Mitosis and Meiosis

Why do cells need to divide? -For repair For growth and development For survival (nutrients – diffusion) For reproduction For renewal (replace old worn out cells)

Cell division and the Cell Cycle The ability of organisms to reproduce their kind is one characteristic that best distinguishes living things from nonliving matter. The continuity of life from one cell to another is based on the reproduction of cells via cell division. This division process occurs as part of the cell cycle - the life of a cell from its origin in the division of a parent cell until its own division into two.

Where are all of the genes for different traits and what is the mechanism for passing these genes to the offspring?

Cell Division and Genetic Material Cell division requires the distribution of identical genetic material - DNA - to two daughter cells. What is remarkable, is the exactnesss with which DNA is passed along, without dilution, from one generation to the next. A cell’s genetic information, packaged as DNA, is called its genome. In prokaryotes (bacteria), the genome is often a single long DNA molecule. In eukaryotes, the genome consists of several DNA molecules. A human cell must duplicate about 3 meters of DNA and separate the two copies such that each daughter cell ends up with a complete genome.

23 chromosome pairs in a male human (karyotype)

Karyotyping -

Cell cycle Process for duplicating chromosomes and other cellular components Growth and nuclear division This is the mechanism that ensures that a complete set of genetic material is passed to daughter cells Nuclear division Mitosis

Cell Cycle

Binary Fission: Cell Division Without Mitosis or Meiosis – occurs in bacteria cells

Cell cycle and mitosis in animal cells

Real Pictures Anaphase Prophase Telophase Metaphase Picture adopted from: http://www.micro.utexas.edu/courses/levin/bio304/genetics/celldiv.html

Real Pictures Anaphase Interphase Prophase Telophase Metaphase This cell is shown, as this is how all cells look before mitosis. Please be aware that Interphase is a phase of the cell cycle, but NOT a stage of mitosis. Mitosis - Early ProphaseTo begin mitosis, the nuclear membrane breaks down, while the chromosomes shorten and thicken (here, a chromosome is two chromatids, bound at a point called the centromere, making an "X" shape). The other structures important for mitosis are also forming (i.e. the centrioles). Mitosis - MetaphaseThe mitotic spindle apparatus has now formed and lies on the poles of the nucleus (but remember, the nuclear membrane has broken down, so there is no distinctly delineated nucleus). The chromosomes are lined up along the cell's equator, also known as the equatorial plate, and are attached to the mitotic spindle apparatus via microtubules (to try and visualize the microtubules extending from the poles to the chromosomes on the equator, think of the Earth - it's as if rope was extending from the chilly north and south poles to the chromosomes basking in the sun at the equator). Here's the confusing part - When the individual chromatids (½ of the "X") are separated from the chromosome (the "X"), they are now each referred to as a chromosome (i.e. In metaphase, the chromosome, composed of two chromatids, separates into the individual chromatids, which are then renamed chromosomes, even though they were only one half of a chromosome only moments before!) - Whew! Mitosis - AnaphaseThe newly formed chromosomes (which were recently chromatids while they were still ½ of the "X") are pulled along the microtubules toward opposite poles of the cell (like Monarch butterflies migrating back to Canada and the southern tip of South America (toward the poles) from Mexico (near the equator). Mitosis - TelophaseThe chromosome have finished their migration to the poles and the mitotic structures breakdown. The plasma membrane of the cell pinches down along the equator creating two separate cells (similar to twisting a long balloon and forcing it to pinch in the middle). At this time, the chromosomes become indistinct (as they are during Interphase), the nuclear membrane forms again and the nucleolus reappears. Metaphase Picture adopted from: http://www.uoguelph.ca/zoology/devobio/210labs/mitosis1.html

Interphase Longest part of the cell cycle Chromosomes are decondensed at this point Chromosomes are replicated and cell growth occurs

Prophase Chromosomes condense Chromosomes are two sister chromatids attached at the centromere The centrosome has duplicated

Homologous chromosome pairs Sister chromatids Centromere Sister chromatids Centromere

Metaphase Mitotic spindle between two centrosomes is formed Chromosomes are aligned along the spindle and the nuclear membrane is broken down

Anaphase The centromere separates and the sister chromatids are pulled to opposite poles in the cell

Telophase The nuclear membrane begins to reform DNA begins to decondense

Cytokinesis The two cells divide One parent cell becomes two daughter cells

Mitosis

Diploid vs. Haploid Haploid = n = half of the necessary genetic info needed to make an organism 2(n) = diploid, complete set of genetic material Homologous chromosomes are 2(n) Humans have a diploid number of 46!

Diploid vs. Haploid Cells that are haploid are: Specialized sex cells such as sperm and egg cells in animals, and pollen and egg cells in plants Cells that are diploid include: EVERYTHING ELSE!! -such as zygote, muscle, blood, skin, many plant cells

Meiosis A process similar to mitosis except that the end product is a haploid (n) gamete Generation of a haploid gamete ensures that there will not be too many chromosome copies in offspring There are two stages of meiosis (I and II)

Meiosis Diploid (2n)  haploid (n) Meiosis and FERTILIZATION = sexual reproduction.

Biological Relationship of Mitosis and Meiosis Zygote 2n ♀ Egg Gametes n Mitosis ♂ Sperm Growth Meiosis Cell Cycle

Meiosis I Differences from mitosis: -centromeres remain intact and pairs separate, not sister chromatids -The chromosome remains in the sister chromatid form

Meiosis II -Similar to mitosis except chromosomes start with sister chromatids from meiosis I. The sister chromatids are not made in interphase like in mitosis.

Meiosis II continued

Ploidy of gametes during meiosis Female Male

Meiosis

Comparison of meiosis and mitosis

Late Prophase I Metaphase I Anaphase I Telophase I Interphase II Prophase II Metaphase II Anaphase II Telophase II Cytokinesis

Syngamy – the union of two gametes

Chromosomes seemed to be the genetic material that carries the factors predicted by Mendel because: They exist in pairs They segregate One of each pair of homologues is inherited One of each pair is inherited from each parent They assort randomly in meiosis

Additional diagrams of Meiosis

Additional diagrams of Meiosis

Variation Meiosis results in variation amongst organisms Crossing over of the chromosomes during Prophase I Random rearrangement of chromosomes during Metaphase I and Anaphase I Random fertilization of gametes (sperm & egg)