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1. Meiosis and chromosome number
2.Steps in meiosis
3.Source of genetic variationa.Independent alignment of homologuesb. Recombination
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Somatic cells are diploid.
• Gametes are haploid, with only one set of chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• human life cycle
• Meiosis creates gametes
• Mitosis of the zygote produces adult bodies
Figure 8.13
MEIOSIS FERTILIZATION
Haploid gametes (n = 23)
Egg cell
Sperm cell
Diploidzygote
(2n = 46)Multicellular
diploid adults (2n = 46)
Mitosis anddevelopment
Meiosis reduces the number of genomes from diploid to haploid
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.14, part 1
MEIOSIS I: Homologous chromosomes separate
INTERPHASE PROPHASE I METAPHASE I ANAPHASE I
Centrosomes(withcentriolepairs)
Nuclearenvelope
Chromatin
Sites of crossing over
Spindle
Sisterchromatids
Tetrad
Microtubules attached tokinetochore
Metaphaseplate
Centromere(with kinetochore)
Sister chromatidsremain attached
Homologouschromosomes separate
Steps in meiosis I
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• In meiosis I, homologous chromosomes are paired
– While paired, they cross over and exchange genetic information (DNA)
– homologous pairs are then separated, and two daughter cells are produced
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.14, part 2
MEIOSIS II: Sister chromatids separate
TELOPHASE IAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II
Cleavagefurrow
Sister chromatidsseparate
TELOPHASE IIAND CYTOKINESIS
Haploiddaughter cellsforming
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Meiosis II is essentially the same as mitosis– sister chromatids of each chromosome separate
– result is four haploid daughter cells
MITOSIS MEIOSISDiploid
somatic cell
Diploid
gameteprecursor
4
1
2
3
5
6
7
2n
2n
2n 2n
2n
2n 2n 1n 1n
2n
2n
2n
1n 1n 1n 1n
division
division
duplication
haploiddiploid
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Each chromosome of a homologous pair comes from a different parent
– Each chromosome thus differs at many points from the other member of the pair
Homologous chromosomes carry different versions of genes (alleles) at corresponding loci
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.16
POSSIBILITY 1 POSSIBILITY 2
Two equally probable
arrangements of chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2 Combination 3 Combination 4
Independent alignment of homologous chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• the exchange of corresponding segments between two homologous chromosomes
Crossing over further increases genetic variability
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.18A
TetradChaisma
Centromere
END OF INTERPHASE
PROPHASE I METAPHASE I ANAPHASE I
MEIOSIS I
Genetic recombination results from crossing over during prophase I of meiosis
TELOPHASE IIANAPHASE II
METAPHASE IIPROPHASE IITELOPHASE I
MEIOSIS
METAPHASE I METAPHASE I
TELOPHASE II
METAPHASE II
INDEPENDENT ASSORTMENT
egg
polarbody
spermatogonium
primaryspermatocyte
secondaryspermatocyte
oogonium
primaryoocyte
secondaryoocyte
polar bodies(will be degraded)
spermatids
meiosis ll
meiosis l
SPERMATOGENESIS OOGENESISa b
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Abnormal chromosome count is a result of nondisjunction
– Either homologous pairs fail to separate during meiosis I
Accidents during meiosis can alter chromosome number
Figure 8.21A
Nondisjunctionin meiosis I
Normalmeiosis II
Gametes
n + 1 n + 1 n – 1 n – 1
Number of chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Or sister chromatids fail to separate during meiosis II
Figure 8.21B
Normalmeiosis I
Nondisjunctionin meiosis II
Gametes
n + 1 n – 1 n n
Number of chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
Figure 8.21C
Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• This karyotype shows three number 21 chromosomes
• An extra copy of chromosome 21 causes Down syndrome
Connection: An extra copy of chromosome 21 causes Down syndrome
Figure 8.20A, B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The chance of having a Down syndrome child goes up with maternal age
Figure 8.20C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Nondisjunction can also produce gametes with extra or missing sex chromosomes
– Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes
Connection: Abnormal numbers of sex chromosomes do not usually affect survival
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Table 8.22
