Download Chapter 10- Molecular Biology of Genes

Chapter 10- Molecular Biology of Genes
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Adenine
AIDS
Anticodon
Bacteriophages
Codon
Cytosine
DNA ligase
DNA polymerase
Double helix
Guanine
HIV
Lysogenic cycle
Lytic cycle
mRNA
Mutagen
Mutagenesis
Mutation
Nucleotides
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Phages
Promoter
Prophage
Reading frame
Retrovirus
Reverse transcriptase
RNA polymerase
rRNA
RNA splicing
Stop codon
Sugar-phosphate backbone
Terminator
Thymine
Transcription
Translation
Triplet code
tRNA
Uracil
Transformation Experiment
• Griffith’s- 1928
– He wanted to learn what was transferred that made a bacteria
become virulent and able to cause pneumonia
– He hypothesized some factor (maybe a gene) was transferred
– 2 strains of bacteria
• S- strain = killed mice, R-strain = mice lived
– Transformation occurred- harmless bacteria was turned into
disease-causing bacteria
Griffith’s Experiment
Next problem:
• 1940’s– scientists knew that DNA and protein made
up chromosomes but they didn’t know which
one was the genetic material
• Much evidence at first pointed to protein
DNA (not protein)
• Hershey and Chase- 1952
– Used bacteriophages- virus that infects bacteria
• They are composed of DNA, RNA, protein coat
• Virus adheres to bacteria and injects genetic information into it, viral genes
act to produce new bacteriophages, cell bursts and new virus come out
– Needed to figure out which part entered the cell so they would
know if genes were made of protein or DNA
• Grew viruses in cultures with radioactive phosphorus or sulfur, used as
markers
• DNA contains no S, protein contains no P
• If S found in bacteria – viral protein was injected
• If P found in bacteria - viral DNA was injected
– Concluded
DNA was the
genetic
material (not
protein)
What do we know about DNA?
• Made up of nucleotides
– Phosphate group
– 5-C sugar (deoxyribose)
– Nitrogen base- A, T, C, G
– Backbone is made up of sugar and phosphate
– Bases stick off the sides
The bases:
• Purines- have 2 rings
– Adenine, Guanine
• Pyrimidines- have 1
ring
– Cytosine, Thymine
There was a race to determine
DNA structure
• Chargaff
– Percentages of A’s and T’s were almost equal
as were percentages of C’s and G’s
• Franklin
– used X-ray diffraction and recorded pattern,
gave clues to the structure
• Pattern showed that it twisted around itself
• Watson and Crick-tried to figure out
structure with models
• When they saw Rosalind’s photos they figured out the structure of DNA,
which solved the mystery of how it carries information and how it was copied
– the double helix “twisted ladder”
• Held together by hydrogen bonds between bases
• This pairing of bases explained Chargraff’s rule
DNA Replication
• Each strand serves as a
template to make a new
complementary strand
• Each DNA strand has 3’ and
5’ end
– DNA can only grow in the 5’  3’
direction
DNA Replication
• Origin of replication- site where DNA replication starts
– Replication proceeds in both directions forming a “bubble”
DNA Replication
• DNA polymerase- adds new nucleotides onto existing
ones
• DNA ligase- joins new pieces of DNA together
• DNA polyermase and ligase are also involved in
proofreading and fixing damaged DNA
• DNA replication involves many other proteins, it is
surprisingly fast and accurate
DNA Replication
How information is transferred
in a cell:
• Central dogma of biology
• DNA  RNA  protein
• Transcription- transfer of information from
DNA to an RNA molecule
• Translation- transfer of RNA information to
a protein molecule
• Beadle and
Tatum’s
experiment
about lack of
growth without
certain
enzymes
yielded the 1
gene = 1
polypeptide
hypothesis
RNA
• Made during transcription
• Single-stranded nucleic acid
• Bases: A, U (uracil), C, G
3 Types of RNA
• mRNA- messenger
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Processed before it leaves the nucleus
G cap and tail chain of A’s is added
Introns are spliced out (RNA splicing)
Introns- intervening sequence in a gene (junk)
Exons- expressed part of a gene
• tRNA- transfer
– match amino acid with correct codons
– One end of tRNA has anticodon (complementary
sequence to codon)
– tRNA’s are available for each amino acid
– Uses ATP to drive binding reaction
• rRNA- ribosomal
– along with proteins make up the ribosome where
protein synthesis takes place
The letters make a code
• Transcription and translation rely on 3letter “words” called codons
• Each codon codes for 1 amino acid
Where does transcription and
translation occur?
• Prokaryotes: both
happen in the
cytoplasm
• Eukaryotes:
transcription- in the
nucleus and
translation- in the
cytoplasm
Transcription
• Occurs in nucleus (we will be discussing eukayotes)
• 1 strand serves as template, RNA polyermase binds and
RNA nucleotides join together to create a n RNA
molecule
• Promoters- sequences that tell RNA pol where to bind
• Stages:
– Initiation- RNA pol binds and starts transcription,
– Elongation- continuation of RNA synthesis,
– Termination- RNA pol reaches terminator sequence
and RNA pol detaches
Translation
• Initiation- mRNA binds to ribosomal subunit
– Initiator tRNA binds to start codon and is carrying Methionine
– Larger ribosome subunit binds and initiator tRNA binds to P site
• Elongation- anticodon on tRNA recognizes codon on
mRNA and pairs in A site
– Peptide bond forms between amino acids in A and P site,
ribosome catalyzes bond
– P site tRNA leaves and A site tRNA moves over, now next tRNA
can bind to A site
• Termination- stops when a stop codon is reached (UAG,
UAA, UGA)
Why do through transcription
and translation?
• DNA  RNA  protein
– Controls the way genotypes produce
phenotypes
What happens if a mistake is
made?
• Mutations- change in the DNA sequence
– bases can be substituted for one another
– Bases can be deleted or inserted
– ** not always a bad thing, also creates new variety that could
possibly benefit
– Reading frame- triplet codon, can be affected by insertions and
deletions
– Mutatgenesis- creation of mutations
– Mutatgen- sources of mutations, chemicals, radiation
Viruses can affect transcription
and translation
• A virus is just a package of genetic information
• Types of viral reproduction
– Lytic cycle- leads to a cell filled with viruses lysing (breaking
open)
– Lysogenic cycle- cell with viral DNA in it’s genome divides
• When phage DNA enters cell it is called a prophage
RNA Viruses
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glycoprotein attaches to host membrane
RNA enters cell
enzymes remove RNA protein coat
complementary strand is made
new strand has instructions to make new proteins and viral RNA
new protein coat assembles on new RNA
lastly viruses leave cell coated in host’s plasma cell membrane
Plant Viruses
• Can stunt growth, lower crop yields
• Plant viruses must pass outer layer of plant, so wind
damage, injury or insects allow for entrance
• Some insects also carry viruses to plants, as well as
pruning shears may hold virus
Animal Viruses
• Influenza, hepatitis, chicken pox, herpes,
mumps, measles, AIDS, polio
How do new ones arise?
• Fragments of cellular nucleic acid that could move from
one cell to another, possible on the surfaces of injured
cells
• Today- mutations of existing viruses is the source of new
ones
• Also passing existing viruses to new hosts, via dust
containing viruses
• RNA viruses mutate faster because there is not DNA
proofreading happening
HIV Virus
• Acquired immune deficiency syndrome
• Human immunodeficiency virus
• Affects WBC’s of immune system, can’t fight off
infection
• Contains 2 RNA copies, instead of usual 1
• It’s a retrovirus, RNA virus that produces DNA using
reverse transcriptase (enzyme that does the
process)
• Viral DNA enters nucleus and then viral DNA is
transcribed and translated making more viral
proteins which then assemble and leave cell to
infect others