Genetics
Meiosis
Sexual Life Cycles
Alternation of meiosis and fertilization is common in most living organisms
Sexual Life
Cycles of
Humans
Meiosis
diploid and haploid stages
n
n
Meiosis II
n
2n
Meiosis I
diploid
n
n
haploid
Meiosis II
n
haploid
Meiosis I
reduction division, separates homologues
n
n
Meiosis II
n
2n
Meiosis I
diploid
n
n
haploid
Meiosis II
n
haploid
Meiosis I

Interphase (precedes Meiosis)


Meiosis I


G1 , S, G2
prophase I, metaphase I, anaphase I, telophase I
Cytokinesis (follows Meiosis)
Interphase
precedes meiosis
 G1 first gap
 S
synthesis
 G2 second gap
Prophase I





longest phase
synapses
tetrad formation
crossing-over
chiasma
Metaphase I

tetrads align at the
metaphase plate
Anaphase I


homologous chromosomes
pull apart
separation accomplished by


disassembly of kinetochore
microtubules
elongation by nonkinetochore
microtubules
homologous chromosomes
separate
Telophase I


Chromosomes reach
the poles
each pole with haploid
number
cleavage furrow
Cytokinesis



Division of cytoplasm
animal cells form a cleavage furrow
plant cells form a cell plate
cleavage furrow
Meiosis II
mitotic division, separates sister chromatid
n
n
Meiosis II
n
2n
Meiosis I
diploid
n
n
haploid
Meiosis II
n
haploid
Meiosis II

prophase II, metaphase II, anaphase II, telophase II

Cytokinesis
Prophase II
spindle apparatus forms
Metaphase II
sister chromatids align at equator
Anaphase II
sister chromatids separate
Telophase II
daughter cells contain half the
number of chromosomes the
initial parent cell had
2n
diploid
n
haploid

vb v
Summary

ghgnfn
Gametogenesis

Spermatogenesis

Oogenesis
Spermatogenesis
Oogenesis
End product is ONE egg and 3
polar bodies
Some Zygote Possibilities

single birth


monozygotic (identical) twins


1 sperm + 1 ovum = 1 zygote
1 sperm + 1 ovum = 1 zygote
dizygotic (fraternal) twins

2 sperm + 2 ova = 2 zygotes
Events that contribute to Variations




independent assortment chromosomes
random fusion of gametes
crossing-over
mutations
Independent assortment of
chromosomes
Random fusion of gametes

A gamete from one individual unites with a
gamete from another individual

1 in 8 million x 1 in 8 million = 64 trillion

64 trillion combinations of chromosomes
Crossing-over
Prophase I
Chromosomes



human = 46 chromosomes (23 pairs)
autosomal chromosomes = 44 (22 pairs)
sex chromosomes = 2 (1 pair)
Female Sex Chromosomes


female = XX
all gametes carry X chromosome

homogametic sex
Male Sex Chromosomes



male = XY
1/2 of gametes carry X, 1/2 carry Y
heterogametic sex

Hemizygous
Activity


Is this cell going through
mitosis or meiosis?
How do you know?





Is this cell going through
mitosis or meiosis?
How do you know?
What specific stage is it in?
What special process is
going on?
What is the importance of it?
The End