Meiosis Summary
Objectives:
1) Explain what haploid, diploid, gamete and crossing over
means
2) Explain the phases of meiosis
3) Explain how meiosis insures variability in offspring.
• Gamete – a sex cells that have a haploid number of
chromosomes
• Haploid – the number of chromosomes in a haploid cell
(gamete)
• Haploid Cell – cells containing a half set of chromosomes or
only one member of each of the 23 sets or pairs of
chromosomes; this means that there is a chromosome from
only one parent for each of the 23 pairs
• Diploid – cells containing twice the number of chromosomes
in a gamete; in the human diploid cell this number is 46; the
symbol is 2n
• Diploid Cell – cells containing a full set of chromosomes; 23
sets or 23x2=46 chromosomes; one member of each of the
23 sets comes from each parent
ASEXUAL REPRODUCTION
Asexual reproduction is based on mitotic
division of the nucleus.
• It may be a unicellular organism reproducing
itself, or:
• Cells of multicellular organisms that break off
to form a new individual
• The offspring are clones—genetically
identical to the parent.
Sexual Reproduction
Sexual reproduction: the offspring are not
identical to the parents.
Sexual reproduction results in shuffling of
genetic information in a population. No two
individuals have exactly the same genes.
There is random selection of half a parents
chromosomes that go into the haploid
gamete; two such gametes fuse—offspring
are not exactly like parents.
Purpose:
i. To produce gametes that have half the
number of chromosomes (23) as the parent
cell.
ii. When a sperm cell (n) unites/fertilizes an egg
(n) cell it forms a new cell called a zygote
(2n).
iii.To ensure genetic diversity through a process
of crossing-over or exchange of genetic
information between non-sister chromatids
Some more terms for you to know
Somatic cells—body cells not specialized for
reproduction.
Each somatic cell contains homologous
pairs of chromosomes with corresponding
genes. Each parent contributes one
homolog.
Meiosis produces…
• In females haploid cells called eggs or ova
• In males haploid cells called sperm
• Meiosis involves 2 stages of nuclear
division, Meiosis I and Meiosis II, occurring
successively
• The first stage (Meiosis I) is significantly
different from Mitosis
• The second stage (Meiosis II) is virtually
identical to Mitosis
• Only Meiosis I starts with interphase
(chromosomes duplicate 46 to 92)
Your Turn
• You will attempt to model the phases of
meiosis using modeling clay.
• In groups of two read through you
instructions carefully.
• Following each step carefully.
• When you are done, you should be able to
explain the phases of meiosis.
Quiz
1. How does the process of “crossing over” in
prophase I, ensure that the resulting cells are
genetically different from the parent cell?
2. What is a tetrad?
3. How is the product of meiosis different from
mitosis?
4. Are the products of meiosis diploid or haploid?
5. What is gametogenesis?
Prophase I
• Chromosomes
condense
• Chromosomes come
together (synapsis)
in homologous pairs
to form tetrads
• Crossing over occurs
Metaphase I
• Chromosomes
(homologous pairs)
line up at equatorial
plate
Anaphase I
• Homologous pairs
separate
• Move to opposite
ends of cell
Telophase I
• Nucleus completes
division
• Nuclear membrane
reforms
• Cleavage furrow
• Chromosomes in
each nuclei are
NOT identical
Prophase II
• New spindle fibers
form
• Chromosomes
condense
Metaphase II
• Chromatids line up
at the equatorial
plate
Anaphase II
• Chromatids separate
• Move to opposite
poles
Telophase II
• Cytoplasm separates
Product
• Result is 4 haploid
cells
– half the number of
chromosomes as
parent cell
Meiosis Summary
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
Your turn
1) With a partner, discuss what two stages are
responsible for creating genetic variability
2) From what you have learned today, what role
does meiosis play in transmitting
genes/characteristics from parents to child?
3) How can you tell (genetically) if a person is a
male or female?
One step further…
Compare mitosis and meiosis describing at least 3
differences.
Mitosis
Meiosis
Purpose
To ensure growth & to repair
cells of the organism
To ensure the procreation
of the species
Type of Cell
produced
Somatic (body) cells
Gametes (sex) cells
-Female: egg/ovum
-Male: sperm
Number of
Nuclear Divisions
1 nuclear division
4 phases
2 nuclear divisions
Meiosis I – 4 phases
Meiosis II – 4 phases
Number of Cells
Produced
2 new identical daughter
4 new non-identical
cells (identical to each other & daughter cells
Number of
chromosomes in
each new cell
46 chromosomes
parent)
Name given to this Diploid cell
-complete set of
type of cell
chromosomes
Symbol given to
this type of cell
2n
23 chromosomes
Haploid cell
-a half set of
chromosomes
n