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MEIOSIS
Reduction-Division
Genetic Recombination
Meiosis
The form of cell division by which GAMETES, with HALF the number __________,
are produced.
DIPLOID (2n)  HAPLOID (n)
Meiosis is ___________ reproduction.
______ divisions (MEIOSIS _ and MEIOSIS __).
Meiosis
Sex cells divide to produce _________ (sperm or egg).
Gametes have HALF the # of chromosomes.
Occurs only in __________ (testes or ovaries).
Male: SPERMATOGENESIS -sperm
Female: OOGENESIS - egg or ova
Spermatogenesis
Oogenesis
Interphase I
 Similar to mitosis interphase.
 ___________________(DNA) replicatein the S phase
 Each duplicated chromosome consistof twoidentical SISTER CHROMATIDSattached at their
CENTROMERES.
 ________________pairsalsoreplicate.
Interphase I
 Nucleus and nucleolus ________.
Meiosis I (four phases)
 Cell division that reduces the chromosome number by one-half.
 Four phases:
a.
b.
c.
d.
Prophase I
 ___________ and most _________ phase (____%).
 Chromosomes condense.
 Synapsis occurs - Homologous ________________come together
to form a tetrad.
 Tetrad is ____ chromosomes
or _____ chromatids (sister and non-sister chromatids).
Non-Sister Chromatids-HOMOLOGS
Prophase I - Synapsis
Homologous Chromosomes
 Pair of chromosomes (__________ and ____________) that are similar in shapeand size.
 Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits.
 Each locus (_________________________) is in the same position on homologues.
 Humans have __________ of homologous chromosomes:
a. First 22 pairs of ___________
b. Last pair of sex chromosomes
Homologous Chromosomes
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Crossing Over
Crossing over may occur between non-sister chromatids at sites called ____________.
Crossing over: segments of nonsister chromatids break and reattach to the other
chromatid.
Chiasmata (___________) are where chromosomes touch each other and
_________________ (crossing over.)
Causes Genetic Recombination
Genetic Recombination
Sex Chromosomes
MEIOSIS I
Homologs separate
Prophase I
Metaphase I
__________ phase
Tetrads align on the _________.
Independent assortment occurs – chromosomes separate randomly causing GENETIC
RECOMBINATION
Metaphase I
n
 Formula: 2
 Example: 2n = 4
then
1n = 2
2
thus 2 = 4
combinations
Question:
 In terms of Independent Assortment -how many different combinations of sperm
could a _____ _______ produce?
Answer
n
 Formula: 2
 Human chromosomes: 2n = 46
 n = 23
23
 2 = ~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 (1n) set of chromosomes.
 Cytokinesis occurs and two haploid daughter cells are formed.
Telophase I
MEIOSIS II
Sister Chromatids Separate
Meiosis II
 No Interphase II or very short
 No ______ Replication
 Remember: Meiosis II is similar to mitosis
Prophase II
 Same as __________ in mitosis
 Nucleus & nucleolus disappear
 Chromosomes condense
 Spindle forms
Metaphase II
Same as ___________ in mitosis
Anaphase II
 Same as Anaphase in ________
 SISTER CHROMATIDS separate
Telophase II
 Same as ___________ in mitosis.
 Nuclei and Nucleoli reform, spindle disappears
 CYTOKINESIS occurs.
 Remember: ________________
DAUGHTER cells are
produced.
 Called __________ (eggs and sperm)
Telophase II
Variation
 Also known as GENETIC RECOMBINATION
 Important to population as the raw material for NATURAL SELECTION.
 All organisms are _____ alike
 Strongest “most fit” survive to reproduce & pass on traits
Question:
 What are the 3 sources of genetic recombination or variation?
Answer:
1. ____________________ (prophase I)
2. ________________________________________ (metaphase I)
3. _______________________________
Question:
 A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its
completion, produce cells containing how many chromosomes?
Answer:
 ____ chromosomes (haploid or 1n)
Karyotype
 An organized picture of the chromosomes of a human arranged in pairs by size
from largest to smallest.
 Pairs 1-22 called AUTOSOMES
 Last pair are SEX CHROMOSOMES
Karyotype
Karyotype
Fertilization
 The fusion of a sperm and egg to form a zygote.
 A zygote is a FERTILIZED ______
Question:
 A cell containing 40 chromatids at the beginning of meiosis would, at its completion,
produce cells containing how many chromosomes?
Answer:
 _____ chromosomes