Meiosis and Sexual Life Cycles http geneticsmadeeasy compreguntaspregunta

Meiosis and Sexual Life Cycles http: //geneticsmadeeasy. com/preguntas/pregunta 30. html

Heredity 4 Heredity: the transmission of traits from one generation to the next 4 Asexual reproduction: clones 4 Sexual reproduction: variation 4 Human life cycle: – – – 23 pairs of homologous chromosomes (46) 1 pair of sex and 22 pairs of autosomes gametes = haploid (1 N) Autosomes = diploid (2 N) fertilization (syngamy) results in a zygote 4 Meiosis: cell division to produce haploid gametes

Alternative life cycles 4 Fungi/some algae – meiosis produces 1 N cells that divide by mitosis to produce 1 N adults (gametes by mitosis) 4 Plants/some algae – Alternation of generations: • 2 N sporophyte produces 1 N spores, by meiosis • spore divides by mitosis to generate a 1 N gametophyte • gametes then made by mitosis which then fertilize into 2 N sporophyte

Meiosis 4 Occurs only in the reproductive organs (gonads) 4 Formation of gametes – spermatogenesis in the testes – oogenesis in the ovaries 4 Allows for variation in offspring 4 Preceded by chromosome replication (chromatid pairs) 4 Followed by 2 cell divisions (Meiosis I & Meiosis II) 4 Results: – 4 daughter cells – 1/2 chromosome number (1 N http: //www. sinauer. com/cooper/4 e/animations 1607. html

Prophase I (1 cell ) 4 Nucleus disappears, spindle fibers form, and the chromatid pairs visible. 4 Chromatid pairs find their homologous pair, forming a tetrad (bivalent). 4 Homologous chromosomes are 2 of the same type of chromosomes. – Crossing over - 2 homologous chromosomes switch pieces of DNA. – Chiasma site of crossing over

Metaphase I (1 cell ) 4 Tetrads meet at the middle of the cell.

Anaphase I (1 cell ) 4 The tetrads split apart. 4 Homologous chromosomes split

Telophase I (2 cells ) 4 Two cells form 4 Each have a full set of chromosomes as chromatid pairs.

Prophase II (2 cells ) 4 Chromatid pairs continue moving to the center of the cell.

Metaphase II (2 cells ) 4 The chromatid pairs meet at the middle.

Anaphase II (2 cells ) 4 The chromatid pairs split apart.

Telophase II (4 cells) 4 Each of the two cells split making a total of 4 cells. 4 Each cell contains 1/2 the normal number of chromosomes. http: //bcs. whfreeman. com/thelifewire/content/chp 09/0902002. html http: //www. sumanasinc. com/webcontent/animations/content/meiosis. html

Meiosis vs. mitosis 4 Synapsis/tetrad/chiasmata/ crossing-over (prophase I) 4 Homologous vs. individual chromosomes line up(metaphase I) 4 Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes. (anaphase I) http: //highered. mcgraw-hill. com/sites/0072437316/student_view 0/chapter 12/animations. html#

Origins of Genetic Variation, I 4 Independent assortment: homologous pairs of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II 4 Combinations possible: – 2 n (n the haploid number of the organism)

Origins of Genetic Variation, II 4 Crossing over (prophase I): – the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) 4 Random fertilization: – 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

http: //www. mhhe. com/biosci/genbio/biolink/j_explorations/ch 10 expl. htm Human chromosomal abnormalities 4 Caused by Nondisjunction – Failure of chromosomes to separate in meiosis 4 Down syndrome results from extra chromosome 21 – Trisomics an extra copy of an autosome – Monosomics one less copy of an autosome – Inc. maternal age inc. risk of occurence

Karyotypes 4 Pictures of homologous chromosomes lined up together. 4 Used to determine if there any chromosomal abnormalities.