CHAPTER 9 MEIOSIS AND SEXUAL REPRODUCTION

9.1 INTRODUCING ALLELES When an orchid or aphid reproduces all by it self, what sort of offspring does it get? By the process of asexual reproduction, one alone produces offspring, and each offspring inherits the same number and kinds of genes as its parent. Genes are stretches of chromosomes made of DNA molecules. The genes for each species contain all the heritable information necessary to make new individuals.

SEXUAL REPRODUCTION Inheritance gets far more interesting with sexual reproduction. The process involves: 1- Meiosis 2- Formation of gametes 3- Fertilization (union of the nuclei of two gametes). In most sexual reproducers, such as humans, the first cell of a new individual contains pairs of genes on pairs of chromosomes. Usually, one of each pair is maternal and the other paternal is origin (Figure 9.2).

By sexual reproduction, offspring inherit new combinations of alleles, which lead to variations in the details of their traits.

9.2 WHAT MEIOSIS DOES Meiosis is a nucleor division process that divides the parental choromosome number in half. Unlike mitosis, meiosis partitions chromosomes into two groups prior to cytoplasm division. Unlike mitosis, it is the first step leading to the formation of gametes. Male and female gametes (form by meiosis of germ cells), such as sperm and eggs, fuse to form a zygote and thennew individual. In most multicelled eukaryotes, cells that form in specialized reproductive structures or organs give rise to gametes. Figure 9.3 shows examples of where the cellular antecedents of gametes originate.

ORIGINATIONOF GAMETES

The chromosome number is the sum total of chromosomes in cells of a given type. If a cell has a diploid number (2n), it has a pair of each type of chromosome, often from two parents. Except for a pairing non-identical sex chromosomes. Each pair has the same length, shape, and assortment of genes, and they line up with each other at meiosis. We call them homologous chromosomes (hommeans alike), Figure 9.4.

HOMOLOGOUS CHROMOSOMES Body cells of humans have 23+23 homologous chromosomes (Figure 9.4). So do the germ cells that give rise to human gametes. After a germ cell finishes meiosis, 23 chromosomes-one of each type-will end up in those gametes. Meiosis halves the chromosome number, so the gametes have a haploid number (n).

9.3 VISUAL TOUR OF MEIOSIS

STAGES OF MEIOSIS/CONT

9.4 HOW MEIOSIS PUTS VARIATION IN TRAITS: CROSSING OVER IN PROPHASE I Prophase I of meiosis is a time of much gene shuffling. Reflect on Figure 9.6a, which shows two chromosomes condensed to threadlike form. All chromosomes in a germ cell condense this way. When they do, each is drawn close to its homologue. Molecular interactions stitch homologues together point by point along their length with little space between a chromatid of one chromosome and chromatid of its homologous partner. The two non-sister chromatids break at the same places along their length, then the two exchange corresponding segments; they swap genes.

KEY EVENTS OF PROPHASE I: CROSSING OVER Crossing over leads to recombination among genes of homologous chromosomes, and to variation in traits among offspring. Every crossover is a chance to swap slightly different versions of hereditary instructions for gene product.

POSSIBLE OUTCOMES FOR THE RANDOM HOMOLOGOUS ALIGHNMENT OF MERELY THREE PAIRS OF CHROMOSOMES AT METAPHASE I

9.5 FROM GAMETES TO OFFSPRING Gametes are not all the same in their details. Human sperm have one tail, opossum sperm have two, and roundworm sperm have none. Crayfish sperm look like pinwheels. Most eggs are microscopic in size, yet an ostrich egg inside its shell is as big as a football. A flowering plant s male gamete is just a sperm nucleus.

GAMETE FORMATION IN PLANTS Seasons vary for plants on land, and so fertilization must coincide with spring rains and other conditions that favor growth of the new individual. That is why life cycles of plants generally alternate between spore production. Plant spores are haploid resting cells, often walled, that develop after meiosis (Figure 9.8a). They originate in reproductive structures of sporophytes, or sporeproducing bodies. Pine trees, corn plants, and all other plants with roots, stems, and leaves are examples of sporophytes. Plant spores stay dormant in dry or cold seasons. When they resume growth (germinate), they undergo mitosis and form gametophytes, or gamete-producing haploid bodies. For example, female gametophytes form on pine cone scales. In their tissues, gametes form by way of meiosis, as chapter 27 explains.

GENERALIZED LIFE CYCLE OF PLANTS AND ANIMALS

GAMETE FORMATION IN ANIMALS In animals, germs cells give rise to gametes. In a male reproductive system, a diploid germ cell develops into a large, immature cell: a primary spermatocyte. This cell enters meiosis and cytoplasmic divisions. Four haploid cells result, and they develop into spermmatids (Figure 9.9). These cells undergo changes that include the formation of a tail. Each becomes a sperm, which is a common type of mature male gamete.

SPERM FORMATION IN ANIMALS

ANIMAL EGG FORMATION In female animals, a germ cell becomes an occyte, or immature egg. Unlike sperm, an occyte stockpiles many cytoplasmic components, and its four daughter cells differ in size and function (Figure 9.10). As an occyte divides after meiosis I, one daughter cell-the secondary occyte-gets nearly all the cytoplasm. The other cell, a first polar body, is small. Later, both of these haploid cells enter meiosis II, then cytoplasm division. One of the secondary oocyte s daughter cells develops into a second polar body. The other receives most of the cytoplasm and develops into a gamete. A mature female gamete is called an ovum (plural, ova) or, more often, an egg.

ANIMAL EGG FORMATION

9.6 MITOSIS AND MEIOSIS-AN ANCESTRAL CONNECTION

END OF CHAPTER 9