tot eh chromosomes

Metaphase I
 Homologous pairs of chromosomes are lined up in the middle

Anaphase I
 Spindle fibers pull chromosomes to opposite ends (chromosomes are still intact)

Telophase 1 &Cytokinesis
 Chromosomes are at opposite ends and the cell splits into 2
 not identical because of crossing over
 each cell now has 46 chromosomes
In between Meiosis 1 and Meiosis 2
 Chromosomes in the 2 cells DO NOT
replicate (duplicate), but they now have 46
chromosomes a piece
Meiosis II
 Prophase II
 Spindle fibers form again
 chromosomes start to move toward metaphase plate (equator)
 Metaphase II
 Individual chromosomes line up in the middle (no longer homologous pairs)
 Anaphase II
 Centromeres of sister chromatids separate
 Sister chromatids move toward opposite poles
 Telophase II & Cytokinesis


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Sister chromatids are now completely separated
Nucleus appears
The result is 4 haploid cells (sperm or egg)
*** for females, only one egg is made the other 3 cells are called polar bodies
which serve to give the egg cell energy as it stays in body for about 28 days
Meiosis vs. mitosis
3 Events in Meiosis I that do not
occur in Mitosis
1) synapsis and crossing over do
not occur in mitosis
2) at metaphase I, homologous
pairs are lined up, but in
mitosis invidivual
chromosomes lined up
3) Anaphase 1 chromosomes
move to opposite poles, but in
mitosis, the sister chromatids
move toward poles
Genetic Variation
Crossing over – during
prophase I the enchange of
genes between homologous
chromosomes (this will make
each sister chromatid in the
tetrad different)
 Independent assortment – in
metaphase I the chromosomes
can pair up with another
chromosome (not their
homolog) before the split
 Random
Fertilization –
n
because each egg can be
fertilized by any of a number of
sperm, this lead to variation .

Origins of Genetic Variation, II
 Crossing over (prophase I):
• the reciprocal exchange of
genetic material between
nonsister chromatids during
synapsis of meiosis I
(recombinant chromosomes)
 Random fertilization:
• 1 sperm (1 of 8 million
possible chromosome
combinations) x 1 ovum (1 of 8
million different possibilities) =
64 trillion diploid combinations!