Sexual Reproduction and Meiosis Reproduction • Organisms can reproduce asexually mitosis, fission, & budding • Organisms can also reproduce sexually – Two sex cells (usually egg and sperm) join together – Males produce sperm, females produce eggs • Sex cells do not have pairs of chromosomes • They are called haploid (single- not paired up: sometimes represented as the letter N). • They only have half the number of chromosomes as body cells. • 23 total chromosomes • All other body cells are diploid (paired up: 2N). – Humans have 23 pairs of chromosomes – 46 chromosomes total • When an egg and sperm join together, it is called fertilization. • The resulting cell is called a zygote • The zygote will begin to undergo cell division http://www.exploratorium.edu/imagingstation/gallery.php?Asset=Zebrafish%20development&Group=&Cat egory=Development&Section=Introduction • The zygote formed following fertilization has a diploid set of chromosomes (46 total). • Half (23) from mom, half (23) from dad. • Sexual reproduction occurs when the genes of two parents are combined to form an offspring. • The offspring is a unique genetic combination of its two parents. So how do we get specialized sex cells with only half the chromosomes? Meiosis • Meiosis is a process that produces haploid sex cells. Why does meiosis happen? • Meiosis allows for the production of sex cells so sexual reproduction can occur. • In meiosis I, the nucleus divides and produces two new cells with one duplicated chromosome • Exactly like mitosis. • In meiosis II, the division happens again and the sister chromatids separate, producing four cells with half (haploid) the number of chromosomes of the original nucleus. http://www.youtube.com/watch?v=kVMb4Js99tA • The end result of meiosis: – A cell undergoes division twice. – This produces four sex cells, with half the number (haploid) of chromosomes of the original cell Mitosis vs. Meiosis http://www.youtube.com/watch?v=Ba9LXKH2ztU Asexual vs. Sexual reproduction • Asexually produced organisms are genetically identical to their parent. • Sexually produced organisms are genetically different from their parent. – This produces unique organisms who may be more (or less) resistant to disease, more (or less) intelligent, more (or less) physical abilities, adaptable to environment, etc. – There is variety among the offspring.