1. Chapter 11
DNA & GENES

2. By the early 1950’s, most scientists were convinced
11-1 The Structure of DNA
By the early 1950’s, most scientists were convinced
That genes were made of DNA.
The problem is that no one knew what it looked like.
Then along came James Watson & Francis Crick.

3. James Watson and Francis Crick

4. A Winding Staircase
Watson and Crick determined that a DNA molecule is a double helix—two strands twisted around each other, like a winding staircase.
Nucleotides are the subunits that make up DNA. Each nucleotide is made of three parts: a phosphate group, a five-carbon sugar molecule, and a nitrogen-containing base

5. The five-carbon sugar in DNA nucleotides is called deoxyribose
Structure of a Nucleotide

6. DNA Double Helix

7. The nitrogen base in a nucleotide can be either a bulky, double-ring purine, or a smaller, single-ring pyrimidine.

8. Discovering DNA’s Structure
Chargaff’s Observations
In 1949, Erwin Chargaff observed that for each organism he studied, the amount of adenine always equaled the amount of thymine (A=T)
Likewise, the amount of guanine always equaled the amount of cytosine (G=C).
However, the amount of adenine and thymine and of guanine and cytosine varied between different organisms

9. X-Ray Diffraction

10. Watson and Crick’s DNA Model
In 1953, Watson and Crick built a model of DNA with the configuration of a double helix, a “spiral staircase” of two strands of nucleotides twisting around a central axis
The double-helix model of DNA takes into account Chargaff’s observations and the patterns on Franklin’s X-ray diffraction photographs.

11. These base-pairing rules are supported by Chargaff’s observations
Pairing Between Bases
An adenine on one strand always pairs with a thymine on the opposite strand, and a guanine on one strand always pairs with a cytosine on the opposite strand
These base-pairing rules are supported by Chargaff’s observations
The strictness of base-pairing results in two strands that contain complementary base pairs

12. In the diagram of DNA below, the helix makes it easier to visualize the base-pairing that occurs between DNA strands

13. The Replication of DNA
When the double helix was discovered, scientists were very excited about the complimentary relationship between the sequences of nucleotides.
Watson and Crick proposed that one DNA strand serves as a template on which the other strand is built.

14. Roles of Enzymes in DNA Replication
The complementary structure of DNA is used as a basis to make exact copies of the DNA each time a cell divided.
The process of making a copy of DNA is called DNA replication
DNA replication occurs during the synthesis (S) phase of the cell cycle, before a cell divides
DNA replication occurs in three steps

15. Step 1- DNA helicases open the double helix by breaking the hydrogen bonds that link the complementary nitrogen bases between the two strands. The areas where the double helix separates are called replication forks

16. Step 2 - At the replication fork, enzymes known as DNA polymerases move along each of the DNA strands. DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base-pairing rules
Step 3 - Two DNA molecules that form are identical to the original DNA molecule

18. Checking for Errors
In the course of DNA replication, errors sometimes occur and the wrong nucleotide is added to the new strand.
An important feature of DNA replication is that DNA polymerases have a “proofreading” role
This proofreading reduces errors in DNA replication to about one error per 1 billion nucleotides

19. In eukaryotic cells, each chromosome contains a single, long strand of DNA
Each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long, each section with its own starting point
With multiple replication forks working in concert, an entire human chromosome can be replicated in about 8 hours

20. Replication Forks

21. Traits, such as eye color, are determined
11-2 RNA & Protein
Traits, such as eye color, are determined
By proteins that are built according to
The instructions specified in the DNA.

22. Decoding the Information in DNA
Proteins, however, are not built directly from DNA. Ribonucleic acid is also involved
Like DNA, ribonucleic acid (RNA) is a nucleic acid—a molecule made of nucleotides linked together

23. RNA differs from DNA in three ways
1. RNA consists of a single strand of nucleotides instead of the two strands found in DNA
2. RNA nucleotides contain the five-carbon sugar ribose rather than the sugar deoxyribose, which is found in DNA nucleotides
3. In addition to the A, G, and C nitrogen bases found in DNA, RNA nucleotides can have a nitrogen base called uracil (U)

24. Comparing DNA and RNA
The instructions for making a protein are transferred from a gene to an RNA molecule in a process called transcription
Cells then use two different types of RNA to read the instructions on the RNA molecule and put together the amino acids that make up the protein in a process called translation

25. The entire process by which proteins are made based on the information encoded in DNA is called gene expression, or protein synthesis

26. Gene Expression

27. Transfer of Information
from DNA to RNA
The first step in the making of a protein, transcription, takes the information found in a gene in the DNA and transfers it to a molecule of RNA
RNA polymerase, an enzyme that adds and links complementary RNA nucleotides during transcription, is required

28. The three steps of transcription are
Step 1 RNA polymerase binds to the gene’s promoter
Step 2 The two DNA strands unwind and separate
Step 3 Complementary RNA nucleotides are added

30. Types of RNA

31. Genetic Code: Three-Nucleotide “Words”
Different types of RNA are made during transcription, depending on the gene being expressed
When a cell needs a particular protein, it is messenger RNA that is made
Messenger RNA (mRNA) is a form of RNA that carries the instructions for making a protein from a gene and delivers it to the site of translation

32. The information is translated from the language of RNA—nucleotides—to the language of proteins—amino acids
The RNA instructions are written as a series of three-nucleotide sequences on the mRNA called codons
The genetic code of mRNA is the amino acids and “start” and “stop” signals that are coded for by each of the possible 64 mRNA codons

34. RNA’s Roles in Translation
Translation takes place in the cytoplasm. Here transfer RNA molecules and ribosomes help in the synthesis of proteins
Transfer RNA (tRNA) molecules are single strands of RNA that temporarily carry a specific amino acid on one end
An anticodon is a three-nucleotide sequence on a tRNA that is complementary to an mRNA codon.

35. Ribosomes are composed of both proteins and ribosomal RNA (rRNA)
Ribosomal RNA (rRNA) molecules are RNA molecules that are part of the structure of ribosomes
Each ribosome temporarily holds one mRNA and two tRNA molecules

36. The seven steps of translation are:
Step 1 The ribosomal subunits, the mRNA, and the tRNA carrying methionine bind together
Step 2 The tRNA carrying the amino acid specified by the codon in the A site arrives
Step 3 A peptide bond forms between adjacent amino acids
Step 4 The tRNA in the P site detaches and leaves its amino acid behind

37. Step 5 The tRNA in the A site moves to the P site
Step 5 The tRNA in the A site moves to the P site. The tRNA carrying the amino acid specified by the codon in the A site arrives
Step 6 A peptide bond is formed. The tRNA in the P site detaches and leaves its amino acid behind
Step 7 The process is repeated until a stop codon is reached. The ribosome complex falls apart. The newly made protein is released

39. THE END