Meiosis Bellwork l Why do your siblings not

Meiosis

Bellwork l Why do your siblings not look exactly like you or your Mom or Dad?

Objectives l Explain the difference between asexual and sexual reproduction l Explain how meiosis creates haploid cells l Describe how sexual reproduction creates genetic variation in the offspring

Key Terminology l Asexual reproduction l Sexual reproduction l Meiosis l Homologous chromosome l Sister chromatid l Crossing over l Independent assortment of chromosomes

Asexual Reproduction Involves mitosis – duplication of cells Offspring formed from a single parent without uniting egg and sperm l Typically, a piece of parent is broken off and grows into a new organism by means of mitotic division l l Ex. prickly pear, African violets, some microorganisms Organisms produced by asexual reproduction are genetically identical l Considered genetic clones

Think-Pair-Share Speculate why you might not want a whole greenhouse of plants that are genetic clones. PROPERTY OF PIMA COUNTY JTED, 2010 3

Individual Variation l Why are you not a clone of your biological parents? l What gives rise to that individual variation?

Sexual Reproduction l Asexual l reproduction Via mitosis only produces clonesgenetically identical offspring l Sexual Reproduction – enables the shuffling of DNA among individuals l produces genetically unique offspring l

Think-Pair-Share Speculate why sexual reproduction is important to the survival of a speciesincluding Humans? PROPERTY OF PIMA COUNTY JTED, 2010 3

Sexual Reproduction l Organisms that reproduce by sexual reproduction l l Human somatic (body) cells l l l alternate between diploid (2 of each chromosomes) and haploid number (1 of each chromosome) contain 46 chromosomes 23 homologous pairs; 2 n =46; diploid Human gametes(egg or sperm) l l contain 23 chromosomes n=23; haploid

Sexual Reproduction (cont. )

Meiosis l Involves the production of haploid cells with unpaired chromosomes l Meiosis comes from the Greek word to diminish

Think-Pair-Share Speculate why you would not want the egg and sperm to be diploid (have a pair of each chromosome). PROPERTY OF PIMA COUNTY JTED, 2010 3

Homologous Chromosomes Two chromosomes that make up a matched pair l One member of the pair donated from the father l One donated from the mother l

Sister Chromatids l Sister chromatidsl an identical duplicated copy of the chromosome donated by the father or by the mother

Meiosis –divided into two major phases l Meiosis I – separate homologous chromosomes l Meiosis II – separate sister chromatids l Each phase is divided into stages similar to mitosis – prophase, metaphase, and telophase

Meiosis I l Interphase l l l duplication of chromosomes Enter into Meiosis I Prophase I l l l Homologous chromosomes line up side by side Chromosomes condense Crossing over event occurs l l exchange segments between maternal and paternal chromosomes Disappearance of nuclear envelope

Meiosis I (cont) l Metaphase I l l Homologous chromosomes line up at equator of cell Homologous chromosomes face opposite poles l What pole the chromosomes face is random l known as independent assortment of chromosomes

Meiosis I (cont) l Anaphase l l I Homologous chromosomes separate At end of anaphase I, each pole has only one member of homologous chromosome (donated by either father or mother)

Meiosis I (cont) l Telophase l l I and cytokinesis Nuclear envelope reappears Cytokinesis occurs

Meiosis II l Little or no intervening interphase between meiosis I and II l Sister chromatids separate l identical to mitosis

Meiosis

Sexual Reproduction Promotes Genetic Variation l Independent assortment of chromosomes = 2 n l l n = number of chromosomes humans 223 = 8 million possible chromosome combinations Crossing over during prophase I l Random Fertilization l l l fusion of egg and sperm to form the zygote 223 x 223 = 70 trillion combinations

3 Minute Paper You have three minutes to write down 2 key points of today’s lesson. Be prepared to share. PROPERTY OF PIMA COUNTY JTED, 2010 24