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

Figure: 09-07 Title: A karyotype displays a full set of chromosomes. Caption: One member of each chromosome pair comes from the individual’s father and the other member from the mother. Each paired set of chromosomes is said to be “homologous,” meaning the same in size and function. (The two chromosomes over the number 1 are a homologous pair, the two over number 2, and so forth.) Homologous chromosomes are not exactly alike, however; the genes on them may differ somewhat, meaning the effects they produce will differ. Because this karyotype set is from a human male, there are 22 pairs of homologous chromosomes and then one X and one Y chromosome (which are not homologous). All the chromosomes are in the duplicated state.

Figure: 10-06 Title: Mobile cells, bearing chromosomes. Caption: Human sperm, colored to show detail.

Gametes have a single set of chromosomes Gametes are haploid, with only one set of chromosomes Somatic cells are diploid.

Multicellular diploid adults (2n = 46) Mitosis and development The human life cycle Meiosis creates gametes Mitosis of the zygote produces adult bodies Haploid gametes (n = 23) Egg cell Sperm cell MEIOSIS FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Figure 8.13

Figure: 10-07 Title: Big difference in size. Caption: A human egg surrounded by much smaller human sperm.

Meiosis reduces the chromosome number from diploid to haploid Chromosomes are duplicated before meiosis, then the cell divides twice to form four daughter cells.

MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Microtubules attached to kinetochore Sites of crossing over Metaphase plate Sister chromatids remain attached Spindle Nuclear envelope Chromatin Sister chromatids Tetrad Centromere (with kinetochore) Homologous chromosomes separate Figure 8.14, part 1

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

MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate Haploid daughter cells forming Figure 8.14, part 2

Meiosis II is essentially the same as mitosis sister chromatids of each chromosome separate result is four haploid daughter cells

MITOSIS MEIOSIS Diploid Diploid 1 somatic cell 2n 2n 2 2n 2n 3 2n 2n 4 gamete precursor somatic cell 2n 2n duplication 2 2n 2n 3 2n 2n 4 2n 2n Figure: 10-01 Title: Meiosis compared to mitosis. Caption: 1. Both mitosis and meiosis begin with diploid cells, meaning cells that contain paired sets of chromosomes. The two members of each pair are homologous, meaning the same in shape and function. Two sets of homologous chromosomes are shown in both the mitosis and meiosis figures.The larger chromosome pairs in each cell represents one homologous pair, while the smaller chromosome pairs represent the other homologous pair. One member of each homologous pair (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other member of the pair (in blue) comes from the father of this person. 2. In both mitosis and meiosis, the chromosomes duplicate. Each chromosome is now composed of two sister chromatids. 3. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, meanwhile, homologous chromosomes -- not sister chromatids -- line up on opposite sides of the metaphase plate. 4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate. 5 In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, in the first of two cell divisions, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on oppositesides of the plate. 6. The sister chromatids of each chromosome then separate. 7. The cells divide again, yielding four haploid cells. division diploid haploid 5 2n 2n 1n 1n 6 division 7 1n 1n 1n 1n

PARENT CELL (before chromosome replication) Site of crossing over MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE PROPHASE I Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication 2n = 4 Chromosomes align at the metaphase plate Tetrads align at the metaphase plate METAPHASE METAPHASE I ANAPHASE I TELOPHASE I ANAPHASE TELOPHASE Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Haploid n = 2 Daughter cells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Figure 8.15

Each chromosome of a homologous pair comes from a different parent 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

Two equally probable arrangements of chromosomes at metaphase I POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

Homologous chromosomes carry different versions of genes at corresponding loci

C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Figure 8.17A, B

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

tetrad Figure: 10-02c Title: Crossing over during prophase I. Caption:

Tetrad Chaisma Centromere Figure 8.18A

How crossing over leads to genetic recombination Coat-color genes Eye-color genes How crossing over leads to genetic recombination Tetrad (homologous pair of chromosomes in synapsis) 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I 3 Separation of chromatids at anaphase II and completion of meiosis 4 Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Gametes of four genetic types

MEIOSIS I END OF INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Figure: 10-02a Title: Meiosis I. Caption:

MEIOSIS TELOPHASE I PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II Figure: 10-02b Title: Meiosis II. Caption: PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

INDEPENDENT ASSORTMENT TELOPHASE II METAPHASE II METAPHASE I METAPHASE I Figure: 10-02d Title: Independent assortment. Caption:

a SPERMATOGENESIS b OOGENESIS spermatogonium oogonium primary spermatocyte primary oocyte meiosis l secondary spermatocyte secondary oocyte Figure: 10-05 Title: Sperm and egg formation in humans. Caption: a. In sperm formation (spermatogenesis), diploid cells called spermatogonia produce primary spermatocytes. The primary spermatocytes are the diploid cells that go through meiosis, yielding haploid secondary spermatocytes. These spermatocytes then go through meiosis II, yielding four haploid spermatids that will develop into mature sperm cells. b In egg formation (oogenesis), cells called oogonia, produced before the birth of the female, develop into primary oocytes. These diploid cells will remain in meiosis I until they mature in the female ovary, beginning at puberty. (Only one oocyte per month, on average, will complete this maturation process.) Oocytes that mature will enter meiosis II, but their development will remain arrested there until they are fertilized by sperm. An unequal meiotic division of cellular material leads to the production of three polar bodies from the original oocyte and one well-endowed egg. The egg can go on to be fertilized, but the polar bodies will be degraded. polar body meiosis ll spermatids polar bodies (will be degraded) egg

Figure: 10-08 Title: Reproduction in bacteria. Caption: There is no fusion of sperm and eggs in bacteria, just a division of one cell into two. Pictured is a Staphylococcus aureus bacterium undergoing such cell division, referred to as binary fission.

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 n – 1 n – 1 Number of chromosomes Figure 8.21A

Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n + 1 n – 1 n n Number of chromosomes Figure 8.21B

Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n + 1 Zygote 2n + 1 Sperm cell n (normal) Figure 8.21C

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 8.20A, B

The chance of having a Down syndrome child goes up with maternal age Figure 8.20C

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

Table 8.22

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

Homologous chromosomes Deletion Duplication Homologous chromosomes Inversion Reciprocal translocation Nonhomologous chromosomes Figure 8.23A, B

Translocation Figure 8.23Bx

Chromosomal changes in a somatic cell can cause cancer A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 9 Reciprocal translocation Chromosome 22 “Philadelphia chromosome” Activated cancer-causing gene Figure 8.23C