DNA Structure and Cell Cycle

NB pg. 38

History of DNA Structure Discovery• Rosalind Franklin used an X-ray method to photograph DNA molecules in 1952.

• Colleague was Maurice Wilkins at King’s College in London (Bad relationship)

• John Randall considered her photos the most beautiful X-rays he had ever taken.

• She came very close to solving the DNA structure, but Watson and Crick beat her to publication.

History of DNA Structure Discovery• Using the X-ray photos taken by Rosalind Franklin, James Watson and Francis Crick were able to figure out the structure of DNA.

• Watson, Crick and Maurice Wilkins received the Nobel Prize for their work on the DNA molecule structure.

• Franklin died at age 37; so, she was not honored.

• By the way, Watson and Crick did not have permission to use Franklins photos.

• DNA is the basis for heredity.• DNA consists of two chains twisted around each other (double helix).• Looks like a spiral staircase or ladder.• The sides of the ladder are made up of deoxyribose (a sugar) and phosphates.

•Structure of DNA

Structure of DNA• Each rung has a pair of nitrogen bases.• Contain the element nitrogen.

• 4 Kinds of Nitrogen Bases that are held together by hydrogen bonds.

1. Adenine (A)2. Cytosine (C)3. Guanine (G)4. Thymine (T)

Rules of Base Pairing

Nitrogen Base

Only Pairs with…

Nitrogen Base

Adenine (A) Thymine (T)

Cytosine (C) Guanine (G)

This pairing rule is key to understanding how DNA replication works.

• DNA replication begins when the two sides of the DNA molecule unwind and separate between the nitrogen bases like a zipper unzipping.• Bases floating in the nucleus pair with the original strand.• Due to base pairing rule the order of the new molecule match the exact order of the old molecule.

•DNA Replication

The Cell Cycle

• The cell cycle is a regular sequence of growth and division that cells undergo.• The cell cycle has 3 phases:

1. Interphase2. Mitosis3. Cytokinesis

Interphase• The period before cell division when the cell grows, makes a copy of its DNA, and prepares to divide into two cells.

1. The cells grows to its full size and produces the structures it needs.

2. An exact copy of the DNA is made by REPLICATION.

3. The cell produces structures that will assist it when it divides in two new cells.

•This is the stage when a cells nucleus divides into two nuclei.•4 stages of Mitosis

1. Prophase2. Metaphase3. Anaphase4. Telophase

Mitosis

PROPHASE

• Chromatin condense to form chromosomes.

• Spindle fibers form a bridge between the ends of the cells.

• Nuclear envelope breaks down.

• The chromosomes line up across the center of the cell. • Each chromosome attaches to a spindle fiber at its centromere.

METAPHASE

Phases of Mitosis

ANAPHASE• The centromeres split, and the chromatids separate.• They are pulled to opposite ends of the cell.• The cell stretches in preparation to become two separate cells.

• The chromosomes begin to stretch out and lose their rod -like appearance. • Nuclear region begins to form around each set of chromosomes.

TELOPHASE

Phases of Mitosis

Cytokinesis •This is the division of the cytoplasm which produces two daughter cells.• Organelles are distributed into each of the two cells.

•At the end of cytokinesis:1. 2 daughter cells (new) have formed

with same number of chromosomes in each.

2. Each cell enters interphase, and the cycle starts again.