Sexual Reproduction and
Meiosis
What would happen without
meiosis?
1. Construct a data table with the headings
Cycle number
Stage
1
1
1
2
2
2
3
3
3
Mitosis
Interphase
Fertilization
Mitosis
Interphase
Fertilization
Mitosis
Interphase
Fertilization
Chromosome
number
What would happen without
meiosis?
1. Model a cell with a pair of chromosomes
2. Demonstrate the cell cycle and mitosis
3. Fuse one of your cells with another
student’s cell (representing fertilization)
4. Repeat the above steps two more times
and record your chromosome numbers
for each stage
What would happen without
meiosis?
• Summarize – How does the chromosome
number in your model change with each
cycle of mitosis and fusion?
• Infer – What must occur when cells fuse in
order for chromosome number to remain
constant?
Organisms have
tens of thousands of
genes
- Genes determine
individual traits
- Genes are lined up
on chromosomes
- One chromosome
can contain a
thousand or more
genes
• Homologous Chromosome
Paired Chromosomes
One set of chromosomes
from your dad, one
set of chromosomes
from your mom.
-The Chromosomes in
body cells of most
plants and animals
occur in pairs
-One chromosome in
each pair came from
the male parent, one
came from the female
parent
-Humans have 46
chromosomes 23
paired chromosomes
Diploid – A cell with
two of each kind of
chromosome
Haploid – A cell with
one of each kind of
chromosome (gametes)
(1 from each parent)
We say the cell is a
diploid cell or 2n
(This supports Mendel’s
conclusion that organisms
have two factors – alleles –
for each trait)
We say the cell is a
haploid or n
(This supports Mendel’s
conclusion that parents give
one allele for each trait to their
offspring)
• Each species of organisms contains a
different number of chromosomes
• The chromosome numbers of a species is
not related to the complexity of the
organism
Homologous Chromosomes – Paired
chromosomes
Each pair of homologous chromosomes has genes
for the same traits
Ex. Pod shape
On homologous chromosomes, these genes are
arranged in the same order
However, because there are different possible
alleles for the same gene, the two chromosomes
in the homologous pairs are not always identical
to each other.
Complete Problem Solving Lab 10.2
(pg.270)
Why Meiosis?
• When cells divide by mitosis, the new cells
have exactly the same number and kind of
chromosomes as the original cell.
What would happen if mitosis were the
only form of cell division?
Meiosis – cell division which produces
gametes containing half the number of
chromosomes as the parent’s body cells
• Meiosis occurs only in the specialized
body cells that produce gametes
– (Male and Female reproductive organs)
• The process of meiosis produces:
– male gametes (sperm) - n, haploid
– female gametes (eggs) – n, haploid
• When the sperm fertilizes the egg, the resulting
cell is a Zygote – 2n diploid
• The zygote then can develop by mitosis into a
multicellular organism
• This process is called Sexual Reproduction
– Two parents produce an offspring that is not identical
to either parent
• What are the benefits to sexual reproduction?
The Phases of Meiosis
Meiosis I
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I
Interphase – just like interphase of Mitosis the cell replicates
its chromosomes
(2 identical sister chromatids held together by a centromere)
Prophase I – The chromosomes coil up and the spindles form.
Each pair of homologous chromosomes come together,
matched gene by gene (forms a four part structure called a
tetrad). This is where crossing over can occur.
Metaphase I – The tetrads are pulled to the middle of the cell
(homologous chromosomes are lined up side by side)
Anaphase I – Homologous chromosomes separate and move
to opposite ends of the cell
Telophase I – The spindle is broken down, the chromosomes
uncoil, and the cytoplasm divides (2 new cells), each with
only half the genetic information of the original cell. (another
cell division is needed because each chromosome is still
doubled – two sister chromatids)
Meiosis II
Prophase II – Spindle fibers form and attach to the
chromosomes
Metaphase II – The chromosomes line up on the
equator
Anaphase II – Sister chromatids are pulled apart
Telophase II – Nuclei re-form, the spindles break
down, the cytoplasm divides, 4 new cells are
formed
Meiosis Videos
• Animation: How Meiosis Works
• Meiosis Square Dance
Meiosis
• Start with 1 diploid cell (2n), end with 4
haploid cells (n)
• Each haploid contains one chromosome
from each homologous pair
• The haploid cells become gametes which
will transmit their genes to offspring
Why are you not identical to
your siblings?
• You each have half of your DNA from your
mom and half from your dad…. Why are
you different from each other?
Genetic Recombination
There are two ways that this occurs
• Crossing over can occur anywhere,
at random, on a chromosome
– Typically, 2-3 crossovers per
chromosome occur during meiosis
• Re-assortment of chromosomes
also occurs during meiosis by
independent segregation of
homologous chromosomes
Can mistakes happen in
meiosis?
Nondisjunction
• The failure of chromosomes to separate
during meiosis
• Animation #1
• Animation #2
Results of Nondisjunction
• Monosomy and Trisomy
– Monosomy refers to a condition in which there is one
chromosome is missing.
– A trisomy has one extra chromosome.
– They are generally lethal, with a few
exceptions
Common Abnormalities from
Nondisjunction
•
•
•
•
•
•
AbnormalityKaryotype
Down SyndromeTrisomy 21
Turner Syndrome X
Triple-X SyndromeXXX
Klinefelter Syndrome XXY
Jacob SyndromeXYY