Chapter 18 Gene Expression & Protein Synthesis Chemistry 20.
Gene Protein Chapter 17. Protein Synthesis / Gene Expression Gene expression: The translation of...
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Transcript of Gene Protein Chapter 17. Protein Synthesis / Gene Expression Gene expression: The translation of...
Gene Protein
Chapter 17
Protein Synthesis / Gene Expression
Gene expression: The translation of information encoded in a gene into protein or RNA. Expressed genes include genes that are transcribed into messenger RNA (mRNA) and then translated into protein, as well as genes that are transcribed into types of RNA such as transfer RNA (tRNA) and ribosomal RNA (rRNA) that are not translated into protein.
Gerrod - 1909
genes dictate phenotype through enzymes that catalyze specific chemical reactions in the cell
symptoms of an inherited disease reflect a person’s inability to synthesize a particular enzyme
one gene - one enzyme hypothesis
Development of the Theory
one gene - one proteinnot all proteins are enzymes
one gene - one polypeptidemany proteins are composed of several
polypeptides, each of which has its own gene
RNA
contains ribose as its sugar
substitutes the base uracil for thymine
consists of a single strand
transcription and translation
Transcription DNA strand provides a template for the synthesis of a complementary RNA strand
Translation the information contained in the order of nucleotides in mRNA is used to determine the amino acid sequence of a polypeptide
Transcription
template strand of DNA provides a template for the sequence of nucleotides in RNA
The complementary RNA molecule is synthesized according to base-pairing rulesexcept that uracil is the complementary base
to adenine
RNA polymerase
separates the DNA strands at the appropriate point
bonds the RNA nucleotides as they base-pair along the DNA template
http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html
At the 5’ end of the pre-mRNA molecule, a modified form of guanine is added, the 5’ cap
At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, the poly(A) tail
RNA splicing Most eukaryotic genes and their RNA transcripts have
long noncoding stretches of nucleotides Spliceosome removes introns and joins exons to
create an mRNA molecule with a continuous coding sequence
What’s the point of splicing?
at least some introns contain sequences that control gene activity in some way
splicing itself may regulate the passage of mRNA from the nucleus to the cytoplasm
one clear benefit of split genes is to enable a one gene to encode for more than one polypeptide.
Fig. 17-11-3RNA transcript (pre-mRNA)
Exon 1 Exon 2Intron
ProteinsnRNA
snRNPs
Otherproteins
5
5
Spliceosome
Spliceosomecomponents
Cut-outintronmRNA
Exon 1 Exon 25
Translation
blocks of three nucleotides, codons, are decoded into a sequence of amino acids
codons are read in the 5’->3’ direction along the mRNA
ribosome adds each amino acid carried by tRNA to the growing end of the polypeptide chain
tRNA
carries a specific amino acid at one end and has a specific nucleotide triplet, an anticodon, at the other
The anticodon base-pairs with a complementary codon on mRNA
http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html
tRNA molecule
binding site for mRNA
The P site holds the tRNA carrying the polypeptide chainThe A site carries the tRNA with the next amino acidDischarged tRNAs leave the ribosome at the E site
Initiation
brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
Codons
Elongation
each amino acid is added to the proceeding one
codon recognition peptide bond formation translocation
Elongation
Termination
Stop codon Releasing factor
polyribosomes
Fig. 17-23Wild-type
3DNA template strand
5
5
53
3
Stop
Carboxyl endAmino end
Protein
mRNA
33
3
55
5
A instead of G
U instead of C
Silent (no effect on amino acid sequence)
Stop
T instead of C
33
3
55
5
A instead of G
Stop
Missense
A instead of T
U instead of A
33
3
5
5
5
Stop
Nonsense No frameshift, but one amino acid missing (3 base-pair deletion)
Frameshift causing extensive missense (1 base-pair deletion)
Frameshift causing immediate nonsense (1 base-pair insertion)
5
5
533
3
Stop
missing
missing
3
3
3
5
55
missing
missing
Stop
5
5533
3
Extra U
Extra A
(a) Base-pair substitution (b) Base-pair insertion or deletion
Producing a protein