DNA and RNA

Genes are made of DNA

• Griffith’s “Transforming Factor” Is the Genetic Material

• Avery Shows DNA Is the Transforming Factor

• Virus Experiments Provide More Evidence

Frederick Griffith

• A 1928 army medical officer trying to find a vaccine against streptococcus pneumoniae bacteria

Griffith’s Experiment• Injected several mice with a harmless strain of

bacteria and other mice with heat-treated deadly bacteria. All the mice survived.

• But when he injected mice with a mixture of the harmless bacteria and the heat-treated deadly bacteria, the mice died.

• The harmless bacteria had been “transformed” by something in the heat-treated deadly bacteria.

Picture of Griffith’s Experiment

Oswald Avery

• In 1944, Oswald Avery repeated Griffith’s experiment to see if the “transforming factor” was DNA or protein.

• When Avery treated the mixture of bacteria to destroy DNA, the transformation did not occur.

• He concluded that DNA was the “transforming factor.”

Martha Chase and Alfred Hershey

• In 1952, Hershey and Chase used viruses to confirm Avery’s conclusions.

• Concluded that DNA was the portion of the phage that carried the genetic information.

Picture of Hershey and Chase’s Experiment

Nucleic acids store information in their sequences of chemical units

• The Building Blocks of DNA

• DNA’s Structure

The Building Blocks of DNA

• Deoxyribonucleic Acid (DNA) is the molecule that stores genetic information.

• DNA is made up of units called nucleotides.

• Each nucleotide has three parts: a sugar, a phosphate group, and a nitrogenous base.

DNA’s Structure

• A nitrogenous base is a single or double ring of carbon and nitrogen atoms with functional groups.

• Nitrogenous bases with single rings are called pyrimidines.

• Nitrogenous bases with double rings are called purines.

• Bonds connect the sugar of one nucleotide to the phosphate of the next nucleotide, forming a sugar-phosphate “backbone.”

Rosalind Franklin and Maurice Wilkins

• In the 1950’s, first to obtain very good x-ray diffraction images of the DNA fibers.

• From these photos, Franklin determined that the DNA molecule must be long and thin.

Franklin and Wilkins’s X-ray Picture of DNA

James Watson and Francis Crick

• In 1953, Watson and Crick discovered the double helix structure of DNA using the information from Franklin and Wilkins.

• Built a model of a DNA molecule with a twisting shape called a double helix.

Erwin Chargaff

• Biochemist who first figured out the equation for the different nitrogen bases.

• Here is what he concluded:

• The amount of (A)denine will always equal the amount of (T)hymine and the amount of (G)uanine will always equal the amount of (C)ytosine.

DNA replication is the molecular mechanism of inheritance

• The Template Mechanism

• Replication of the Double Helix

The Template Mechanism

• DNA is copied before a cell divides.

• DNA replication is the process of copying the DNA molecule.

• During DNA replication, the two strands of the double helix separate.

• The cell uses each strand as a template, or pattern, for making a complementary strand.

• Nucleotides are added to the new strand, according to the rules of base pairing.

Replication of the Double Helix

• DNA replication begins at the origins of replication and proceeds in both directions, producing “bubbles.”

• Eventually, all the bubbles merge, resulting in two separate daughter DNA molecules.

A gene provides the information for making a specific protein

• One Gene, One Polypeptide

• Information Flow: DNA to RNA to Protein

• The Triplet Code

One Gene, One Polypeptide

• In the 1940s, George Beadle and Edward Tatum worked with bread mold Neurospora crassa to study the relationship between genes and proteins.

• Proposed the “one gene-one enzyme” hypothesis, stating that each gene controls the production of a specific enzyme.

• Scientists now know that most genes code for the production of a single polypeptide.

Structure of RNA

• Ribonucleic Acid (RNA) is a nucleic acid whose sugar is ribose rather than deoxyribose of DNA.

• Another difference is that RNA contains a nitrogenous base called uracil (U) instead of thymine of DNA.

• And RNA typically forms a single, sometimes twisted strand, not a double helix like DNA.

Information Flow: DNA to RNA to Protein

1. DNA sequence is converted to the form of a single-stranded Ribonucleic Acid (RNA) molecule in a process called transcription.

2. RNA moves from the nucleus to the cytoplasm.

3. Translation converts the nucleic acid sequence into a sequence of amino acids.

What are the rules for translating the nucleotide sequence of RNA into an

amino acid sequence?• American biochemist Marshall

Nirenberg began cracking this code in the early 1960s.

• He built an RNA molecule that only had uracil nucleotides, called “poly U.” Because it contained only uracil nucleotides, it contained only one type of codon: UUU, repeated over and over.

• He concluded that the RNA codon UUU codes for the amino acid phenylalanine (Phe).

The Triplet Code

• A codon, or three-base sequence, codes for one amino acid.

• The order of the codons in the RNA determines the order in which amino acids will be put together to form a polypeptide.

• There are 64 codons and 20 amino acids.

• Certain codons signal protein synthesis to start and stop.

There are two main steps from gene to protein

• Transcription: DNA to RNA

• Editing the RNA Message

• Translation: RNA to Protein

• Review of Protein Synthesis

Transcription: DNA to RNA

• Messenger RNA (mRNA) is transcribed from the DNA molecule.

• An enzyme called RNA polymerase joins the RNA nucleotides together during transcription.

Editing the RNA Message

• The mRNA molecule contains sections, called introns, that do not code for amino acids.

• The process of RNA splicing removes the introns and joins the coding regions, which are called exons.

• After RNA splicing is complete, the mRNA molecule moves from the nucleus to the cytoplasm.

Translation: RNA to Protein

• Transfer RNA (tRNA) is a molecule with an amino acid binding site at one end and a three-base anticodon at the other end.

• An anticodon is a sequence that is complementary to a codon in the mRNA.

• The tRNA anticodon binds to the mRNA codon, bringing and amino acid into position to be added to the polypeptide.

• This process takes place on a ribosome, which is made up of ribosomal RNA (rRNA).

Review of Protein Synthesis

• What is the overall significance of transcription and translation?– The proteins that form

from the polypeptides determine the appearance and functioning of the cell and of the whole organism.

Mutations can change the meaning of genes

• How Mutations Affect Genes

• What Causes Mutations?

How Mutations Affect Genes

• A change in the nucleotide sequence of DNA is called a mutation.

• A mutation can be a change in a single nucleotide or a change in a large part of a chromosome. For example, sickle cell disease.

• Mutations include substitutions, in which one nucleotide is replaced by another. Other mutations are insertions or deletions, in which one or more nucleotides are added to or removed from a gene. Insertions and deletions usually have more serious effects than substitutions.

What Causes Mutations?

• Mutations can be caused by errors in crossing over or DNA replication.

• A physical or chemical agent that causes a mutation is called a mutagen.

• Most mutations are harmful changes, but occasionally a mutation occurs that is beneficial.

• Mutations that are present in gametes, or sex cells, can be passed to an organism’s offspring.

More Information

Websites:• The DNA Files• DNA Interactive• You Try It: DNA Workshop