W HAT DOES DNA LOOK LIKE ? Unit 5: Seventh Grade

F OR MANY YEARS, THE STRUCTURE OF A DNA MOLECULE WAS A PUZZLE TO SCIENTISTS In the 1950’s, two scientists deduced the structure while experimenting with chemical models. They later won the Nobel Prize for helping to solve this puzzle.

W HAT IS DNA? B REAKING IT DOWN As you know, inherited characteristics are determined by genes. Genes are passed from one generation to the next. Genes are parts of chromosomes. Chromosomes are structures in the nucleus of most cells. Chromosomes are made of protein and DNA So, DNA is the material that makes up your chromosomes that determine your inherited characteristics

W HAT D OES DNA STAND FOR ? DNA stands for: Dexoyribo Nucleic Acid But is written out as deoxyribonucleic acid

T HE PIECES OF THE PUZZLE Scientists knew that the material that makes up our genes must do two things: Give instructions for building and maintaining cells Must be able to be copied each time a cell divides so that each cell could contain identical genes Scientists eventually discovered that DNA is made of subunits called nucleotides

N UCLEOTIDES : S UBUNITS OF DNA DNA is made of subunits called nucleotides. A “subunit” is a subdivision of a larger unit. For example: a subdivision you live in is a smaller unit of a city or town. It takes many subunits working together to make up a whole, functioning unit. Nucleotides are the subunits that make up DNA

N UCLEOTIDES : T HE SUBUNITS OF DNA Although it may look complicated, the DNA in a cell is really just a pattern made up of different parts called nucleotides. Nucleotides are molecules that, when joined together, make up DNA

N UCLEOTIDES : T HE S UBUNITS OF DNA Nucleotides are made up of a sugar, a phosphate, and a base. All nucleotides are the same except for the base. There are four different kinds of bases: Adenine Thymine Guanine Cytosine

T HE F OUR N UCLEOTIDES Each of the four bases has a different shape. Scientists often refer to a nucleotide’s base by the first letter of it’s base A = Adenine T = Thymine G= Guanine C = Cytosine Annie Takes Good Care

N UCLEOTIDES AND DNA Imagine a set of blocks that has only four shapes, or an alphabet that has only four letters. DNA is a long string of these blocks or letters.

C HARGAFF ’ S RULES Edwin Chargaff, a biochemist, discovered that the amount of adenine (A) always equaled the amount of thymine (T). A = T He also found out that the amount of guanine (G) equals the amount of cytosine (C) G= C Annie Takes Good Care

C HARGAFF ’ S RULES These findings are known as Chargaff’s Rules At the time, these findings were thought to be relatively unimportant, but later on, Chargaff’s rules helped scientists understand the structure of DNA

W ATSON AND C RICK After looking at x-ray images of DNA showing that DNA has a spiral shape, scientists James Watson and Francis Crick concluded that DNA must look like a long, twisted ladder. This discovery eventually helped explain how DNA is copied and how it functions in a cell.

DNA’ S DOUBLE STRUCTURE DNA looks like a twisted ladder. This shape is known as a double helix. The sides of the ladder are made of alternating sugar and phosphate parts. The rungs are made of a pair of bases.

R UNGS OF THE LADDER The rungs of the ladder are made of the bases adenine, thymine, guanine, and cytosine. Adenine on one side of the rung always pairs with thymine (A, T) Annie Takes Guanine always pairs with cytosine on a rung (G,C) Good Care

M AKING C OPIES OF DNA These pairings of the bases allows the cell to replicate, or make copies of, DNA. Each base only bonds with only one other base. Furthermore, pairs of bases are complementary to each other. For example GCAC will bond with CGTG Ex: TGCA will bond with:

H OW COPIES ARE MADE During replication, a DNA molecule is split down the middle, where the bases meet. As the bases are exposed, complementary nucleotides are added to each side of the split ladder. Two new DNA molecules are formed. Half of each molecule is the old DNA Half of each molecule is the new DNA

W HEN COPIES ARE MADE DNA is copied every time the cell divides Each new cell gets a complete copy of all the DNA.