1. Meiosis and chromosome number Steps in meiosis Source of genetic variation Independent alignment of homologues b. Recombination

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

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

MEIOSIS I: Homologous chromosomes separate Steps in meiosis I 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 (DNA) 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

Each chromosome of a homologous pair comes from a different parent Homologous chromosomes carry different versions of genes (alleles) at corresponding loci 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 Independent alignment of homologous chromosomes 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

Crossing over further increases genetic variability the exchange of corresponding segments between two homologous chromosomes

Tetrad Chaisma Centromere Figure 8.18A

MEIOSIS I END OF INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Figure: 10-02a Title: Meiosis I. Caption: Genetic recombination results from crossing over during prophase I of meiosis

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 Sperm and egg formation in humans. 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. 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: 10-07 Title: Big difference in size. Caption: A human egg surrounded by much smaller human sperm.

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

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

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