1 Meiosis and chromosome number Life cycle and

1. Meiosis and chromosome number Life cycle and ploidy levels 2. Steps in meiosis 3. Source of genetic variation a. Independent alignment of homologues b. b. recombination

• Gametes are haploid, with only one set of chromosomes • Somatic cells

• The human life cycle Haploid gametes (n = 23) Egg cell • Meiosis creates gametes Sperm cell MEIOSIS • Mitosis of the zygote produces adult bodies FERTILIZATION Diploid zygote (2 n = 46) Multicellular diploid adults (2 n = 46) Mitosis and development Figure 8. 13 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Meiosis reduces the chromosome number from diploid to haploid • Chromosomes are duplicated before meiosis, then the cell divides twice to form four daughter cells. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Microtubules attached to Spindle kinetochore Sites of crossing over Chromatin Sister chromatids Tetrad Figure 8. 14, part 1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase plate Centromere (with kinetochore) ANAPHASE I Sister chromatids remain attached Homologous chromosomes separate

• In meiosis I, homologous chromosomes are paired – While paired, they cross over and exchange genetic information – homologous pairs are then separated, and two daughter cells are produced Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate Figure 8. 14, part 2 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Haploid daughter cells forming

• Meiosis II is essentially the same as mitosis – sister chromatids of each chromosome separate – result is four haploid daughter cells Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Diploid MITOSIS MEIOSIS 1 gamete precursor 2 n duplication somatic cell Diploid 2 n 2 n 3 2 n 2 n 2 n 4 2 n division diploid 2 n 2 n haploid 5 1 n 1 n 6 division 7 1 n 1 n

MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE ANAPHASE TELOPHASE 2 n Chromosome replication 2 n = 4 Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together 2 n Daughter cells of mitosis Figure 8. 15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS I METAPHASE I ANAPHASE I TELOPHASE I Daughter cells of meiosis I Haploid n=2 No further MEIOSIS II chromosomal replication; sister chromatids separate during anaphase II n n Daughter cells of meiosis II

Genetic variation among offspring is a result of 1) Independent orientation of chromosomes in meiosis 2) random fertilization • 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Figure 8. 16 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Combination 3 Combination 4

Homologous chromosomes carry different versions of genes at corresponding loci Copyright © 2003 Pearson

Coat-color genes Eye-color genes Brown Black C E c e White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Figure 8. 17 A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings C E c e Chromosomes of the four gametes

Crossing over further increases genetic variability • Crossing over is the exchange of corresponding segments between two homologous chromosomes • Genetic recombination results from crossing over during prophase I of meiosis, which increases variation further Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

tetrad

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Coat-color genes • How crossing over leads to genetic recombination Eye-color genes Tetrad (homologous pair of chromosomes in synapsis) 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma 3 Separation of homologous chromosomes at anaphase I 4 Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Figure 8. 18 B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gametes of four genetic types

MEIOSIS I END OF INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

MEIOSIS TELOPHASE I PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

INDEPENDENT ASSORTMENT TELOPHASE II METAPHASE I

a SPERMATOGENESIS b OOGENESIS spermatogonium oogonium primary spermatocyte primary oocyte meiosis l secondary spermatocyte secondary oocyte meiosis ll polar body spermatids polar bodies (will be degraded) egg

8. 21 Accidents during meiosis can alter chromosome number • Abnormal chromosome count is a result of nondisjunction – Either homologous pairs fail to separate during meiosis I Nondisjunction in meiosis I Normal meiosis II Gametes n+1 n– 1 Number of chromosomes Figure 8. 21 A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

– Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n+1 n– 1 n Number of chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings n Figure 8. 21 B

• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n+1 Zygote 2 n + 1 Sperm cell n (normal) Figure 8. 21 C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

8. 20 Connection: An extra copy of chromosome 21 causes Down syndrome • This karyotype shows three number 21 chromosomes • An extra copy of chromosome 21 causes Down syndrome Figure 8. 20 A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• The chance of having a Down syndrome child goes up with maternal

8. 22 Connection: Abnormal numbers of sex chromosomes do not usually affect survival • 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Table 8. 22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

8. 23 Connection: Alterations of chromosome structure can cause birth defects and cancer • Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer – Four types of rearrangement are deletion, duplication, inversion, and translocation Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Deletion Duplication Homologous chromosomes Inversion Reciprocal translocation Nonhomologous chromosomes Copyright © 2003 Pearson Education,

• Translocation Figure 8. 23 Bx Copyright © 2003 Pearson Education, Inc. publishing

• Chromosomal changes in a somatic cell can cause cancer – A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 9 Chromosome 22 Reciprocal translocation “Philadelphia chromosome” Activated cancer-causing gene Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8. 23 C