Sexual Reproduction Science 9, 2016

Introduction Usually involves two individuals Doesn’t always have to involve a male and a female Occurs in species we may not think of having definite male and female individuals (plants and coral) Scientific Definition Is the union of two sex cells (gametes) to produce a new individual

Sex Cells Gametes from females are called eggs Gametes from males are called sperm Union of the sperm cell with the egg cell is called fertilization

Meiosis Cell division process that produces gametes Divided into two processes Meiosis I: Chromosomes are duplicated and then cell divides Meiosis II: Cell divides again without chromosomes being duplicated Result of meiosis is four daughter cells with half the number of chromosomes of the parent

Sexual Reproduction in Animals Most animals reproduce sexually Joining of the two gametes creates a new single cell known called a zygote Zygote has a full set of chromosomes and is the first cell of a new individual Zygote then divides into two cells, beginning a process called cleavage New multicellular life form (embryo) develops through continued mitotic division

Development of the Embryo Offspring will show some traits of male parent and some of female parent Although it may look more like one parent, it will not be identical to either parent Depending on the species, the embryo may develop differently For most mammals it happens inside the female For most other animals, it occurs in an egg

Critical Role of Meiosis During sexual reproduction, the male and female gametes unite to form a zygote Gametes are haploid They have half the number of chromosomes of a normal cell If the gametes were diploid (like every other cell) then the zygote would inherit twice as many chromosomes as it needs Incorrect number of chromosomes can cause problems with normal cell function and growth

Hermaphrodites Can produce both male and female gametes Usually mate with other individuals, but can fertilize themselves in times of environmental stress Example: Common garden worms and slugs

Sequential Hermaphrodites Born one sex but then become the opposite sex Finding Nemo: Clownfish travel in packs that consist of a large reproductive female, a smaller reproductive male, and numerous small, non reproductive males If the female dies or leaves, the reproductive male becomes a female and the largest of the non-reproductive male becomes the new reproductive male

Sexual Reproduction in Plants Ovules contain the female gametes of a plant and are found in the pistil (female part of plant) Pollen contains the male gametes of the plant and is found on the stamen (male structure of plant) Most plants produce both female and male gametes However, some produce only male gametes and others only female

Pollination Pollination occurs when pollen is transferred from the anther of the stamen to the stigma of the pistil Self pollination: when the transfer is within the same plant Cross pollination: when the pollen of one plant is carried to the stigma of another by wind, water or animals like bees A gamete in the pollen grain and a gamete in an ovule join and a zygote is formed Zygote undergoes many cell divisions to produce an embryo Embryo is produced inside a protective covering called a seed

Advantages of Sexual Reproduction Offspring produced through sexual reproduction show variation because they have a mix of traits from both parents Variation helps a species survive by giving it the ability to adapt to a changing environment

Disadvantages of Sexual Reproduction Requires a great deal amount of energy Flowering plants produce lots of pollen to be blow by the wind The embryo requires nurturing Limits the number of offspring