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• Watson and Crick – 1st to propose
structure of DNA.
• Requires precise transmission
during replication.
http://www.ncbe.reading.ac.uk/DNA50/Resources/wc1993.gif
• Prior to Watson, Griffith tested
transmission.
• Griffith - injected live bacterial
strains into mice.
• Mixed R strain of bacteria
(harmless) with heat-killed S
strain (harmful) and injected it.
• After mouse died, removed strain
from mouse.
• Substance eventually found to be
DNA - supported by injecting
bacteria into viruses.
• Viruses consist of DNA
(sometimes RNA) enclosed by
protective coat of protein.
• To replicate - virus infects host
cell; takes over cell’s metabolic
machinery.
• Viruses that specifically attack
bacteria - bacteriophages
(phages)
http://www.monografias.com/trabajos5/virus/Image164.gif
• Transformation - change in
genotype and phenotype due to
assimilation of foreign substance
(now DNA) by cell.
http://www.swbic.org/products/clipart/images/bacteriophage.jpg
• Hershey and Chase labeled
protein and DNA - injected them
into bacteria.
• Hershey and Chase concluded
that DNA, not protein, is
responsible for transmission.
• DNA doubles prior to mitosis.
• 1940’s - DNA made of bases
(adenine, thymine, cytosine,
guanine)
• Also known that sugar of one
nucleotide attached to phosphate
of another - forms backbone of
DNA.
• Chargaff’s rules - even amount of
thymine and adenine. (and guanine
and cytosine)
• Watson 1st to figure DNA in
helix shape + specific distance
between nucleotides.
• Partnered with Crick – came up
with double stranded model of
DNA - double helix.
Fig. 16.5
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Found purine (A, G) has to pair
with pyrimidine (T, C) to achieve
distance needed.
• Knew A - T (2 H bonds), C - G (3
H bonds
• Each gene found to have unique
sequence of nitrogen bases DNA strands not all the same.
http://academy.d20.co.edu/kadets/lundberg/dna_wallpaper/dna800x600.jpg
• Each strand of DNA can be
template to make more DNA.
• Cell copies DNA - each strand
forms as template to determine
new complementary bases.
• Nucleotides pair in
complementary fashion, one by
one.
• Semiconservative replication each DNA molecule has one
parent strand and one daughter
strand.
• Even though process is amazingly
quick, only about 1 in a billion
nucleotides copied wrong.
• Proteins and enzymes also part of
process, not just nucleotides.
http://www.bio.miami.edu/dana/250/nucleotides.jpg
• Origins of replication - where
replication begins.
• Bacteria - 1 site - replication is
bubble moving along DNA.
• Eukaryotes - many origins of
replication on each chromosome.
• Origin sites - DNA strands
separate forming replication
“bubble” with replication forks at
each end.
• Elongation of DNA catalyzed by
1DNA polymerase.
• Polymerase adds complementary
bases to growing strand of new
DNA.
• 2Helicase - untwists double helix
of DNA at replication fork.
• 3Single-strand binding proteins
help keep strands apart from one
another during replication.
• Strands of DNA - antiparallel.
• Sugar-phosphate backbones run
in opposite directions.
• Each end of strand labeled either
5’ end or 3’ end.
• Nucleotides only be added to 3’
end.
• DNA strand can only elongate
from 5’ end to 3’ end.
• Replication fork - problem system because strands run in
opposite directions (antiparallel)
http://www.mie.utoronto.ca/labs/lcdlab/biopic/fig/11.16.jpg
• 1 parent strand (leading strand one that runs 3’ to 5’) used as
template to keep complementary
strand continuous.
• Other strand (lagging strand one that runs 5’ to 3’) copied
from fork in small segments Okazaki fragments.
http://www.biology.arizona.edu/molecular_bio/problem_sets/nucleic_acids/graphics/repfork1.gif
• Fragments “glued” together by
4DNA ligase to form backbone
(made of sugar and phosphate) of
single DNA strand.
• Polymerase adds nucleotides to
strands, cannot start whole
process.
• Done by a piece of RNA - primer.
http://www.biologie.uni-hamburg.de/b-online/library/bio201/primase.jpg
• Once primer formed, polymerase
adds DNA nucleotides to growing
daughter strand of DNA.
• After, 5DNA polymerase
(different) replaces original RNA
with new complementary DNA
nucleotides - no RNA left in
strand.
• Replication fork, leading strand
copied continuously into fork
from single primer.
• Lagging strand copied away from
fork in short segments, each
requiring new primer.
• Original errors in reading of
template occur.
• Enzyme (DNA polymerase)
removes mistake and replaces it.
• Some things can alter DNA
outside of body.
http://library.thinkquest.org/C0123260/basic%20knowledge/images/basic%20knowledge/DNA/polymerase%201.jpg
• X-rays, UV rays can alter DNA
after replication.
• Mistakes can be fixed after DNA
synthesis - cell continually
monitors DNA.
• 1Mismatch repair - special
enzymes fix incorrectly paired
nucleotides - happens in certain
types of cancers.
http://www.sinauer.com/cooper4e/sample/Figures/Chapter%2006/highres/CELL4e-Fig-06-24-0.jpg
• 2Nucleotide excision repair nuclease cuts out segment of
damaged strand.
• Xeroderma pigmentosa (genetic
disease) cannot go through
process.
• Disease prevents person from
going in sun - UV rays interfere
with DNA replication (more
susceptible to skin cancer - can’t
fix mistakes)
http://162.129.70.33/images/xeroderma_pigmentosa_2_040620.jpg
• Ends of DNA strand can break
down from constant replication.
• Ends of chromosomal DNA
molecules – telomeres - special
nucleotide sequences.
• Telomeres protect genes from
being eroded through multiple
rounds of DNA replication.
• When telomeres shorten,
telomerase uses piece of RNA to
lengthen telomere.
• Telomerase has life span to
certain tissues or organism.
• Important for telomerase to be
in gamete cells so they can pass
long telomeres on to zygote.
• Active telomerase in body cells
can be responsible for cancer
cells because cells keep dividing.
• Proteins - link between genotype
(what DNA says) and phenotype
(physical expression)
• Beadle and Tatum – 1st to make
connection between genes and
enzymes that carry out genes
(bread mold experiments)
http://fig.cox.miami.edu/~cmallery/150/gene/17x3.jpg
• Bridge between DNA, proteins RNA.
• RNA similar to DNA - sugar
ribose; contains uracil instead of
thymine.
• RNA single-stranded.
http://gibk26.bse.kyutech.ac.jp/jouhou/image/nucleic/rna/rna_bb_st.gif
• Nucleotides found in DNA and
RNA - code - determines order
of amino acids.
• 2 steps - transcription and
translation.
• Transcription - DNA serves as
template for complementary RNA
strand.
http://www.ktf-split.hr/glossary/image/nucleotide.gif
• Transcription produces mRNA
strand (messenger RNA).
• Translation uses mRNA sequence
to determine order of amino
acids - creates polypeptide.
http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/graphics/Transcription.02.GIF
• Bacteria - transcription and
translation occur at once.
• Eukaryotes, most transcription
occurs in nucleus, translation
occurs at ribosome.
• Before primary transcript can
leave nucleus - modified during
RNA processing before enters
cytoplasm.
• Genetic code - triplet code series 3 nitrogen bases that code
for specific amino acid.
• 64 possible combinations of
nitrogen bases.
• Only 20 amino acids = each amino
acid has more than 1 code.
http://www.dls.ym.edu.tw/lesson/gen.files/codon.jpg
• 61 of 64 codes specific to an amino
acid.
• Other 3 - stop codons - determine
when process stops.
• Specific code that signals start of
translation - also codes for amino
acid.
• Start begins correct reading frame
of polypeptide.
• Transcription, 1 DNA strand template strand, provides
template for ordering sequence
of nucleotides in RNA transcript.
• Translation, blocks of 3
nucleotides, codons, decoded into
sequence of amino acids.
• Possible to take genetic code of 1
organism, place it into another nearly universal.
• Firefly gene for luminescence
transplanted into tobacco plant.
• Bacteria can be inserted with specific
genes to synthesize genes in large
amounts.
Synthesis and Processing of
RNA
• mRNA transcribed from template
of original gene.
• RNA polymerase separates DNA
strands and bonds RNA bases
along complementary strand.
• Bases can only be added to 3’ end.
http://www.csu.edu.au/faculty/health/biomed/subjects/molbol/images/7_9.jpg
• Specific sequences determine
where transcription starts and
where it ends.
• Promoter sequence – initiates;
terminator ends.
• Presence of promotor determines
which strand of DNA helix is
template.
• Proteins (transcription factors)
recognize promotor region (TATA
box) and bind to promotor.
http://www.nslij-genetics.org/pic/promoter.gif
• After binding, RNA polymerase
binds to transcription factors.
• RNA polymerase starts
transcription.
• RNA polymerase moves along nucleotides added to 3’ end.
• Single gene can be transcribed
simultaneously by several RNA
polymerases at a time.
• Growing strand of RNA trails off
from each polymerase.
• RNA splicing - removal of large
portion of RNA molecule because
most eukaryotic genes and RNA
transcripts have long noncoding
(introns) stretches of nucleotides
between coding regions (exons)
http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/introndeletion.gif
• RNA splicing removes introns,
joins exons to create mRNA
molecule with continuous coding
sequence.
• Splicing done by spliceosome.
• Translation - cell interprets
codons along mRNA molecule.
• Transfer RNA (tRNA) transfers
amino acids from cytoplasm’s pool
to ribosome.
• Ribosome adds each amino acid
carried by tRNA to growing end
of polypeptide chain.
• tRNA links mRNA codon with
amino acid.
• tRNA arriving at ribosome
carries specific amino acid at 1
end, has specific nucleotide
triplet, anticodon, at other.
• Anticodon base-pairs with
complementary codon on mRNA.
http://bioweb.uwlax.edu/GenWeb/Molecular/Theory/Translation/ribosome.jpg
• tRNA synthesized like other
forms of RNA.
• Once in cytoplasm, each tRNA
used repeatedly to pick up and
drop off that amino acid.
• Anticodons recognize more than
one codon.
• Rules for base pairing between
3rd base of codon and anticodon
relaxed (wobble).
http://www.geneticengineering.org/chemis/Chemis-NucleicAcid/Graphics/tRNA.jpg
• Each ribosome has 3 binding sites
for tRNA molecules.
• P site holds tRNA carrying
growing polypeptide chain.
• A site carries tRNA with next
amino acid.
• Discharged tRNAs leave ribosome
at E site.
http://nobelprize.org/educational_games/medicine/dna/a/translation/pics/translation2.gif
• 1Initiation brings together
mRNA, tRNA with 1st amino acid.
• 2Elongation - each amino acid
added to previous one.
• 3 steps of elongation continue
codon by codon to add amino
acids until polypeptide chain
completed.
Fig. 17.18
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 3Termination - 1 of 3 stop codons
reaches A site.
• Release factor binds to stop
codon, breaks bond between
polypeptide and tRNA in P site frees polypeptide.
• 2 types of ribosomes active in
process.
• 1Free ribosomes suspended in
cytosol synthesize proteins in
cytosol.
• 2Bound ribosomes attached to
endoplasmic reticulum.
Fig. 17.21
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Bacteria and eukaryotes have
differences in details of
processes.
• Eukaryotic RNA polymerases
differ from prokaryotic; require
transcription factors.
• Differ in how transcription
terminated.
• Ribosomes also different.
• Prokaryotes can transcribe and
translate same gene
simultaneously.
• Mutations - changes in genetic
material of cell (or virus).
• Chemical change in 1 base pair of
gene causes point mutation.
• Occur in gametes or cells
producing gametes - may be
transmitted to future
generations.
http://staff.jccc.net/PDECELL/evolution/mutations/mutypes.gif
• If it results in replacement of
pair of complementary
nucleotides with another
nucleotide pair - base-pair
substitution.
• Can have little or no impact on
protein function (silent
mutations).
http://fajerpc.magnet.fsu.edu/Education/2010/Lectures/26_DNA_Transcription_files/image008.jpg
• Missense mutations - code for
different amino acid.
• Nonsense mutations - code for
“stop” - leads to malfunctioning
protein.
Fig. 17.24
Copyright © Pearson Education, Inc., publishing as Benjamin Cummings
• Insertions and deletions additions or losses of nucleotide
pairs in gene.
• Unless these mutations occur in
multiples of 3 - cause frameshift
mutation.
Fig. 17.24
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Mutations can occur in many ways during DNA replication, DNA
repair, or DNA recombination.
• Mutagens - chemical or physical
agents that interact with DNA to
cause mutations (high-energy
radiation - X-rays UV light).