Chapter 12
Biology
Sylvia S. Made
Michael Windelspecht
Chapter 12
Molecular Biology of the Gene
Lecture Outline
See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes.
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Slide 1
Outline
12.1 The Genetic Material
12.2 Replication of DNA
12.3 Gene Expression: RNA and the Genetic Code
12.4 Gene Expression: Transcription
12.5 Gene Expression: Translation
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Slide 2
The Genetic Basis of Eye, Hair, and Skin Coloration
All life on Earth contains the four bases of DNA: A, G, C, and T.
These bases are molecules and code for biological parts such as the proteins that make skin, bones, and eyes.
Different combinations of these bases make up genes.
MC1R is a gene that contributes to skin, hair, and eye color in humans and is found in the nucleus of every cell.
Some of the cells in skin become specialized pigment-making cells called melanocytes.
Humans have variations in their MC1R that come from their ancestry.
Differences in the DNA base sequence can alter gene expression, which affects how much melanin is produced.
Everyone has melanin genes, but each individual’s gene expression is determined from information in their DNA.
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Slide 3
12.1 The Genetic Material
Frederick Griffith investigated virulence of Streptococcus pneumoniae.
He concluded that virulence could be passed from a dead strain to a nonvirulent living strain.
Transformation
Further research by Avery et al.
Discovered that DNA is the transforming substance
DNA from dead cells is incorporated into the genome of living cells.
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Slide 4
The Genetic Material (1)
DNA and proteins were the candidates for the hereditary material.
Proteins contain 20 amino acids that can be sequenced in different ways.
DNA and RNA each contain only four types of nucleotides.
Requirements for the genetic material:
Must be able to store genetic information
Must be stable and able to be replicated accurately during cell division and transmitted from generation to generation
Must be able to undergo mutations to provide genetic variability
Researchers showed that DNA can fulfill all these functions.
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Slide 5
The Genetic Material (2)
Griffith’s Transformation Experiment
Mice were injected with two strains of pneumococcus, an encapsulated (S) strain and a non-encapsulated (R) strain.
The S strain is virulent (the mice died); it has a mucous capsule and forms “shiny” colonies.
The R strain is not virulent (the mice lived); it has no capsule and forms “dull” colonies.
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Slide 6
Griffith’s Transformation Experiment
Injected live S strain has capsule and causes mice to die.
Injected live R strain has no capsule and mice do not die.
Injected heat-killed S strain does not cause mice to die.
Injected heat-killed S strain plus live R strain causes mice to die.
Live S strain is withdrawn from dead mice.
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Slide 7
The Genetic Material (3)
Avery, MacLeod, and McCarty’s Experiment
Scientists argued that DNA lacked variability to be able to store genetic information.
Avery and colleagues used enzymes that
eak down DNA (DNase), or RNA (RNase), or protein (protease) in separate experiments to digest the substance which allowed Streptococcus to produce a capsule and become virulent.
The only enzyme that had an effect was the DNase, which prevented the “transformation” from occu
ing.
These experiments show that DNA was the transforming substance.
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Slide 8
The Genetic Material (4)
Hershey and Chase’s Experiment
They used a virus called a T phage, composed of radioactively labeled DNA and coat proteins (in two separate experiments), to infect E. coli bacteria.
They discovered that radioactively labeled DNA, but not protein, ended up inside the bacterial cells, causing them to become transformed.
Only the genetic material could have caused this transformation.
This experiment showed that only DNA, and not protein, was the genetic material.
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Slide 9
The Genetic Material (5)
Transformation of organisms today:
The result is the so-called genetically modified organisms (GMOs).
Invaluable tool in modern biotechnology today
Commercial products that are cu
ently much used
Green fluorescent protein (GFP) used as a marker
A jellyfish gene codes for GFP.
The jellyfish gene is isolated and then transfe
ed to a bacterium, or the em
yo of a plant, pig, or mouse.
When this gene is transfe
ed to another organism, the organism glows in the dark.
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Slide 10
The Genetic Material (6)
Viral DNA is labeled (yellow).
When bacteria and viruses are cultured together, radioactive viral DNA enters bacteria.
Agitation in blender dislodges viruses. Radioactivity stays inside bacteria.
Centrifugation separates viruses from bacteria and allows investigator to detect location of radioactivity.
Viral capsid is labeled (yellow).
When bacteria and viruses are cultured together, radioactive viral capsids stay outside bacteria.
Agitation in blender dislodges viruses. Radioactivity stays outside bacteria.
Centrifugation separates viruses from bacteria and allows investigator to detect location of radioactivity.
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Slide 11
The Genetic Material (7)
DNA contains:
Two nucleotides with purine bases
Adenine (A)
Guanine (G)
Two nucleotides with pyrimidine bases
Thymine (T)
Cytosine (C)
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Slide 12
The Genetic Material (8)
Chargaff’s Rules:
The amounts of A, T, G, and C in DNA:
Are constant among members of the same species
Vary from species to species
In each species, there are equal amounts of:
A and T
G and C
All of this suggests that DNA uses complementary base pairing to store genetic information.
Each human chromosome contains, on average, about 140 million base pairs.
The number of possible nucleotide sequences is .
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Slide 13
Nucleotide Composition of DNA
Purine nucleotides
Pyrimidine nucleotides
c. Chargaff’s data
Table 12.1 DNA Composition in Various Species (%)
Species A T G C
Homo sapiens (human) 31.0 31.5 19.1 18.4
Drosophila melanogaster (fruit fly) 27.3 27.6 22.5 22.5
Zea mays (corn) 25.6 25.3 24.5 24.6
Neurospora crassa (fungus) 23.0 23.3 27.1 26.6
Escherichia coli (bacterium) 24.6 24.3 25.5 25.6
Bacillus subtilis (bacterium) 28.4 29.0 21.0 21.6
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Slide 14
The Genetic Material (9)
X-Ray diffraction:
Rosalind Franklin studied the structure of DNA using X-rays.
She found that if a concentrated, viscous solution of DNA is made, it can be separated into fibers.
Under the right conditions, the fibers can produce an X-ray diffraction pattern.
She produced X-ray diffraction photographs.
This provided evidence that DNA had the following features:
DNA is a helix.
Some portion of the helix is repeated.
A colleague of Franklins, Wilkins, showed her photo to James Watson, who understood its significance about DNA’s structure.
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Slide 15
X-Ray Diffraction of DNA
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Slide 16
The Genetic Material (10)
The Watson and Crick Model (1953)
Double helix model is similar to a twisted ladder.
Sugar-phosphate backbones make up the sides.
Hydrogen-bonded bases make up the rungs.
The two DNA strands are antiparallel.
Information stored in DNA must be read in the 5 prime to 3 prime direction so DNA is replicated in a 5 prime to 3 prime direction.
Complementary base pairing ensures that a purine is always bonded to a pyrimidine (A with T, G with C).
They received a Nobel Prize in 1962.
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Slide 17
Watson and Crick Model of DNA
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Slide 18
12.2 Replication of DNA
DNA replication is the process of copying a DNA molecule.
Semiconservative replication: Each strand of the original double helix (parental molecule) serves as a template (mold or model) for a new strand in a daughter molecule.
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Slide 19
Semiconservative Replication
The mechanism of DNA replication
The products of replication
Robert Brooker, et al, Biology, 2e. New York, NY: McGraw-Hill Education, Copyright © 2011 McGraw-Hill Education. All rights reserved. Used with permission
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Slide 20
Replication of DNA (1)
Replication requires the following steps:
Unwinding, or separation, of the two strands of the parental DNA molecule by the DNA helicase enzyme
Single-stranded binding proteins (SSB) attach to newly separated DNA and prevent helix from re-forming.
Complementary base pairing between a new nucleotide and a nucleotide on the template strand
DNA primase places short primers on the strands to be replicated.
Polymerase recognizes RNA and begins DNA synthesis.
The two strands are replicated differently: leading and lagging strands.
Joining of nucleotides in the lagging strand by DNA ligase form the new strand.
Each daughter DNA molecule contains one old and one new strand.
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Slide 21
Replication of DNA (2)
Prokaryotic Replication
Bacteria have a single circular loop of DNA.
Replication moves around the circular DNA molecule in both directions.
It produces two identical circles.
The process begins at the origin of replication.