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From Gene to Protein
Chapter 17
OVERVIEW
• DNA
RNA
Protein
• Transcription
–Synthesis of mRNA from DNA
• Translation (change of
language)
–Synthesis of protein
(polypeptide) from mRNA
(uses tRNA)
Figure 17.2 Overview: the roles of transcription and translation in the flow of genetic
information
Figure 17.3 The triplet code
CODONS
• Three base nucleotides that
eventually code for a specific
amino acid
• There are 64 codons
• Marshall Nirenberg (1961)
deciphered first codon
–Found UUU codes for
phenylalanine
• Amino acids abbreviated by first
3 letters of name or designated
single letter
• AUG = methionine (met) or
start
• UAA, UAG, and UGA = stop
codons
• Nearly universal language
among all living organisms
Figure 17.4 The dictionary of the genetic code
RNA vs. DNA
• Ribose instead of deoxyribose
• Uracil instead of thymine
• Single strand vs. double strand
TRANSCRIPTION
Making pre-mRNA
INITIATION
• Promoter – site where RNA
polymerase attaches to DNA and
starts transcription
• Transcription factors –
proteins that mediate the
binding of RNA polymerase (in
eukaryotes – huge role in gene
regulation)
• TATA box – sequence of
nucleotides (TATAAAA) that is
part of promoter region and
binds to transcription factors
• RNA polymerase attaches to
promoter, helix unwinds, and
elongation begins
Figure 17.6 The stages of transcription: initiation, elongation, and termination
Figure 17.6 The stages of transcription: initiation, elongation, and termination
Figure 17.7 The initiation of transcription at a eukaryotic promoter
ELONGATION
• RNA polymerase adds
complementary nucleotides in 5’
to 3’ direction
• About 60 nucleotides per
second
Figure 17.6 The stages of transcription: initiation, elongation, and termination
TERMINATION
• Elongation stops at or just
following a stop codon
Figure 17.6 The stages of transcription: initiation, elongation, and termination
RNA PROCESSING
• In eukaryotes…
• The 5’ end of pre-mRNA is capped
with guanine
• Poly(A) tail – several adenine added
to 3’ end
– Protects end
– Signal for future ribosome
attachment
– Help to get mRNA out of nucleus
– Help prevent degradation
Figure 17.8 RNA processing; addition of the 5 cap and poly(A) tail
RNA SPLICING
• In eukaryotes…
• Large portions of mRNA do not code for parts of
a protein
• Introns – noncoding segments
• Exons – coding segments
• snRNPs (small nuclear ribonucleoproteins)
combine with proteins to make spliceosome
• Spliceosomes cut at ends of introns and
rejoins remaining exons together (recognize
special sequences)
• Ribozymes – mRNA that catalyzes its own
intron removal (not all enzymes are proteins)
Figure 17.9 RNA processing: RNA splicing
Figure 17.10 The roles of snRNPs and spliceosomes in mRNA splicing
WHY INTRONS?
• Split genes can code for
different proteins or different
regions of same polypeptide
• Introns increase the cross over
frequency between 2 alleles
which increases diversity
Figure 17.11 Correspondence between exons and protein domains
TRANSLATION
mRNA to protein
Figure 17.12 Translation: the basic concept
tRNA
• Shorter than mRNA
• Shaped like an “L”
• A specific amino acid
attaches to 3’ end
• Loop region contains
anticodon
Figure 17.13a The structure of transfer RNA (tRNA)
Figure 17.13b The structure of transfer RNA (tRNA)
• Aminoacyl-tRNA
synthetases - bind correct
amino acid to a tRNA
–There are 20 of these
synthetases so each amino
acid has its own enzyme
–Driven by hydrolysis of ATP
Figure 17.14 An aminoacyl-tRNA synthetase joins a specific amino acid to a tRNA
RIBSOMES
• Made of ribosomal RNA (rRNA) and protein
• Two subunits: large and small
– Join only when translation occurring
• Three binding sites for tRNA
– E = exit site
• tRNA leaves ribosome
– P = peptidyl-tRNA binding site
• Holds growing polypeptide
– A = Aminoacyl-tRNA binding site
• Holds next tRNA with next amino acid
Figure 17.15 The anatomy of a functioning ribosome
Figure 17.16 Structure of the large ribosomal subunit at the atomic level
INITIATION
• Small ribosomal subunit binds
to mRNA at 5’end.
• Initiator tRNA (with met) binds
to P site (H bonds between
anitcodon and codon).
• Large ribosomal subunit
attaches with help of proteins
• GTP supplies energy.
Figure 17.17 The initiation of translation
ELONGATION
• H bonds between codon and
anticodon (in A site)connect
next tRNA with next amino acid
• GTP needed.
• A ribozyme catalyzes peptide
bond between first and second
amino acid.
• Peptide attached to second
tRNA.
• mRNA moves through ribosome
so that the first tRNA leaves via
E site and second tRNA moves
to P site.
• Then the third tRNA comes in to
A site.
• Movement requires GTP.
• Process continues like a
conveyer belt.
Figure 17.18 The elongation cycle of translation
• Wobble effect – third base of
anticodon can pair with
noncomplementary base of
codon (a U can bind to a A or
G)
• Inosine (I) can bond with U, C,
or A
TERMINATION
• When a stop codon reaches the
A site there is no matching
anticodon on a tRNA.
• Release factor protein binds
instead.
• Polypeptide released by
hydrolysis.
• Ribsome disassembles.
Figure 17.19 The termination of translation
Misc.
• Polyribosomes - several ribosomes can
translate the same mRNA strand
• All synthesis of all proteins begins in
cytoplasm
• Signal peptide sends protein to ER
• Signal peptide is recognized by signal
recognition particle (SRP)
• Proteins are transorted via rough ER
and can be modified in Golgi body (ex.
Removal of first met)
Figure 17.20 Polyribosomes
Figure 17.21 The signal mechanism for targeting proteins to the ER
Prokaryote vs. Eukaryote
• Prokaryotes
– No introns or TATA box
– No 5’ G cap or poly A tail
– Translation begins before mRNA is
completely made (remember no nucleus)
• Eukaryotes
– Introns and TATA box
– Cap and tail (protection for exiting nucleus)
– mRNA must leave nucleus before
translation can start
Figure 17.22 Coupled transcription and translation in bacteria
MUTATIONS
• Mutation – a change in DNA sequence
• Point Mutations cause:
– missense mutations no change in amino acid(s)
– nonsense mutations changes amino acid and
therefore protein
• Two types of Point Mutations
– Base pair substitutions replacement of nucleotide
– Insertions and Deletions -additions or losses of
one or more nucleotides
• Frameshift mutation - occurs when number of
nucleotides inserted or deleted is not 3 or a
multiple of 3
• Mutation rate is ~1 nucleotide altered in every 1010
Figure 17.23 The molecular basis of sickle-cell disease: a point mutation
Figure 17.24 Categories and consequences of point mutations: Base-pair insertion
or deletion
Figure 17.24 Categories and consequences of point mutations: Base-pair
substitution
MUTAGENS
• Physical or chemical agents
cause DNA to mutate
–X-rays
–UV light
–Radiation
–Most carcinogens
A gene is more than just a
protein maker.
• A gene is a region of DNA whose final product
is protein or RNA
• Types of RNA made include
– mRNA, tRNA, rRNA, snRNA, SRP RNA (part
of signal recognition particle), snoRNA
(small nucleolar RNA helps process prerRNA), and siRNA (small interfering RNA)
and miRNA (micro RNA) both involved in
gene regulation
Figure 17.25 A summary of transcription and translation in a eukaryotic cell