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

53

35

Transcriptionfactors

53

35

Promoter

53

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