Chapter 2 3 protein biosynthesis

Protein biosynthesis

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The Central Dogma of Life

replication

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Protein Synthesis
• Genetic information: the form of specific
sequences of nucleotides along the DNA strands
• The DNA inherited leads to specific traits by
dictating the synthesis of proteins
• Protein Synthesis: includes two stages:
transcription and translation

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Transcription and Translation

• Transcription

– the synthesis of RNA under the direction of DNA
– Produces messenger RNA (mRNA)

• Translation

– the actual synthesis of a polypeptide under the direction
of mRNA
– Occurs on ribosomes

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Transcription and Translation
• In prokaryotes, transcription & translation
occur together
TRANSCRIPTION

DNA

mRNA
Ribosome

TRANSLATION
Polypeptide

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(a) Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
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Transcription and Translation
• In a eukaryotic cell, the nuclear envelope separates
transcription from translation
• Extensive RNA processing occurs in the nucleus
Nuclear
envelope

DNA

TRANSCRIPTION

Pre-mRNA

RNA PROCESSING

mRNA

Ribosome
TRANSLATION

(b) Eukaryotic cell. The nucleus provides a separate
compartment for transcription. The original RNA
transcript, called pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.

Polypeptide

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Transcription
• Product/catalysis/materials/
steps
• Transcription is the DNAdirected synthesis of mRNA
• RNA synthesis

– catalyzed by RNA polymerase

– Follows the same base-pairing
rules as DNA, except that in

RNA, uracil substitutes for
thymine
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TRANSCRIPTION - OVERVIEW
 Transcription
- when a cell requires a particular protein  specific mRNA synthesized
- first, a section of DNA containing the gene unwinds
- only one of the DNA strands copied (at the initiation point: sequence TATAAA)
- RNA polymerase: moves along DNA template in the 3’-5’direction  replicates DNA sequence
into a complementary sequence of mRNA
- mRNA synthesized using complementary base pairing with uracil (U) replacing thymine (T)
 moves out of the nucleus to ribosomes in the cytoplasm, acts as the template for protein
biosynthesis (translation) and the DNA re-winds
 released at the termination point


RNA
• Types/functions
• RNA is single stranded, not double stranded like DNA
• RNA is short, only 1 gene long, where DNA is very long and contains
many genes
• RNA uses the sugar ribose instead of deoxyribose in DNA
• RNA uses the base uracil (U) instead of thymine (T) in DNA.

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Synthesis of an RNA Transcript
• The stages of
transcription:
– Initiation
– Elongation
– Termination

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Synthesis of an RNA Transcript - Initiation
• Promoters signal the
initiation of RNA synthesis
• Transcription factors help
eukaryotic RNA polymerase
recognize promoter
sequences
• A crucial promoter DNA
sequence is called a TATA
box.

1 Eukaryotic promoters

TRANSCRIPTION

DNA

RNA PROCESSING

Pre-mRNA

mRNA
TRANSLATION

Ribosome
Polypeptide

5
3

Promoter

3
5

T A T A A AA
AT AT T T T

TATA box

Start point
2

Template
DNA strand

Several transcription
factors

Transcription
factors

5
3

3 Additional transcription

factors

RNA polymerase II

5
3

Transcription factors

3
5

5

RNA transcript
Transcription initiation complex

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3
5

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Synthesis of an RNA Transcript - Elongation
•
•

RNA polymerase synthesizes a single strand of RNA against the DNA template
strand (anti-sense strand), adding nucleotides to the 3’ end of the RNA chain
As RNA polymerase moves along the DNA, it continues to untwist the double
helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA
nucleotides
Non-template
strand of DNA

Elongation

RNA nucleotides

RNA

polymerase

3

C

C

A

A T

3 end

T

U
C A

E

G

C

A

T

A

G

G

T

Newly made
RNA

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T

U

A

T

Direction of transcription
(“downstream”)

5

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C

G

5

T

G

A

A

A

A

C

G

Template
strand of DNA

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Synthesis of an RNA Transcript - Termination
•

Specific sequences in the DNA signal
termination of transcription (TAA, TAG, TGA)

•

When one of these is encountered by the
polymerase, the RNA transcript is released
from the DNA and the double helix can zip
up again.

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Post Termination RNA Processing
•

Most eukaryotic mRNAs aren’t ready to be translated into protein directly after
being transcribed from DNA. mRNA requires processing.

•

RNA processing occur in the nucleus. After this, the messenger RNA moves to the
cytoplasm for translation.

•

The cell adds a protective cap to one end, and a tail of A’s to the other end.
These both function to protect the RNA from enzymes that would degrade

•

Most of the genome consists of non-coding regions called introns
– Non-coding regions may have specific chromosomal functions or have regulatory
purposes
– Introns also allow for alternative RNA splicing

•

Thus, an RNA copy of a gene is converted into messenger RNA by doing 2 things:
– Add protective bases to the ends
– Cut out the introns

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Alteration of mRNA Ends
• Each end of a pre-mRNA molecule is modified in a
particular way
– The 5 end receives a modified nucleotide cap
– The 3 end gets a poly-A tail
A modified guanine nucleotide
added to the 5 end
TRANSCRIPTION

50 to 250 adenine nucleotides
added to the 3 end

DNA

Pre-mRNA

RNA PROCESSING

mRNA

5

Protein-coding segment

G P P P

Polyadenylation signal
AAUAAA

3

AAA…AAA

Ribosome
TRANSLATION

5 Cap
Polypeptide

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5 UTR

Start codon Stop codon

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3 UTR

Poly-A tail

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RNA Processing - Splicing
• The original transcript
from the DNA is called premRNA.
• It contains transcripts of
both introns and exons.
• The introns are removed
by a process called splicing
to produce messenger
RNA (mRNA)

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RNA Processing - Splicing
• Ribozymes are catalytic RNA molecules that function
as enzymes and can splice RNA
• RNA splicing removes introns and joins exons
5 Exon Intron
TRANSCRIPTION

RNA PROCESSING

DNA

Pre-mRNA

Intron

Exon

Exon

3

Pre-mRNA 5 Cap
1

Poly-A tail
30

31

Coding
segment

mRNA

Ribosome

104

105

146

Introns cut out and
exons joined together

TRANSLATION

Polypeptide

mRNA 5 Cap

1
3 UTR

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Poly-A tail

146

3 UTR
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RNA Processing - by spliceosomes
• RNA Splicing can also be carried out by spliceosomes

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Alternative Splicing (of Exons)
• How is it possible that there are millions of human
antibodies (proteins) when there are only about
30,000 genes?
• Alternative splicing refers to the different ways
the exons of a gene may be combined, producing
different forms of proteins within the same genecoding region
• Alternative pre-mRNA splicing is an important
mechanism for regulating gene expression in
higher eukaryotes

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Protein – modular structure
• Proteins often have a modular architecture
consisting of discrete structural and functional
regions called domains
• In many cases, different exons code for the
different domains in a protein
DNA

Gene
Exon 1

Exon 2

Intron

Intron

Exon 3

Transcription
RNA processing
Translation
Domain 3
Domain 2
Domain 1
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Polypeptide

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