Key Biscayne Seagrass Field Trip Weds. March 14th (Rago’s classes) $15 cash reserves spot– get 2 permission slips MAX of 70 students Must go in to water up to waist with CLOSED TOED SHOES

Meiosis and Genetic Variations (still Ch. 8 notes – Sections 8.12-8.23) *MEIOSIS ONLY OCCURS IN PRODUCTION OF GAMETES!

GENETICALLY IDENTICAL OFFSPRING! Asexual Reproduction Many single-celled organisms reproduce by splitting, budding, binary fission, etc. GENETICALLY IDENTICAL OFFSPRING! Some multicellular organisms can reproduce asexually, produce clones.

Sexual reproduction Fusion of 2 gametes (fertilization) to produce a single zygote. Introduces greater genetic variation, allows genetic recombination. With exception of self-fertilizing organisms (e.g. some plants), zygote is a combination of gametes from 2 different parents.

Human Chromosomes ALL human body (somatic) cells have 23 pairs of homologous chromosomes 22 pairs of autosomes 1 pair of sex chromosomes EXCEPTION: GAMETES! (reproductive cells)

Fig. 11.3, p.171

Review of Chromosome Numbers Gametes (sperm/ova cells): Each gamete is haploid (half the full set of chromosomes) contains 22 INDIVIDUAL autosomes and 1 sex chromosome; Haploid number in humans “n” = 23 Fertilization: results in zygote with 2 haploid sets of chromosomes - now diploid. Diploid cell; 2n = 46. (n=23 in humans) Most cells in the body produced by mitosis. Only gametes are produced by meiosis!

sex chromosome combinations possible in the new individual diploid germ cells in female diploid germ cells in male meiosis, gamete formation in both female and male: eggs sperm X × Y X × X fertilization: X X X XX XX Y XY XY sex chromosome combinations possible in the new individual Fig. 11.2, p.170

Chromosome numbers: All are even numbers – diploid (2n) sets of homologous chromosomes! All even, as all are diploid, contain pairs of chromosomes. Ploidy = number of copies of each chromosome. Diploidy

Chromosomes Combining Once fertilization occurs, cells begin dividing and reproducing through MITOSIS. But how are the HAPLIOD gametes formed? THROUGH MEIOSIS!

Meiosis – key differences from mitosis Meiosis reduces the number of chromosomes by half. ALL daughter cells produced are genetically different from parent and each other. Meiosis involves two divisions, Meiosis I and Meiosis II. Meiosis I involves: Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids). In Metaphase I, homologous pairs line up at metaphase plate. In Anaphase I, sister chromatids do NOT separate. Overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome.

Homologues Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues.

KEY DIFFERENCE! Homologous pairs separate first

CROSSING OVER!

Sister Chromatids separate at centromeres

Meiosis 1 First division of meiosis Prophase 1: Each chromosome duplicated and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. Metaphase 1: Homologous chromosomes pair up and align in the middle. 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 middle of cell. Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. Telophase 2: Cell division is complete. Four haploid daughter cells, all genetically different, are formed.

Mitosis vs. Meiosis Just meiosis! Comparing Mitosis and meiosis

Meiosis creates genetic variation During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations

Independent assortment

In humans… e.g. 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!

Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!! Mitosis after fertilization Fertilization envelope

Crossing over Chiasmata – sites of crossing over synapsis- Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes.

Alterations in chromosome number and individual chromosomes Many mutations can occur during mitosis or meiosis that will affect the chromosome numbers or alter the information on individual chromosomes. Mutations can be harmful or beneficial to the organism Types of mutations include: Nondisjunction Deletion Duplication Inversion Translocation

Trisomy 21- Cause of Down Syndrome Nondisjunction animation

Various animations showing chromosomes alterations

Review Questions  What happens as homologous chromosomes pair up during prophase I of meiosis? How does metaphase of mitosis differ from metaphase I of meiosis? What is the sole purpose of meiosis? What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis?

Compare mitosis and meiosis on the following points: a. number of daughter cells produced. b. the amount of DNA in the daughter cells in contrast to the original cell. c. mechanism for introducing genetic variation. 6. What is a zygote and how is it formed?

SHOW ME MEIOSIS! Using your phone and your magnetic chromosomes, show me ALL of meiosis, beginning with Interphase and ending with cytokinesis after meiosis 2. Take a picture of the chromosomes at every stage. Use chalk to draw nuclear envelopes, spindles and centrioles. Use labels in photos for phase names. When finished, raise your hand for me to check your photos for completion. IF IT’S INCORRECT you need to DELETE photos and start AGAIN!

Onion Root Tip Visuals of Cells undergoing mitosis Data Table % of Cell Cycle = # of cells/total # of cells Phase # of Cells on Card % of Cell Cycle Interphase Prophase Metaphase Anaphase Telophase TOTALS: