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Chapter 17. Central Dogma of Molecular Biology From Genes to Protein. One gene – one polypeptide hypothesis. One gene dictates the production of a single polypeptide. Prokaryotic Cell. Central Dogma of Molecular Biology. transcription. mRNA.

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Chapter 17

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Chapter 17 l.jpg

Chapter 17

Central Dogma of Molecular Biology

From Genes to Protein

One gene – one polypeptide hypothesis

One gene dictates the production of a single polypeptide


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Prokaryotic Cell

Central Dogma of Molecular Biology

transcription

mRNA

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

ribosome

translation

polypeptide

Eukaryotic Cell

transcription

mRNA

1o transcript

RNA Processing

mRNA

2o transcript

translation

ribosome

polypeptide


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Directional Triplet-code Instructions for a polypeptide

3’

5’

DNA

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

Transcription

3’ to 5’ direction on DNA

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

mRNA

5’

3’

Translation

5’ to 3’ direction on RNA

polypeptide

trp

phe

gly

ser


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3 RNA Players Involved

Central Dogma

tRNA = 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

mRNA = DNA transcript


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Transcription3 steps

Initiation

  • Initiation

  • Elongation

  • Termination

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


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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


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Termination

1) 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.

Transcription video

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


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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


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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


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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.


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Structure of a Ribosome

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

P = Peptidyl-tRNA binding site

A = Aminoacyl-tRNA binding site

A = tRNA brings in new AA

E= Free t-RNA detaches from ribosome

E = Exit Site

tRNA Anticodon

Large subunit

Small subunit

E P A

mRNA codon

Ribosome=

rRNA + protein


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Translation

Initiation

  • Initiation

  • Elongation

  • Termination

- 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.


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Elongation

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

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

A peptide bond forms between the two adjacent AA

Ribosome slides down the 3’ end of the mRNA


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Termination

Ribosome encounters a “stop” codon:

UAA, UAG, or UGA

Release factor facilitates the release of both ribosomal subunits

Translation Video


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Watch these real-time videos on DNA transcription and translation…Totally Cool Dude!

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


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Redundancy in AA

  • Notice…several codons code for the same AA

     Evolutionary advantage?

Redundancy helps to minimize errors in protein synthesis due to mutations

Other ways of increasing redundancy:

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


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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.


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