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Chapter 13
Meiosis and Sexual Life Cycles
Inheritance/Heredity
When traits are passed down from one
generation to the next, we say they are
inherited.
 The offspring differ from one generation
to the next and we use the term
variation to describe these differences.

Genetics
When people study genetics, they are
studying the heredity and inherited
variations.
 For example, when we say we have our
parent’s nose or eyes, we are actually
saying that we have their segments of
DNA (genes)

Genes

Genes are
regions of our
genome that
program the
specific traits we
see (phenotype)
such as eye color
or nasal
appearance.
So, how does this happen?

The exact replication of our genes/DNA
in our gametes which produces copies
that can be passed from one generation
to the next.
So, how does this happen?

Remember, our
gametes are the
sperm and egg that
unite during
fertilization and give
rise to the new
individuals.
So, how does this happen?
The new individuals now have traits of
both parents.
 Remember, the chromosome that is
inherited is tightly coiled and contains a
few hundred to a few thousand genes.

The Locus

The locus is the region of the
chromosome where a particular gene is
located.
Modes of Reproduction
When discussing heredity, there are a
couple of modes of reproduction that
need to be discussed.
 Sexual and asexual.

Sexual Reproduction
2 parents “mate” and give rise to
offspring that have a variety of
combinations of genes which are
inherited by the new offspring.
 They are not genetically identical to the
parents.

Asexual Reproduction
A single parent gives rise to the
offspring.
 In the process, all of the parent’s genes
are passed on to the offspring and a
clone is formed.

Asexual Reproduction

A hydra budding is
an example of
asexual
reproduction.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.

Movie
The Sexual Lifecycle--Humans
Somatic cells are the cells other than
the sperm and egg.
 Each one contains 46 c-somes (23
pairs, 1 pair from each parent).
 Each parent contributes 22 autosomes
and 1 sex c-some.

Homologous Chromosomes
Homologous c-somes are c-somes of
the same number.
 Each homologous c-some contains
genes that control the same inherited
characteristic.
 For example, if the gene that codes for
height is found on one c-some, it is
found at the exact same locus on the
homologous c-some.

Homologous Chromosomes--Sort
Of…
The only two c-somes that don’t really fit
the idea of being homologous are the
sex c-somes, the X and the Y.
 The two sets of genes on these csomes are more different than they are
alike.
 They determine the sex of the
individual: XX-female; XY-male

Chromosome NumberAutosomes
When discussing c-some number, we
use an “n” to represent this idea.
 Any cell having both copies of c-somes
from the parent are said to be diploid
which is 2n.
 In humans, 2n=46.

Chromosome Number-Sex
Chromosomes
The gametes of sexually reproducing
organisms contain a single c-some set
and are said to be haploid.
 n=23 (22 autosomes + 1 sex c-some).

Fertilization
When 2 haploid cells fuse together, a
diploid zygote is formed.
 The zygote now contains a mix of genes
from the parents.
 It will divide mitotically and give rise to a
mature organism.

Sexual Maturity

As the organism reaches sexual
maturity, its gonads will begin producing
haploid cells through a process called
meiosis.
Meiosis

Meiosis will allow the chromosome
number to remain constant throughout
the generations.
Meiosis and Fertilization

The process of meiosis and fertilization
are common to sexually reproducing
organisms.
3 Main Types of Life Cycles in
Sexually Reproducing Organisms
1. Animals
 2. Plants and some algae
 3. Fungi and some protists

Animals
In animals, meiosis occurs in the
production of gametes and the gametes
are the only haploid cells.
 The gametes unite during fertilization.
 Mitosis gives rise to a diploid,
multicellular organism.

Plants and Some Algae

Alternation of Generations
– A complex reproductive event where a
plant gives rise to spores which gives rise
to gametophytes.

The gametophytes give rise to gametes
that fertilize forming a zygote that
becomes a new plant.
Most Fungi and Some Protists
The adult is a haploid, multicellular
organism.
 Mitosis produces gametes that fertilize
forming a zygote.
 Meiosis then produces haploid cells
which divide mitotically to form another
haploid, multicellular organism.

3 Main Types of Life Cycles in
Sexually Reproducing Organisms
All 3 result in meiosis and mitosis.
 All 3 accomplish the same thing:
chromosome halving and doubling
which results in genetic variation among
offspring.

Meiosis
Meiosis reduces the ploidy.
 Provides for genetic variation.
 Keeps the c-some number constant
through the generations.

Meiosis
Meiosis can be broken into two parts:
Meiosis I and Meiosis II.
 Meiosis I separates homologous csomes.
 Meiosis II is essentially a mitotic
division.

Meiosis

Meiosis halves the
number of
chromosomes.
Interphase I
Generally this is
where the
chromosomes
and the
centrosomes
replicate.
 Movie

Meiosis I

Prophase I:
Homologous csomes condense;
crossing-over takes
place (a source of
variation; nuclear
envelope breaks
down
 Movie
Meiosis I

Metaphase I:
Homologous pairs of
c-somes line up on
the metaphase
plate.
 Movie
Meiosis I

Anaphase I: The csomes (each now
having 2
chromatids) of each
homologous pair
separate and move
to the opposite
poles of the “cell.”
 Movie
Meiosis I

Telophase I: Csomes arrive at the
spindle poles.
 Cytokinesis:
Cytoplasm divides
producing 2 cells,
each having 1/2 the
original number of csomes.
 Movie
Interkinesis
In some cells, the c-somes relax and
the nuclear envelope reforms.
 Other cells go directly to meiosis II.

Meiosis II Sister Chromatids
Separate

Prophase II:
Chromosome
condense again, the
nuclear envelope
disappears and the
mitotic spindle
reforms.
 Movie
Meiosis II Sister Chromatids
Separate

Metaphase II: Csomes line along the
metaphase plate.
 Movie
Meiosis II Sister Chromatids
Separate

Anaphase II: Sister
chromatids separate
and migrate as
individual c-somes
to the spindle poles.
 Movie
Meiosis II Sister Chromatids
Separate

Telophase II: Csomes arrive at the
poles, the spindle
breaks down, the
nuclear envelope
reforms.
 Cytokinesis: The
cytoplasm divides.
 Movie
Meiosis Overview
When viewing
this movie, try
to think about
what is
happening
during each
phase.
 Movie

Mitosis and Meiosis, A
Comparison
The biggest difference is that in mitosis,
genetically identical cells are formed
and ploidy is conserved.
 In meiosis, genetically different cells
are formed and the ploidy is reduced.

3 Main Events Important to
Meiosis
1. Synapsis and Crossing Over
 2. Tetrad Formation
 3. Separation of Homologues

1. Synapsis and Crossing Over
In prophase I, duplicated homologues
line up and become physically attached.
 Crossing over occurs where genetic
information is rearranged between the
non-sister chromatids comprising the
tetrad.
 An “X” shaped region appears called
the chiasma at the site of crossing over.

2. Tetrad Formation

The paired tetrads align along the
metaphase (I) plate rather than the
individual replicated c-somes (as in
mitosis).
3. Separation of Homologues

At anaphase I of meiosis, the duplicated
c-somes of each homologous pair move
to opposite poles, sister chromatids
remain attached (instead of separating
as they do in mitosis).
3 Mechanisms that Produce
Genetic Variation
1. Independent Assortment
 2. Crossing Over
 3. Random Fertilization

1. Independent Assortment
Homologous c-somes can be positioned
independently at the metaphase plate.
 Maternal and paternal c-somes sort
independently of every other pair.

1. Independent Assortment
Independent assortment give a large
number of possible daughter cells.
 The cells are dividing in two and are
sorting independently.
 The number of combinations is 2n.
 “n” is the haploid number (23 in humans)
 223 possible combinations (8,000,000+)

2. Crossing Over
Gene mixing. Non-sister chromatids
trade places.
 Taking crossing over into account
makes the number of possible
combinations endless.

3. Random Fertilization

The different combinations of gametes
produced during meiosis will randomly
fertilize upon mating. This produces the
endless amount of gamete
combinations.
Creation of Genetic Variation

Movie
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.