Protein Synthesis. DNA is in the form of specific sequences of nucleotides along the DNA strands The...
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Transcript of Protein Synthesis. DNA is in the form of specific sequences of nucleotides along the DNA strands The...
Protein Synthesis
Protein Synthesis
• DNA is in the form of specific sequences of nucleotides along the DNA strands
• The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins
• The process by which DNA directs protein synthesis, gene expression includes two stages, called transcription and translation
*Transcription and Translation*
• Cells are governed by a cellular chain of command– DNA RNA PROTEIN
• Transcription (DNA to RNA)– Is the synthesis of RNA under the direction of DNA– Produces messenger RNA (mRNA)
• Translation (RNA to PROTEIN)– Is the actual synthesis of a polypeptide, which occurs under
the direction of mRNA– Occurs on ribosomes
*Transcription and Translation in Eukaryotes*
• In a eukaryotic cell the nuclear envelope separates transcription from translation
Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA.
(b)
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
mRNA
DNA
Pre-mRNA
Polypeptide
Ribosome
Nuclearenvelope
*Transcription (Part 1 of Protein Synthesis)*• Transcription is the DNA-
directed synthesis of RNA
RNA synthesis Is catalyzed by?
RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides
How does it work?Follows the same base-pairing rules as DNA, except that in RNA, Uracil substitutes for Thymine
*RNA*
Table 17.1
• RNA is single stranded, not double stranded like DNA• RNA is short, only 1 gene long• RNA uses the sugar ribose instead of deoxyribose in DNA• RNA uses the base uracil (U) instead of thymine (T)
*Synthesis of an RNA Transcript*
• The stages of transcription are– Initiation
– Elongation
– Termination
PromoterTranscription unit
RNA polymerase
Start point
53
35
35
53
53
35
53
35
5
5
Rewound
RNA
RNA
transcript
3
3Completed RNA transcript
Unwound
DNA
RNA
transcript
Template strand of DNA
DNA
1 Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5 3 . In the wake of
transcription, the DNA strands re-form a double helix.
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
• Promoters signal the initiation of RNA synthesis
• Transcription factors help eukaryotic RNA polymerase recognize promoter sequences
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
T A T AAA AATAT T T T
TATA box Start point TemplateDNA strand
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35
Transcriptionfactors
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35
Promoter
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355
RNA polymerase IITranscription factors
RNA transcript
Transcription initiation complex
Eukaryotic promoters1
Several transcriptionfactors
2
Additional transcriptionfactors
3
*Synthesis of an RNA Transcript – Initiation*
Synthesis of an RNA Transcript - *Elongation*
Elongation
RNApolymerase
Non-templatestrand of DNA
RNA nucleotides
3 end
C A E G C A A
U
T A G G T TA
AC
G
U
AT
CA
T C C A A TT
GG
3
5
5
Newly madeRNA
Direction of transcription(“downstream”) Template
strand of DNA
• RNA polymerase synthesizes a single strand of RNA against the DNA template 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
• Specific sequences in the DNA signal termination of transcription
• 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.
*Synthesis of an RNA Transcript – Termination*
Transcription Overview
• Transcription of RNA processing occur in the ________. After this, the messenger RNA moves to the _________ for translation.
• The cell adds a protective cap to one end, and a tail to the other end. Why?
• **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
*Post Termination RNA Processing*
*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 nucleotideadded to the 5 end
50 to 250 adenine nucleotidesadded to the 3 end
Protein-coding segment Polyadenylation signal
Poly-A tail3 UTRStop codonStart codon
5 Cap 5 UTR
AAUAAA AAA…AAA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
G P P P
53
RNA Processing - Splicing
• The original transcript from the DNA is called pre-mRNA.
• It contains transcripts of both introns and exons.
• The introns are removed by a process called splicing to produce messenger RNA (mRNA)
RNA Processing
• 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
Figure 17.12
GeneDNA
Exon 1 Intron Exon 2 Intron Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 1
Domain 2
Polypeptide
Homework
Worksheet on Transcription