Chromosomes Gene – segment of DNA that carries instructions for a specific trait. DNA in cells is usually spread out so transcription can take place. During cell division, chromosomes form. Chromosomes are DNA and protein (histones) coiled into a rod. After DNA replicates, two chromatids are formed that attach at a point called a centromere.

Each organism has a certain number of chromosomes.  Chromosomes occur in pairs called homologous chromosomes – each is same size and has same genes.  Diploid – 2N – cell that has two of each homologous chromosome Human is 46  Haploid – 1N- only 1 of a homologous pair 23 is Human

DIPLOID CELLS Diploid Cells = Cells with the full set of chromosomes Paired chromosomes Half of our chromosomes come from each parent (23 from each parent) Somatic (non-sex) cells are diploid Created by mitosis Ex: Skin, Muscle, Nerve, Blood Cells

HAPLOID CELLS Haploid Cells = Cells with ½ the total number of chromosomes Gametes (sex cells) are the only haploid cells Ex: Sperm, Egg, Pollen Created by meiosis Chromosome number reduced by 1/2

Somatic Cells - regular body cells (diploid in humans) Gametes - cells used in sexual reproduction, these cells are haploid. Why do gametes have only 23? if had 46, egg and sperm would unite during fertilization and new individual would have 92 Future generations would have an impossibly large amount.

Cell Division – Mitosis & Cell Division  Interphase – G1 – growth – cell doubles in size S - synthesis – DNA Replication G2 – growth Mitosis Prophase   Metaphase Anaphase Telophase Cytokinesis in plants,

Cell Division – Mitosis & Cell Division  Interphase – G1 – growth – cell doubles in size S - synthesis – DNA Replication G2 – growth Mitosis Prophase  chromosomes coil  nuclear membrane disappears  centrioles move to opposite ends  spindle fibers form Metaphase  chromosomes line up in the middle Anaphase  chromatids move towards opposite poles Telophase  nuclear membranes form Cytokinesis  cytoplasm splits and forms two new cells in plants, new cell wall forms

Interphase

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Crash Course: MITOSIS (11 min) STOP POINT MITOSIS

form of duplication using only mitosis. Asexual Reproduction form of duplication using only mitosis. produces only genetically identical offspring called clones it is rapid, allowing for quick spread of an organism Examples: plant cuttings, Amoeba, bacteria, Sexual Reproduction new individual creates by fusing two haploid cells (fertilization), creates variation Zygote – cell created when sperm and egg unite contains two sets of chromosomes

Meiosis converts a diploid cell into a haploid gamete. Before meiosis, cell goes through Interphase, duplicating its chromosomes

Meiosis I Prophase I : chromatid pairs pair up with their homologous pair, forming a tetrad.

Meiosis I Prophase I : chromatid pairs pair up with their homologous pair, forming a tetrad. Crossing over - pieces of two homologous chromosomes, switch places and end up on the other chromosome allows for genetic diversity

Meiosis I Metaphase I : Tetrads line up along the center. Independent assortment – homologous chromosomes line up randomly Anaphase I : tetrads split and move in opposite directions. Telophase I/Cytokinesis: Two cells form. End product of Meiosis I is two haploid (n) cells.

Meiosis II Prophase II : chromatid pairs move toward the center Metaphase II : The chromatid pairs line up along the center of the cell. Anaphase II : The chromatid pairs split moving the individual chromosomes toward the opposite poles. Telophase II/Cytokinesis : Each of the two cells splits. All are haploid

The haploid cells may develop into gametes The haploid cells may develop into gametes. In most male animals, all four cells develop into sperm. In females, one develops into an ootid, and three into polar bodies.