Transcript Protein synthesis and mut ppt

PROTEIN
SYNTHESIS
Protein Synthesis: overview
 DNA
is the code that controls everything in
your body
 In order for DNA to work the code that it
contains must be transcribed and
translated into proteins
 Two hypotheses
 One gene-one enzyme hypothesis (Beadle
and Tatum)
 One gene-one polypeptide (protein)
hypothesis
Overview of Protein Synthesis
 Transcription:
synthesis of RNA under the
direction of DNA (mRNA)
 Translation:
 actual synthesis of a polypeptide
under the direction of mRNA

The Triplet Code
 The
genetic instructions for a polypeptide
chain are ‘written’ in the DNA as a series of
3-nucleotide ‘words’
 Each triplet in DNA codes for a specific
amino acid in a protein
 Codons
 Are what we call the 3- nucleotides in DNA
which code for amino acids
Process of Transcription





RNA polymerase: pries DNA apart and hooks
RNA nucleotides together from the DNA code
Promoter region on DNA: where RNA polymerase
attaches and where initiation of mRNA begins
Terminator region: sequence that signals the end
of transcription
Transcription unit: stretch of DNA transcribed
into an RNA molecule
Structure of mRNA
 Complimentary to DNA except Thymine is
replaced by another base called Uracil
 Ribose sugar in backbone
Process of Transcription

Initiation- transcription factors mediate the
binding of RNA polymerase to an initiation
sequence (TATA box)
 Elongation- RNA polymerase continues
unwinding DNA and adding nucleotides to the 3’
end- 60 nucleotides per second
 Termination- RNA polymerase reaches
terminator sequence
 Prokaryotes stop right away
 Eukaryotes go 20-35 nucleotides further than
termination sequence
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
Translation

Message coming from the mRNA is now
translated in the cytoplasm into a polypeptide
 tRNA is the interpreter that reads the codon from
the mRNA through its anti-codon and carries the
correct amino acid that matches.
 Structure of tRNA
 About 80 nucleotides long forming and 3
dimensional shape that looks like and L
 Each is unique and has its own anti-codon
 Has an amino acid attachment site
How are tRNA’s attached to the
amino acid?
 Through
the use of Aminoacyl-tRNAsynthetases
 Enzyme that joins the correct amino
acids with the tRNA through the
hydrolysis of ATP
 Happens freely in the cytoplasm
Figure 17.14 An aminoacyl-tRNA synthetase joins a specific amino acid to a
tRNA
Steps in Translation

Initiation




Brings together mRNA, tRNA, and the two
sub units of the ribosome
mRNA begins being read at the start
signal and will go in a 5’ to 3’ direction
mRNA, tRNA, and small ribosomal unit
bind first
Then, large ribosomal unit attaches to the
tRNA with the tRNA sitting in the P site
and the A site is ready for the next tRNA
Figure 17.17 The initiation of translation
Steps in Translation
 Elongation





mRNA strip is continuously read and as
this happens tRNA’s are bringing amino
acids and making a polypeptide
tRNA binds to A site
tRNA in P site transfers polypeptide to
tRNA in A site
Free tRNA in P site moves into E site and
exits
tRNA in A site moves to P site and new
tRNA comes into A site
Figure 17.18 The elongation cycle of translation
Steps in Translation
 Termination



Elongation continues until a stop codon is
reached at the A site of the ribosome
Protein called release factor binds to the
A site
Due to the addition of water to the
polypeptide it gets released and the
translation unit breaks down
Figure 17.19 The termination of translation
Polyribosomes
 Typically
a single mRNA strand is used to
make many copies of the polypeptides it
codes for simultaneously
 Many ribosome's can be bonded to the
same mRNA strip all at once
 Polypeptides with specific destinations
 Some polypeptides need to leave the cell
 Therefore they are made in bound
ribosome's on the ER and other
membrane bound organelles for transport
Figure 17.20 Polyribosomes
Figure 17.21 The signal mechanism for targeting proteins to the ER
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 pre-rRNA), 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