1. DNA Replication
How DNA Makes Copies of Itself
Before a cell divides, its DNA is replicated (duplicated.) Because the two strands of a DNA molecule have
complementary base pairs, the nucleotide sequence of each strand automatically supplies the information
needed to produce its partner. If the two strands of a DNA molecule are separated, each can be used as a
pattern or template to produce a complementary strand. Each template and its new complement together
then form a new DNA double helix, identical to the original.
Before replication can occur, the length of the DNA double helix about to be copied must be unwound. In
addition, the two strands must be separated, much like the two sides of a zipper, by breaking the weak
hydrogen bonds that link the paired bases. Once the DNA strands have been unwound, they must be held
apart to expose the bases so that new nucleotide partners can hydrogen-bond to them.
The enzyme DNA polymerase then moves along the exposed DNA strand, joining newly arrived nucleotides into
a new DNA strand that is complementary to the template. Figure 1 shows the process part way through.
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Replication
Transcription
Introduction (from
Wikipedia):
Cell division is essential for an
organism to grow, but when a cell
divides it must replicate the DNA in its
genome so that the two daughter
cells have the same genetic
information as their parent.DNA
provides a simple mechanism for
replication.
In transcription, the codons of a
gene are copied into messenger
RNA by RNA polymerase
Base Pairing:
Since there are 4 bases in 3-letter
combinations, there are 64 possible
codons (43 combinations).
This enzyme makes
thecomplementary strand by
finding the correct base
throughcomplementary base
pairing, and bonding it onto the
original strand.
Purpose:
The purpose of replication is to
conserve the entire genome for next
generation.
The purpose of transcription is
to make RNA copies of
individual genes that the
cell can use in the biochemistry.

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Replication
Transcription
Codons:
These encode the twenty standard
amino acids, giving most amino acids
more than one possible codon. There
are also three 'stop' or 'nonsense'
codons signifying the end of the
coding region; these are the UAA,
UAG and UGA codons.
DNA polymerases can only
extend a DNA strand in a 5′ to
3′ direction,
different mechanisms are used
to copy the antiparallel strands
of the double helix. In this way,
the base on the old strand
dictates which base appears on
the new strand.
Result:
In replication, the end result is two
daughter cells.
while in transcription, the end
result is a protein molecule.
Product:
Replication is the duplication of twostrands of DNA.
Transcription is the formation
of single, identical RNA from
the two-stranded DNA.
Enzymes:
The two strands are separated and
then each
strand's complementaryDNA
sequence is recreated by
anenzyme called DNA polymerase.
In transcription, the codons of a
gene are copied into messenger
RNA by RNA polymerase.This
RNA copy is then decoded by a
ribosome that reads the RNA
sequence by base-pairing the
messenger RNA to transfer
RNA, which carries amino acids.
Definition:
DNA replication is the replication of a
strand of DNA into two daughter
strands, each daughter strand
contains half of the original DNA
double helix.
Transcription is the synthesis of
RNA from a DNA template. The
structure of DNA is not altered
as a result of this process
EnzymesRequired:
DNA Helicase, DNA Polymerase.
Transcriptase (type of DNA
Helicase), RNA polymerase.
Products:
One strand of DNA becomes 2
daughter strands.
mRNA, tRNA, rRNA and noncoding RNA( like microRNA)
Product processing:
In eukaryotes complementary base
pair nucleotides bond with the sense
In eukaryotes, a 5’ cap is added,
a 3’ poly A tail is added and

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Replication
Transcription
or antisense strand. Thesre are then
connected with phosphoester bonds
by DNA helix to create a complete
strand.
Methods to measure and
detect:
introns are spliced out.
Meselson and Stahl Experiment
RT-PCR, DNA microarray, In-situ
hybridization, Northern blot,
RNA-Seq.
Translation process
The translation process is divided into three
steps:
Initiation: When a small subunit of a ribosome
charged with a tRNA+the amino acid
methionine encounters an mRNA, it attaches
and starts to scan for a start signal. When it
finds the start sequence AUG, the codon
(triplet) for the amino acid methionine, the
large subunit joins the small one to form a
complete ribosome and the protein synthesis
is initiated.
Elongation: A new tRNA+amino acid enters the
ribosome, at the next codon downstream of
the AUG codon. If its anticodon matches the
mRNA codon it basepairs and the ribosome
can link the two aminoacids together.(If a
tRNA with the wrong anticodon and therefore
the wrong amino acid enters the ribosome, it
can not basepair with the mRNA and is
rejected.) The ribosome then moves one
triplet forward and a new tRNA+amino acid
can enter the ribosome and the procedure is
repeated.
Termination: When the ribosome reaches one
of three stop codons, for example UGA, there
are no corresponding tRNAs to that sequence.

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Replication
Transcription
Instead termination proteins bind to the
ribosome and stimulate the release of the
polypeptide chain (the protein), and the
ribosome dissociates from the mRNA. When
the ribosome is released from the mRNA, its
large and small subunit dissociate. The small
subunit can now be loaded with a new
tRNA+methionine and start translation once
again. Some cells need large quantities of a
particular protein. To meet this requirement
they make many mRNA copies of the
corresponding gene and have many ribosomes
working on each mRNA. After translation the
protein will usually undergo some further
modifications before it becomes fully active.
DNA Polymerase - Elongates a new DNA strand at a replication fork. Adds nucleotides one by one to
the new and growing DNA strand.
DNA ligase - Joins sugar-phosphates of Okazaki fragments. Okazaki fragments are found on the
lagging strand.
Primase - Starts an RNA chain from scratch that will eventually be replaced by DNA nucleotides
(remember nucleotides come from DNA polymerase).
Helicase - Untwists the double helix at replication forks to make two parental strands available as
template strands.
Topoisomerase - Relieves strain of ahead of the replication fork due to untwisting of the double helix.
These five enzymes are the most basic needed for DNA replication as has been described above. As I
said earlier there are many more enzymes that can be utilized depending on species. Also, if you go
more in depth and want to know about proofreading and repairing DNA, even MORE enzymes crop
up. I would suggest reading your text book to get a better idea of what is going on. For instance, if we
are talking about replication of the lagging strand of DNA, the order of enzymes being put to work
would be as follows:

5. DNA Transcription
DNA consists of four nucleotide bases [adenine (A), guanine (G), cytosine (C), and thymine (T)] that
are paired together (A-T and C-G) to give DNA its double helical shape. Nucleotide base sequences
are the genetic code or instructions for protein synthesis.
There are three main steps to the process of DNA transcription.
RNA Polymerase Binds to DNA
DNA is transcribed by an enzyme called RNA polymerase. Specific nucleotide sequences tell RNA
polymerase where to begin and where to end. RNA polymerase attaches to the DNA at a specific area
called the promoter region.
Elongation
Certain proteins called transcription factors unwind the DNA strand and allow RNA polymerase to
transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA
(mRNA). The strand that serves as the template is called the antisense strand. The strand that is not
transcribed is called the sense strand.
Like DNA, RNA is composed of nucleotide bases. RNA however, contains the nucleotides adenine,
guanine, cytosine, and uracil (U). When RNA polymerase transcribes the DNA, guanine pairs with
cytosine and adenine pairs with uracil.
Termination
RNA polymerase moves along the DNA until it reaches a terminator sequence. At that point, RNA
polymerase releases the mRNA polymer and detaches from the DNA.
Since proteins are constructed in the cytoplasm of the cell, mRNA must cross the nuclear membrane
to reach the cytoplasm. Once in the cytoplasm, ribosomes and another RNA molecule called transfer
RNA work together to translate mRNA into a protein. This process is called translation. Proteins can
be manufactured in large quantities because a single DNA sequence can be transcribed by many RNA
polymerase molecules at once.