Download DNA replication proceeds in a semi conservative fashion, where the

DNA replication proceeds in a semi conservative fashion, where the two complementary DNA strands
of the parent molecule unwind and each strand acts as a template for building a new strand based on
base-pairing rules, A to T and G to C.
Major Steps:
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Begins at a specific point in the DNA molecule called the origin of replication site: in eukaryotic
chromosomes multiple origins of replication could be present simultaneously;
The enzyme helicase unwinds and separates a portion of the DNA molecule to be replicated,
creating a replication “bubble”: at the end of each replication “bubble” is a replication fork, a Yshaped region, where new DNA strands are elongating;
Single-strand binding proteins react with and stabilize the separated single-stranded sections of
the DNA molecule;
Enzymes called topoisomerases correct “overwinding” ahead of the replication fork by
breaking, swiveling, and rejoining the DNA parent strands;
An enzyme called primase assembles an RNA primer ( a short sequence of RNA nuleotides,
complementary to a small, initial section of the DNA strand) at the origin of replication site;
DNA polymerase III synthesizes new DNA in the 5’->3’ direction continuously, towards the
unwinding, creating the so called leading DNA strand;
The other parent DNA strand is replicated discontinuously creating the so called DNA lagging
strand, with the formation of short DNA segments called Okazaki fragments, each requiring a
separate RNA primer;
DNA polymerase I removes and replaces the RNA primers with appropriate DNA sequences ;
The enzyme ligase seals the nicks and bonds together separate DNA fragments into a
continuous complementary strand;
DNA transcription is the synthesis of messenger RNA (mRNA) which is complementary to the template
DNA strand, A to U and G to C.
Major steps:
I.
Initiation:
Transcription factors assist RNA polymerase to locate and bind to the promoter on the doublestranded DNA molecule: the transcription factors associated with the RNA polymerase are referred to
as the transcription initiation complex;
II.
Elongation:
As the bound RNA polymerase moves along the DNA, it untwists the double helix;
The RNA polymerase synthesizes complementary single-stranded mRNA in the 5’->3’ direction;
III.
Termination:
In prokaryotes, the RNA polymerase stops transcription at the end of the terminator sequence;
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In eukaryotes, the RNA polymerase continues transcription after the cleavage of the pre-mRNA
from the growing RNA chain: the RNA polymerase eventually falls off the DNA;
In prokaryotes , no additional modification of the newly synthesized mRNA occurs.
In eukaryotes, post transcriptional modifications take place in the nucleus to the pre-mRNA
before it goes into the cytoplasm to participate in DNA translation.
DNA translation is the synthesis of protein based on the mRNA message decoded by the appropriate
transfer RNA (tRNA) molecules binding to the ribosome associated with the mRNA strand.
Major steps:
I.
Initiation:
The small ribosomal subunit binds with mRNA and a special initiator tRNA;
The small subunit moves along the mRNA until it reaches the start codon (AUG);
Proteins called initiation factors bring in the large ribosomal subunit , the initiator tRNA
occupies the P site;
II.
Elongation:
During codon recognition in the 5’->3’ direction, a new tRNA binds complementary through its
anticodon to the codon on the mRNA in the A site of the ribosome;
During peptide bond formation, a protein/RNA complex called peptidyl tansferase catalyzes
the formation of a new peptide bond between the growing polypeptide chain in the P site of
the ribosome and the amino acid in the A site;
As a result, the polypeptide chain moves from the P site to the A site;
The ribosome translocates in the 5’->3’ direction to the next codon;
The empty (uncharged) tRNA occupies the E site and leaves the ribosome;
the growth of the polypeptide chain is one amino acid at a time;
The next codon is positioned in the A site: repeated rounds of codon recognition, petide bond
formation, and translocation occur.
III.
Termination:
A stop codon ( UAA, UAG, and UGA) appears in the A site;
No tRNA corresponds to the stop codon, therefore a protein called a release factor binds to the
A site;
The polypeptide chain is cleaved from the tRNA completing the synthesis of the polypeptide;
The empty tRNA is released from the ribosome;
The two ribosomal subunits dissociate from the mRNA chain.