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Lecture 11

BIL 150 Lecture Notes - Lecture 11: Ribosomal Rna, Small Nuclear Rna, Rna Interference


Department
Biology
Course Code
BIL 150
Professor
Gaines Michael
Lecture
11

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Chapter 17 and 18: Gene Expression/Regulation of Gene Expression
Protein synthesis: an overview
oTranscription is the synthesis of RNA using information in the DNA
Just as DNA strand provides a template for making a new
complementary strand during DNA replication, it also can serve
as a template for assembling a complementary sequence of
RNA nucleotides
For a protein-coding gene, the resulting RNA molecule is a
faithful transcript of the gene’s protein-building instructions, this
type of RNA molecule is called messenger RNA (mRNA)
mRNA carries a genetic message from the DNA to the
protein-synthesizing machinery of the cell
oTranslation is the synthesis of a polypeptide using the information in
the mRNA
During this stage, the cell must translate the nucleotide
sequence of an mRNA molecule into the amino acid sequence
of a polypeptide
The sites of translation are ribosomes, molecular complexes
that facilitate the orderly linking of amino acids into polypeptide
chains
Types of RNA
oMessenger RNA (mRNA) is a strand of RNA complementary to DNA
that conveys the genetic information in DNA to the ribosomes during
protein synthesis
Sequences of three nucleotides, called codons, in mRNA
specify particular amino acids to incorporate into a protein
oTransfer RNA (tRNA) is a type of RNA that binds to a specific amino
acid and bears a set of three nucleotides (the anticodon)
complementary to the mRNA

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The function of tRNA is to transfer amino acids from the
cytoplasmic pool of amino acids to a growing polypeptide in a
ribosome
oRibosomal RNA (rRNA) is the most abundant type of RNA, which
together with proteins (ribosomal subunits) forms the structure of
ribosomes
oPrimary transcript (pre – mRNA) is the initial RNA transcript from any
gene, including those specifying RNA that is not translated into protein
Serves as a precursor to mRNA
oSmall nuclear RNA (snRNA) is RNA that combines with protein to
form splicesome
oSmall nucleolar RNA (snoRNA)
oSignal recognition particle RNA (srpRNA) is a protein-RNA complex
that recognizes a signal peptide as it emerges from the ribosome
oPiwi-associated RNA (piRNA) is a class of 30-nucleotide long RNA
molecules that interfere with the expression of genes
They are linked to the Argonaute family of proteins
They are transcribed from repeat regions of the genome
originally thought not to be transcribed
oSmall interfering RNA (siRNA) is a class of 20-25 nucleotide-long
RNA molecules that interfere with the expression of genes
These are produced as part of the RNA interference (RNAi)
process by the enzyme Dicer
oMicro RNA (miRNA) is a small, single stranded RNA molecule that
binds to a complementary sequence in mRNA molecule sand directs
associated proteins to degrade or prevent translation of the target
mRNA
The Genetic Code
oThe flow of information from gene to protein is based on a triplet code

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The genetic instructions for a polypeptide chain are written in
the DNA as a series of nonoverlapping, three-nucleotide words
oDuring transcription, the gene determines the sequence of nucleotide
bases along the length of the RNA molecule that is being synthesized
oFor each gene, only one of the two DNA stands is transcribed, called
the template strand
Provides the template for the sequence of nucleotides in an
RNA transcript
omRNA 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
Takes 300 nucleotides along an mRNA strand to code for the
amino acids in a polypeptide that is 100 amino acids long
AUG (MET) = start codon
UAA, UAG, UGA = stop codons
oThere is redundancy in the genetic code, but no ambiguity
Although codons GAA and GAG both specify glutamic acid,
neither of them ever specifies any other amino acid
oThe genetic code is nearly universal, shared by organisms from the
simplest bacteria to the most complex plants and animals
CCG is translated as the amino acid proline in all organisms
whose genetic code has been examined
Transcription
oMessenger RNA, the carrier of information from DNA to the cell’s
protein-synthesizing machinery, is transcribed from the template strand
of a gene
RNA polymerase pries the two strands of DNA apart and joins
together RNA nucleotides complementary to the DNA template
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