Meiosis
IB Biology
Match up the key words to their definitions and copy them into your notes!
Gene locus (loci)
Diploid
Allele
Homologous pair
Gamete
Haploid
Definitions
• Chromosome pairs, one from each parent, that are similar in
length, gene position and centromere location.
• A cell that contains one complete set of chromosomes.
• A cell that contains two sets of chromosomes. One set of
chromosomes is donated from each parent.
• Location of a gene on a chromosome
• Different forms of the same gene
• Sex cells that unite during sexual reproduction to form a new
cell called a zygote
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
n=23
diploid (2n)
n=23
meiosis I
meiosis II
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two identical
sister chromatids attached at their centromeres.
• Centriole pairs also replicate.
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Meiosis I (four phases)
• Cell division that reduces the chromosome number
by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes come
together to form a tetrad.
• Tetrad is two chromosomes or four chromatids
(sister and nonsister chromatids).
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are similar
in shape and size.
• Homologous pairs (tetrads) carry genes controlling the same
inherited traits.
• Each locus (position of a gene) is in the same position on
homologues.
• Humans have 23 pairs of homologous chromosomes.
a.
b.
22 pairs of autosomes
01 pair of sex chromosomes
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister chromatids
break and reattach to the other chromatid.
• Chiasmata (chiasma) are the sites of crossing over.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Sex Chromosomes
XX chromosome - female
XY chromosome - male
Prophase I
spindle fiber
aster
fibers
centrioles
Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example:
2n = 4
then n = 2
thus
22 = 4 combinations
Metaphase I
OR
metaphase plate
metaphase plate
Question:
• In terms of Independent Assortment -how
many different combinations of sperm could
a human male produce?
Answer
• Formula: 2n
• Human chromosomes:
2n = 46
n = 23
• 223 = ~8 million combinations
Anaphase I
• Homologous chromosomes separate and move
towards the poles.
• Sister chromatids remain attached at their
centromeres.
Anaphase I
Telophase I
• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells
are formed.
Telophase I
Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
metaphase plate
metaphase plate
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember:
four haploid daughter cells
produced.
gametes = sperm or egg
Telophase II
Meiosis
n=2
sex cell
sperm
n=2
n=2
2n=4
haploid (n)
n=2
n=2
diploid (2n)
n=2
meiosis I
meiosis II
Variation
• Important to population as the raw material for
natural selection.
• Question:
What are the three sexual sources of
genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!
Question:
• A cell containing 20 chromosomes (diploid) at the
beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?
Answer:
• 10 chromosomes (haploid)
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Question:
• A cell containing 40 chromatids at the beginning of
meiosis would, at its completion, produce cells
containing how many chromosomes?
Answer:
• 10 chromosomes
Now
complete
your
Meiosis
worksheet