Mitosis versus Meiosis - the division of 1 cell into 4 daughter cells (egg or sperm) containing ½ the genetic material - haploid (n) = contains 1 copy of the genome Mitosis - the division of 1 cell to produce 2 genetically identical daughter cells - diploid (2n) = contains 2 copies of the genome 2n 46 chrom 2n 46 chrom n (23) n (23) 2n (46) 2n (46) n 23 n 23 n 23 n 23

Development of male and female gametes The Formation of sex cells during meiosis is referred to as gametogenesis Sperm and egg production are different: In males 4 viable sperm are produced  Spermatogenesis In females 3 of the cells produce are known as polar bodies and do not survive. Only one egg is formed  Oogenesis

Differences The cyctoplasm of the female oocyte does not divide equally; one of the daughter cells called an ootid, receives most of the cyctoplasm while the other cells called polar bodies (“nurse cells”) die and are reabsorbed to provide nutrients Sperms show equal division of cytoplasm; all 4 daughter cells become viable sperm

Spermatogenesis Occurs in the seminiferous tubules where diploid spermatogonia, stem cells that are the precursors of sperm. Spermatogonia divide by mitosis to produce more spermatogonia or differentiate into spermatocytes

Spermatogenesis Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process.

Sketch Spermatogeneis showing meiotic divisions 46 Spermatogonium 2. 1° Spermatocyte 3. 2° Spermatocyte 4. Spermatid 5. Sperm 1st Meiotic division 46 23 23 2nd Meiotic division 23 23 23 23

Oogenesis Egg formation takes place in the ovaries. the initial steps in egg production occur prior to birth. Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes

Sketch Oogeneis showing meiotic divisions 46 Oogonium 2. Oocyte a) 2° Oocyte b) 1st polar body a) Ootid b) polar bodies 5. Ovum 46 1st Meiotic division 23 23 2nd Meiotic division

By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed. By the time she is born, 1–2 million of these remain. Each has begun the first steps of the first meiotic division (meiosis I) and then stopped.

No further development occurs until years later when the female becomes sexually mature. Then the primary oocytes recommence their development, usually one at a time and once a month. Unfertilized oocyte

The primary oocyte grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.

Meiosis: Male and Female Differences

In humans, 22 of the 23 are homologous; these are the autosomes Thomas Hunt Morgan discovered that having 2 rod shaped (X chromosome) indicated female and 1 rod with 1 hooked shaped chromosome (Y chromosome) identified males The X and Y are not homologous, they are the sex chromosomes

Meiosis and Variation Unlike mitosis, meiosis does not produce identical cells The cells produced only have half the number but the chromosomes therefore only ½ the genetic information What chromosomes end up in what cell all depend upon how the chromosomes line up in Metaphase I

Meiosis and Variation If the two blue chromosomes line up on the same side and the two red chromosomes line up on the same side Then the daughter cells will have either the genetic information from the red or blue chromosome

Meiosis and Variation If 1 red and 1 blue line up on the same side The daughter cells will have genetic information from both red and blue chromosomes

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 TELOPHASE Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together ANAPHASE I TELOPHASE I Haploid n = 2 Daughter cells of meiosis I No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II 2n 2n Daughter cells of mitosis n n n n Daughter cells of meiosis II

Abnormalities during Meiosis The movement of the chromosomes in a dividing cell is so precise that only 1 in every 100,000 divisions will contain an error Non-disjunction = an error during meiosis where sister chromatids fail to come apart resulting in gametes that are missing or have extra chromosomes

Non-disjunction One daughter cell will be missing one of the chromosomes (22) Other daughter cell will contain an extra chromosome (24) In humans, non-disjunction produces gametes with 22 or 24 chromosomes

Non-disjunctions When gamete with 24 chromosomes joins a normal gamete with 23 chromosomes, the zygote will contain 47 (instead of 46). This zygote will have 3 chromosomes in place of the normal pair = trisomy When the gamete with 22 chromosomes joins a normal gamete with 23 chromosomes, the zygote has 45; this zygote will have 1 chromosome in place of the normal pair = monosomy

KARYOTYPES The best way to study non-disjunctions is by looking at karyotypes Karyotypes are an inventory of an individuals chromosomes A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes

Preparation of a karyotype Packed red And white blood cells Hypotonic solution Fixative Blood culture Stain White Blood cells Centrifuge 3 2 1 Fluid Centromere Sister chromatids Pair of homologous chromosomes 4 5 Figure 8.19

Non-disjunctions A number of disorders are caused by non-disjunctions Down’s Syndrome Edward’s Syndrome Patau’s Syndrome Turner’s Syndrome Kleinfelter’s Syndrome Other severe abnormalities

Non-disjunctions The risk of chromosomal abnormalities increases with maternal age because the egg cells are older Women over the age of 35, who have children increase their chance exponentially

Karyotype for Down’s Syndrome Trisomy 21

Down’s Syndrome Trisomy 21 Cause: Non-disjunction of chromosome #21 Individual has 47 chromosomes Karyotype: - has 3 chromosome #21 Symptoms: - mentally & physically delayed, large forehead, large space between eyes

What does this karyotype tell us?

Patau’s Syndrome (trisomy) Cause: - non-disjunction of chromosome # 13 Karyotype: - has 3 chromosome #13 Symptoms: - child with multiple and severe abnormalities, and severe mental retardation -  head very small, eyes absent or very small - hairlip, cleft palate, usually malformations of internal organs - most cases child dies soon after birth

What does this karyotype tell us?

Edward’s Syndrome: Trisomy 18 Cause: - Non-disjunction of chromosome #18 Karyotype: -has 3 of chromosome #18 Symptoms: -very small and weak, head flattened, hands short with very little development - severe mental handicap with life expectancy of less than one year

What does this Karyotype tell us?

Klinefelter’s Syndrome (Trisomy 23) Cause: Non-disjunction sex chromosomes Karyotype: - 2 X chromosomes and a Y chromosome (XXY) Symptoms: Appears male at birth but upon puberty releases high levels of female hormones Sterile males, breast development, slightly feminized physique Affects 1/1000 male births

What does this karyotype tell us?

Turner Syndrome (Monosomy) Cause: Non-disjunction of X chromosome in females Karyotype: - females have a single X chromosome (XO) Symptoms: sterile females, short stature, underdeveloped gonadal structures Affects 1/5000 female births

Non-disjunction in Males XX XY X X O XY XXY XO Klinefelter Turner

Non-disjunction in females XX XY O XX X Y XXY XO Klinefelter Turner

Other possible abnormalites caused by non-disjunction these infants do not live very long – a few days at the most; many are premature and stillborns These photos may disturb you, you are not required to watch