Key Concepts What events take place during the three stages of the cell cycle? How does the structure of DNA help account for the way in which DNA copies itself?

 Cell cycle  Interphase  Replication  Mitosis  Chromosome  cytokinesis

 Cell Cycle – the regular sequence of growth and division that cells undergo ◦ Cells grow, divide into 2 cells called daughter cells  Interphase is the period before cell division where the cell grows, makes a copy of its DNA, and prepares to divide into 2 cells

Growing – during first part grows to full size and produces structures it needs. – Makes copies of both mitochondria and chloroplasts Copying DNA – makes exact copy of DNA in a process called replication (each daughter cell must have a complete set of DNA Preparing for division- cell produces structures it will use to divide into 2 cells

 The stage during which the cell’s nucleus divides into 2 new nuclei  One copy of the DNA is distributed to each daughter cell  Mitosis has 4 parts or phases: ◦ Prophase ◦ Metaphase ◦ Anaphase ◦ Telophase

 Chromatin condenses to form chromosomes  Chromosomes are double-rod structures. They are double because the DNA in the cell has replicated ( made a copy of itself)  Each identical rod is called a chromatid  The two chromatids are  held together by  a centromere

 Chromosomes line up across the center of the cell.  Each chromosome attaches to a spindle fiber at centromere

 Centromeres split. 2 chromatids separate and are drawn by spindle fibers to opposite sides of the cell  The cell stretches out

 Chromosomes stretch out. Lose rod-like appearance  New nuclear envelope forms around each region of chromosomes

 Final stage of cell cycle  The cytoplasm divides. The organelles are distributed into each of the 2 new cells  Starts at same time as telophase  Ends and the cell goes into interphase  In animals, cell membrane pinches in  In plants, a cell plate forms across the middle of the cell. Gradually develops into new cell membranes. Then new cell walls form around the membrane

 Structure was discovered through the work of 3 scientists:  Rosalind Franklin – used an x-ray method to photograph DNA molecules in 1952  James Watson and Francis Crick – used photographs to create a 3-D model showing the helical (spiral) structure of DNA in 1953

 Looks like a twisted ladder or spiral staircase  Sides are made of a sugar – deoxyribose alternating with a phosphate  Rungs are pairs of molecules called nitrogen bases. 4 types: adenine (A), thymine (T), guanine (G), and cytosine (C)  Adenine only pairs with thymine  Guanine only pairs with cytosine

 2 sides of DNA molecule unwind and separate  Nitrogen bases that are floating in nucleus pair up with bases on either side.  Because of the way in which N bases pair with one another, the order of bases in the new strands of DNA are exactly the same as the original strand  Ends with 2 strands of DNA

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