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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