What Are The Odds…?
Non-Fatal Injuries

Zipper: 1: 2,600

Toilet: 1: 6,500

Alarm Clock: 1: 350,000

Bed: 1: 400
Fatal Injuries 1

Snake Bite: 1: 36,000,000

Dog Bite: 1: 20,000,000
Fatal Injuries 2

Ebola: 1: 14,000,000

Appendicitis: 1: 700
Fatal Injuries 3

Earth Destroyed By Meteor: 1: 20,000

You Alone Are Killed By Meteor:
1: 150,000,000,000
What Are The Odds That There Is An Exact
Duplicate of YOU?
Unlikely…
 3.2
billion nucleotides in human genome
 97% of this is intron (non-coding), dropping the number
of coding base pairs to 210,000
 Two parents = 2^210,000
 1 chance in 10^63,000
 However,
born.
only ~108 billion humans have EVER been
Probability and Genetics
Probability: The
determination of certain
outcomes based upon the
number and type of
possible outcomes
 Genetics: The
determination of heritable
outcomes due to the
passage of DNA

If the central dogma dictates that DNA is
copied in its entirety, and mitosis ensures
that each daughter cell gets the same
information as the previous, why do
organisms need to be different?
Genetic Variety
 Genetic
variety ensures
that
populations/species
continue to survive
despite changes in their
ecosystems
 Populations that do not
change genetically
must have environments
that rarely change
Sources of Genetic Variety
 Mutations
Point
Mutations (Single Nucleotide
Polymorphisms)
Frameshift/Nonsense Mutations
Chromosomal Mutations
7 Billion Individuals v. 3 Million SNP’s
HOW IS SUCH VARIETY POSSIBLE IF
99.9% OF OUR GENES ARE
IDENTICAL?
Sources of Genetic Variety v 2.0
Sexual
Reproduction
Builds In
Genetic
Variety
Sexual v. Asexual Reproduction
Asexual Reproduction
Def: Reproduction requiring
only one parent/genetic
source
 i.e. binary fission,
parthenogenesis, budding
 PRO: Quick, Fast, Low
Requirements
 CON: Low Genetic Variety

Sexual Reproduction
Def: Reproduction requiring
two parents/genetic
sources
 i.e. conjugation, fertilization
 PRO: Genetic Variety
 CON: Need water, partners
and GAMETES

GAMETES

Def: Specialized
reproductive cells
(i.e. sperm, ovum)
Gametes contain half the
genetic content of other
cells in the organism (yet still
contain all of the genes)
 To produce another
generation, gametes must
be fused together to
produce a ZYGOTE

Diploid v. Haploid
 Somatic
(body) cells are
DIPLOID (two sets of
chromosomes)
 In each diploid cell, there
are pairs of homologous
chromosomes containing
the same number and
type of genes but NOT
necessarily coding for
identical proteins
 GAMETES
are
HAPLOID (only one
set of
chromosomes)
How does a cell go from diploid to
haploid and yet retain all of the
genes necessary for the organism?
Meiosis
To
develop
gametes, a cell
must undergo
MEIOTIC DIVISION
(i.e. MEIOSIS)
How is meiotic division different
from mitotic division?
Mitosis v. Meiosis

Mitosis occurs in somatic
(body) cells

One diploid cell produces
two diploid cells

Daughter cell is
genetically identical to
parent cell

Meiotic division only
occurs in ovary or testes
cells

Meiosis contains two sets
of steps/divisions

One diploid cell produces
four haploid cells

All resultant cells have
half the genetic material
as the original cell
Meiotic Cell Division
What Is The Significance of The
Differences In Prophase?
Tetrads/Bivalents

NON-SISTER HOMOLOGOUS
CHROMOSOMES move next
to each other, forming
groups of four called
TETRADS or BIVALENTS
SYNAPSIS
 The
non-sister homologous
chromosomes, pressed
together as tetrads, may
“swap” genes between their
NON-SISTER HOMOLOGOUS
CHROMOSOMES
 Creates “hybrid”
chromosomes carrying the
same traits but different
“versions”
What Is The Significance of The
Differences In Metaphase?
Meiotic Metaphase 1
Unlike meiosis, the centromeres
of the sister chromatids are not
lined up directly on the
equatorial plane
 Homologous pairs are arranged
side by side on either side of
the equator, with one spindle
fiber attaching to each
centromere.
 Because they are not on the
equator, there are two ways to
place the homologous pairs

Meiotic Anaphase (1)
Homologous
pairs
separate, but
the sister
chromatids
DO NOT.
Meiotic Telophase & Interkinesis

Same actions as mitotic
telophase

However, after cell has
divided into two diploid
cells, DNA REPLICATION
STOPS

This period of time is called
INTERKINESIS
Why is the lack of DNA
replication during interkinesis
essential in the formation of
gametes?
Meiosis II = Mitosis
Prophase II
Metaphase II
As
in mitotic
division, the sister
chromatids line up
above each other
with the
centromeres
directly on top of
the equator
Anaphase II

Now that spindle fibers
have been attached to
each side of the
centromere, the retraction
of the spindle fibers caused
the centromere to break
MEIOSIS
Telophase II & Cytokinesis
 Four
cells are created
by the second
anaphase division
 Resultant cells have
less cytoplasm but
also half the genetic
material (HAPLOID))
Oogenesis v. Spermatogenesis
The meiotic division of testes
cells produces four viable
spermatids
 The meiotic division of an
ovary cell/ovum is unequal
in terms of cytokinesis
 Only one of four egg cells is
viable

Why does the double division of meiosis
actually increase genetic diversity?
Lab: Modeling Meiotic Division
 With
your lab partner(s), build two sets of homologous
chromosomes,
The first pair of homologues should contain 10 genes (i.e.
beads), 5 on each side of the centromere.
The second pair of homologues should contain 6 genes,
3 on each side of the centromere
Use two different colors to differentiate between the
homologues.
 Replicate both homologues, connecting each to its sister
chromosome with a magnetic centromere. You should
have 8 total chromosomes at this point
Lab: Modeling Meiotic Division Part 2
Move the homologous chromosomes & their
attached sister chromatids through the various
steps of meiotic division
 Include synapsis of two genes in the first
homologous pair and one gene in the second
homologous pair as part of Prophase 1
 Your lab group will be checked for
understanding. One step of the processes
(mitosis AND meiosis) will be selected
 Complete the analysis questions #1-10

Modeling Meiosis Analysis Questions

1) How does meiosis accurately resemble the old adage, “The
more things change, the more they stay the same”? Be
specific in what is changing and what is continuous

2) Why is a consistency of chromosome number an important
prerequisite of successful sexual reproduction?

3) Spores are cells produced by meiosis that can
independently grow into a new plant. If spores are produced
by meiosis, why is the new generation not the product of
sexual reproduction?
Modeling Meiosis Analysis Questions

4) Why is sexual reproduction the most efficient way of
cont.
bringing genetic variety to a population?

5) The ovaries and testes of pre-pubescent humans must grow
and develop, yet do not produce sperm or eggs. How is
cellular division different during these years than post-puberty?

6) Monera and Protista are both single celled organisms, yet
while there are 20,000 different Monera, there are 100,000
different Protista. Why?
Modeling Meiosis Analysis Questions
cont.
 7) How does the fact that horses and
donkeys have different numbers of
chromosomes (48 and 50) explain why
mules are sterile?
 8) Why is there no synapsis during mitotic
division?
 9) Why is there no synapsis between sister
chromatids?
 10) Patau Syndrome is a fatal genetic
condition caused by three copies of
chromosome 15. Explain how this condition
occurs in every cell of the fetus, while
neither parent has the condition.