Chapter 17

Central Dogma of Molecular Biology

From Genes to ProteinOne gene – one polypeptide hypothesisOne gene dictates the production of a single polypeptide

transcription

transcription

Transcription: synthesis of mRNA under the direction of DNA

- one step in prokaryotic cells

- Two steps in eukaryotic cells

1. creating a 1o transcript

2. RNA processing (editing) to create 2o transcript

Translation: synthesis of a polypeptide under the direction of mRNA

translation

translation

RNA Processing

Central Dogma of Molecular BiologyProkaryotic Cell

Eukaryotic Cell

mRNA

1o transcript

mRNA

polypeptide

polypeptide

ribosome

ribosome

mRNA

2o transcript

Directional Triplet-code Instructions for a polypeptide

Transcription

3’ to 5’ direction on DNA

Translation

5’ to 3’ direction on RNA

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

3’

3’

5’

5’

DNA

mRNA

polypeptide

3 RNA Players Involved

mRNA = DNA transcripttRNA = type of RNA that carries the amino acids to the ribosome

rRNA = RNA that, combined with protein, makes up the structural component of a ribosome

Central Dogma

Transcription3 steps

1) Initiation

2) Elongation

3) Termination

Initiation

Transcription begins at a “Promotor” region of DNA, recognized by a TATA box sequence

A transcription factor binds to help RNA polymerase (RNA pol.) to bind to the DNA

RNA pol. binds and begins to unwind DNA

Elongation

RNA pol unzips 10-20 DNA bases at a time in a 3’ 5’ direction and base pairs with the DNA using RNA nucleotides

Base-pair occurs at a rate of 60 nucleotides/second

A-U; G-C base-pair rules

New 1o transcript (initial copy) of mRNA peels away from DNA template

Termination1) RNA sequence, not DNA sequence, is used to end transcription.

Once RNA pol. creates the AAUAAA termination sequence, the primary mRNA transcript peels away from the DNA template.

mRNA

2) RNA processing occurs: (creating of 2o transcript)

- 5’ cap of guanine nucleotides are added to protect mRNA from hydrolytic enzymes and to provide a starting site for ribosomes in translation

- 3’ poly A tail is added to protect mRNA from hydrolytic enzymes and to help with mRNA export from the nucleus

Transcription video

TerminationRNA Processing Continues

Primary transcript of mRNA is long and includes introns (non-coding regions) and exons (coding regions)

Evolutionary Significance? Non-coding RNA regions were the result of non-coding DNA regions. Longer DNA increased chances of X-over during meiosis.

During RNA processing, introns must be cut out (spliced) before a functional polypeptide can be made

mRNA

TerminationRNA Processing Continues

3) RNA splicing

A “spliceosome” complex recognizes intron sequences and deletes them

Spliceosomes are made up of special RNA called snRNA + various proteins

2o mRNA transcript is now ready

t-RNA Activation for Translation

tRNA = brings in the corresponding AA coded by the mRNA to the ribosome for polypeptide (protein) synthesis

Aminoacyl-tRNA synthase (enzyme) catalyzes the binding of a specific amino acid to a free tRNA.

Activated t-RNA is now ready for translation.

Structure of a Ribosome

Large subunit

Small subunit

Ribosome=

rRNA + protein

E P A

A = Aminoacyl-tRNA binding site

P = Peptidyl-tRNA binding site

E = Exit Site

tRNA Anticodon

mRNA codon

A = tRNA brings in new AA

P= Creates peptide bonds between AA and holds the growing polypeptide chain

E= Free t-RNA detaches from ribosome

Translation

1) Initiation

2) Elongation

3) Termination

Initiation

- Small ribosomal subunit attaches near the 5” end of mRNA at AUG start codon

- tRNA carrying AA Methionine attaches to AUG start codon

- Large ribosomal subunit attaches to mRNA w/ tRNA occupying the “P” site.

Elongation

New tRNA brings in another amino acid, based on the codon on the mRNA. Base-pairing occurs.

A peptide bond forms between the two adjacent AA

Ribosome slides down the 3’ end of the mRNA

Free tRNA exits and can return to bind with other AA for repeat deliveries

Termination

Ribosome encounters a “stop” codon:

UAA, UAG, or UGA

Release factor facilitates the release of both ribosomal subunits

Translation Video

http://www.dnai.org/a/index.html

Watch these real-time videos on DNA

transcription and translation…Totally

Cool Dude!

Redundancy in AA

- Notice…several codons code for the same AA

Evolutionary advantage?

Redundancy helps to minimize errors in protein synthesis due to

mutations

Although A pairs with U and G pairs with C, U can sometimes pair with G wobble effect

Other ways of increasing redundancy:

Endo-membrane System

Recall that glyco-proteins (made by attached ribosomes) are processed by the ER and golgi before being incorporated within vesicles (ie. lysosomes) , exported from the cell, or incorportated into the plasma membrane.