Transcript Chapter 10

Chapter 10: The genetic code
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Genes
•
A single gene provides the genetic instructions for
one polypeptide
• There is a specific relationship between the DNA
sequence of the gene and the amino acid
sequence of the protein
• The process of converting DNA information into
protein molecules is gene expression
• The information is deciphered using the genetic
code
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RNA
•
•
RNA has the same primary structure as DNA. It
consists of a sugar-phosphate backbone, with
nucleotides attached to the 1' carbon of the sugar
The differences between DNA and RNA are that
– RNA has a hydroxyl group on the 2' carbon of the sugar
(the difference between deoxyribonucleic acid and
ribonucleic acid)
– RNA uses the pyrimidine base uracil (U) to pair with
adenine (A)
– RNA exists as a single-stranded molecule
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Transcription
•
DNA acts as a template for the synthesis of RNA in
a process called transcription
• Only one strand of DNA is used as the template
• Like DNA replication, transcription proceeds 5’ 3’
on the strand being produced
• Nucleotides are added according to the
complementary base pairing rules
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Fig. 10.2a: Transcription of DNA by RNA
polymerase
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Fig. 10.2b: The structure of RNA polymerase,
shown in the act of transcribing DNA to produce
RNA
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Initiation
•
RNA polymerases do not require priming
• The polymerase binds just downstream of the
promoter
• Elongation of the RNA continues by addition of
complementary nucleotides until a termination
signal is reached
• The transcribed region is called a transcription unit
with the RNA being a primary transcript
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10-7
Fig. 10.4: Formation of the first phosphodiester
bond to initiate transcription
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Transcription
•
Transcription generates three major RNAs
– messenger RNA (mRNA) determines the amino acid
sequence of the protein during translation
– ribosomal RNA (rRNA) is one of the components of the
ribosome involved in translation
– transfer RNA (tRNA) is a small RNA that can bind an
amino acid at one end, and mRNA at the other end. It
acts as an adaptor to carry the amino acid elements of a
protein to the appropriate place as coded for by the
mRNA
(cont.)
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Transcription (cont.)
•
In prokaryotes transcription occurs in the cytoplasm
• Since many bacterial genes are arranged in
operons (Chapter 11) the RNA transcripts are
polycistronic
• Transcription and translation are often coincident
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Fig. 10.5: Coincident transcription and
translation in prokaryotes
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Eukaryotic transcription
•
Eukaryotic transcription occurs in the nucleus
• Eukaryotic genes are usually monocistronic—
coding for a single polypeptide
• The eukaryotic nucleus has an organelle called the
nucleolus which is the site for ribosomal RNA
synthesis
• The three major classes of RNA are transcribed by
different RNA polymerases
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mRNA processing
•
Both ends of the primary RNA transcript are
modified before transport to the cytoplasm
– A methyl guanidine cap is added to the 5’ end
– A poly adenine or poly A tail is added to the 3’ end of the
transcript
•
One of the most important stages in RNA
processing is RNA splicing. In many genes, the
DNA sequence coding for proteins, or ‘exons’,
may be interrupted by stretches of non-coding
DNA, called ‘introns’
(cont.)
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mRNA processing (cont.)
•
•
In the cell nucleus, the DNA that includes all the
exons and introns of the gene is first transcribed
into a complementary RNA copy called 'nuclear
RNA’ or nRNA
Introns are then removed from nRNA by a process
called RNA splicing
– splicing occurs at specific sequences on the intron–exon
boundaries
– exons are joined together
– the edited sequence is the final mRNA
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Fig. 10.6: Post-transcriptional processing of
eukaryotic mRNA transcripts
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The genetic code
•
mRNA transfers information to protein in the form
of a code defined by a sequence of nucleotide
bases
• Each amino acid is specified by three nucleotides
called a codon
• Since RNA is constructed from four types of
nucleotides, there are 64 possible codons (4x4x4).
• Three of these codons called stop codons specify
the termination of the polypeptide chain
(cont.)
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The genetic code (cont.)
•
That leaves 61 codons to specify only 20 different
amino acids
• Therefore, most of the amino acids are
represented by more than one codon
• Thus, the genetic code is to be degenerate
• Particularly in the third nucleotide position, a base
change often does not change the amino acid
specified
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Fig. 10.7: The genetic code
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Reading frames
•
•
In order for the protein to be synthesised,
translation must start and stop correctly
The region of the mRNA used to encode the
amino acid sequence is called the open reading
frame
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Fig. 10.8: The concept of reading frames
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Frame shift mutations
•
Each mRNA potentially has three reading frames
but only one gives the correct sequence for the
protein
• The reading frame is set by the position of the start
codon (AUG)
• Within a gene, small deletions or insertions of a
number of bases not divisible by 3 will result in a
frame shift in the mRNA during translation
(cont.)
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Fig. 10.9: Frameshift mutations
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Reading frame mutations (cont.)
•
A nonsense mutation creates a stop codon where
none previously existed
– this shortens the resulting protein, possibly removing
essential regions
•
A missense mutation changes the code of the
mRNA
– for example if an AGU is changed to an AGA, the protein
will have an arginine instead of serine
– the shape or function of the protein may be altered
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Translation
•
•
•
During protein synthesis, ribosomes move along
the mRNA molecule and read its sequence one
codon at a time from the 5' end to the 3' end
Each amino acid is specified by the mRNA's codon
Codons pair with a sequence of three
complementary nucleotides (anticodon) carried by
a particular tRNA
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Transfer RNA (tRNA)
•
•
tRNAs have an anticodon at one end and an
amino acid at the other
They act as adaptor molecules to bring the correct
amino acid to the mRNA codon
(cont.)
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Fig. 10.10: Transfer RNA structure
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Transfer RNA (tRNA) (cont.)
•
Each tRNA only binds the appropriate amino acid
for its anticodon and is recharged after depositing
its amino acid into the growing chain
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Fig. 10.11: Activation of RNA by
aminoacylation
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Ribosomes
•
•
Consist of two protein subunits, large and small,
and associated rRNAs
Provide a precise method of aligning codons and
tRNAs to ensure amino acids are synthesised in
the correct order
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Fig. 10.12: Ribosomes
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Polypeptide synthesis
•
Initiation
– the ribosomes form from subunits
– tRNA methionine binds to start codon
– Large subunit of ribosome binds to form A and
P sites
(cont.)
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Fig. 10.14: The three phases of protein
synthesis
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Polypeptide synthesis (cont.)
•
Elongation
– begins with the formation of a peptide bond between the
methionine and the second amino acid
– the process involves the addition of a sequence of amino
acids, specified by the codons
– as each new amino acid is brought into position, a
peptide bond is formed with the preceding amino acid
– translation proceeds 5’ to 3’ along the mRNA
– the peptide is synthesised from the amino (NH2) end to
the carboxyl (COOH) end
(cont.)
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Polypeptide synthesis (cont.)
•
Termination
– when the ribosome arrives at a stop codon the elongation
process stalls because there is no tRNA for stop codons
– termination factors remove the last amino acid from its
tRNA
– the ribosome separates into its two subunits and leaves
the mRNA
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Fig. 10.15a: Initiation
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Fig. 10.15b: Elongation
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Fig 10.15c: Termination
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Protein processing
•
•
•
•
Polypeptide synthesis is only the first step in the
production of a mature protein
The protein may be further modified by the
addition of chemical residues
Proteins are targeted to particular organelles by
the addition of signal sequences which bind to
receptors at the correct location
Proteins for secretion are sorted and packaged in
the Golgi apparatus
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Fig. 10.16: Pathways of targeting in
eukaryotic cells
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