1. Meiosis and chromosome number 2.Steps in meiosis 3.Source of genetic variation a.Independent...
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Transcript of 1. Meiosis and chromosome number 2.Steps in meiosis 3.Source of genetic variation a.Independent...
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