Transcript Replication, Transcription and Translation

DNA – The Molecule
Deoxyribonucleic Acid
Deoxyribonucleic Acid
• DNA is made of a phosphate group, a
simple sugar and a nitrogenous base.
• Nitrogen Base- Organic Ring structure that
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contains one or more Nitrogen atoms
Four Nitrogen Bases of DNA
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–
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Note: No Oxygen on the #2 Carbon
Guanine- double ring structure
1
2
Adenine
Cytosine – single ring structure
Base Pairings
• Adenine pairs with Thymine
• Cytosine pairs with Guanine
• Adenine and Guanine are double ring
structures called purines
• Cytosine and Thymine are single ring
structures called pyrimadines
Ribonucleic Acid - RNA
• RNA is similar in structure to DNA.
• RNA is single stranded as opposed to
double stranded (DNA)
• RNA has a ribose sugar as opposed to a
deoxyribose sugar (DNA) see figure.
• The base pairs are A,C,G,and U
– Uracil replaces Thymine.
The Double Helix
• The spiral shape of DNA is called a double
helix. It basically resembles a twisted
ladder.
• The uniqueness of a strand of DNA is due
to the order of the base pairs.
• Every time a cell reproduces, it must make
an exact copy of its DNA.
Replication
• The process in which DNA is copied.
• Replication must be perfect, the method of
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replication is described as being semiconservative.
semi-conservative- each new strand of DNA is
composed of half original and half new material.
Opposed to having one completely new strand
of DNA and one completely original strand of
DNA
Chargaff’s Rule
• The amount of purines = the amount of
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pyrimadines.
Therefore, in each species the amount of A=T
and the amount of C=G.
The amount of A,T, C and G in DNA varies from
species to species.
Both of these aspects of Chargaff’s rule helped
to determine and understand the process of
replication
DNA Replication-the process
• Replication is similar
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to unzipping a zipper.
An enzyme breaks the
hydrogen bonds
between the base
pairs.
Then as the molecule
unzips, free
nucleotides pair up
with each of the
strands.
Steps in Replication
• 1. Unwinding- The old strand of
parent DNA is unwound and
unzipped.
– Helicase- enzyme that unwinds DNA.
• 2. Complementary base pairing- A
pair with T and C with G.
– An RNA primer is added to initiate the
process.
– DNA polymerase adds the base pairs,
and removes the RNA primer.
Replication (continued)
• 3. Proofreading the strand- Most
mistakes in pairings, and even some
insertion of the wrong base pair or a
deletion are corrected in the final
step.
– Polymerase is the enzyme that is
involved in this process.
• 4. The phosphate groups attach to
the next ribose group with the help of
the enzyme Ligase.
It’s not THAT simple
• During the replication process,
nucleotides are joined in a 5’ to 3’
direction. (See drawing)
• Only one strand of DNA runs in a
5’to3’ direction.
• In order for replication to occur
quickly both strands must be copied
at once.
• This is accomplished by having a
leading and lagging strand.
Leading Strand
• As the molecule unzips the new DNA is
synthesized going in the 5’ to 3’ direction
heading toward the replication fork.
Lagging Strand
• The second strand works slower. It also
must go in the 5-3 direction, but it goes
away from the replication fork. Thus,
leaving gaps of unpaired nucleotides.
• Okazaki Fragments- Pieces of replicated
DNA found on the lagging strand.
Responsible for speeding up the rate of
replication.
DNA Replication- the figure
Polymerase Chain Reaction (PCR)
• Technique used to produce several
identical copies of DNA using the enzyme
polymerase.
• See Seven Daughters of Eve
Variability in DNA
• DNA can vary as a result of mutations and
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“jumping genes” called transposons.
Transposons- moveable genetic elements.
These are sections that control or suppress the
expression of information.
Because transposons have the ability to move
from one part of the chromosome to another
they can extremely alter the phenotype.
Transposons cause:
• Localized mutations. These are
mutations that may occur in some cells
but not others.
• Are a source of chromosomal
mutations. translocation, deletion
and inversions
• Can leave a copy of themselves before
they jump.  duplication.
• Have been shown to be the basis of
bacterial resistance to antibiotics.
Variability (cont.)- MUTATION
• Mutations in the base pair sequence
– Mutagen- environmental substance that
causes mutations.
• Types of mutations
– Frameshift mutations- An insertion or
deletion of a nucleotide which results in the
changing of the genetic code.
– Point mutation- Change in a specific
nucleotide which alters the codon. See page
247 for info on mutations and sickle cell anemia.
Chapter 16- Gene expression
• We have already discussed similarities/
differences of DNA and RNA.
• What is the location and function of DNA?
Location: nucleus, function: “directions” for
protein synthesis
• What is the location and function of the
ribosomes?
• Does anyone see a problem?
Location: cytoplasm, function:
protein production
Directions for the product are NOT at the factory!
The Messenger
• We must have a messenger to carry the
“info” of DNA to the factory” (ribosome)
Transcription
Translation
• DNA  mRNA  ribo.protein
The Central Dogma of
Molecular Biology
• The sequence of nucleotides in DNA codes
for a sequence of nucleotides in RNA
which directs the order of Amino Acids in
a polypeptide (protein).
The players
• First the three types of RNA
– Messenger RNA (mRNA)- takes a message
from DNA in the nucleus to the ribosome in
the cytoplasm.
– Ribosomal RNA (rRNA)- along with some
proteins, makes up the ribosome which
synthesizes polypeptides (proteins).
– Transfer RNA (tRNA)- brings Amino Acids to
the ribosome.
The processes- The Game
don’t be a hater
• Transcription- Process where a DNA strand
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serves as a template for the information on
mRNA.
Translation- Process where the sequence
of codons in mRNA determines the
sequence of Amino Acids in a polypeptide.
– Codon- three nucleotides in DNA or RNA which
codes for a particular amino acid or termination
of translation. See fig 17.4 pg 329
Transcription
• DNA TAC TGC CTG GCC ACT
• When rewritten as RNA
• RNA AUG ACG GAC CGG UGA
The Code: the language
Translation-decoding the RNA
• AUGACGGACCGGUGA
• AUG- methionine
• ACG- threonine
• GAC- aspartate
• CGG- arginine
• UGA- stop
Transcription- the details
• Transcription is started by the promoter
region.
• When RNA is produced it is often shorter
than the template strand of DNA.
• mRNA is processed and so that only
particular sections of DNA end up in the
mature segment of mRNA.
Transcription
• Only one strand of DNA is the template for
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mRNA production. It is called the template
strand.
The template strand contains the promoter
region. There can be multiple promoter
regions on one strand. As a result, RNA
production can occur at multiple sites on a
DNA strand.
The strand that is not used for mRNA
production is called the inactive strand.
The Processing of mRNA
Introns-Portions of primary mRNA which
are removed from mRNA.
Introns can be thought of as DNA which
does not bind to RNA. Whatever
proteins are coded for by the bases on
these sections of DNA will NOT be
produced.
The presence of introns suggests
that there is more DNA than is necessary.
The role of introns is still under investigation.
Exons- Portions of DNA which bind to
RNA. Whatever traits these segments
code for will be expressed.
mRNA
• mRNA that still contains introns is called
primary mRNA.
• A complex of special protiens called
spliceosomes remove the introns, leaving
behind just the exons on a strand of
mRNA.
• The mature mRNA leaves the nucleus
through a pore in the nuclear membrane.
Translation requires three steps
• 1. Initiation- A small ribosomal unit
attaches to the start codon (AUG). The
first tRNA called the initiator tRNA pairs
with the codon. Then a large ribosomal
unit attaches and translation begins.
– Anticodon- A group of three bases on the
tRNA that is the compliment to the three
bases of the codon on the strand of mature
mRNA.
Structure of tRNA- note the
anticodon
Translation step 2: Chain
elongation
• Amino acids are added in a long chain to form a
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peptide.
Elongation occurs when the tRNA moves from
one site on the ribosome to another. (see figure
16.10).
The amino acids brought in by previous tRNA is
added to the current tRNA. Thus extending the
chain of amino acids in a specific order as
specified by the mRNA.
Chain elongation
Step 3. Chain termination
• The stop codon does not code for a
particular amino acid and causes peptide
production to cease.
• The tRNA is cleaved and the peptide
leaves the ribosome.
Transcription and Translation