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Meiosis - is derived from the Greek word meion, which means to reduce.
6.1 - Meiosis
• Meiosis is an important aspect of
sexual reproduction
(2 parents; offspring not genetically identical)
• The process of meiosis also shuffles genetic
information (DNA) and results in variation in
genes. Therefore producing a variation in
offspring
Diploid, Haploid & Chromosomes
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Normal body cells have a diploid (2n) chromosome number,
meaning chromosomes occur in pairs (46 chromosomes).
In humans, the male and female each contribute half (haploid
(n)), 23 chromosomes.
Gametes & Fertilization

Gametes – specialized cells necessary for reproduction

Females – egg, Males – sperm
When fertilization takes place:
23 (egg,) + 23 (sperm) = 46 (zygote)

The zygote goes on to develop into an embryo,
and on into a complete individual.
Meiosis

occurs in reproductive organs and only produce gametes with half the
chromosome compared to body cells
23 chromosome
23 Chromosomes
46 chromosomes
23 Chromosomes
23 pairs,
46 chromosomes
23 chromosomes
Meiosis I - Homologous Pair Separate
Prophase I - Homologous chromosomes pair up, and non-sister
chromatids exchange genetic material. This process is called
crossing over.
Metaphase I - Homologous chromosomes pair up at the equator.
(one chromosome from each parent)
Anaphase I - Homologous chromosomes separate and are pulled
to opposite poles by the spindle fibres. (sister chromatids move to
opposite poles )
Telophase I - One chromosome from each homologous pair is at
each pole of the cell.
Interkinesis - results in 2 daughter cells with ½ # of chromosomes.
This stage between cell divisions the cell will grow and make
proteins as in interphase of mitosis. Unlike interphase in mitosis,
there is no replication of DNA during this stage.
Crossing over
Is when sister chromatids
exchange DNA during the
Prophase
Crossing over creates a
number of genetic variations
in gametes
Independent Assortment
A shuffling of the 23 homologous
pairs, in which there are two
possibilities for how chromosomes will
sort themselves into daughter cells.
 “independent assortment” leads to
variations in species as there are more
than 8 million combinations possible
for the 23 pairs in any egg or sperm
cell.
 70 trillion different zygotes are
possible from one sperm and one egg
cell.

Meiosis II
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- Separates sister chromatids
Meiosis II is like mitosis because in both processes the chromatids of each chromosome are pulled
to opposite poles.
Each daughter cell inherits one chromatid from each chromosome resulting in four haploid cells
each with half the N of chromosomes.
Prophase II - There is one chromosome of the homologous pair in each cell.
Metaphase II - The X-shaped chromosomes form a single line across the middle of the cell.
Anaphase II - Sister chromatids move to opposite poles of the cell. Once they separate, each sister
chromatid is considered to be a chromosome.
Telophase II - Spindle fibres begin to disappear, and a nuclear membrane forms around each set
of chromosomes.
Cytokinesis - In cytokinesis, the two daughter cells are separated.
Male and Female Gametes

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In males, all 4 cells resulting from meiosis develop into sperm
In females, 1 cell gets most of the cytoplasm and becomes the egg
while other 3 die, due to unequal cytoplasm and organelles
Differences in
Mitosis and Meiosis
Mitosis
 # daughter chromosomes =
# parent chromosomes
 Two identical daughter
cells
 Occurs in all cells except
reproductive cells
 Produces diploid cells
 Diploid cells = 2n
Review of
Mitosis vs.
Meiosis
Comparison animation
Meiosis
 # daughter chromosomes =
half the # parent chromosomes
 Four non-identical daughter
cells
 Occurs only in reproductive
cells
 Produces haploid cells
 Haploid cells = n
Chromosome
Mutation
Chromosome Mutations
Types:
inverted,
deleted,
duplicated or
moved to
another spot
Sometimes occur spontaneously
Chromosome changes during meiosis can cause changes in the genetic information.
Occur because of mutagens
Chromosome changes, sometimes leading to genetic disease or death, can be cause by mutagens
such as radiation or chemicals
Can occur if there is failed separation of chromosomes in meiosis
Failed separation means that a gamete may end up with no chromosome or too many of a
chromosome - Zygotes that result from these gametes rarely survive, and if they do, they will have
serious genetic disorders
Genetic
Disorders
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By using a karyotype,
geneticists
can study an individual’s
chromosomes
Chromosomes are identified
and paired by size,
centromere location and
banding patterns.
Certain genetic disorders or
syndromes occur when there
are specific chromosomes
extra or missing
Down syndrome usually
occurs when there is an extra
21st chromosome
Sexual
Reproduction
Chapter 6.2
(18 Ways to make a baby)
Sexual reproduction brings non-identical gametes together to form a
new organism - it occurs in 3 stages:
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Mating - the process by which gametes are brought together at same place and
same time
Fertilization - process by which egg and sperm join to form a new organism
(zygote)
Development - the process by which an organism develops as an embryo
Methods of Fertilization
External or Internal Fertilization

In order for either of these methods to produce a successfully
developing embryo, certain conditions must be met:
1.
enough nutrients.
2.
temperature must not be too cold or too hot.
3.
Proper moisture so that embryo
does not dry out.
4.
protected
from predators and items
in environment that can
potentially harm it.
External Fertilization

In external fertilization, sperm and egg join outside parents
Advantages
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Very little energy required to mate
Large numbers of offspring produced
Offspring can be spread widely in
the environment - less competition
between each other and parents
Disadvantages
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Many gametes will not survive
Many eggs will not be fertilized
Offspring are often not protected
by parents, so many of them die
Frog Eggs
External Fertilization Example
Internal Fertilization

In internal fertilization, sperm and egg join inside parents, embryo
is nourished inside mother
Advantages
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Embryo protected from predators
Offspring more likely to survive,
as many species will protect their
young while they mature
Disadvantages
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Much more energy required to find mate
Fewer zygotes produced, resulting in
fewer offspring
More energy required to raise and care for offspring
Pollination

Most plants transfer male gametes as pollen. Pollen can
be carried by wind or other organisms.
Human
Reproductive
System
Once ovulated, the egg is only viable
for about 24 hours if not fertilized.
 Fertilization occurs in the top one
third of the Fallopian tubes.
 After fertilization, the zygote
continues to move down the
fallopian tube.
 As it does, development begins.
 It takes the embryo 7 - 12 days
to get to the uterus and implant
in the endometrium.

Embryonic Development

Embryonic development is the early development of an
organism - in humans, it is the first three months after fertilization
Stages

End of the first week - ball of cells
called morula

By end of second week it is a
hollow ball called a blastula
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3-4 weeks embryo is
known as a gastrula
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Cells at this stage are stem cells,
and have the ability to develop
into any kind of cell
develops 3layers:
ectoderm (skin, nerves),
mesoderm (muscles, bones), and
endoderm (lungs, liver, digestive
system lining)
“more blasting gas”
Gastrula
Embryonic Development
Period of the Embryo
 For the first three months,
organogenesis occurs and it
is termed an embryo.
 This is a 6 week embryo.
Can you guess which embryos are:
chicken, fish, human, rabbit,
salamander, and tortoise
Fetal Development

The cell layers now differentiate into the organs and
tissues of a baby - this is divided into 3 trimesters.
First Trimester (0-12 weeks)
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Organ systems begin to develop
and form. Bone cells form.
Second Trimester (12-24 weeks)
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Rapid growth from 12-16 weeks.
Third Trimester (24+ weeks)
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Continued growth, especially of brain. Fat begins
to deposit at 32 weeks to keep baby warm at birth.
This is a 4
month fetus.
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The third trimester, period of the baby, is marked by growth and
maturation of all organs and organ systems.
The baby is capable of life outside the womb during this period.
Review: Life cycle of animals
Sexual Reproduction:
Advantages and Disadvantages
6.3 Assisted Reproductive Technologies
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Infertility is the inability of a couple to have a baby
Assisted reproductive technologies involve removing
eggs from the woman, fertilizing them, and returning
them to the uterus.
Reproductive technologies help childless couples, but
carry a higher risk of birth defects.
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Also creates the problem of “unwanted” embryos.
What should be done with them?
Types of Assisted Reproductive Technologies
1.
2.
3.
4.
Artificial Insemination - donor sperm is placed in the female.
In vitro fertilization (IVF) - egg and sperm are collected and
fertilization takes place in a dish. Embryo(s) then placed in
female’s uterus.
Gamete intrafallopian transfer (GIFT) - eggs and sperm are
collected, mixed, then injected into the woman’s fallopian tubes.
Intracytoplasmic Sperm Injection (ICSI) - a single sperm is
injected directly into an egg.