Download Ch. 12 DNA

DNA
12-1
Originally, people thought protein was genetic material because it was
complex. However, the work of 4 scientists showed it was really
DNA…
1928- Frederick Griffith
* was trying to find a vaccine for Streptococcus pneumoniae - the
bacteria that caused pneumonia
*Vaccine - substance prepared from weakened or killed org. that is
introduced to the body to build immunity
* was working with 2 strains of bacteria: S strain and R strain
1.) Strain enclosed in a polysaccharide capsule
~protected from body's defenses
~makes it virulent…able to cause disease
~bacteria grow smooth edged colonies in a petri dish (S)
2.) Strain lacks the poly. capsule
~ does NOT cause disease
~ bacteria grow rough edged colonies in a petri dish (R)
He tested both strains on mice…
**S strain caused disease while the R strain didn't
So to make a vaccine - heat-killed the S strain…raised the temp. to
where the bacteria couldn't divide
Mice with the heat-killed lived!
Next, mixed heat-killed S with R bacteria and the mice died!
* S strain - made you sick! = mice died
* R strain - you were "all right" = mice lived
* heat killed the S bacteria = mice lived
*R strain + heat-killed S strain = mice died
* discovered the R strain had undergone a transformation when it
acquired the genetic material from the heat-killed S strain
COOL! But what does this mean?
1944 - Oswald Avery - discovered DNA was responsible for
transformation
* was trying to determine what was responsible for
transformation… DNA, RNA, or protein
* used enzymes to destroy each of these 3 in turn and discovered
that when protein and RNA were destroyed, transformation
still took place.
* when DNA was destroyed, transformation stopped.
* thus proving DNA was responsible for transformation and was
the genetic material
However, most scientists were still not convinced…
1952 - Hershey and Chase
* supported Avery's findings by showing how DNA was passed on
to other cells
Used a bacteriophage - virus that infects bacteria - T2 phage
Looks like this:
Protein coat
Head
DNA
Tail
Here's how it works…
*T2 phage attaches to a bacterium - injects its DNA
notice the head and tail stay out of
the bacterium!
* Phage DNA then replicates and tells the bacterium to make more
phages
* when the phages mature, they burst out of the bacterium to seek
new bacteria to attack!
Hershey and Chase used the natural properties of the T2 phage to prove
that DNA was genetic material…
* DNA has phosphorous - tagged it with radioactive phosphorous = 32P
~grew one batch of the phage in a nutrient broth with the 32P
*The protein coat has sulfur in it - tagged it with radioactive sulfur - 35S
~grew one batch in a nutrient broth with the 35S
After tagging the batches they put one sample of T2 with 35S in a vial
with E.coli and let it attach to the bacteria
They used a blender to separate the phages from the infected bacteria
They centrifuged the mix and separated it into 2 layers
When they looked in the microscope, no protein tag was in the bacteria!
They repeated this with the tagged DNA and got different results…
After examination, most of the E.coli were infected with the 32P tagged
DNA
Also, the new generation of phages also had the tagged DNA!
This was concrete proof that DNA was genetic material! YAY!
Shape of DNA - 12 -2
DNA is made up of nucleotides linked together in a chain
Each nucleotide is made of 3 parts:
1.) Phosphate group
2.) 5 C sugar molecule - called deoxyribose
3.) Nitrogen base
Looks like:
Different nitrogen bases are what make each nucleotide unique!
4 bases
1.) Adenine
2.) Guanine
Purines - each have a double ring of C and N
3.) Thymine
4.) Cytosine
Pyrimidenes - each have a single ring of C and N
In 1949, Erwin Chargaff noticed that the amount of Adenine always
equaled the amount of Thymine …. same goes for Guanine and
Cytosine
* Total amounts of these bases vary by organism
* Called base-pairing
* He basically said that how the base pairs are arranged, determine
what genes they are
1953 - Watson and Crick developed a 3-D model of DNA and it looked
like a twisted or spiral staircase == double helix
* sides made of alternating sugar and phosphorous units
* "rungs" made of 1 purine and 1 pyrimidene held together by H
bonds
~ always form like this b/c only A can bind with T and only
G can bind with C
~ this makes the 2 strands complementary
Ex: if one strand has a sequence like this:
ATTGCTACGG
the other strands sequence is:
TAACGATGCC
How is DNA copied?
By way of replication! - this synthesizes a new DNA strand
Done in steps:
1.) DNA must be unwound
- helicase - enzyme that splits the double helix by breaking
the H bonds -- "unzips" the helix
2.) Other enzymes hold the strands apart and keep them from
twisting
3.) DNA polymerase (another enzyme) attaches at the replication
fork and moves down each strand adding new nucleotides to
the exposed bases
~ nucleotides added according to the base-pairing rules
~ this forms 2 new DNA strands!
~ DNA polymerase stays attached until it reaches a signal
that tells it to stop
Errors can be made! But these are corrected immediately
~ DNA polymerase can "erase" errors by replacing any base that
was incorrectly paired - called "proofreading"
~ Will not move on unless it is correctly paired
~ This prevents errors in replication…
errors made maybe about 1 in 1 billion!!
Replication begins anywhere along the strand - not at one end!
* this saves time
* many replication forks work at once to shorten replication time
from weeks to minutes!
*forks split and move in both directions to do this
*Human DNA has a replication fork about every 100,000 nucleotides!
From Genes to Proteins!
After discovering genes, how does this info
translate into traits?
Answer (sort of): the genes tell us what proteins
to make and these lead to the traits!
But how does DNA determine proteins?
Well… proteins are NOT made directly from genes!
~ genes act like instructions for building
proteins
*working instructions are made of
RNA - ribonucleic acid
How is RNA different from DNA?
1.) Single strand of nucleotides
2.) Has the 5 C sugar ribose instead of
deoxyribose
3.) Has the base Uracil (U) instead of
Thymine (T)
*U is complementary to A!!!
RNA present in 3 forms:
1.) mRNA - messenger
~ RNA copy of a gene used as a blueprint for a
protein
~ carries and delivers the message to the site
of
translation
~ serves as a template for AA assembly
2.) rRNA - ribosomal
~ plays a structural role in ribosomes during
translation
3.) tRNA - transfer
~ translates or interprets the mRNA
sequences
into AA sequences
All of these are used for gene expression*2 stages:
1.) Transcription - info from DNA transferred to
mRNA
2.) Translation - info from mRNA used to make
protein
Like this:
Transcription:
* This re-writes the info from DNA and makes
mRNA
~ in eukaryotes… takes place in the nucleus
~ in prokaryotes… takes place in the cytoplasm
*Begins with RNA polymerase binding to a
promoter
~signal made by a specific sequence of DNA =
start codon
*RNA polym. unwinds the DNA and uses ONE SIDE
as a template
*RNA polym. then makes a new RNA strand by
adding the correct nucleotides
** uses U instead of T to pair with A
**
Ex: if the DNA strand's sequence was
ATTGCGAATCC
the complementary mRNA strand would be:
UAACGUUAGG
* Just like DNA replication, the phosphate group
and the 5 C sugar make the backbone of the mRNA
* The mRNA hangs off the DNA like a tail
*Once the mRNA is through being paired, the DNA
strand recombines to form the double helix!
*Transcription will continue until it reaches a stop
or "terminator" codon.
*This is where the RNA polym. stops adding bases
and leaves the DNA strand.
*The new mRNA strand can now go to the next
stage of gene expression!
When transcription is finished, the mRNA leaves
the nucleus through nuclear pores and enters the
cytoplasm
*It is now ready to be translated!
The instructions are written as codons - 3
nucleotide sequences
* each codon codes for an AA or a terminator
codon
Ex: the codon UUU codes for the AA phenylalanine
Stop codons are ALWAYS one of these:
UAA, UAG, UGA
Table 9-1 is the genetic code for all 20 AAs!
For the most part, the genetic code is universal…
~ this means we theoretically come from one
common ancestor!
Translation
* Recall that this is info from mRNA used to
make proteins!
This stage takes place in the cytoplasm!
tRNA is waiting for the mRNA to arrive…
tRNA looks like this:
The anticodon matches up with a codon on the
mRNA
Stop codons have no matching tRNA so translation
stops!
Opposite the anticodon is where the AA attaches
To make a protein, several steps take place…
1.) mRNA is held between the large and small parts
of a ribosome
2.) within the lg. ribosome, there are 2 sites:
P site
A site
3.) the start codon - AUG - must line up in the
first site - called the P site.
4.) the A site is empty right now waiting for the
tRNA to match with the start codon
5.) the matching tRNA comes into the P site with
its AA attached to it - always MET
6.) once this happens, the ribosome is now working
and the codon under the A site can receive its
tRNA that matches the mRNA's codon
7.) Enzymes help bond the two AA with peptide
bonds
8.) The tRNA in the P site now leaves without its
AA and the tRNA in the A site moves over to the
P site with the growing protein chain - this also
moves the strand of mRNA b/c they're still
attached!
9.)The A site is now ready to get a new tRNA
10.) the process continues until there is a stop
codon on the mRNA in the A site
11.) The ribosome then falls apart and the new
protein is released into the cell!
This process happens quickly and often…
Once one ribosome is done with a protein,
another ribosome can do it again by hooking up to
the start codon and making another protein.
Mutations
12-4
We know that a mutation is a kind of a change in genetic
material…
Kinds of mutations:
Gene mutations~
1.) Point mutation - involve one or few nucleotides
~ occur at a single point on the DNA strand
~ can include subs, insertions, or deletions
~ subs are usually harmless or affect only one AA
~ insertions or deletions can result in another
mutation…
2.)Frameshift mutation - when a nucleotide is added or
deleted, the whole way the strand is read can change!
~ is bad b/c will change every AA that follows on
that strand!
EX: Original DNA strand: TAC GCA TGG AAT
mRNA strand:
AUG CGU ACC UAA
AA's that result:
Met Arg Thr Leu
if DNA has a sub:
mRNA strand:
AA's that result:
if DNA has an insertion:
mRNA strand:
AA's that result:
TAC GTA TGG AAT
AUG CAU ACC UAA
Met His Thr Leu
TAT CGC ATG GAA T
AUA GCG UAC CUA A
Ile Ala Try Leu
Chromosomal mutations:
~involve changes in the structure or number of
chromosomes
*Deletions, inversion, translocations, duplications.
( we already covered this!)
* These types of mutations are usually harmless, but
some can have some adverse affects…
~genetic disorders such as
*sickle cell anemia
*cystic fibrosis
*cancers
*Some can have benefits!
~some have been known to cause a resistance to
HIV!
~some result in polypoloidy - an org. has an extra set
of chrom - usually seen in plants
* can make that plants stronger and larger
ex; bananas and citrus fruits
*Also these mutations are the source for genetic
variability within species1