Download Lecture 17 Protein synthesis pp101-110

Lecture 20
Protein synthesis
1
3.12 Proteins are made from amino acids linked by
peptide bonds
• Amino acids, the building blocks of proteins,
have an amino group and a carboxyl group
– Both of these are covalently bonded to a central
carbon atom
– Also bonded to the central carbon is a hydrogen
atom and some other chemical group symbolized by
R
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Amino
group
Carboxyl
group
Leucine (Leu)
Hydrophobic
Serine (Ser)
Aspartic acid (Asp)
Hydrophilic
3.12 Proteins are made from amino acids linked by
peptide bonds
• Amino acid monomers are linked together to
form polymeric proteins
– This is accomplished by an enzyme-mediated
dehydration reaction
– This links the carboxyl group of one amino acid to
the amino group of the next amino acid
– The covalent linkage resulting is called a peptide bond
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3.13 A protein’s specific shape determines its
function
• A polypeptide chain contains hundreds or
thousands of amino acids linked by peptide
bonds
– The amino acid sequence causes the polypeptide to
assume a particular shape
– The shape of a protein determines its specific
function
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Protein Synthesis
• DNA serves as master blueprint for protein
synthesis
• Genes are segments of DNA carrying
instructions for a polypeptide chain
• Triplets of nucleotide bases form the genetic
library
• Each triplet specifies coding for an amino acid
Genetic information flows:
DNA  RNA  polypeptide
Remember for further information
Roles of the Three Types of RNA
• Messenger RNA (mRNA) – carries the genetic
information from DNA in the nucleus to the
ribosomes in the cytoplasm
• Transfer RNAs (tRNAs) – bound to amino acids
base pair with the codons of mRNA at the
ribosome to begin the process of protein
synthesis
• Ribosomal RNA (rRNA) – a structural
component of ribosomes
From DNA to Protein
Nuclear
envelope
DNA
Transcription
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Polypeptide
Figure 3.33
From DNA to Protein
DNA
Figure 3.33
From DNA to Protein
Transcription
DNA
Figure 3.33
From DNA to Protein
DNA
Transcription
Pre-mRNA
RNA Processing
mRNA
Figure 3.33
From DNA to Protein
Nuclear
envelope
DNA
Transcription
Pre-mRNA
RNA Processing
mRNA
Figure 3.33
From DNA to Protein
Nuclear
envelope
DNA
Transcription
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Polypeptide
Figure 3.33
DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
DNA strand
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
Transcription:
• RNA Polymerase, An enzyme that oversees the
synthesis of RNA Unwinds the DNA template
(17 base pair at a time)
• Adds complementary ribonucleoside
triphosphates on the DNA template
• Joins these RNA nucleotides together
• Encodes a termination signal to stop
transcription
RNA polymerase
Transcribes the DNA code into RNA code
DNA =
G
A
T
C
A
T
T
A
G
RNA =
C
U
A
G
U
A
A
U
C
10.9 Transcription produces genetic messages in
the form of RNA
– Overview of transcription
– The two DNA strands separate
– One strand is used as a pattern to produce an RNA chain,
using specific base pairing
– For A in DNA, U is placed in RNA
– RNA polymerase catalyzes the reaction
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10.9 Transcription produces genetic messages in
the form of RNA
 Stages of transcription
– Initiation: RNA polymerase binds to a promoter, where
the helix unwinds and transcription starts
– Elongation: RNA nucleotides are added to the chain
– Termination: RNA polymerase reaches a terminator
sequence and detaches from the template
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5
RNA nucleotides
RNA
polymerase
Direction of
transcription
Newly made RNA
Template
strand of DNA
RNA polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
1
Initiation
2
Elongation
3
Termination
Completed
RNA
Area shown
in Figure 10.9A
Growing
RNA
RNA
polymerase
Transcription makes a
mRNA copy of DNA
RNA polymerase
DNA of gene
Promoter
DNA
1. RNA polymerase enzyme
binds to DNA (Initiation)
2. RNA polymerase catalyzes
the production of mRNA,
using DNA as a template
(Elongation)
3. RNA polymerase unbinds
from DNA and RNA
production stops
(Termination)
Figure 10.9B
Terminator
DNA
1 Initiation
Area shown
In Figure 10.9A
2 Elongation
3 Termination
Completed RNA
Growing
RNA
RNA
polymerase
Initiation of Translation
• A leader sequence on mRNA attaches to the
small subunit of the ribosome
• Methionine-charged initiator tRNA binds to
the small subunit
• The large ribosomal unit now binds to this
complex forming a functional ribosome
10.8 The genetic code is the Rosetta stone of life
– Characteristics of the genetic code
– Triplet: Three nucleotides specify one amino acid
– 61 codons correspond to amino acids
– AUG codes for methionine and signals the start of transcription
– 3 “stop” codons signal the end of translation
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10.8 The genetic code is the Rosetta stone of life
– Redundant: More than one codon for some amino acids
– Unambiguous: Any codon for one amino acid does not
code for any other amino acid
– Does not contain spacers or punctuation: Codons are
adjacent to each other with no gaps in between
– Nearly universal
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Genetic Code
• RNA codons code
for amino acids
according to a
genetic code
Figure 3.35
Strand to be transcribed
DNA
Strand to be transcribed
DNA
Transcription
RNA
Start
codon
Stop
codon
Strand to be transcribed
DNA
Transcription
RNA
Start
codon
Polypeptide
Met
Translation
Lys
Phe
Stop
codon
10.10 Eukaryotic RNA is processed before leaving
the nucleus
– Messenger RNA (mRNA) contains codons for protein
sequences
– Eukaryotic mRNA has interrupting sequences called
introns, separating the coding regions called exons
– Eukaryotic mRNA undergoes processing before leaving
the nucleus
– Cap added to 5’ end: single guanine nucleotide
– Tail added to 3’ end: Poly-A tail of 50–250 adenines
– RNA splicing: removal of introns and joining of exons to
produce a continuous coding sequence
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Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
Comparison
•
•
•
•
•
•
•
•
DNA
Deoxyribonucleic Acid
Double stranded
Deoxyribose
GATC
Long in length
Nucleus – always
Coding
1 type
•
•
•
•
•
•
•
•
RNA
Ribonucleic Acid
Single stranded
Ribose
GAUC
Shorter in length
Nucleus  cytosol
Messenger
3 Types
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