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8.1 Identifying DNA as the Genetic Material
KEY CONCEPT DNA was identified as the genetic material through a series of experiments.
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8.1 Identifying DNA as the Genetic Material
Griffith finds a ‘transforming principle.’
• Griffith experimented with the bacteria that cause pneumonia.
• He used two forms: the S form (deadly) and the R form (not deadly).
• A transforming material passed from dead S bacteria to live R bacteria, making them deadly.
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8.1 Identifying DNA as the Genetic Material
Avery identified DNA as the transforming principle.
• Avery isolated and purified Griffith’s transforming principle.
• Avery performed three tests on the transforming principle.– Qualitative tests showed DNA was present.– Chemical tests showed
the chemical makeupmatched that of DNA.
– Enzyme tests showedonly DNA-degradingenzymes stoppedtransformation.
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8.1 Identifying DNA as the Genetic Material
Hershey and Chase confirm that DNA is the genetic material.
• Hershey and Chase studied viruses that infect bacteria, or bacteriophages.
• Tagged DNA was found inside the bacteria; tagged proteins were not.
– They tagged viral DNA with radioactive phosphorus.
– They tagged viral proteins with radioactive sulfur.
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8.1 Identifying DNA as the Genetic Material
KEY CONCEPT DNA structure is the same in all organisms.
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8.1 Identifying DNA as the Genetic Material
DNA is composed of four types of nucleotides.
• DNA is made up of a long chain of nucleotides.• Each nucleotide has three parts.
– a phosphate group– a deoxyribose sugar– a nitrogen-containing base
phosphate group
deoxyribose (sugar)
nitrogen-containingbase
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8.1 Identifying DNA as the Genetic Material
• The nitrogen containing bases are the only difference in the four nucleotides.
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8.1 Identifying DNA as the Genetic Material
Watson and Crick determined the three-dimensional structure of DNA by building models.
• They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.
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8.1 Identifying DNA as the Genetic Material
• Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.
– Franklin’s x-ray images suggested that DNA was a double helix of even width.
– Chargaff’s rules stated that A=T and C=G.
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8.1 Identifying DNA as the Genetic Material
TAC
G
Nucleotides always pair in the same way.
• The base-pairing rules show how nucleotides always pair up in DNA.
• Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width.
– A pairs with T– C pairs with G
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8.1 Identifying DNA as the Genetic Material
• The backbone is connected by covalent bonds.
hydrogen bond covalent bond
• The bases are connected by hydrogen bonds.
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8.1 Identifying DNA as the Genetic Material
KEY CONCEPT DNA replication copies the genetic information of a cell.
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8.1 Identifying DNA as the Genetic Material
Replication copies the genetic information.
• A single strand of DNA serves as a template for a new strand.
• The rules of base pairing directreplication.
• DNA is replicated during theS (synthesis) stage of thecell cycle.
• Each body cell gets acomplete set ofidentical DNA.
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8.1 Identifying DNA as the Genetic Material
Proteins carry out the process of replication.
• DNA serves only as a template. • Enzymes and other proteins do the actual work of
replication.– Enzymes unzip the double helix.– Free-floating nucleotides form hydrogen bonds
with the template strand.nucleotide
The DNA molecule unzips in both directions.
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8.1 Identifying DNA as the Genetic Material
– Polymerase enzymes form covalent bonds between nucleotides in the new strand.
– DNA polymerase enzymes bond the nucleotides together to form the double helix.
DNA polymerase
new strand nucleotide
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8.1 Identifying DNA as the Genetic Material
• DNA replication is semiconservative.
original strand new strand
Two molecules of DNA
• Two new molecules of DNA are formed, each with an original strand and a newly formed strand.
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8.1 Identifying DNA as the Genetic Material
There are many origins of replication in eukaryotic chromosomes.
• DNA replication starts at many points in eukaryotic chromosomes.
Replication is fast and accurate.
• DNA polymerases can find and correct errors.
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8.1 Identifying DNA as the Genetic Material
KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule.
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8.1 Identifying DNA as the Genetic Material
RNA carries DNA’s instructions.
• The central dogma states that information flows in one direction from DNA to RNA to proteins.
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8.1 Identifying DNA as the Genetic Material
• The central dogma includes three processes.
• RNA is a link between DNA and proteins.
replication
transcription
translation
– Replication– Transcription– Translation
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8.1 Identifying DNA as the Genetic Material
• RNA differs from DNA in three major ways.
– RNA has a ribose sugar.– RNA has uracil instead of thymine.– RNA is a single-stranded structure.
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8.1 Identifying DNA as the Genetic Material
Transcription makes three types of RNA.
• Transcription copies DNA to make a strand of RNA.
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8.1 Identifying DNA as the Genetic Material
• Transcription is catalyzed by RNA polymerase.
– RNA polymerase and other proteins form a transcription complex.
– The transcription complex recognizes the start of a gene and unwinds a segment of it.
start site
nucleotides
transcription complex
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8.1 Identifying DNA as the Genetic Material
– RNA polymerase bonds the nucleotides together.– The DNA helix winds again as the gene is transcribed.
– Nucleotides pair with one strand of the DNA.
DNA
RNA polymerase moves along the DNA
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8.1 Identifying DNA as the Genetic Material
– The RNA strand detaches from the DNA once the gene is transcribed.
RNA
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8.1 Identifying DNA as the Genetic Material
• Transcription makes three types of RNA.
– Messenger RNA (mRNA) carries the message that will be translated to form a protein.
– Ribosomal RNA (rRNA) forms part of ribosomes where proteins are made.
– Transfer RNA (tRNA) brings amino acids from the cytoplasm to a ribosome.
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8.1 Identifying DNA as the Genetic Material
The transcription process is similar to replication.
• Transcription and replication both involve complex enzymes and complementary base pairing.
• The two processes have different end results.– Replication copies
all the DNA;transcription copiesa gene.
– Replication makesone copy;transcription canmake many copies.
growing RNA strands
DNA
onegene