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Genetics Notes: Meiosis
Meiosis: Reduction-Division / Genetic Recombination
 Similar in many ways to mitosis
 Several differences however
 Involves 2 cell divisions
 Results in 4 cells with 1/2 the normal genetic information.
 Vocabulary:
 Diploid (2N) - Normal amount of genetic material
 Haploid (N) - 1/2 the genetic material.
 Gamete – haploid sex cell
 The form of cell division by which GAMETES, with HALF the number of
CHROMOSOMES, are produced.
 DIPLOID (2n) 
HAPLOID (n)
 Meiosis is SEXUAL reproduction.
 TWO divisions (MEIOSIS I and MEIOSIS II).
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Sex cells divide to produce GAMETES (sperm or egg).
Gametes have HALF the # of chromosomes. (haploid)
Occurs only in GONADS (testes or ovaries).
Male: SPERMATOGENESIS –produces sperm
Female: OOGENESIS – produces egg or ova
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Interphase 1
 Similar to mitosis interphase.
 CHROMOSOMES (DNA) replicate in the S phase
 Each duplicated chromosome consists of two identical SISTER
CHROMATIDS attached at their CENTROMERES.
 CENTRIOLE pairs also replicate.
Meiosis 1
 1st Cell division reduces the chromosome number by one-half.
 Four phases:
a.
Prophase I
b.
Metaphase I
c.
Anaphase I
d.
Telophase I
Prophase 1
 Longest and most complex phase (90%).
 Chromosomes condense.
 Synapsis occurs - Homologous chromosomes come together
to form a tetrad.
 Tetrad is two homologous chromosomes
or four chromatids (sister and non-sister chromatids).
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Homologs contain DNA that codes for the same genes , but
different versions of those genes
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Genes occur at the same loci
Homologous Chromosomes:
 Pair of chromosomes (maternal and paternal) that is 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 homologs.
 Humans have 23 pairs of homologous chromosomes:
a.
First 22 pairs are autosomes
b.
Last pair are sex chromosomes
Crossing-Over:
 Crossing-over may occur between non-sister chromatids at sites
called chiasmata.
 Crossing-over: segments of nonsister chromatids break and
reattach to the other chromatid.
 Chiasmata (chiasma) are where chromosomes touch each other
and exchange genes (crossing-over.)
 Causes Genetic Recombination
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Sex Chromosomes:
Female: XX (homologous chromosomes)
Male: XY (NOT homologous)
Metaphase 1
 Shortest phase
 Tetrads align on the equator.
 Independent assortment occurs – The chromosomes align
independently, for example, the maternal chromosomes align
randomly, not necessarily on the same side of the plate.
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Formula for determining the number of gamete combinations:
Formula: 2n
Example:
2n = Diploid
2n = 4 (there are 4 chromosomes in the diagram)
then
1n = 2
thus
22 = 4
there are 4 possible different gamete combinations
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Question:
In terms of Independent Assortment, how many different combinations
of sperm could a human male produce?
Answer:
 Formula: 2n
 Human chromosomes: 2n = 46 (Human’s diploid number is 46)
 n = 23
 223 = ~8 million combinations
Anaphase 1
 Homologous chromosomes separate and move towards the poles.
 Sister chromatids remain attached at their centromeres.
Telophase 1
 Each pole now has haploid (1n) set of chromosomes.
 Cytokinesis occurs and two haploid daughter cells are formed.
Meiosis II
 No Interphase II or very short
 No DNA Replication
 Remember: Meiosis II is similar to mitosis
Prophase II
 Same as Prophase in mitosis
 Nucleus & nucleolus disappear
 Chromosomes condense
 Spindle forms
Metaphase II
 Same as Metaphase in mitosis
 Chromosomes (not homologs) line-up on the equator
Anaphase II
 Same as Anaphase in mitosis
 SISTER CHROMATIDS separate
Telophase II
 Same as Telophase in mitosis.
 Nuclei and Nucleoli reform, spindle disappears
 CYTOKINESIS occurs.
 Remember: FOUR HAPLOID DAUGHTER cells are
produced.
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 Called GAMETES (eggs and sperm)
Genetic Variation
 Also known as GENETIC RECOMBINATION
 Important to population as the raw material for NATURAL SELECTION.
 All organisms are NOT alike
 Strongest, or “most fit” survive to reproduce & pass-on traits
Question:
What are the 3 sources of genetic recombination or variation?
Answer:
1. CROSSING-OVER (prophase I)
2. INDEPENDENT ASSORTMENT (metaphase I)
3. RANDOM FERTILIZATION
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 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
Male (XY)
Female (XX)
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Karyotype: Down Syndrome
3 of chromosome #21
Female: XX
Fertilization:
 The fusion of a sperm and egg to form a zygote.
 A zygote is a FERTILIZED EGG (*diploid cell)
Explain how a person gets Down Syndrome:
Nondisjunction occurs in which chromosome pair 21 doesn’t separate
during meiosis and a gamete is produced that carries an extra
chromosome 21. When that gamete is used in fertilization, the child has 3
chromosome 21 instead of 2.
Question
A cell containing 40 chromatids at the beginning of meiosis would, at its
completion, produce cells containing how many chromosomes?
Answer
10 chromosomes
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