An Introduction to Meiosis

There are two types of reproduction: sexual and asexual (some organisms might involve both in their life cycles) During asexual reproduction, the organism inherits all of its chromosomes from a single parent – so the offspring is 100% genetically identical to the parent Ex -Bacteria, hydra, starfish Sexual reproduction involves the joining of sex cells (doesn’t necessarily require two parents, since some can self-fertilize) What benefits and problems would each form of reproduction have?

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 (a gene is a small piece of DNA that codes for a specific trait). Each chromosome has hundreds or thousands of genes, each of which play a role in determining the characteristics and functions of the cell

Each of your body cells have 46 chromosomes Each parent gives 23 chromosomes, making 23 pairs (46 individual strands) The chromosomes that make up a pair (one from each parent) are called homologous chromosomes They are the same length and have the same centromere position (holding them together) and they carry genes that control the same inherited traits Ex – the gene for earlobe type will be located at the same position on both homologous chromosomes); they would each code for earlobe type, they may not code for the same TYPE of earlobe

Gametes are sex cells (like an egg or sperm) that have half the number of chromosomes as our regular body cells– in humans each gamete has 23 chromosomes (instead of 46). The symbol n can be used to represent the number of chromosomes in a gamete A cell with n number of chromosomes is called a haploid cell (haploos means single in greek) When one haploid gamete combines with another it’s called fertilization – when that happens, the cell will now contain a total of 2n chromosomes (n from the female and n from the male parent) A cell that has a 2n number of chromosomes is called a diploid cell

Gametes (your sex cells) are formed during a process called meiosis (remember, body 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)

Very similar to mitosis Interphase – happens BEFORE meiosis; DNA is replicated (copied) Prophase I – the copied chromosomes become visible and form sister chromatids (draw and label them); they 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

Cytokinesis – usually occurs, but not always (remember, during the cell cycle cytokinesis isn’t really a part of mitosis) pinches in the cell; the cell may go into interphase again before the 2 nd set of division, but the DNA isn’t replicated

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

It gives us genetic variation/diversity

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

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. Materials: Textbook page 273, paper, colored pencils.