MEIOSIS Human sperm contacts an egg. Note the difference in size. Chapter 9

Why must organisms reproduce? Reproduction is not essential to the life of an individual. However, it is essential for species survival.

A. Types of Reproduction 1. Asexual Reproduction involves only 1 parent offspring are genetically identical to the parent (clones) advantageous when organism is well-adapted to a stable environment Ex. binary fission, parthenogenesis Asexual reproduction - cell doubles its contents & then divides into two identical individuals. Binary fission is common in single-celled organisms like amoeba. Parthenogenesis - reproduction without a mate; eggs of female divide by mitosis to yield viable offspring. Common in insects (aphid, bees), salamanders, lizards, snakes, turkeys, roundworms & flatworms.

2. Sexual Reproduction involves 2 parents parents produce haploid (1n) gametes which fuse to form a diploid (2n) zygote (first cell of offspring). offspring are genetically diverse advantageous in a changing environment Haploid gametes are produced by a type of cell division called meiosis. Sexual reproduction - the combination of genetic material from 2 individuals (parents) to create genetically diverse offspring. Haploid - nuclei contain only one set of chromosomes. Ex. Human gametes (eggs & sperm) contain 23 chromosomes. Diploid - nuclei contain two sets of chromosomes. Human body cells (somatic cells) contain 46 chromosomes (two sets of 23).

In humans, where would germ cells be located? B. Meiosis Type of cell division that halves the number of chromosomes. involves 2 nuclear divisions (meiosis I & meiosis II) 1 diploid (2n) cell  4 haploid (1n) cells occurs in germ cells In humans, where would germ cells be located? In humans, germ cells would be located in the testes (male) & ovaries (female)

1. Meiosis I (reduction division) Germ cell replicates its chromosomes in S phase of interphase. 1. Meiosis I (reduction division) Prophase I replicated chromosomes condense spindle apparatus forms & fibers attach to chromosomes nucleolus disappears nuclear membrane breaks down synapsis & crossing over occurs Meiosis I is called reduction division because it reduces the number of chromosomes (separates homologous chromosomes). Human germ cells have 46 chromosomes, so at the completion of meiosis I, resulting daughter cells will have only 23 chromosomes.

Synapsis - homologous chromosomes (homologs) pair up. Homologs - chromosomes that carry genes for the same traits in the same sequence. A diploid human cell has 23 homologous pairs. Homologs carry genes for the same traits (ie. hair color, eye color, finger length). However, they may carry different forms (alleles) of the same gene. For example: Both chromosomes in diagram carry genes for the same traits (same letters of alphabet); however, each homolog carries a different version of the gene (upper case letters vs. lower case letters). [‘A’ may code for black hair, while ‘a’ codes for blond hair] One chromosome of each homologous pair was inherited from the female parent (maternally derived), while the other chromosome was inherited from the male parent (paternally derived).

Crossing over - non sister chromatids of homologs exchange chromosomal material. Chiasma - visible point of cross over between homologs. Crossing over helps to generate genetic diversity by mixing up parental traits. Since each homolog comes from a different parent, crossing over results in chromosomes that have some genes from the mother & some genes from the father.

Animal cell in prophase I

paired homologous chromosomes line up along equator of spindle Metaphase I paired homologous chromosomes line up along equator of spindle Note: if this were mitosis rather than meiosis, would see 4 chromosomes in line rather than 2 pairs of chromosomes in line.

homologous chromosomes separate & move to opposite poles Anaphase I homologous chromosomes separate & move to opposite poles Note: centromeres do NOT part; sister chromatids remain attached.

spindle apparatus breaks down chromosomes decondense Telophase I spindle apparatus breaks down chromosomes decondense nuclear membranes partially reform around two nuclei Note: these two cells are haploid because they contain 1 rather than 2 sets of chromosomes (one #1 & one #2). Cytokinesis usually occurs between meiosis I & II

2. Meiosis II (equational division) Prophase II chromosomes condense spindle apparatus forms & fibers attach to chromosomes nuclear membrane breaks down Metaphase II chromosomes line up single-file along equator of spindle Meiosis II produces four cells from the two formed in meiosis I.

Anaphase II centromeres part; sister chromatids (now called chromosomes) separate & move toward opposite poles Telophase II spindle apparatus breaks down chromosomes de-condense nuclear membranes reform nucleoli reappear Cytokinesis divides two cells into four nonidentical cells.

Meiosis II Four non-identical haploid cells Prophase II Metaphase II Anaphase II Telophase II

Differences between mitosis & meiosis. 1 nuclear division results in 2 diploid cells that are genetically identical no synapsis & crossing over of homologs chromosomes align individually (metaphase) occurs in somatic cells occurs throughout life cycle used for growth, repair & asexual reproduction Meiosis 2 nuclear divisions results in 4 haploid cells that are genetically diverse synapsis & crossing over of homologs (during Prophase I) homologous pairs align (metaphase I) occurs in germ cells in humans, completed only after sexual maturity used for sexual reproduction

C. Gametogenesis in Humans 1. Spermatogenesis (meiosis in the ) Gametogenesis - formation of gametes (meiosis). Spermatogenesis occurs in seminiferous tubules of testes. Spermatogonia are (2n) germ cells. Have the ability to undergo mitosis (to replace themselves) & meiosis (give rise to sperm). Will not undergo meiosis until puberty. 1. Spermatogonium (2n) grows, accumulates cytoplasm & replicates its chromosomes. Is now called a primary (1o) spermatocyte (2n). Note: several spermatogonia may be attached to each other by bridges of cytoplasm & they may undergo meiosis simultaneously. 2. Primary spermatocyte divides by meiosis I to form two secondary (2o) spermatocytes (1n). 3. Secondary spermatocytes divide by meiosis II to form two spermatids (1n). 4. Spermatids undergo period of maturation (eject most of cytoplasm & grow a flagellum) to form spermatozoa (sperm).

Structure of human sperm: Head nucleus - 23 chromosomes acrosome - contains enzymes that help sperm penetrate egg Midpiece mitochondria Tail Sperm are stored in the epididymis until ejaculation.

2. Oogenesis (meiosis in the ) Oogenesis occurs in ovaries. 1. Oogonia are (2n) germ cells. Oogonia divide by mitosis up to the 3rd month post-conception. Before she is born, these oogonia grow, accumulate cytoplasm & replicate their chromosomes, developing into primary (1o) oocytes (2n). At birth, she possesses about 1 million 1o oocytes in her ovaries. These oocytes began dividing (meiosis I), but they were arrested in Prophase I. [only 400,000 remain at puberty] 2. Beginning at puberty & continuing until menopause, one or a few complete meiosis I, forming a large secondary (2o) oocyte & a very small 1st polar body (unequal cytokinesis). 3. 2o oocyte begins dividing by meiosis II, but is arrested in Metaphase II. The 2o oocyte is ovulated & enters oviduct. 4. Meiosis II will not be completed unless a sperm penetrates the 2o oocyte’s outer membrane. At completion of meiosis II, a large ovum & small 2nd polar body are formed. Sperm & ovum nuclei fuse to form zygote. 1st polar body may or may not divide. All polar bodies degenerate. Thus, only 1 functional cell results (ovum).

Comparison of Oogenesis & Spermatogenesis a woman is born with all the primary oocytes she is ever going to produce. [when she is 40 years old, her oocytes are 40 years old] oogenesis begins before birth, but is not completed until fertilization occurs (sperm stimulates completion of meiosis II). oogenesis is not a continuous process (halted twice - prophase I & metaphase II). women ovulate over short period of lifetime (puberty to menopause). She will ovulate ~ 400 secondary oocytes in her lifetime. Spermatogenesis: spermatogenesis begins at puberty & is a continuous process (spermatogonium -> sperm takes 74 days). sperm are produced from puberty till death.