Chromosomes and Cell Reproduction Chapter 6

chromosomes Section 6-1

Bacterial cells divide to reproduce  Bacteria reproduce by binary fission  Binary fission is a form of asexual reproduction that produces identical offspring  There are two stages:  DNA is copied  Bacteria divides

Eukaryotic cells form chromosomes  DNA is organized into individual units called genes  A gene is a segment of DNA that codes for a protein or RNA  As a cell preps to divide, DNA is copied and coils up into chromosomes  The two exact copies of DNA that make up a chromosome are called chromatids  The chromatids are attached at a point called a centromere

Chromosome Number and Structure Affect Development  Each human somatic cell (non-sex cell ) has 2 copies of 23 chromosomes  Each of the 23 pairs of chromosomes consist of 2 homologous chromosomes  Homologous chromosomes have a similar size, shape, and genetic content  Each homologue comes from each parent

Chromosomes  A somatic cell is diploid, because it contains 2 sets of chromosomes  A gamete, which has one set of chromosomes, is said to be haploid  The fusion of 2 haploid cells is called fertilization and results in a zygote or a fertilized egg

Chromosomes determine sex  Autosomes are chromosomes that aren’t involved in determining gender  The sex chromosome is 1 of 23 pairs of chromosomes and determines gender  Y = Boy, X= Girl  Males carry XY, females carry XX

Change in Chromosome number  The presence of all 46 chromosomes is essential for normal development

The Cell Cycle Section 6-2

The Cell Cycle  The cell cycle is a repeating sequence of cellular growth and division  A cell spends 90% of its time in the first 3 phases of the cycle called interphase  There are 5 phases of the cell cycle

Cell Cycle 1. First Growth (G1) phase: cells grow rapidly and carry out routine functions. If a cell isn’t dividing, it remains in G1 2. Synthesis (S) phase: DNA is copied. 3. Second Growth (G2) phase: Nucleus prepares to divide 4. Mitosis: nucleus of cell is divided 5. Cytokinesis: Cytoplasm divides

Cell Cycle is Controlled  The cell has checkpoints and feed back signals that trigger the next phase of the cell cycle  Proteins control the cell cycle in eukaryotes

Cell Cycle is Controlled  Cell Growth (G1) checkpoint: this checkpoint decides if the cell will divide.  Nerve and muscle cells remain in this stage and never divide  DNA Synthesis (G2) checkpoint: DNA replication is checked by repair enzymes and triggers mitosis  Mitosis Checkpoint: this triggers the exit from mitosis and signals the beginning of growth cycle

When Control is Lost  Certain genes make the proteins that regulate cell growth  Mutations can disrupt the regular function of these proteins  Cancer is the uncontrolled growth of cells  It is a disorder of cell division

Mitosis and Cytokinesis Section 6-3

In Mitosis, Chromatids are Pulled by Microtubules  During mitosis, the nucleus divides into two nuclei, each contains a complete set of chromosomes  Spindles are cell structures made of centrioles and microtubules that move chromosomes during division

Forming the spindle  Animal cells have centrioles that sit at a right angle to each other  Centrioles are replicated to make 2 pairs  Centrioles move to opposite poles of the cell and spindles form  Made of microtubules  Plant cells do not have centrioles but still form spindles

Separating chromatids by attaching spindle fibers  The spindles attach to the poles of the cell and to the centromere  The chromatids may now separate  The spindle fibers break down by the pole and “reel” the chromatids closer to the pole

Mitosis 1. Prophase: chromosomes coil up and become visible while spindles form 2. Metaphase: chromosomes line up at center of cell and the spindle fibers attach to the chromatids and poles 3. Anaphase: centromeres divide and chromatids move toward the poles 4. Telophase: a nuclear envelope forms around the chromosomes at the pole and spindles disappear

Cytokinesis  As mitosis ends, cytokinesis begins  Cytoplasm is divided in half and cell membranes grow to enclose each cell  Animal cells use proteins to pinch the membrane in half  Plant cells fuse vesicles together to make a cell plate and cell walls form on each side to allow cell to split