Regents Living Environment – Mr. Skolnick Chapter 21 – Meiosis & Sexual Reproduction

n This is an onion cell in interphase. n How can you tell that this is not prophase?

n Which stage is this? How can you tell it is not interphase? n Name all organelles you can see here.

Background Information n Sexual reproduction: when the chromosomes of 2 parents combine to produce offspring. n The chromosomes that combine during sexual reproduction are contained in special reproductive cells called gametes. n Sperm and Eggs are gametes.

n Meiosis is a type of cellular reproduction that produces gametes. n Differences between meiosis and mitosis: 1 Daughter cells produced from meiosis have half the normal chromosome number, and are not all alike. 2 Mitosis has one parent cell dividing to produce 2 identical daughter cells; meiosis has one parent cell divide 2 times to produce 4 daughter cells with half the normal chromosome number.

Chromosome Number n A cell with one complete set of chromosomes (has all the information needed for organism to function properly) is called a haploid (n) cell. n Gametes are haploid cells. In humans, gametes have 23 chromosomes each. n Except for gametes, all your body cells contain 2 complete sets of chromosomes (46).

n Any cell that contains 2 complete sets of chromosomes is called a diploid (2n) cell. n When an egg cell and a sperm cell of the same type of organism join to produce a new individual, the process is called fertilization. n The zygote is the single cell that results from fertilization. n It has 2 complete sets of chromosomes, one from each gamete.

n The chromosomes in the zygote exist in pairs. n For every chromosome that was in the egg, there is a matching chromosome from the sperm. n Matching pairs of chromosomes in a diploid cell are called homologous pairs.

n Illustration: the process by which a single parent diploid cell (both homologous chromosomes) divides to produce four daughter haploid cells (one homologous chromosome of the pair). n Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. n Meiosis involves a reduction in the amount of genetic material.

n Meiosis comprises two successive nuclear divisions with only one round of DNA replication. n Four stages can be described for each nuclear division.

First division of meiosis: n Prophase 1: Each chromosome duplicates and remains closely associated. These are called sister chromatids. Spindle forms as centrioles move apart. n Synapsis=Each pair of sister chromatids lines up with its homologous pair, forming a tetrad.

Interphase

Prophase I

n Metaphase 1: Homologous chromosomes align at the equatorial plate. Crossing-over can occur during this stage. n Anaphase 1: Homologous pairs separate with sister chromatids remaining together.

Metaphase I

Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

n Interkinesis: This very brief stage in between meiosis I and meiosis II finds the genetic material in the form of chromatin surrounded by a nuclear membrane. n What was once one cell in Prophase I is now two cells with the genetic material divided evenly between the two cells. n All following steps occur in two cells at the same time. Each cell now only has one copy of each chromosome. These cells are haploid.

Second division of meiosis: n Prophase 2: – DNA does not replicate.

Metaphase 2: Chromosomes align at the equatorial plate. Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.

Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

Summary of Meiosis n One parent cell produces four daughter cells. n Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different. n Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.

Sexual Reproduction in Simple Organisms n Advantages of Sexual Reproduction: n Offspring are not identical to either parent. How is this good? n Increasing the amount of variation in members of a species increases the possibility that some individuals are more likely to survive environmental changes.

Conjugation n Simplest type of sexual reproduction: takes place in protists and other simple organisms. n No distinct sexes: there are usually different mating types or strains. (+,-) n A bridge of cytoplasm forms between two cells, and an exchange or transfer of nuclear material takes place. n Conjugation in Bacteria: mating types are donor and recipient. n Conjugation in Spirogyra (type of algae): mating types are: active and passive. n Conjugation in Paramecia: mating types are: +,-

Sexual Reproduction in Animals n Reproductive Systems – Two sexes: male and female. – ♂ and ♀ gonads (sex organs): produce gametes. n Hermaphroditism – No separate sexes: each individual has both testes and ovaries. – Examples: earthworms, snails, and hydras. – Self-fertilization is rare: individuals exchange sperm with other individuals.

Gametogenesis n Meiosis in females and males: – Spermatogenesis – Oogenesis

Fertilization and Zygote Formation n Sperm (♂) + ovum (♀) = zygote n n 2n n For fertilization to take place, there must be a fluid medium so that the sperm can swim to the egg. n External fertilization: the gametes fuse outside the body of the female. – Takes place in animals that breed in water. – To overcome the hazards, large numbers of eggs and sperm are released. – Examples: aquatic invertebrates, most fish (but not sharks), and many amphibians. n Internal fertilization: the gametes fuse inside the body of the female. – Found most often in animals that reproduce on land, and some aquatic animals, such as sharks and lobsters. – Fewer eggs are needed because they are well protected.

n Parthenogenesis: development of an unfertilized egg into an adult animal without fertilization. n Examples: many insects, such as bees, wasps, aphids, and ants. n Bees: – Unfertilized eggs → male drones. – Fertilized eggs → female workers or queens.