Reduction Division Production of Gametes Meiosis Reduction Division Production of Gametes

Vocabulary Meiosis Homologous Chromosomes - chromosomes in a biological cell that pair (synapse) during meiosis Crossing over - when two chromosomes break and then reconnect but to the different end piece. The result is an exchange of genes, called genetic recombination Chromatid - is one of two identical strands of DNA making up a chromosome that are joined at their centromeres Centromeres – holds two chromatids together to make a chromosome Tetrad - consists of two homologous chromosomes attached together each composed of two sister chromatids. Haploid – (n) half the normal number of chromosomes Diploid – (2n) the normal number of chromosomes Loci - the position of a gene on a chromosome

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.

Gametes have a single set of chromosomes Gametes (egg and sperm) are haploid, with only one set of chromosomes Somatic (body) cells are diploid. Meiosis reduces the chromosome number from diploid to haploid

Meiosis creates gametes Mitosis of the zygote produces adult bodies The human life cycle Meiosis creates gametes Mitosis of the zygote produces adult bodies Figure 8.13

MEIOSIS I: Homologous chromosomes separate 1. Chromosomes are duplicated in meiosis I (4n) 2. Crossing over occurs (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

3. Then cell divides once to form two cells that are diploid (2n) and then those two cells divide again to form four daughter cells which are haploid (n) gametes. (meiosis II) 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 number of chromosomes

Mitosis chromosome number

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 is essentially the same as mitosis without the doubling of the chromosomes The two daughter cells divide sister chromatids of each chromosome separate result is four haploid daughter cells each with ½ the genetic information

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 Homologous chromosomes carry different versions of genes 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 Crossing over mixes the genes from the parents increasing variation

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 This increases genetic variability

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

Spermatogenesis vs Oogenesis Spermatogenesis goes though meiosis and produces 4 sperm for every one follicle cell. Oogenesis goes though meiosis and produces ONLY 1 mature egg cell and 3 polar bodies for every one follicle cell. The polar bodies will be broken down and reabsorbed.

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

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