Download Molecular Genetics

Molecular Genetics
Ch. 12 - 14
DNA & RNA – The Genetic Material
• Griffith - was the 1st to
identify genetic material –
exhibited transformation
with encapsulated and
non-encapsulated bacteria
– was able to prove that
DNA is the genetic material
• Avery - repeated
Griffith’s work –
discovered that DNA is
the nucleic acid that
stores & transmits the
genetic information
from 1 generation of an
organism to the next
• Hershey & Chase - used bacteriophages & E. coli to
prove that DNA is the genetic material (phosphorus
was injected into the bacterium and not sulfur)
Getting started
• What they already knew…
– The protein coat contains sulfur and very little
phosphorus.
– The DNA contains phosphorus but no sulfur.
• The setup…
– Tagged some bacteriaphages with radioactive
sulfur.
– Tagged some with radioactive phosphorus.
Scientists were
finally convinced.
Thanks USA!
• Watson & Crick used Rosalind
Franklin’s Photo 51 to discover the
3-dimensional double helix that
represents the shape of DNA. It
looks like a spiral staircase with 2
strands of nucleotides wrapped
around the axis. Each rung is a
purine or pyrimidine held together
by a hydrogen bond. The strands
are complements of each other.
The Structure of DNA
• DNA is a long, thin molecule made of subunits called
nucleotides_ that are linked together like a
chain. The 3 parts of a nucleotide are the
1. 5-Carbon Sugar
2. Phosphate group
3. Nitrogen base
• The sugar molecule and the phosphate are the same for all
nucleotides – only the bases differ.
Chargaff is the scientist who discovered that the amount of
adenine equals the amount of thymine & the amount of
guanine equals the amount of cytosine. In DNA, Adenine
always pairs with thymine and cytosine always pairs with
guanine. Purines always bond to pyrimidines in DNA & RNA.
DNA Orientation
• One side of the DNA
molecule has an
orientation of sugar
that goes from 5’ to
3’ while the other
strand reads from 3’
to 5’.
Chromosome Structure
• In prokaryotes – it’s in a ring in the cytoplasm
• In eukaryotes – the DNA coils around histones
(beadlike) & forms a nucleosome. The
nucleosomes condense into chromatin fibers
that coil into a chromosome.
SEMICONSERVATIVE REPLICATION
• When DNA is replicated, that means it is synthesizing
a new strand. The DNA must 1st be unwound. This
is accomplished by DNA helicases that break the
hydrogen bonds. The replication fork is the point
where it unzips.
• At the fork, DNA polymerases move along
strands of DNA adding complementary bases
to exposed bases.
• Two 1/2 new, ½ old double helices form that
eventually get the signal to detach. The DNA
polymerase also has the ability to “proofread”
the strand to check for mistakes.
original strand
Two molecules of DNA
new strand
• The molecules may have many forks
throughout the molecule moving in separate
directions that causes it to form bubbles.
There are many origins of replication in eukaryotic chromosomes.
BASE PAIRING
• DNA polymerase
catalyzes the addition of
the new nucleotides to
the new DNA strand
starting at the 3’ end.
• DNA ligase links the
pieces together into a
single DNA strand.
• RNA contains the
working instructions
for the cell. It consists
of a single strand of
nucleotides (DNA is
double), a 5 carbon
sugar called ribose
(DNA had
deoxyribose), and
uracil rather than
thymine as a base.
DNA
(2 strands)
RNA
(1 strand)
3 Types of RNA
• mRNA - (messenger) – complementary to Dna
– carries message from DNA to direct protein
synthesis (codon)
• rRNA – (ribosomal) – associates with protein to
form ribosomes
• tRNA – (transfer) – transports amino acids to
ribosomes – acts as a translator by picking up
the appropriate amino acids & recognizing the
appropriate codons in mRNA (like a concierge) –
has anticodon site & amino acid
• All 3 types of RNA are essential for processing the
info from DNA into proteins. This process is called
gene expression. This occurs in 2 stages:
1. Transcription - the info in DNA is transferred to
mRNA
2. Translation - the info in mRNA is used to make a
protein
This may be summarized as
• DNA ---------> RNA ---------> protein
Transcription
Translation
• Transcription in eukaryotes occurs in the nucleus and
in prokaryotes occurs in the cytoplasm. It begins
when RNA polymerase binds to a promoter
region. A promoter is a specific sequence of DNA
that acts as a “start” signal for transcription.
•
The molecule begins to unwind and the double helix is
separated. This exposes the N-base. Only 1 strand of DNA
serves as the template. The polymerase moves down the
strand like a train on a track and pairs the exposed base to
its complementary RNA nucleotide.
• These are linked with covalent bonds. It can
work at a rate of 60 nucleotides/second. This
continues until it reaches a stop signal on the
terminator. The enzyme is released from DNA
and RNA is released into the cell for the next
stage.
RNA SPLICING
• Much of the DNA of an organism does not code for
protein. Most genes are interrupted by long
segments of nucleotides that have no coding
information (kind of like fillers in medications). The
noncoding sequences are called introns_ while those
that code for amino acids are called exons because
they are expressed. Introns are believed to add
genetic flexibility to organisms by shuffling. In a long
sequence, the introns are chopped out and a short
_exon sequence is made that actually forms the
protein.
• mRNA is built after transcription. It leaves the
nucleus through pores and enters the
cytoplasm.
One Gene-One Polypeptide Hypothesis –
The function of a gene is to dictate the
production of a specific enzyme.
One More Time!
Step 1: Hydrogen bonds
between complimentary
bases break
DNA “unzips”
Step 2: DNA strands
pull apart from each other
Step 3:
RNA nucleotides
in the cell match
up with only one
side of the
“unzipped” DNA
each “unzipped’
strands forms a
template for a
mRNA strand
RNA nucleotide
Step 4:
RNA nucleotides
continue to match
up with
“unzipped” DNA
until the message
is completely
transcribed
mRNA strand
One side of DNA strand
mRNA strand
Step 5:
mRNA strand
breaks off
from the DNA
strand
One side of DNA strand
Step 6:
mRNA strand
leaves the
nucleus for
the ribosome
Step 7: Once the mRNA
leaves, the DNA “zips”
back together
TRANSLATION
• The equipment for translation is located in the
cytoplasm where tRNA is found. tRNA consists of 3
loops. One of the loops has a 3-nucleotide sequence
called an anticodon that is complementary to the
genetic code. This enables tRNA to bind to mRNA
through hydrogen bonding. The 3 stop codons are
UAG, UAA, UGA.
A ribosome has 3 binding sites. The first site holds
mRNA so that its codons are accessible to tRNA. The
A site holds a tRNA molecule that is carrying its
specific amino acid. The P site holds a tRNA
molecule that is carrying its specific amino acid
attached to the growing protein chain.
Steps of Translation
•
•
•
•
1. “Start” codon initiates building.
2. Codons match with anticodons.
3. Peptide bonds form between amino acids.
4. “Stop” codon stops the building once it hits
the A site.
• 5. Polypeptide is released.
• Codons contain the
instructions for building
proteins. They consist
of a 3-nucleotide
sequence that
corresponds either to
an amino acid or a stop
signal. The genetic
code represents the
amino acids and stop
signals that are coded
by each of the possible
mRNA codons.
DNA firefly
Central Dogma
• First transcribe the DNA sequence to mRNA and then
translate into a protein
5' GGGAACGATGCCCCTTAA 3´
• Use your codon chart
1) Which amino acid does the mRNA codon
AAU code for?
2) Which amino acid does the tRNA anticodon
CGC code for?
3) Which amino acid does the DNA sequence
TAC code for?
PROKARYOTIC GENE REGULATION
• Every cell must be able to regulate when particular
genes are used. Otherwise, there would be no order
to the cell. Every function that an organism carries
out is the controlled expression of genes.
An operon is a cluster of genes that codes for
proteins with related functions. The promoter is
where the RNA polymerase first binds to the DNA. It
initiates transcription. The operator controls RNA
polymerase’s access to the structural genes. It acts
like a switch, turning the operon on or off. When a
repressor is bound to the operator, the operon is
turned off. Transcription can resume when an
inhibitor removes the repressor.
2 Types of Operons
• Repressible – transcription is normally
repressed (off), but when the metabolite
(tryptophan) is low, it’s turned back on (trp
operon) – allows bacteria to stop making
essential molecules that are already present
to save energy & materials
• Inducible - (lac operon) – repressor is normally
present, but is turned off by an inducer such
as lactose so that transcription can occur
Eukaryotic Gene Regulation
• Hox genes – (homeobox genes) – lay out the
general body plan of the organism – code for
transcription factors that determine what
body part goes where
• A chromosome puff is a region of intense
transcription in eukaryotes. It is believed to
form in order to make the genes in that region
more accessible to RNA polymerase.
MUTATIONS
• Changes in the organism’s hereditary information are
known as mutations. Not all mutations are bad –
some may lead to positive changes. Mutations in
gametes_ may be passed on to the offspring
whereas somatic cell mutations are not. Some
mutations may alter the chromosome, but point
mutations change only 1 or a few nucleotides. There
are 3 types of these:
1. _Substitutions - replace 1 nucleotide with another
2. Insertions or deletions - 1 or more nucleotides are
added or deleted
• 3. Duplication - duplicates sequence over & over
(responsible for several disorders)
THE BIG FAT CAT ATE THE RAT AND GOT ILL
THE IGF ATC ATA TET HER ATA NDG OTI LL
(deletion)
• Mutagens are
environmental agents
that cause
mutations. Examples
include radiation, UV
light, &
chemicals. Carcinogens
are cancer-causing
agents.
• p53 is an example of a
gene that can protect
cells from mutations
such as in cancers.
• Mutations
• Transposons are genes that have the ability to
move from one chromosomal location to
another. They are sometimes called “jumping
genes.” When they move from one location to
another, they may inactivate a gene or cause
mutations. These may influence
evolution. These were discovered by Barbara
McClintock in the 1950s.
• Multigene families are clusters of almost
identical sequences of genes. These families
are used to trace evolution. An example of
this in the human body is with hemoglobin,
which has 12 different genes in 2 different
families that are active in different times of
life.