Figure 17.4

DNAmolecule

Gene 1

Gene 2

Gene 3

DNA strand(template)

TRANSCRIPTION

mRNA

Protein

TRANSLATION

Amino acid

A C C A A A C C G A G T

U G G U U U G G C U C A

Trp Phe Gly Ser

Codon

3 5

35

Four Major Steps1. Initiation

2. Elongation

3. Termination

4. Posttranscriptional Modification

Parts of a Genepromoter - DNA sequences which indicate the

location of a gene

promoters are located upstream from the DNA region that contains the information to be transcribed into mRNA

promoter transcription region termination sequence

gene

Orientation About a Gene

RNA polymerase – transcription enzyme (synthesizes mRNA in 5’ 3’ direction)

uses upstream, promoter region to determine where to start mRNA transcription

0positive numbersnegative numbers

upstream downstream

start transcription

InitiationdsDNA (double stranded DNA) needs to be opened

for mRNA to be made

promoter regions are often sequences of A’s and T’s2 H-bonds between A&Teasier to break than 3 H-bonds between G&Cprokaryotic genes have a TATA box

RNA polymerase opens the dsDNA

Initiation

transcription factors - numerous protein factors are involved in starting transcription

some of these proteins help control how often genes are transcribed

Figure 17.8

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

T A T A AA AA T A T 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

Elongation

RNA polymerase synthesizes mRNA in the 5’ 3’ directionno primer is

necessary

template strand - only one strand of the DNA is transcribed

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

Elongation Nomenclature

the template strand is the antisense strand

5’ A T T A C G A T C T G C A C A A G A T C C T 3’

5’ A U U A C G A U C U G C A C A A G A U C C U 3’

3’ T A A T G C T A G A C G T G T T C T A G G A 5’SENSE STRAND

ANTISENSE STRAND

DNA

DNAmRNA

TerminationRNA polymerase stops transcribing once it reaches

the termination sequenceenzyme dissociates with DNA strand and binds to

another promoter sequence

termination sequences differ between prokaryotes and eukaryotes

Transcription

Transcription Animation

Transcription Animation

Transcription

Figure 17.7

PromoterTranscription unit

RNA polymerase

Start point

53

35

35

53

53

35

53

35

5

5

Rewound

RNA

RNA

transcript

3

3

Completed 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.

Posttranscriptional Modification mRNA of eukaryotic cells need to be modified before

moving into the cytoplasm

primary transcript – initial eukaryotic mRNA transcript, before modification

1. 5’ cap – 7-methylguanosine triphosphate2. poly-A tail – approx. 200 adenine ribonucleotides are

added at the end

modifications prevent cellular enzymes from breaking down mRNA before it is translated into protein

5’ 3’

5’ cap added by capping enzyme complex

primary transcript

3’ poly-A tail added by poly-A polymerase

AAAAAAA

mG

mG

Modifications: Introns / Exonseukaryotic genes are longer than prokaryotic

geneswe carry extra “junk” DNAmost of this “junk” DNA signals when and how often

genes should be transcribed regulatory DNA

primary transcript is longer than necessaryexons – RNA sequences that will be expressed; helps

makes the proteinintrons – interfering RNA sequences; need to be

removed before translation

AAAAAAAmG

exon exon exonintron intron intron intron

introns removed by spliceosome proteins

AAAAAAAmG

exon exon exonmRNA transcript

Spliceosome ComplexSpliceosomes are a series of small

nuclear ribonucleoproteins (snRNP) that work together to remove introns.

snRNPs recognize specific sequences on the intronscuts out intron sequencessplices exon sequences together

RNA transcript (pre-mRNA)

Exon 1 Intron Exon 2

Other proteinsProteinsnRNA

snRNPs

Spliceosome

Spliceosomecomponents

Cut-outintronmRNA

Exon 1 Exon 2

5

5

5

1

2

3

Classwork/HomeworkSection 5.3 – pg. 249 #1-5, 7-9