Science 9 6.1 - Meiosis

Objectives By the end of the lesson you should be able to: Describe the process of meiosis Compare and contrast meiosis and mitosis Explain why meiosis is needed

MEIOSIS Sexual reproduction – 2 parents – genetic differences The process of Meiosis results in the production of special cells called gametes Gametes have half the number of chromosomes as body cells

MEIOSIS Cell division occurs twice in Meiosis Once at the end of Meiosis I and again at the end of Meiosis II In Meiosis I, matching chromosomes called homologous chromosomes separate In Meiosis II, sister chromatids separate

MEIOSIS The process of meiosis shuffles genetic information and results in variation of the gametes Results in genetic diversity

IMPORTANT Terms: Haploid: half the total number of chromosomes found in gametes (in humans = 23) Diploid: total number of chromosomes found in body cells (in humans = 46)

Meiosis AKA: Germ cell division or sex cell division Purpose: to produce gametes (i.e. sex cells: sperm and egg) Products: 4 haploid cells each genetically different  this creates genetic variability

Fertilization 2 gametes come together; in humans an egg and sperm come together Each has 23 chromosomes, thus together there are 46 in the zygote The zygote will grow to become the developing embryo 23 + Gametes: why is the sperm smaller than the egg? 23 46 Zygote: will grow (by mitosis) into a baby in ~9 months

MEIOSIS: REDUCING CHROMOSOME NUMBER

Number of DNA STRANDS Sequence 46 92 23 All genetically different Parent Cell Daughter Cells

CHROMOSOME SEQUENCE Meiosis Video

Meiosis – Prophase I Chromosomes coil and become visible. Nuclear membrane disappears. Spindle fibers form. Homologous chromosomes move towards each other and pair.

Meiosis – Metaphase I Pairs of homologous chromosomes move to the midline of the cell. These pairs are aligned on either side of the metaphase plate.

Meiosis – Anaphase I Homologous chromosome pairs are pulled away from each other towards opposite ends of the cell. Paired chromatids remain attached.

Meiosis - Telophase I Paired chromatids may uncoil and become less visible. Nuclear membranes may form around each chromosome cluster. Spindle fibers disappear. Cytokinesis occurs.

Meiosis II Phase II of meiosis is the mitotic division of haploid cells. Although there will be four new non-identical cells at the end of meiosis II, all other processes for this division are similar to mitosis.

Meiosis II – Prophase II Paired chromatids coil. Spindle fibers form. Nuclear membrane disappears.

Meiosis II – Metaphase II Paired chromatids line up in the middle of the cell. The chromatids are attached to spindle fibers.

Meiosis II – Anaphase II Each pair of chromatids split at the centromere to form two independent chromosomes. Chromosomes move to opposite ends of the cell.

Meiosis II – Telophase II Chromosomes uncoil and become less visible. Nuclear membranes form around four new nuclei. Spindle fibers disappear. Cytokinesis occurs. Meiosis Square Dance

Meiosis - Cytokinesis Same process as in mitosis. Four non-identical cells are formed. These cells are called haploid cells (n). # chromosomes = half the # of chromosomes of parent cell.

Meiosis Events Meiosis I Matching chromosome pairs (homologous chromosomes)  move to opposite poles of the cell - two daughter cells result. Meiosis II Chromatids of each chromosome are pulled apart - the end result is four haploid cells, each with half the number of chromosomes. These develop into gametes.

CROSSING OVER Important event that occurs between each chromosome pair in meiosis I There is an exchange of segments of DNA and picks up new genetic information

INDEPENDENT ASSORTMENT This occurs when homologous pairs of chromosomes separate at the equator and move towards opposite poles of the cell. Video Mitosis vs Meiosis

FERTILIZATION There are more than 8 million combinations possible for these 23 pairs in any egg or sperm cell When fertilization occurs, 70 trillion different zygotes are possible from the combination of one sperm cell and one egg cell!

Gamete Formation Males produce 4 sperm for every round of meiosis Females produce one egg for each round of meiosis: 3 of the 4 “eggs” produced die off - called polar bodies All the cytoplasm, nutrients and energy gets put into one egg

Chromosome Mutations Sometimes occur spontaneously Chromosome changes during meiosis can cause changes in the genetic information. Parts of chromosomes can be inverted, deleted, duplicated or moved to another spot. Occur because of mutagens Chromosome changes, sometimes leading to genetic disease or death, can be caused by mutagens such as radiation or chemicals. Can occur if there is failed separation of chromosomes in meiosis Failed separation means that a gamete may end up with no chromosome or too many of a chromosome. Zygotes that result from these gametes rarely survive, and if they do, they will have serious genetic disorders.

CHROMOSOME MUTATION IN MEIOISIS

DIAGNOSING GENETIC DISORDERS A geneticist can prepare a different type of picture for individuals that show all chromosomes called karyotype

DIAGNOSING GENETIC DISORDERS By analyzing karyotypes, geneticists can determine whole chromosome mutations Can diagnose genetic disorders or syndromes Down syndrome – most frequent genetic disorder Caused by an extra 21st chromosome