What is DNA? What are the subunits of DNA?

1. Journal Entry: What is DNA? What are the subunits of DNA? • Objectives: • List 3 important events that led to understanding the structure of DNA. • Describe the basic structure of a DNA molecule. • Explain how DNA molecules can be copied.

2. The Pieces of the Puzzle • DNA stands for deoxyribonucleic acid. • DNA is the genetic material that determines inherited characteristics. • Nucleotides: The Subunits of DNA • DNA is made of subunits called nucleotides. • A nucleotide consists of a sugar, a phosphate, and a base.

3. The Pieces of the Puzzle, continued • Chargaff’s Rule • Erwin Chargaff found that the amount of adenine in DNA always equals the amount of thymine, and the amount of guanine always equals the amount of cytosine. (A = T, C= G) • Franklin’s Discovery • Chemist Rosalind Franklin was able to make images of DNA molecules by using X-ray diffraction. • X-ray diffraction: X rays of a DNA molecule projected on film. • Showed that DNA has a spiral shape.

4. The Pieces of the Puzzle, continued • Watson and Crick’s Model • James Watson and Francis Crick used Chargaff’s and Franklin’s research to build a model of DNA. • The model, which looked like a long, twisted ladder, eventually helped explain how DNA is copied and how it functions in the cell.

5. DNA’s Double Structure • The Double Helix • The shape of DNA is known as a double helix. • The two sides of the ladder are made of alternating sugar parts and phosphate parts. • The rungs of the ladder are made of a pair of bases.

6. Making Copies of DNA • How Copies Are Made • During replication, a DNA molecule is split down the middle, where the bases meet. • The bases on each side of the molecule are used as a pattern for a new strand. • When Copies Are Made • DNA is copied every time a cell divides. • Each new cell gets a complete copy of all the DNA.

7. Making Copies of DNA, continued • 1. Unwinding: Old strands that make up the parental DNA molecule are unwound and “unzipped”. A special enzyme called helicase unwinds the molecule. • 2. Complementary base pairing: New complementary nucleotides are paired together. • 3. Joining: The complementary nucleotides join to form new strands.

8. Unraveling DNA • In a cell that has a nucleus, the strands of DNA (chromatin) and proteins are bundled into chromosomes. • A gene consists of a string of nucleotides that give the cell information about how to make a specific trait.

9. Genes and Proteins • Groups of 3 bases form the code for an amino acid. • Many amino acids strung together form a protein. • Proteins and Traits • Proteins act as chemical triggers for many of the processes within cells. • Proteins help determine traits. • A gene is a segment of DNA that specifies the amino acid sequence of a protein.

10. RNA • Help from RNA • Another type of molecule that helps make proteins is called RNA, or ribonucleic acid. • RNA is so similar to DNA that RNA can serve as a temporary copy of a DNA sequence. • Genes pass genetic information onto RNA molecules which are more directly involved in protein synthesis. • The nucleotides in RNA, however, contain the sugar ribose and the bases adenine, cytosine, guanine, and uracil (which replace thymine).

11. RNA, continued • There are 3 major classes of RNA. • Messenger RNA (mRNA): Takes a message from DNA in the nucleus to the ribosomes in the cytoplasm. • Transfer RNA (tRNA): Transfers amino acids to the ribosomes. • Ribosomal RNA (rRNA): Makes up the ribosomes, where amino acids are collected to form proteins (many amino acids form a protein).

12. Journal Entry: Unscramble the following words: tpsoneir neesg Now think of 3 words you associate with each of the above words and use them all in a paragraph that highlights what you know about DNA. • Objectives: • Explain the relationship between DNA, genes, and proteins. • Outline the basic steps in making a protein. • Describe three types of mutations, and provide an example of a gene mutation. • Describe two examples of uses of genetic knowledge.

13. From DNA to RNA to Protein • The Making of a Protein • The first step in making a protein is to copy one side of the segment of DNA containing a gene. • This copy is called messenger RNA (mRNA). • A ribosome is a cell organelle composed of RNA and protein. • A ribosome uses mRNA, transfer RNA (tRNA), and amino acids to make proteins.

14. Protein Synthesis (Transcription and Translation)

17. Changes in Genes • Mutations • Changes in the number, type, or order of bases on a piece of DNA are known as mutations.

18. Changes in Genes, continued • Do Mutations Matter? • There are three possible consequences to changes in DNA: an improved trait, no change, or a harmful trait. • How Do Mutations Happen? • Mutations happen regularly because of random errors when DNA is copied. • Any physical or chemical agent that can cause a mutation in DNA is called a mutagen.

19. An Example of Substitution • A mutation, such as a substitution, can be harmful because it may cause a gene to produce the wrong protein. • A simple change in an amino acid can cause a disease such as sickle cell anemia.

21. Uses of Genetic Knowledge • Genetic Engineering • Scientists can manipulate individual genes within organisms. • This kind of manipulation is called genetic engineering. • Genetic Identification • Your DNA is unique, so it can be used like a fingerprint to identify you. • DNA fingerprinting identifies the unique patterns in an individual’s DNA.