BIOL10005 Lecture Notes - Lecture 13: Central Dogma Of Molecular Biology, Dna Replication, Phosphodiester Bond
Lecture 13 - Monday 22 August 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
LECTURE 13
TRANSCRIPTION & THE GENETIC CODE
CHAPTER 11 - THE GENETIC CODE
THE CENTRAL DOGMA
•The central dogma of gene expression asserts that the amino acid sequence is not synthesised
directly from the DNA sequence but rather is a 2 stage process.
•The first stage, transcription, involves synthesis of an RNA intermediate from a DNA template.
•The second stage, translation, is the stage in which DNA acts as the template for the synthesis of
a polypeptide.
•This is an enzymatic process in which a ribosome moves along the RNA and catalyses the
synthesis of a polypeptide.
•The ribosome ‘reads’ the nucleotide sequence of the RNA and converts it into a specific
amino acid sequence, determining the properties of the polypeptide.
•Basically, one language (sequence of nucleotides) is translated into another (sequence of
amino acids).
•A gene is a specific DNA sequence that directs the synthesis of a polypeptide via an RNA
intermediate.
•While all polypeptides (and hence all the functional proteins they form) are encoded by genes, not
all genes code for proteins; some genes code for RNA molecules that are not translated but are
part of the protein synthesis machinery.
TRANSCRIPTION
•The first stage of gene expression, transcription, involves synthesis of an RNA molecule whose
sequence is complementary to the sequence of one strand of a segment of DNA.
•Copying involves separating the DNA strands and synthesizing a new RNA strand in which
complementary base pairing determines the bases incorporated.
•There are important differences between DNA replication and transcription.
•The structure of RNA differs from that of DNA in two ways;
•1. It is composed of ribonucleotides instead of deoxyribonucleotides.
•2. Uracil (U) is used in place of thymine (T) as the pyrimidine base that pairs with adenine
(A).
•RNA polymerase is an enzyme used to synthesise RNA, and it differs from DNA polymerase in
that it does not need a primer to begin the synthesis of an RNA chain.
•Transcription involves transient separation of the DNA duplex strands and binding of an RNA
polymerase which then moves along the template DNA incorporating ribonucleotides and
extending the growing RNA chain in a 5’ to 3’ direction.
INITIATION OF TRANSCRIPTION
•The RNA polymerase moves along the DNA double helix without making transcripts until it
reaches and binds to a specific sequence called the promoter, which itself is not transcribed but
promotes initiation of transcription.
•Initiation occurs by base pairing of the first specified ribonucleoside triphosphate with its
complementary base adenine on the DNA template, and this repeats over.
•RNA polymerase then catalyses the formation of a phosphodiester bond between the two
ribonucleotides and synthesis of the new RNA molecule is initiated.
•The RNA product does not remain base paired to template DNA but dissociates as RNA
polymerase moves along the template DNA.
•This process continues until a specific termination signal on the DNA is reached, which relies on a
specific set of nucleotides in DNA, and when a termination signal is encountered, RNA !
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