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 chromatids. Each half is a chromatid. • The point where each chromatid crosses or attaches to the other chromatid is called the centromere. • Chromatids only occur during S phase of interphase. • 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. • Karyotype – a map of the chromosomes.
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. • Animals only have centrioles from the centrosomes. Pg. 150 • Spindle fibers form between the two centrioles. • Nuclear membrane & nucleolus break down and disappear.
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 chromatids in half. • Now each chromatid is at opposite ends.
Telophase • 4th phase of mitosis • The chromatins reach the opposite poles of the cell. • The spindle fiber & centrioles disappear • The chromatids 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. • Mitosis Video
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. • Meiosis Video
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 prophase/metaphase of Meiosis I. This causes a mixing of genetic material. • Independent assortment – is the random separation of chromosomes during prophase/metaphase of Meiosis I. This causes genetic variation of maternal and paternal chromosomes. • Cell Division Video • Mitosis vs. Meiosis