Ch. 8 Cell Reproduction What is cell division? Mitosis What is a zygote? It forms when the male and female gametes unite. All cells come from preexisting cells DNA is inside a rod-shaped structure called a? Chromosomes

Chromosomes Every chromosome consists of two identical parts called sister chromatins. Each half is a chromatid. The point where each chromatid crosses or attaches to the other chromatid is called the centromere. Every cell has a certain # of chromosomes in the cell. (Pg. 146) Ex. Humans=46, chicken=78, cat=32, chimp=48, dog=78 Pg. 145  Histones are proteins that DNA wraps around in eukaryotic cells.

Why do all organisms have an even # of chromosomes? Chromosomes are in pairs called homologous chromosomes. A diploid (2N) cell has both chromosomes that make up the homologous pair. A haploid (1N) cell has only 1 half of the homologous pair. Sex chromosomes determine the sex of the offspring (X or Y). Autosomes are all the chromosomes except the sex chromosomes. We have 44 or 22 pair.

Cell Cycle 3 parts to the cell cycle 1. Interphase – resting & growth stage (3 parts – G1, S, & G2) 2. Mitosis – cell division (4 parts) 3. Cytokinesis- produces 2 daughter cells Pg. 149 There are approximately 100 trillion cells in an average human. Some cells divide every 20 to 30 minutes in a animal embryo.

4 Stages of Mitosis Prophase – 1st stage of mitosis Chromosomes start to coil and shorten. Centrioles form and migrate to each end. Spindle fibers form between the two centrioles.

Metaphase 2nd phase of mitosis The chromosomes are attached to the spindle fibers. The spindle fibers arrange the chromosomes in a line at the middle of the cell.

Anaphase 3rd stage in mitosis The centromeres are pulled apart as the spindle fiber shortens and separates the sister chromatins in half. Now each chromatin is at opposite ends.

Telophase 4th phase of mitosis The chromatins reach the opposite poles of the cell. The spindle fiber & centrioles disappear The chromatins uncoil The nucleolus reappears and a new nuclear envelope forms around each set of chromosomes.

Cytokinesis The plasma membrane pinches off and 2 new identical cells are formed. The 2 new cells are called daughter cells.

Interphase(1st part of the cell cycle) Resting and growth stage The chromatins are duplicated in order to form a sister chromatin, which is two identical chromatins. The cell is preparing to undergo mitosis.

Meiosis Reduction of Division  Meiosis Meiosis I produces 2 daughter cells, each with half the homologous chromosome. Go from one cell with 46 chromosomes (2N) to two daughter cells with 23 (1N) chromosomes. Draw. Pg. 154-155 Meiosis II occurs after meiosis I. The 2 daughter cells divide to produce 4 haploid cells. These haploid cells are gamete cell, either egg cells or sperm cells.

Differences in Meiosis vs. Mitosis 1. Form synapsis in prophase – homologous chromosomes pair up. Each pair of homologous chromosomes is called a tetrad. 2. Crossing-over occurs when the chromatids twist and reattach to the opposite homologous chromosome in the tetrad. Pg. 154. 3. The outcome of Meiosis I is 2 new cells with half the number of chromosomes as the original cell, but the 2 new cells have two copies of the chromosomes because of replication before Meiosis I. Meiosis II does not have replication, so the final product is 2 new cells (4 total now) with half the chromosomes (haploid – 1N).

Genetic recombination is the result of crossing-over during metaphase of Meiosis I. This causes a mixing of genetic material. Independent assortment – is the random separation of chromosomes during anaphase of Meiosis I. This causes genetic variation of maternal and paternal chromosomes.