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Chapter 16 The Molecular Basis of Inheritance and Chapter 17 Gene…
Chapter 16 The Molecular Basis of Inheritance and Chapter 17 Gene Expression: From Gene to Protein
Concept 16.1 DNA is the Genetic Material
The Search for the Genetic Material: Scientific Inquiry
T. H. Morgan's group showed that genes exist as parts of chromosomes: DNA and protein.
Biochemists identified proteins as a class of macromolecules w/ great heterogeneity & hereditary material
Evidence that DNA can Transform Bacteria
In 1928, F. Griffith developed vaccine against pneumonia.
He used two strains of bacterium, one pathogenic & one nonpathogenic to study
streptococcus pneumonia
Figure 16.2: Griffith Expt
Two Strains: S (smooth/with capsule) strain caused pneumonia & R (rough/no capsule) strain are nonpathogenic.
Griffith injected mice w/ two strains
Results
Living S cells caused mouse to die
Living R cells caused mouse to stay healthy
Heat-killed S Cells - healthy mouse
Mixture of Heat-killed S cells & icing R cells - dead mouse
In blood sample, living S cells were found
Conclusion
: living R bacteria transformed into pathogenic S bacteria by unknown, heritable substance from dead S cells that enabled R cells to make capsules.
Transformation
: A change in genotype and phenotype due to the assimilation to external DNA by a cell.
Evidence That Viral DNA can program Cells
Bacteriophage or Phages
Virus
DNA/RNA enclosed by a protective coat, that is protein.
DNA is the genetic, material of phage know as T2
T2 typically infects
E. Coli
T2 is composed of DNA & protein
Could quickly turn E. Coli cell into a T2 producing factory that released many copies of new phages when cell ruptures.
Hershel & Chase found that the phage DNA entered the host cells but the protein did not.
Results: DNA inside the cells played an ongoing role during injection process.
Concluded that the DNA injected by the phage must be the molecule carrying the gene info that makes the cells produce new viral DNA & proteins.
Is Protein or DNA the genetic material of phage T2?
1) Mixed radioactively labeled phages w/bacteria. The phages infected the bacterial cells.
Agitated the mixture in a blender to free phage parts outside the bacteria from the cells.
Centrifuged the mixture so that bacteria formed a pellet at the bottom of test tube; free phages & phage parts, which are lighter, remained suspended in the liquid.
Measure the radioactivity in the pellet & the liquid.
Additional Evidence that DNA is the Genetic Material
DNA: Nitrogenous Base, Pentose Sugar called deoxyribose & a phosphate group.
Bases can be adenine (A), thymine (T) guanine (G), or cytosine (C)
Chargaff reported that the base composition of DNA varies from one species to another.
EX: Sea urchins = 32.8 % base A, Human DNA = 30.4 % bass A nucleotides, only 24.7% bacterium E. Coli have base A.
In Chargaff's analysis, sea urchin percentages: A = 32.8%; T=32.1%; G = 17.7%; C= 17.3%
Building a Structural Model of DNA:
Scientific Inquiry
Figure 16.5
Each DNA nucleotide monomer consists of a nitrogenous base (T, A, G or C, sugar deoxyribose (blue) & a phosphate group (yellow).
Phosphate group of one nucleotide is attached to the sugar of the next by a covalent bond, forming a "backbone" of alternating phosphates & sugars from which base projects.
A polynucleotide strand has directionality from 5' end to 3' (w/the -OH group of the sugar), 5' and 3' refer to the #s assigned to the carbons in the sugar ring.
James Watson & Crick were the first to solve puzzle of DNA structure w/ X-Ray crystallography.
Spots & Smudges in the image were by the X-Rays that were diffracted as they passed thru aligned fibers of purified DNA
Double Helix
The two sugar-phosphate backbones are
antiparallel
running in opposite directions.
Adenine & Guanine are purine (Nitrogenousbases w/organic rings)
Cytosine & thymine are pyrimidine (Nitrogenous Bases w/a single ring).
The amount of A will equal the amount of T.
The amount of G will equal the of C.
Concept 16.2 Many Proteins work together in DNA replication & repair
The Basic Principle: Base Pairing a Template Strand
Each Chain acts as a template for the formation on to itself of a new companion chain so the two pairs of chains exist.
When DNA is copied, each strand serves as a template for ordering nucleotides into a new, complementary, strand.
Watson's & Crick's model represents that when a double helix replicates, each of the two daughter molecules will have one old strand, from the parent molecule & one new strand.
The conservative Model: Two parent strands reassociate after acting as templates for new strands, thus restoring the parent double helix.
Semiconservative Model: Two strands of the parental molecule separate & each functions as a template for synthesis of a new complementary strand.
Dispersive Model: Each strand of
both
daughter molecules contain a mixture of old & synthesized DNA.
DNA Replication Compare/Contrast between Prokaryotes and Eukaryotes
Prokaryotes
Proteins that initiate DNA replication recognize this sequence & attach to the DNA, separating the 2 strands & opening up a replication "bubble".
Then Proceeds in both directions until molecule is copied.
Replication bubble has a
replication fork
at each end (y-shaped region where DNA are being unwound).
Helicases
untwist double helix at replication forks, making parental strands as template strands.
single-stranded binding proteins
bind to unpaired DNA strands to keep them from repairing
**Okazaki Fragments: 1,000-2,000 nucleotides in E.Coli
DNA polymerase iii & i
The untwisting of double helix causes a tighter twist & strain on Y-Fork.
Topoisomerase
:enzyme helps relieve strain by breaking swiveling & rejoin strands.
Eukaryotes
Multiple replication bubbles form & fuse. Speeding up the copying of the very long DNA molecules.
Eukaryotic DNa replication proceeds in both directions of each origin.
Replication Fork
at each end (Yshaped region where DNA are being unwound).
Helicases
untwist double helix at the replication forks, making parental strands as template strands.
single-stranded binding proteins
bind to unpair DNA strands to keep them from repairing.
Okakzaki Fragments
100-200 nucloeotideslong.
The untwisting of the double helix causes tighter twist & strain on Y-Fork
Topoisomerase
: enzyme helps relieve strain by breaking, swiveling, and rejoin strands
Synthesizing a New DNA Strand
Initial Nucleotide Chains: RNA chain called a
primer
& synthesized by
primase
Primase begin w/a single nucleotide & adds RNA nucleotide one at a time using parental DNA strand as template.
The new DNA will start from 3' end of RNA primer.
DNA polymerases
catalyze synthesis of new DNA by adding nucleotides to 3' end of a preexisting chain.
Antiparallel Elongation
Antiparallel: opposite direction of each other
New strands must be antiparallel to template strands
New DNA strands elongate in 5 --> 3 direction.
leading strand
: only one primer is required for DNA pol iii to synthesize the entire leading strands.
DNA pol III must work along template strand in direction "away" from Y-fork.
lagging strand
: strand elongating in direction;
okazaki fragments
are segments of the lagging strands.
Figure 16.14
Addition of a nucleotide to a DNA strand.
DNA polymerase catalyzes the addition of a nucleotide to the 3' end of a growing DNA strand, with release of two phosphates
Figure 16.15
Synthesis of the leading strand during DNA replication
After RNA primer is made, DNA pol III starts to synthesize the leading strand.
The leading strand is elongated continuously in the 5 ---> 3 direction as the fork progresses.
Figure 16.16
Synthesis of the lagging strand
Figure 16.17
Table 16.1 Bacterial DNA Replication Proteins and Their Functions
Protein: Helicase
Function: Unwinds parental double helix at replication forks
Protein: single-stranded binding protein
Function: Binds to and stabilizes single-stranded DNA until it is used as a template
Protein: Topoisomerase
F: Relieves overwinding strain ahead of Y-Forks by breaking, swiveling, and rejoining DNA strands
P: Primase
F: Synthesizes an RNA primer at 5' end of leading strand and at 5' end of each Okazaki fragment of lagging strand
P: DNA pol III
F: Using parental DNA as a template, synthesizes new DNA strand by adding nucleotides to an RNA primer or a pre-existing DNA strand
P: DNA pol I
F: Removes RNA nucleotides of primer from 5' end and replaces them with DNA nucleotides added to 3' end of adjacent fragment.
P: DNA ligase
F: Joins Okazaki fragments of lagging strand; on leading strand, joins 3' end of DNA that replaces primer to rest of leading strand DNA.
Telomeres
Nucleotide sequences in Eukaryotic chromosomal DNA
Contain NO genes
consists of multiple repetitions of one short nucleotide sequence
EX: 6-nucleotide sequence TTAGGG is repeated b/w 100 and 1,000times
Two protective functions
telomeric DNA prevent the staggered ends of the daughter molecule from activating the cell's systems for monitoring DNA damage.
telomeric DNA acts as a buffer zone to protect against the organism's genes shortening.
Concept 17.5 Mutations of one or a few nucleotides can affect protein structure and function
Mutations
are the ultimate source of new genes
huge diversity of genes
Point Mutations
changes in a single nucleotide pair of a gene
Types of Small-scale Mutations
Substitutions
nucleotide pair substitution
is the replacement of one nucleotide and its partner with another pair of nucleotides.
silent
missense mutation
Insertions and Deletions
Concept 17.4 Translation is the RNA-directed synthesis of a polypeptide:
a closer look
Figure 17.19 The initiation of translation
Figure 17.20 The Elongation Cycle of Translation
Figure 17.21 The Termination of Translation
Structure of tRNA
Figure 17.22 The signal mechanism for targeting proteins to the ER
Figure 17.23 Polyribosomes
may either be free or bound
seen with electron microscope
transcribe multiple mRNAs from the same gene.
Concept 17.1 Genes specify proteins via transcription and translation
Transcription
synthesis of RNA using info in the DNA
mRNA
carries genetic message from DNA to protein-synthesizing machinery in the cell.
Translation
synthesis of a polypeptide using info in the mRNA
ribosomes
site of translation
molecular complexes that facilitate the orderly linking of amino acids into polypeptide chains.
Concept 17.2 Transcription is the DNA-directed synthesis of RNA:
a closer look
Figure 17.8
The Stages of transcription: initiation, elongation, and termination
Figure 17.9 The initiation of transcription at a eukaryotic promoter.
Figure 17.10 Transcription Elongation
Concept 17.3 Eukaryotic Cels modify RNA after transcription
Alteration of mRNA Ends
Split Genes and RNA Splicing
RNA Splicing
large portions of RNA molecules are removed & the remaining portions are reconnected.
similar to editing a movie
Introns
intervening sequences
Exons
are eventually expressed usually by being translated into amino acid sequences.
spliceosome
the removal of intron is accomplished by a large complex made of proteins and small RNAs
