Big Idea 3 Essential knowledge 3.A.2 (c) Ch. 13 - Meiosis Big Idea 3 Essential knowledge 3.A.2 (c)

Overview Each of you have specific traits that have been given to you by your parents (hair color, height, etc) The instructions for each trait are located on chromosomes (tightly coiled strands of DNA), which are found in the nucleus of your cells –do you remember how many you have in each nucleus of every cell? The DNA that makes up your chromosomes is arranged in segments called genes. Each chromosome has hundreds or thousands of genes, each of which play a role in determining the characteristics and functions of the cell

Diploid vs. haploid review Diploid – you have 2 complete sets of every genes (two complete sets of the chromosomes) Ex – 2 copies of chrom. 1, 2 copies of chrom. 2, etc. All of your body cells are diploid (2n) Haploid – one set of all the chromosomes All of your sex cells When haploid cells combine, they form a diploid zygote, which eventually forms you

Meiosis - Overview Gametes (your sex cells) are formed during a process called meiosis (remember, body –somatic- cells are formed during mitosis) Meiosis is a type of cell division that reduces the number of chromosomes by half – from 2n to n (sometimes called a reduction division) It occurs in the reproductive structures of organisms that reproduce sexually It involves two consecutive (back-to- back) cell divisions called meiosis I and meiosis II It makes 4 different daughter cells (instead of 2 identical cells)

Meiosis - Overview Two gametes combine in a process called fertilization This forms a zygote The zygote goes through mitosis, which eventually forms a new organism That new organism forms gametes, which undergo meiosis

Meiosis I Very similar to mitosis Interphase – happens BEFORE meiosis; DNA is replicated (copied) G1, S, and G2 phases

Meiosis 1 cont. Prophase I – the copied chromosomes become visible and form sister chromatids , which thicken and condense and form homologous pairs in a process called synapsis. During synapsis, a process called crossing over occurs; this is when segments of chormosomes are exchanged. Metaphase I – the pairs of chromosomes line up at the equator of the cell, spindle fibers attach to the centromeres (during mitosis single chromosomes line up, but in mitosis they line up in pairs) Anaphase I – the homologous chromosomes separate. Each member of the pair is pulled by spindle fibers to opposite poles of the cell (chromosome number is reduced from 2n to n); remember, during mitosis the sister chromatids split, but during meiosis the chromatids stay together Telophase I – the homologous chromosomes (made of two sister chromatids) reach the opposite poles; each pole contains only one member of the original pair

Meiosis 1 cont. Cytokinesis – usually occurs, and pinches in the cell to form a “cleavage furrow”; the cell may go into interphase again before the 2nd set of division, but the DNA isn’t replicated

Meiosis II Prophase II – spindle fibers appear and the chromosomes condense again Metaphase II – chromosomes move to the equator and spindle fibers are attached (during meiosis I it’s a diploid # of chromosomes, now it’s a haploid #) Anaphase II – the sister chromatids are pulled apart at the centromere by the spindle fibers and they move toward the opposite poles of the cell Telophase II – the chromosomes reach the poles and the nucleus and nuclear membrane reform Cytokinesis – the membranes pinch off and you are left with 4 haploid cells, each with n number of chromosomes and each different because of crossing over that occurred in meiosis I

Importance of Meiosis This process gives us our variation Why would variation by important?

Mitosis Vs. Meiosis Two divisions Four haploid (n) cells formed. Daughter cells are not genetically identical. Occurs in reproductive cells (gametes) Genetic diversity. One Division Two identical cells formed per cycle. Genetically identical daughter cells. Occurs only in body cells. Cell growth and repair

You are special snowflakes! Independent Assortment – This principle states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization basically, it states that you can get billions of possibilities of having your chromosomes sort the way they do– think of a dihybrid cross Crossing over – each of your chromosomes are going to be different because of this process Random fertilization – think of the chances of having 2 people meet, then having a specific sperm fertilize a specific egg, then make it successfully through cell division without any major problems to produce you!

Assignment Task: Create a cartoon drawing that shows the stages of meiosis. Requirements: Illustrations of the 8 stages and final products. A brief description of events taking place at each stage.