Journal
•Why are you not
identical to your
siblings?
Meiosis
Chromosome Number
– Chromosomes—those strands of DNA
and protein inside the cell nucleus—are
the carriers of genes.
– The genes are located in specific
positions on chromosomes.
Diploid Cells
•
A body cell in an adult fruit fly has
eight chromosomes, as shown in the
figure.
•
Four of the chromosomes come from
its male parent, and four come from its
female parent.
•
These two sets of chromosomes are
homologous, meaning that each of the
four chromosomes from the male parent
has a corresponding chromosome from
the female parent.
• A cell that contains both sets of
homologous chromosomes is diploid,
meaning “two sets.” The diploid number
of chromosomes is sometimes
represented by the symbol 2N.
• For the fruit fly, the diploid number is 8,
which can be written as 2N = 8, where N
represents twice the number of
chromosomes in a sperm or egg cell.
• For a human the diploid number is 46 or
2n=46.
Haploid Cells
– Some cells contain only a single set of chromosomes,
and therefore a single set of genes. Such cells are
haploid, meaning “one set.”
– The gametes of sexually reproducing organisms are
haploid.
– For fruit fly gametes, the haploid number is 4, which
can be written as N = 4. The haploid number in
humans is 23 or n=23.
How many sets of genes do
multicellular organisms inherit?
• The diploid cells of most adult organisms contain two
complete sets of inherited chromosomes and two
complete sets of genes.
• The number of chromosomes is reduced from 46 (23
pairs) to 23 during the process of meiosis.
• In a human egg or sperm, there are 23 chromosomes. A
human egg is haploid (has 23 chromosomes) and a
sperm is haploid (has 23 chromosomes).
• Upon fertilization, the new baby now has the correct
human number of 46 chromosomes in each of its
somatic cells. Fertilization of the egg by the sperm
restores the diploid number of 46 chromosomes.
• Understanding the
process of meiosis is
fundamental to
understanding human
health and development
Mitosis
• A normal cell with, with 2 copies
of each chromosome
• Daughter cells are exact copies
of the mother cell with 2 copies
of each chromosome
• This is the usual process of
division by which cells in out
bodies renew themselves
What is Meiosis?
• Meiosis is the production of sperm and egg
cells. These cells are "Gametes" or "Sex"
cells. Each cell has to go through the division
process twice in order for the cell to end up
with half the number of chromosomes.
• Meiosis usually involves two distinct
divisions, called meiosis I and meiosis II. By
the end of meiosis II, the diploid cell
becomes four haploid cells.
Meiosis
•The purpose of
meiosis is to reduce
the normal diploid
cells to haploid cells
called gametes
• In humans, special
haploid cells resulting
from meiosis are eggs
(female) or sperm (male)
• In yeast, the haploid cells
are called spores
• To make a diploid again—
haploid gametes produced
by each parent will fuse to
form a zygote (during
fertilization)
• Offspring receive one copy of
each chromosome from each
parent
• Depending on the
recombination events that
occurred to produce a
gamete, genetic information
may be rearranged
• This explains why siblings get
different combinations of genes
from their parents, which is why
they look related but not identical
Meiosis vs. Mitosis
1. In Meiosis, two cell divisions take place (4
daughter cells produced)
2. During meiosis, chromatids may exchange
similar parts, so they are NOT identical when
distributed
3. In meiosis, chromosomes duplicate only ONCE,
even though they divide twice
4. During meiosis, chromosomes are distributed into
daughter cells randomly, not equally as it is in
mitosis
Types of Reproduction
• Sexual reproduction involves the fusion of
two gametes (parent cells) to make a
genetically different offspring. Two
gametes fusing creates a ZYGOTE (new
individual, embryo) through the process of
fertilization.
• Asexual Reproduction occurs when one
parent donates all chromosomes to an
offspring and there is no union of gametes
Meiosis explains
Mendel's Results
• Meiosis explains the physical basis of Mendel's
results. In Anaphase I, when the segregation of
chromosomes occurs, Mendel's observation that
each parent gives one allele for each trait at
random to each offspring, regardless of whether
the allele is expressed, is explained. It explains
Mendel's observation that factors, or genes, for
different traits are inherited independently for each
other.
Steps of
Meiosis
•To carry out meiosis
cells duplicate their
chromosomes but
follow this by 2 rounds
of cell division, instead
of one like mitosis
st
1
Division of
Meiosis
• Separates duplicated
chromosomes from each
other
• Reduces the number of
chromosomes in each cell
nd
2
Step of
Meiosis
• Allows for chromosome copies to
separate
• Offspring from meiosis
have ½ the number of
chromosomes as their
parent cell, because they
receive just one copy of
each chromosome rather
than 2
Phases of
Meiosis
Remember
• Meiosis is the cell division
that produces gametes
containing half the number
of chromosomes as a
parent’s body cell.
Meiosis contains 2
separate divisions
•Meiosis I
•Meiosis II
Meiosis I
• Interphase I
–The cell first carries
out its usual
metabolic activities
& replicates its
chromosomes
Prophase I
• Each chromosome consists of
2 identical sister chromatids,
held together by a centromere
• Chromosomes coil up & a
spindle forms
• Each pair of
chromosomes comes
together to from a 4-part
structure called a tetrad
Tetrad
• Made up of 2 homologous
chromosomes, each of
which is made up of 2
sister chromatids
Crossing Over
• Chromatids of homologous chromosomes
cross over each other
• Crossed sections—containing alleles—are
exchanged
• Produces new combinations of alleles in the
cell
Metaphase I
• Tetrads line up on the
middle of the spindle
Anaphase I
• Homologous chromosomes
separate and move to
opposite ends of the cell
because the centrometers do
not split
Telophase I
• Spindle breaks down
• Chromosomes uncoil
• Cytoplasm divides so there are
2 new cells
• The chromatids are
Still attached.
Meiosis I Summary
• Meiosis I results in two cells, called daughter
cells, each of which has four chromatids, as
it would after mitosis .Because each pair of
homologous chromosomes was separated,
neither daughter cell has the two complete
sets of chromosomes that it would have in a
diploid cell. The two cells produced by
meiosis I have sets of chromosomes and
alleles that are different from each other and
from the diploid cell that entered meiosis I.
Meiosis II
• Newly formed cells undergo a
short interphase in which the
chromosomes do not
replicate
• Sometimes the cells go from
Meiosis I right into Meiosis II
Prophase II
• Each cell contains one member
of each homologous pair.
• A spindle forms in each of the
two new cells
Metaphase II
• Chromosomes, still made
up of sister chromatids,
line up at the middle of
the cell.
Anaphase II
• The centromere of each
chromosome splits, allowing sister
chromatids to separate and move
to opposite poles of the cell
Telophase II
• Nuclei reform
• Spindles break down
• Cytoplasm divides
•Meiosis results in 4
haploid cells
•Each cell containing
half of the
chromosomes of the
parent cell
Gametes to Zygotes
• The haploid cells produced by meiosis II are
gametes.
– In male animals, these gametes are called
sperm. In some plants, pollen grains contain
haploid sperm cells.
– In female animals, generally only one of the cells
produced by meiosis is involved in reproduction.
The female gamete is called an egg in animals
and an egg cell in some plants.
• Fertilization—the fusion of male and
female gametes—generates new
combinations of alleles in a zygote.
The zygote undergoes cell division by
mitosis and eventually forms a new
organism.
• Spermatogenesis
– Meiosis in a male organism, producing 4
sperm cells
• Oogenesis
– Meiosis in a female organism, producing 1
egg and 3 inactive polar bodies
Identical Twins
• A single egg is fertilized to
form one zygote
• The zygote divides to form
two separate embryos
• Meiosis Stages
• Unique Features of Meiosis