Transcript Document

The Molecule of Life
Important Scientists in the Discovery
Of DNA
• Rosalind Franklin &
Maurice Wilkins
– X-ray
crystallographic
images of DNA
• Photo 51
• James Watson, Francis
Crick
– Used x-ray
crystallography images
& models to figure out
double helix structure
Basic Structure
• Chains of nucleotides,
phosphodiester bonds
Dehydration
reaction
Basic Structure
• Double helix, 2 strands of antiparallel DNA nucleotides
• Ladder-like
– Sugar,P on outside (sides of
ladder)
– Bases on inside (rungs of ladder)
• Base pairing– Always A/T, G/C
(purine/pyrimidine)
– H bonds between bases
• Entire molecule twisted in helix
shape
3’
5’
3’
5’
DNA replication is semiconservative
The instructions For
making each new strand
Are contained in the old,
Parent strand!!!
http://www.lewport.wnyric.org/JWANAMAKER/animations/DNA%20Replication%20-%20long%20.html
The Players
• Helicase-enzyme which “unzips” the helix by
breaking hydrogen bonds
– Begins at origin of replication, creates “Y” replication
fork
• DNA Polymerase-enzyme which builds the new
strands of DNA
– “reads” base on parent strand, adds complementary
base to new strand
– Only moves in one direction!!!!-moves 3’(OH) to 5’(P)
• Leading strand-built continuously, in one piece,
toward the replication fork
• Lagging strand-built in pieces called Okazaki
fragments, away from the fork
• DNA ligase-puts Okazaki fragments together
Origin of replication-where DNA synthesis starts
Helicase first binds here
Steps in
DNA synthesis….
1.
2.
3.
4.
Helicase unzips helix at
origin of rep., forming
replication fork
DNA polymerase reads
bases on leading strand,
places complementary
nucleotides in place as it
moves toward rep. Fork
On lagging strand, DNA
polymerase builds new
strand in Okazaki
fragments-DNA ligase
joins them together
This continues until the
ends of the parent
strands are reached
Why can’t the
lagging
strand be
built continuously?
• The replication fork
continues to grow
and….
• DNA polymerase
can’t go “backwards”
DNA repair
• DNA polymerase
proofreads its
work as it goes
along & fixes
most mistakes!!!!
Transcription
DNA to RNA
DNA vs.RNA: differences
• DNA (deoxyribose
nucleic acid) and
RNA (ribose nucleic
acid) are both
nucleotide polymers.
These molecules are
very similar but there
are some distinct
differences between
them.
• Both molecules are helical:
– DNA is a double helix
– RNA is a single helix.
• DNA bases:
– Adenine (A), Thymine (T),
Cytosine (C) & Guanine (G)
• RNA bases:
–
A, G and C but T is replaced
with Uracil (U)
• DNA has one less oxygen on
the 5 carbon sugar than RNA;
thus the difference in their
names. Deoxyribose simply
refers to a ribose sugar lacking
an oxygen molecule.
DNA vs. RNA molecular difference
• The lack of one oxygen
molecule on the DNA 5
carbon sugar
RNA types
• 1. Ribosomal RNA (rRNA): make up
ribosomes
• 2. Transfer RNA (tRNA): transport
amino acids to ribosomes
• 3. Messenger RNA (mRNA): copied from
DNA, conveys information from
chromosomes to ribosomes
DNA vs. RNA: Similarities
• Both essential in protein
synthesis.
– Transcription: DNA is
transcribed into Messenger
RNA (mRNA).
– Translation: mRNA is
translated into a polypeptide
chain with the aid of
Ribosomal RNA (rRNA)
and Transfer RNA (tRNA).
Transcription Essentials
• Transcription occurs in nucleus.
• Transcription: production of mRNA copy of the
DNA gene.
– Think of DNA as instructions to build hardware
(proteins), unfortunately, these instructions are in
another language and incomprehensible to the workers
that will eventually assemble the hardware. This is
where mRNA will come into the picture - to provide
new instructions that will be used by the workers.
Steps of Transcription
1. Initiation: DNA is unzipped and the enzyme
RNA polymerase runs along the template strand of
the DNA.
– The template strand of DNA can be identified
by finding the promoter region: nucleotide
sequence T A C at the 3’ end (If the strand is
written backwards it may look like C A T at the
3’ end). This identifies that strand as the
template and the other strand, the information
strand, will not be used in this transcription
(this does not mean, however, that it may not
be used in future transcription processes).
Steps of Transcription
2. Elongation: As the RNA polymerase runs along
the DNA template strand it will add the
complementary RNA nucleotides to the DNA
nucleotides.
– This means that G will be paired with C, and
visa versa, and A (DNA) will be paired with U
(RNA - rather than T in DNA replication) and
T (DNA) paired with A (RNA).
Steps of Transcription
3. Termination: Transcription continues until RNA
polymerase reaches a DNA region called the
termination signal: nucleotide sequence that
marks the end of a gene.
• When the single helix mRNA strand is
complete, RNA polymerase releases the DNA
andnew RNA molecule. The DNA will re-zip
into the double helix.
The Process of Transcription
Diagrams of Transcription
Processing the Products of
Transcription
• In eukaryotes, once the mRNA is transcribed it
will then be processed.
– A cap and tail will be added to the ends of the mRNA
strand.
– The strand will be spliced.
• The introns (non-coding regions) will be removed
• The exons (coding regions) will be spliced together
– The completed mRNA strand has groups of three
nucleotides known as codons (for example, A U G is
the codon in mRNA that was transcribed from T A C).
These groups of three will code for a particular amino
acid in translation (A U G will code for the start amino
acid, methionine, in translation).
Translation
From RNA to Protein
The Process of Translation
• Protein Synthesis: Translation
Translation
• Translation occurs when the mRNA strand moves
out of the nucleus and into the cytoplasm to a
ribosome.
– At this point mRNA, rRNA and tRNA all come
together.
• The rRNA consists of two parts, the large ribosomal unit and
the small ribosomal unit.
• On the large ribosomal unit are two sites- the A site and the P
site. These will be the sites of polypeptide synthesis and
elongation.
– The rRNA is like the factory of translation and tRNA is
the worker.
Terminology for Translation
The tRNA molecules have an amino acid
attachment site and carries an anticodon.
• Anticodon: the 3 nucleotide sequence on t-RNA
which the ribosome must fit against m-RNA to
ensure that the correct amino acid is placed in the
growing protein during translation.
• The tRNA will pick up the appropriate amino acid
in the cytoplasm that is coded for by the mRNA
codon that its anticodon matches.
– A lock and key process.
General Steps of Translation
• Initiation: tRNA is bonded to mRNA, rRNA polymerase
binds to mRNA strand.
• Elongation: Ribosome reads mRNA chain in three
nucleotide groups (codon) & inserts another tRNA.
– tRNA anti-codon (with amino acid) binds to mRNA codon.
• Translocation: the ribosomal unit physically moves
(translocates) 3 bases (a new codon: AUG) along the
mRNA in the 5' ---> 3' direction.
• Termination: tRNA recognizes release factors of
nonsense codon. Newly completed polypeptide is released
from ribosome.
Specifics of Initiation
• 1. Initiation:
– The large (top) and small
(bottom) ribosomal units must be
bound together to the strand of
mRNA with the help of rRNA
polymerase.
– The ribosomes position
themselves so that the "start"
codon sequence AUG on the
mRNA is exposed.
– A tRNA unit with the anticodon
sequence UAC bonds to the
exposed "start" codon. This first
tRNA only carries the amino acid
methionine (met) which is now
set in place.
Specifics of Elongation
•
2. Elongation:
– tRNA and its associated amino acids
bond to the complementary codon on
mRNA to elongate the polypeptide
chain.
– As the large ribosomal unit sets in
place, a second codon on mRNA is
exposed (in this case, the codon is
CAU) (A site).
– Elongation factors assists the 2nd
tRNA to bond to this newly exposed
codon.
– The newly arrived amino acid (his) is
lined up next to the 1st amino acid
(met).
– An enzyme binds both amino acids
via dehydration synthesis (loss of
water) bonding.
Amino Acid Chart
Intermediate Step
• 3. When the 1st two
amino acids are bonded,
the first tRNA leaves
the mRNA/ ribosomal
complex.
Specifics of Translocation
• 4. Translocation:
– Ribosomal unit physically
moves (translocates) 3 bases
(a new codon: AUG) along
the mRNA in the 5' ---> 3'
direction.
– When the new codon is
exposed, another elongation
protein assists the new tRNA
and its associated amino acid
(Ser) bind to the codon. After
this occurs, an enyme binds
the amino acids His and Ser
via dehydration synthesis.
Specifics of Termination
•
5. Termination:
–
–
–
The diagram to the right illustrates the
ribosomal complex after it has been
translocated down the mRNA many codon
sequences. The ribosome has constantly
read the mRNA in the 5' ---> 3' direction.
The result is a growing chain of amino
acids, all bonded together to make a
polypeptide chain.
When a codon with the nonsense sequence
UAA, UAG (seen here), or UGA is
exposed, that is a signal that translocation
is to stop.
The stop codon is not bonded to a
complementary anticodon sequence on a
tRNA. Rather, a protein known as a release
factor binds at the A site. The release factor
ultimately will help release the finished
polypeptide chain in the next step.
Release
• 6. The release factor
prevents further reading
of the mRNA message.
– The polypeptide molecule is
released from the ribosomal
units.
– The mRNA and the large and
small ribosomal units are thus
free to begin the translation
process again.
Protein Synthesis
RNA – ribonucleic acid
• RNA nucleotides are composed of a sugar,
a phosphate group, and a nitrogen base
• Ribose is the sugar
Nitrogen Bases
•
•
•
•
Cytosine
Adenine
Guanine
T nope Uracil
Single strand
• RNA is single stranded not double stranded
like DNA
3 types of RNA
• mRna messenger – single long chain that
carries the message from DNA in the
nucleus to the ribosomes in the cytoplasm
(Transcription)
mRNA Codon
• Nucleotides are
arranged into
groups of three.
• Also called
triplets – 3
nitrogen bases
Amino acids
• Each codon
represents one of
the 20 different
amino acids
– AUG
– CGA
– UGA
3 types of RNA
• tRNA transfer – hair
pin chain that carries
amino acids to help build
proteins
• tRNA contains an anticodon on one end and an
amino acid on the other
3 types of RNA
• rRNA ribosomal – rRNA and protein make
the ribosome
Problem
• DNA is in the nucleus……
• Proteins are made on the ribosomes in the
cytoplasm……
• How does the information get from the
DNA in the nucleus to the ribosomes in the
cytoplasm?
Transcription
• The process where
genetic information
from DNA is
“downloaded” to
messenger RNA
This happens in the nucleus only!
• The DNA never leaves the nucleus!
Transcription
• Works just like DNA replication, but produces
only one strand of mRNA
• The DNA is unzipped by the enzyme “Helicase”
• RNA Polymerase reads the DNA and pairs
complimentary RNA nucleotides to the RNA
strand
• The mRNA leaves the nucleus
• DNA zips back up
Transcription cont.
• The strand of mRNA is complimentary
because it is constructed from the DNA
nucleotide sequence!
• Remember: Uracil replaces Thymine in the
mRNA sequence
Translation
• The process of making a protein from the
mRNA code……..
• The mRNA leaves the nucleus and goes to
the ribosome
• The mRNA goes through the ribosome and
is read by the ribosome
• The ribosome recognizes a codon and send
a message to the tRNA
Translation
• tRNA carries an amino acid that is floating
in the cytoplasm over to the ribosome
• The tRNA anti-codon binds to the mRNA
codon while linking the amino acids
together……….
• Thus creating a polypeptide, amino acid
chain (protein)
Make a protein
• At the ribosomes, the tRNA gives the amino acid
away to build protein
• Then goes back into the cytoplasm to look for a free
amino acid and repeats
• The ribosome keeps reading the mRNA taking the
amino acids from tRNA and sticking them together to
make a protein chain
• This all happens very quickly!