LESSON # 19 MEIOSIS

TWO TYPES OF CELL DIVISION 1- MITOSIS The number of chromosomes in the daughter cells is identical to that of the mother cells. It takes place in most cells (somatic cells) 2- MEIOSIS The number of chromosomes in the daughter cells is the half of that of the mother cells. It takes place only in sex cells (gametes)

Reproductive Cells or Gametes are Haploid Cells Somatic Cells are Diploid Cells Reproductive Cells or Gametes Sperm Egg are Haploid Cells

DIPLOID: Two sets of chromosomes. Somatic human cells are DIPLOID cells because they have 23 pairs of chromosomes ( 46 chromosomes ). (23) DIPLOID: Two sets of chromosomes. Diploid cells: 2n cells. In this case, 2(23) cells. Chromosomes in every pair have the same structure and function and are called homologous chromosomes. (23) Reproductive human cells are HAPLOID cells because they have 23 chromosomes. HAPLOID: One set of chromosomes Haploid cells: 1n cells. In this case, 1(23) cells.

(23) (23) MITOSIS Haploid Haploid During MITOSIS, the number of chromosomes does NOT change. Therefore, haploid cells will produce haploid cells. Haploid

MITOSIS Diploid Diploid During MITOSIS, the number of chromosomes does NOT change. Therefore, diploid cells will produce diploid cells. Diploid

Diploid (2n cell) MEIOSIS I During MEIOSIS I, a single diploid cell divides to produce two haploid repro-ductive cells. The number of chromo-somes is reduced to the half. Sister chromatids remain together. Gametogonium They are the cells that produce the gametes (oocyte and sperm). (2n cell) Diploid (23) MEIOSIS I Gamete (Haploid) (23)

During MEIOSIS II, each haploid gamete produces two haploid gametes During MEIOSIS II, each haploid gamete produces two haploid gametes. Sister chromatids separate like in mitosis. (23) (23) MEIOSIS II Haploid Haploid During MEIOSIS II, the number of chromosomes does NOT change. Therefore, haploid cells will produce haploid cells. Haploid

+ Diploid Haploids (2n cell) Fertilization Sperm Egg Haploids + Fertilization 1n cell 1n cell During FERTILIZATION, two haploid gametes combine to produce a diploid embryo. (2n cell) Diploid

+ + Somatic Cells undergo MITOSIS and cell division. Gametogonia: They are cells, which undergo meiosis to produce haploid reproductive cells or gametes. Reproductive Cells or Gametes: They are specialized cells, which come together to produce offspring. Reproduction: It is the process by which the reproductive cells come together to produce offspring. Haploid Diploid + chromosomes 46 + 23 chromosomes chromosomes

The cells that give rise to sperm and egg are diploid cells. When the cells that give rise to sperm and egg cells divide, the result is sperm or egg cells that have only half of the usual somatic number of chromosomes. Diploid (2n) Spermatogonium Oogonium Haploid (1n)

Sex Human Cells (haploid) Somatic Human Cells (diploid) Sperma- togonia Meiosis Haploid Haploid + Haploid (Diploid) Meiosis Haploid Oogonia (Diploid) Diploid zygote Diploid Somatic Human Cells (diploid) Mitosis

Meiosis It is the process by which a single diploid cell divides to produce haploid reproductive cells. 1n 2n Homologous means same in size and function.

The Steps in Meiosis

Homologous chromosomes Meiosis Homologous chromosomes 2n 1n One diploid cell gives rise two haploid cells. MEIOSIS-I MEIOSIS-II 1n Each haploid cell gives rise two haploid cells.

Paternal chromosome Maternal chromosome DNA I Metaphase Metaphase

Homologous chromosomes 2n MITOSIS 2n MEIOSIS I Homologous chromosomes Homologous chromosomes 2n 1n

2n MEIOSIS-I 1n MEIOSIS-II 1n

MEIOSIS I Prophase I 1- Homologous chromosomes links as they condense forming tetrads. 2- Crossing over occurs. Maternal chromosomes Paternal chromosomes 2n 2n 2n

Crossing over is the first important source of genetic variation. MEIOSIS I It is the process by which non-sister chromatids exchange genetic material. CROSSING OVER: Crossing over is the first important source of genetic variation.

Metaphase I or 1- Homologous chromosomes align at metaphase plate. MEIOSIS I Metaphase I 1- Homologous chromosomes align at metaphase plate. 2- Independent assortment occurs. Prophase I or

Independent Assortment: MEIOSIS I It is the random distribution of homologous chromosome pairs during the metaphase of meiosis I. Independent Assortment: Green eyes & blond hair Black eyes & black hair Green eyes & black hair Independent Assortment is the second important source of genetic variation. Black eyes & blond hair

SOURCES OF GENETIC VARIATION MEIOSIS I SOURCES OF GENETIC VARIATION Crossing over is the first important source of genetic variation. Independent assortment is the second important source of genetic variation.

MEIOSIS I Anaphase I Microtubules separate the homologous chromosomes (sister chromatids remain together). 1n 2n

MEIOSIS I Telophase I Two haploid cells result from Cytokinesis 2n 1n

MEIOSIS I GENETIC VARIATION

MEIOSIS I Prophase I Metaphase I Anaphase I Telophase I 1- Homologous chromosomes links as they condense forming tetrads. 2- Crossing over occurs. Prophase I Crossing over is the process by which non-sister chromatids exchange genetic material Crossing over is the first important source of genetic variation. Metaphase I 1- Homologous chromosomes align at metaphase plate. 2- Independent assortment occurs. Independent assortment is the second important source of genetic variation. Anaphase I 1- Microtubules separate the homologous chromosomes (sister chromatids remain together). Telophase I 1-Two haploid cells result from Cytokinesis.

Meiosis and Sex Outcome Diploid DNA replication Diploid Haploid Haploid

The X chromosome has about 1500 genes. The Y chromosome has about 50 genes.

Meiosis and Sex Outcome XX GAMETOGONIA XY MOTHER FATHER Diploid X MEIOSIS I X Y Haploid MEIOSIS II X X Y X X X Y FERTILIZATION XX XY Female Diploid Male