Transcription.translation lecture

TRANSCRIPTION / TRANSCRIPTION / TRANSLATIONTRANSLATION

Take Notes on 17R

Transcription- process that makes mRNA from DNA

The Central DogmaThe Central Dogma

AP Biology 2007-2008

Transcription

fromDNA nucleic acid language

toRNA nucleic acid language

Only a specific part of DNA is Tx at one time, NOT the entire strand of DNA

Beadle & Tatum 1941 | 1958

George Beadle Edward Tatum

Nobel Prize for their discovery that genes act by regulating definite

chemical events

one gene : one enzyme hypothesis

one gene : one polypeptide

Enzymes are polypeptides- updated findings are:

Transcription

Making a single strand of mRNAtranscribed DNA strand = template stranduntranscribed DNA strand = coding strand

• same sequence as RNA

synthesis of complementary RNA strand at transcription bubble

Enzyme used: RNA polymerase

template strand

rewinding

mRNA RNA polymerase

unwinding

coding strand

DNAC C

C

C

C

C

C

C

C CC

G

GG

G

G G

G G

G

G

GAA

AA A

A

A

A

A

A A

A

AT

T T

T

T

T

T

T

T T

T

T

U U

5

35

3

3

5build RNA 53

Transcription

animation

3 Processes of TranscriptionIn Nucleus1. Initiation

DNA unzipped into 2 separate strands by DNA Helicase.

Promoter region identified, Initiation Complex Made

2. Elongation

Free floating RNA NITROGEN BASES in the nucleus pair up w/unzipped DNA NITROGEN BASES by RNA Polymerase

Ex: DNA template: 3’ –C G T- 5’

3. Termination

Complement pairing is done at the end of gene

-a single strand of RNA is released

Which gene will be Tx?1. Initiation

• Promoter region– binding site before beginning of gene

– TATA box catches attention

At Promoter Region, Transcription Factors make Initiation Complex

• Initiation complex– transcription factors bind to promoter region

suite of proteins which bind to DNA turn on or off transcription

– trigger the binding of RNA polymerase to DNA

2. ElongationMatching bases of DNA & RNA• Match RNA bases to DNA

bases on one of the DNA strands

U

A G GGGGGT T A C A C T T T T TC C C CA A

U

UU

U

U

G

G

A

A

A C CRNA

polymerase

C

C

C

C

C

G

G

G

G

A

A

A

AA

5' 3'

3. TerminationRNA Polymerase reaches the end of the gene

and the RNA P, DNA, and new mRNA dissociate.

mRNA Strand Needs Processing:

After Transcription

• In our DNA there are sequences that are not needed to make proteins.

• Due to complementing, these region end up in mRNA after Tx.

• These regions (aka=some junk)must be removed before Tl.

Eukaryotic genes have junk= mRNA have junk!

• Eukaryotic genes are not continuous– exons = the real gene

expressed / coding DNA

– introns = the junkIn between sequence

eukaryotic DNA

exon = coding (expressed) sequence

intron = noncoding (inbetween) sequence

intronscome out of RNA!

After TranscriptionmRNA ProcessingmRNA Processing

After Transcription Review

Which mRNA sequences are removed?

Why are they removed?

mRNA Processing

Result:Genetic code from DNA is transferred to

mRNA

The code obtained from DNA lets the mRNA know which amino acids to pick up:

• code is a set of 3 nitrogen bases = Codon

Overall Process of Transcription

mRNADNA

RNAPolymerase

TranscriptionTranscriptionAdenine (DNA and RNA)Cystosine (DNA and RNA)Guanine(DNA and RNA)Thymine (DNA only)Uracil (RNA only)

AP Biology 2007-2008

Translation

fromnucleic acid language

toamino acid language

TranslationTranslation• Process that converts mRNA into

protein

• Proteins are made of monomers of AA

• Tl occurs on ribosomes

• Where are ribosomes?

Ribosomes Facilitates coupling of tRNA anticodon to mRNA codon

• organelle & enzyme!!

Structureribosomal RNA (rRNA) & proteins

2 subunits large small E P A

Protein synthesis/quiz

Ribosomes

Met

5'

3'

UUA C

A G

APE

A site (acceptor site) • holds the newest tRNA carrying the next

amino acid to be added to chain

P site (peptidyl-tRNA site) • holds tRNA carrying growing polypeptide

chain

E site (exit site)• empty tRNA leaves ribosome

from exit site

Protein synthesis 2

tRNA: remindertRNA: reminder

• tRNA = transfer RNA• picks up the correct amino acids

and carries them to the mRNA strand forming the protein

• Location of anticodon, the complement to the mRNA codon

3 Steps of Translationmaking proteins

1. Initiation2. Elongation

3. Termination

Building a polypeptideInitiationbrings together mRNA, ribosome subunits, initiator tRNA

Elongationadding amino acids based on codon sequence

Terminationend codon 123

Leu

Leu Leu Leu

tRNA

Met MetMet Met

PE AmRNA5' 5' 5' 5'

3' 3' 3'3'

U UA AAACC

CAU UG G

GUU

A AAAC

CC

AU UG GGU

UA

AAAC

CC

AU UG GGU U

A AACCA U UG G

G AC

ValSer

AlaTrp

releasefactor

AA A

CCU UGG 3'

How translation works

Step 1: Initiationa)Ribosome attaches to the mRNA

b) Start codon is always AUG

Step 2: Elongation

a) tRNA anticodon matches with codon on mRNA

b) Peptide bonds are formed between the AA

Elongation (continued)

c) AA released from tRNA

d) Ribosome moves down the mRNA and continues adding AA

Step 3: Terminationa)The ribosome reaches one of three stop

codons (UAA, UAG, UGA)b) The mRNA and protein (polypeptide chain) is

released

The Decoder

This allows humans to translate the mRNA code into proteins

There are 20 Amino Acids

mRNA Start codon

Ribosome

Methionine

PhenylalaninetRNA

Lysine

Nucleus

TranslationTranslation

mRNA

mRNACodon on

mRNA Translation direction

Amino AcidtRNA

tRNARibosome

Growing polypeptide chain

mRNA

 Translation (continued)Translation (continued)

Anticodon on tRNA

The Decoder

This allows humans to translate the mRNA code into proteins

Prokaryote vs. Eukaryote genesProkaryotesDNA in cytoplasmcircular chromosomenaked DNAno intronsTx & Tl can occur simultaneously

EukaryotesDNA in nucleuslinear chromosomesDNA wound on histone proteinsintrons vs. exonstime & physical separation b/w Tx & Tl: ~1 hr from DNA to protein