BISC 1111 Chapter Notes - Chapter 17: Rna Splicing, Start Codon, Polyadenylation

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Samuel Mohebban Chapter 17 Notes Exam 2
1
Gene expression is the process by which DNA directs the synthesis of proteins
o 2 stages
1) Transcription
2) Translation
A eukaryotic gene can code for a set of closely related polypeptides via a process called
alternative splicing.
The bridge between DNA and protein synthesis is the nucleic acid RNA.
RNA contains ribose instead of deoxyribose as its sugar and has the nitrogenous base
uracil rather than thymine, and is usually single stranded.
Transcription is the synthesis of RNA using information in the DNA.
o DNA strand acts as a template for assembling a complementary sequence of RNA
nucleotides.
Messenger RNA (mRNA)- carries a genetic message from the DNA to
the protein-synthesizing machinery of the cell.
Translation is the synthesis of a polypeptide using the information in the mRNA.
o The cell must translate the nucleotide sequence of an mRNA molecule into the
amino acid sequence of a polypeptide.
o The sites of translation are ribosomes- molecular complexes that facilitate the
orderly linking of amino acids into polypeptide chains.
Transcription occurs in the nucleus, and mRNA is then transported to the cytoplasm,
where translation occurs.
The transcription of a protein-coding eukaryotic gene results in pre-mRNA, and further
processing yields the finished mRNA.
o The initial RNA transcript from any gene, including those specifying RNA that is
not translated into protein, is more generally called a primary transcript.
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Samuel Mohebban Chapter 17 Notes Exam 2
2
Triplet code- the genetic instructions for a polypeptide chain are written in the DNA as
a series of non-overlapping, three nucleotide-words
The Template strand provides the pattern, or template, for the sequence of nucleotides
in an RNA transcript.
An mRNA molecule is complementary to its DNA template, not identical.
The mRNA nucleotide triplets are called codons, and they are customarily written in the
5’ 3’ direction.
During translation, the sequence of codons along an mRNA molecule is decoded, or
translated, into a sequence of amino acids making up a polypeptide chain.
There are 64 codons total, and the three codons that do not designate amino acids are
stop signals, or termination codons, marking the end of translation.
AUG is a common start codon, and it codes for the amino acid methionine.
Transcription
An enzyme called RNA polymerase pries the two strands of DNA apart and joins
together RNA nucleotides complementary to the DNA template strand, thus elongating
the RNA polynucleotide.
o Can only assemble a polynucleotide in its 5’ 3’ direction.
o Don’t need a primer like DNA polymerase.
The DNA sequence where RNA polymerase attaches and initiates transcription is known
as the promoter.
o In bacteria, the sequence that signals the end of transcription is called the
terminator.
The stretch of DNA downstream from the promoter that is transcribed into an RNA
molecule is called a transcription unit.
o Promoter sequence is upstream from the terminator.
The three stages of transcription
o Initiation
o Elongation
o Termination
In eukaryotes, a collection of proteins called transcription factors mediate the binding
of RNA polymerase and the initiation of transcription.
Only after transcription factors are attached to the promoter does RNA polymerase II
bind to it to form a transcription initiation complex.
o TATA box is a crucial promoter DNA sequence.
As RNA polymerase moves along the DNA, it untwists the double helix for pairing with
RNA nucleotides.
o The enzyme adds nucleotides to the 3’ end of the growing RNA molecule as it
continues along the double helix.
o The new RNA molecule peels away from its DNA template, and the DNA double
helix re-forms.
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Samuel Mohebban Chapter 17 Notes Exam 2
3
The mechanism of termination differs between bacteria and eukaryotes.
o In bacteria, transcription proceeds through a terminator sequence in the DNA.
The terminator functions as the termination signal, causing the
polymerase to detach from the DNA and release the transcript, which
requires no further modification before translation.
o In eukaryotes, RNA polymerase II transcribes a sequence on the DNA called the
polyadenylation signal sequence- which specifies a polydenylation signal in
the pre-mRNA.
During RNA processing, both ends of the primary transcript are altered.
Each end of a pre-mRNA molecule is modified in a particular way.
o The 5’ end, which is synthesized first, receives a 5’ cap- a modified form of a
guanine G nucleotide added onto the 5’ end after transcription of the first 20-
40 nucleotides.
o At the 3’ end, and an enzyme then adds 50-250 more adenine (A) nucleotides
forming a Poly-A tail.
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