DNA, RNA and protein synthesis
The genetic material is stored in the form of DNA in most organisms. In humans, the nucleus of each cell contains 3 × 109
base pairs of DNA distributed over 23 pairs of chromosomes, and each
cell has two copies of the genetic material. This is known collectively
as the human genome. The human genome contains around 30 000 genes, each
of which codes for one protein.
Large stretches of DNA in the human genome are transcribed but do not code for proteins. These regions are called introns and make up around 95% of the genome. The nucleotide sequence of the
human genome is now known to a reasonable degree of accuracy but we do
not yet understand why so much of it is non-coding. Some of this
non-coding DNA controls gene expression but the purpose of much of it is
not yet understood.
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| figure1:Concept of Central Dogma |
DNA replication
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| figure1:Process synthesis DNA replication |
Each
time a cell divides, each of its double strands of DNA splits into two
single strands. Each of these single strands acts as a template for a
new strand of complementary DNA. As a result, each new cell has its own
complete genome. This process is known as DNA replication.
Replication is controlled by the Watson-Crick pairing of the bases in
the template strand with incoming deoxynucleotide triphosphates, and is
directed by DNA polymerase enzymes. It is a complex process,
particularly in eukaryotes, involving an array of enzymes.
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| figure2:DNA replication |
DNA biosynthesis proceeds in the 5′- to 3′-direction. This makes it
impossible for DNA polymerases to synthesize both strands
simultaneously. A portion of the double helix must first unwind, and
this is mediated by helicase enzymes.
The leading strand
is synthesized continuously but the opposite strand is copied in short
bursts of about 1000 bases, as the lagging strand template becomes
available. The resulting short strands are called Okazaki fragments
(after their discoverers, Reiji and Tsuneko Okazaki). Bacteria have at
least three distinct DNA polymerases: Pol I, Pol II and Pol III; it is
Pol III that is largely involved in chain elongation. Strangely, DNA
polymerases cannot initiate DNA synthesis de novo, but require a
short primer with a free 3′-hydroxyl group. This is produced in the
lagging strand by an RNA polymerase (called DNA primase) that is able to
use the DNA template and synthesize a short piece of RNA around 20
bases in length. Pol III can then take over, but it eventually
encounters one of the previously synthesized short RNA fragments in its
path. At this point Pol I takes over, using its 5′- to 3′-exonuclease
activity to digest the RNA and fill the gap with DNA until it reaches a
continuous stretch of DNA. This leaves a gap between the 3′-end of the
newly synthesized DNA and the 5′-end of the DNA previously synthesized
by Pol III. The gap is filled by DNA ligase.
Mistakes in DNA replication
DNA
replication is not perfect. Errors occur in DNA replication, when the
incorrect base is incorporated into the growing DNA strand. This leads
to mismatched base pairs, or mispairs. DNA polymerases have proofreading activity, and a DNA repair
enzymes have evolved to correct these mistakes. Occasionally, mispairs
survive and are incorporated into the genome in the next round of
replication. These mutations may have no consequence, they may result in
the death of the organism, they may result in a genetic disease or
cancer; or they may give the organism a competitive advantage over its
neighbours, which leads to evolution by natural selection.
Transcription
Transcription is the process by which DNA is copied (transcribed)
to mRNA, which carries the information needed for protein synthesis.
Transcription takes place in two broad steps. First, pre-messenger RNA
is formed, with the involvement of RNA polymerase enzymes. The process
relies on Watson-Crick base pairing, and the resultant single strand of
RNA is the reverse-complement of the original DNA sequence. The
pre-messenger RNA is then "edited" to produce the desired mRNA molecule
in a process called RNA splicing.
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| figure3:Simplified representation of the formation of pre-messenger RNA (orange) from double-stranded DNA (blue) in transcription. |
Formation of pre-messenger RNA
The mechanism of transcription has parallels in that of DNA replication.
As with DNA replication, partial unwinding of the double helix must
occur before transcription can take place, and it is the RNA polymerase
enzymes that catalyze this process.
Unlike DNA replication, in
which both strands are copied, only one strand is transcribed. The
strand that contains the gene is called the sense strand, while the complementary strand is the antisense strand. The mRNA produced in transcription is a copy of the sense strand, but it is the antisense strand that is transcribed.
Ribonucleotide
triphosphates (NTPs) align along the antisense DNA strand, with
Watson-Crick base pairing (A pairs with U). RNA polymerase joins the
ribonucleotides together to form a pre-messenger RNA molecule that is
complementary to a region of the antisense DNA strand. Transcription
ends when the RNA polymerase enzyme reaches a triplet of bases that is
read as a "stop" signal. The DNA molecule re-winds to re-form the double
helix.
RNA splicing
The
pre-messenger RNA thus formed contains introns which are not required
for protein synthesis. The pre-messenger RNA is chopped up to remove the
introns and create messenger RNA (mRNA) in a process called RNA
splicing.
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| figure4:RNA splicing |
Translation
The
mRNA formed in transcription is transported out of the nucleus, into
the cytoplasm, to the ribosome (the cell's protein synthesis factory).
Here, it directs protein synthesis. Messenger RNA is not directly
involved in protein synthesis − transfer RNA (tRNA) is required for
this. The process by which mRNA directs protein synthesis with the
assistance of tRNA is called translation.
The ribosome is a very large complex of RNA and protein molecules. Each three-base stretch of mRNA (triplet) is known as a codon,
and one codon contains the information for a specific amino acid. As
the mRNA passes through the ribosome, each codon interacts with the anticodon
of a specific transfer RNA (tRNA) molecule by Watson-Crick base
pairing. This tRNA molecule carries an amino acid at its 3′-terminus,
which is incorporated into the growing protein chain. The tRNA is then
expelled from the ribosome.
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| figure5:Translation |
source:http://www.atdbio.com/content/14/Transcription-Translation-and-Replicationhttp://www.atdbio.com/content/14/Transcription-Translation-and-Replication
