Reproduction BSCS Ch 12

Reproduction The ability to reproduce is one of the unifying characteristics of all living things. Sexual reproduction produces offspring that are genetically different from their parents. Asexual reproduction produces offspring genetically identical to their parent.

Asexual Reproduction Fission, budding, and fragmentation are some of the mechanisms that allow organisms to reproduce asexually. Asexual reproduction allows an organism to rapidly produce many offspring without the time and resources committed to courtship, finding a mate, and mating.

In sexual reproduction new individuals are produced by the fusion of gametes to form a zygote. Sperm are male gametes. Ova (ovum singular) are female gametes. Sexual reproduction offers the benefit of generating genetic variation among offspring, which enhances the chances of the population's survival. Costs of this process include the need for two individuals to mate, courtship rituals, as well as a number of basic mechanisms described later. Sexual Reproduction

Human Male Reproductive System Human reproduction employs internal fertilization. Gonads are sex organs that produce gametes. Male gonads are the testes, which produce sperm and male sex hormones.

Testes are suspended outside the abdominal cavity by the scrotum, a pouch of skin that keeps the testes away from the body to be at an optimal temperature for sperm development. Seminiferous tubules are inside each testis, and are where sperm are produced. Sperm production begins at puberty with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature and are stored.

Sperm pass through the vas deferens which connects to a short ejaculatory duct that connects to the urethra. The urethra passes through the penis and opens to the outside. Secretions from the seminal vesicles add fructose and prostaglandins to sperm as they pass. The prostate gland secretes a milky alkaline fluid. The bulbourethral gland secretes a mucus-like fluid that provides lubrication for intercourse. Sperm and secretions make up semen.

Human Female Reproductive System Female gonads are the ovaries, which produce eggs (ova) and female sex hormones. The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. Each egg begins oogenesis as a primary oocyte.

A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of eggs. After puberty the ovary cycles between a follicular phase (maturing follicles) and a luteal phase (presence of the corpus luteum). The ovarian cycle lasts usually 28 days. During the first phase, the oocyte matures within a follicle. At midpoint of the cycle, the oocyte is released from the ovary in a process known as ovulation.

The secondary oocyte passes into the oviduct (fallopian tube or uterine tube). The oviduct is connected to the uterus. The uterus has an inner layer, the endometrium, in which a fertilized egg implants. At the lower end of the uterus the cervix connects the uterus to the vagina. The vagina is the birth canal.

The Menstrual Cycle Menstrual cycles vary from between 15 and 31 days. The first day of the cycle is the first day of blood flow (day 0) known as menstruation. During menstruation the uterine lining is broken down and shed as menstrual flow.

Around day 14 (midcycle) hormones trigger the maturation and ovulation of an egg. If pregnancy does not occur, the drop in hormones causes the sloughing off of the inner lining of the uterus by a series of muscle contractions of the uterus.

Chromosome Numbers All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number. These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined. The gametes contain the haploid number (n) of chromosomes.

Chromosome Numbers in Some Species (Diploid) Buffalo60 Cat 38 Human46 Dog 78 Horse64 Apple 34 Potato 48

Gamete Formation: Meiosis During the formation of gametes, the number of chromosomes is reduced by half (haploid), and returned to the full amount (diploid) when the two gametes fuse during fertilization. Chromosomes that carry the same genes are termed homologous chromosomes.

Meiosis Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells.

Meiosis I Interphase: Before meiosis begins, genetic material is duplicated. First division of meiosis –Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage.

Crossing Over When chromosomes pair in the early prophase of the first division of meiosis (Meiosis I), a crossover occurs between two non-sister chromatids. This results in an exchange of genetic material between the maternal and paternal chromosomes. The drawing provided shows a crossover between the non-sister chromatids of two, homologous chromosomes.

Crossing Over Animation

–Metaphase 1: Homologous chromosomes align at the equatorial plate. –Anaphase 1: Homologous pairs separate with sister chromatids remaining together. –Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Meiosis II Second division of meiosis: Gamete formation –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.

Meiosis Animation

Meiosis and Spermatogenesis During meiosis, 4 sperm are produced by the two nuclear divisions. These four sperm each have the haploid number of chromosomes. The sperm are equal in size.

Meiosis and Oogenesis Only one egg is produced by meiosis. The ova need to contain a lot of nutrients (to get the embryo through its first set of divisions), so these cells need to be big. So, the cytoplasm is not divided equally between the 4 resulting cells. The other cells are called polar bodies and they break down and disintegrate.

Prokaryotic Reproduction Prokaryotes are much simpler in their organization than are eukaryotes. The usual method of prokaryote cell division is termed binary fission. The prokaryotic chromosome is a single DNA molecule that first replicates, then attaches each copy to a different part of the cell membrane. When the cell begins to pull apart, the replicate and original chromosomes are separated. Following cell splitting there are then two cells of identical genetic composition

Binary Fission

Sexual Reproduction in Plants Many plants reproduce sexually. The advantage to the plant is that its offspring have a selection of genes from two parents, so each individual's genes are different. The offspring are not identical, and there is variety in the species.

Plant Reproductive Structures Flowering plant's sexual organs consist of: 1.Male: the stamen, or male sex structure, consisting of a filament and a pollen- bearing anther at the tip 2.Female: the pistil or female sex structure, consisting of ovary and ovule, style, and stigma at the tip. (The pistil is also sometimes called the carpel.)

Pollination An insect or the wind carries pollen grains from the anther of another flower. The pollen grains land on the stigma and a pollen tube grows down through the style to the ovary. The nucleus of the pollen grain passes down the tube. It fertilizes the egg cell inside the ovule. The fertilized egg cell develops into an embryo. The ovary becomes the fruit and the ovule becomes a seed - from which (once dispersed) the offspring plant will grow.