Meiosis and Sexual Reproduction
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Students know and understand the
characteristics and structure of living things,
the processes of life, and how living things
interact with each other and their
environment.
Benchmark 3.10: Cell reproduction/division
has various processes and purposes (mitosis,
meiosis, binary fission)
Assessment objective 3.10a: Compare and
contrast the purposes and processes of
mitosis, meiosis, and binary fission.
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Gamete
Zygote
Diploid
Haploid
Homologous chromosomes
Meiosis
Crossing-over
Independent assortment
Life cycle
Sperm
Ovum
Video time: 67:48min.
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What are chromosomes?
Chromosomes are thread-like structures
located inside the nucleus of animal and plant
cells. Each chromosome is made of protein
and a single molecule of deoxyribonucleic
acid (DNA). Passed from parents to offspring,
DNA contains the specific instructions that
make each type of living creature unique.
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Reproduction is the
process of
producing
offspring.
Organisms are
reproduced by two
or sometimes just
one parent.
This is known as
sexual and asexual
reproduction.
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In asexual
reproduction, there
is only one parent.
The offspring is
genetically identical
to its parent.
Types include:
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Binary fission
Fragmentation
Budding
Parthenogenesis
Binary fission (1:00)
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Budding in hydra (1:26)
Hydra is a genus of
small, fresh-water
animals of the phylum
Cnidaria and class
Hydrozoa, native to the
temperate and tropical
regions. Biologists are
especially interested in
Hydra because of their
regenerative ability -they appear not to age
or die of old age.
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Fragmentation or
clonal fragmentation in
multicellular or
colonial organisms is a
form of asexual
reproduction or
cloning in which an
organism is split into
fragments. Each of
these fragments
develop into mature,
fully grown individuals
that are clones of the
original organism.
Budding and fragmentation (2:00)
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Parthenogenesis is a
type of asexual
reproduction in which
the offspring develops
from unfertilized eggs.
It is particularly
common amongst
arthropods and
rotifers, can also be
found in some species
of fish, amphibians,
birds, and reptiles, but
not in mammals.
Komodo dragons (4:40)
Sexual reproduction
takes place with two
parents where the
offspring is genetically
different from the
parents. This generally
takes place in eukaryotic
cells.
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Each parent produces
a gamete
(reproductive cell)
that together form a
zygote during
fertilization.
Because the offspring
have both parent’s
genetic material, it is
not exactly like either
parent but a mixture
of the two genetic
packages.
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Gametes have 23
chromosomes.
Oogenesis is the
creation of an ovum
(egg cell). It is the
female process of
gametogenesis.
Typically, human
females only produce
one functional egg in
the ovaries.
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If one egg is
fertilized and splits
in two then that
makes identical
twins.
If more than one
egg is fertilized
then that will
produce fraternal
twins with different
genetic traits.
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Spermatogenesis is
the process by which
male spermatogonia
develop into mature
spermatozoa.
Spermatozoa are the
mature male gametes
in many sexually
reproducing
organisms.
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Germ cells are
specialized for sexual
reproduction and only
they can produce
gametes.
Somatic cells are body
cells that do not
participate in sexual
reproduction.
What would be the
advantage of sexual
reproduction as
compared to asexual
reproduction?
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Each chromosome
has 1000’s of genes
that determine an
organism’s
development and
function.
In humans, each cell
has two copies of 23
chromosomes for a
total of 46.
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When fertilization
happens, 2 cells
combine to form a
zygote, that has 46
chromosomes also.
How can they be the
same?
The gametes that
form a zygote have
only one copy of
each chromosome or
one set of 23. This
keeps the
chromosome number
at a constant 46.
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Diploid Cells (1:04) have two
sets of chromosomes.
Haploid Cells (1:03) have
one set of chromosomes.
Gametes are haploid cells.
The symbol n represents
the number of
chromosomes in one set.
Thus, humans are n = 23.
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Each diploid cell has
pairs of chromosomes
made up of two
homologous
chromosomes.
Homologous
chromosomes are
similar in size, shape
and genetic makeup.
In humans, 23
chromosomes come
from Mom and 23 come
from Pops.
These chromosomes can
carry different forms of
genes, depending on
how they go together.
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Sex chromosomes
determine the sex of
the offspring.
Males have one XY
chromosome and
females have an XX
chromosome.
Autosomes are
chromosomes that do
NOT determine the sex
of an individual.
A meiosis I, B meiosis II, C interphase, D prophase I, E metaphase I,
F anaphase I, G telophase I, H prophase II, I metaphase II,
J anaphase II, K telophase II, L four haploid cells
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Prophase I:
◦ Chromosomes condense
◦ Nuclear envelope breaks
down
◦ Homologous
chromosomes pair.
◦ Chromatids exchange
genetic information in a
process called crossing
over.
Prophase 1 (1:08)
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Pairs of
homologous
chromosomes move
to the equator of
the cell.
Metaphase 1 (0:17)
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Pairs of homologous
chromosomes
separate and the
spindle fibers pull
the chromosomes of
each pair to opposite
poles of the cell. The
chromatids remain
joined at their
centromeres. The
genetic material
recombines.
Anaphase 1 (0:24)
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Chromosomes
gather at the poles.
The cytoplasm
divides.
Telophase 1 (0:12)
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The two cells from
Meiosis I now enter
the prophase II
phase of Meiosis II.
New spindles form
around the
chromosomes.
Prophase II (0:29)
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Chromosomes line
up at the equator.
Metaphase II (0:13)
Centromeres divide,
and chromatids
move to opposite
poles.
Anaphase II (0:13)
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A nuclear envelope
forms around each
set of
chromosomes. The
cells divide.
Telophase II (0:13)
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Mitosis makes new cells
that are used during
growth, development,
repair, and asexual
reproduction.
Meiosis makes cells that
enable an organism to
reproduce sexually and
are found only in
reproductive structures.
The main difference is
that in mitosis, the
genetic material is not
rearranged.
Comparing Mitosis and Meiosis (8:02)
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Three key components of
genetic variation are:
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Crossing over
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Independent assortment
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Random fertilization
Chromosomal Basis of Life (1:57)
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During Prophase I,
homologous
chromosomes line up
next to each other.
One arm of a
chromatid crosses
over the other arm of
a chromatid,
chromosomes break
at the crossover and
reform with the new
piece from the other
chromosome.
Increasing Genetic Variation:
Crossing Over (2:19)
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During metaphase,
chromosomes line up at
the center of the cell.
As this is random, any
two pairs of
chromosomes can line up
in either of two equally
probable ways.
Since there are 23 pairs
of chromosomes in
humans that separate
independently, there are
223 or 8 million possible
gene combinations in
gametes from a single
cell.
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Fertilization is a random
process that adds to
genetic variation.
A zygote is formed by the
random joining of two
gametes.
Because fertilization of an
egg by a sperm is random,
the number of possible
outcomes is squared.
In humans, the possibility
is 223 x 223 or 70 trillion
combinations—that’s a lot
of variation!!
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In diploid life cycles,
meiosis in germ cells
of a multicellular
diploid organism
results in the
formation of haploid
gametes.
Males produce
gametes called sperm.
Females produce
gametes called eggs,
or ova. Plural = ovum
Three Basic Patterns of
Sexual Life Cycles (2:19)
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This life cycle
happens in most
fungi and some
protists.
In haploid life cycles,
meiosis in a diploid
zygote results in the
formation of the first
cell of a multicellular
haploid individual.
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Plants and most
multicellular protists
have a life cycle that
alternates between a
haploid phase and a
diploid phase called
alternation of
generations.
Sporophytes are the
multicellular diploid
phase in the life cycle of
plants.
A spore forms a
multicellular
gametophyte.
Moss Reproduction (1:23)