Protein Synthesis:

DNA Transcription into mRNA, RNA Processing

and mRNA Translation into Protein

How does DNA work?What is a gene?

A gene is a segment of DNA that codes for a certain characteristic or trait. Ex) Blue eyes, Black hair, Dark skin, etc.

How do you get from a sequence of DNA to having black hair? To have black hair, your body must produce vast amounts of the dark pigment melanin.

How do you make melanin? Your cells must have the basic materials to build melanin and the enzymes needed to carry out the synthesis of melanin. Similarly, most jobs the cell needs to carry out depend upon the use of enzymes or other PROTEINS.

So, to get from a gene (or genotype) to a specific trait (phenotype) requires the action of specific proteins. The way genes work is every gene codes for the creation of a different protein, each with its own function.

Gene X --> Protein X --> Trait X

From Gene to ProteinWhere is the DNA in a eukaryotic cell? Where are proteins made?

Problem: DNA is in the _____________ while proteins are made in the ________________ (at structures called ______________).

In addition, DNA is too large to fit through the nuclear pores…frustrating!

How do we get the information in our DNA (our genes) out to the cytoplasm so we can make proteins?

Solution: Use a messenger!In the cell, that messenger is RNA (mRNA).

nucleuscytoplasm

ribosomes

Gene to Protein in Two Steps

Step 1: _____________ (in nucleus)

Makes copie(s) of a segment of DNA (a gene) in the form of mRNA.

Step 2: ____________ (at ribosome)

Translates the sequence of nucleotides in mRNA into a sequence of amino acids (a protein!).

transcription

translation

Transcription

Translation

DNA mRNA Protein

How is a DNA sequence transcribed into an RNA sequence?

Same as DNA-DNA base pairing rules except… (look left)

DNA RNA

C pairs with ____ G ____ T ____ A ____

G

CA

U

TRANSLATION: From nucleic acid to protein

• How is the information for making a protein encoded?• There are ___ different amino acids used to build proteins.

But, there are only ___ RNA nucleotides.• If we read the RNA one base at a time, that would mean

we could only code for ___ amino acids.For example:

Base Amino Acid– Adenine --> glycine– Cytosine --> tryptophan– Guanine --> alanine– Uracil --> phenylalanine

204

4

We wouldn’t have enough bases to code for 20 amino acids!

Why a triplet code??

• What if we “read” the RNA two bases at a time? How many unique two-letter code words could we make using four bases?

AG GG CG UGAC GC CC UCAU GU CU UUAA GA CA UA

– This give us 42 = ___ unique code words….Is this enough to code for all 20 amino acids? _____

• So what about three-letter code words? That should give us 43= ___ unique code words… Enough?

16

No!

64

Yes, more than enough…

The Genetic Code, Revealed

A set of three nucleotides in an RNA sequence, called a codon, codes for the addition of an amino acid in a polypeptide.

GUG = _____

ACU = ________

UUA = ________

AUG = ________

UAA,UGA,UAG = __________

The Genetic Code, Revealed

A set of three nucleotides in an RNA sequence, called a codon, codes for the addition of an amino acid in a polypeptide.

GUG = _____

ACU = ________

UUA = ________

AUG = ________

UAA,UGA,UAG = __________

valine

threonine

leucine

Methionine or START codon

STOP codons

The Genetic Code(how to translate the 64 mRNA codons)

Phe = Phenylalanine

Leu = Leucine

Ile = Isoleucine

Met = Methionine

Val = Valine

Ser = Serine

Pro = Proline

Thr = Threonine

Ala = Alanine

Tyr = Tyrosine

His = Histadine

Gln = Glutamine

Asn = Asparagine

Lys = Lysine

Asp = Aspartic Acid

Glu = Glutamic Acid

Cys = Cysteine

Trp = Tryptophan

Arg = Arginine

Gly = Glycine

The 3 letter Amino Acid abbreviations

TranscriptionCopying DNA

into RNA

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Binding & Initiation Details

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Elongation Detail

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Transcription: Copying DNA into RNA

1. Initiation: RNA polymerase binds to the promoter sequence at the start of a gene. – The promoter sequence frequently includes a sequence

of TATAAAA (the “TATA box”). Not all promoter sequences are identical.

2. Elongation: RNA polymerase moves along the template strand, connecting the RNA nucleotides by bonding the sugar-phosphate backbone.– Note that RNA is synthesized from its 5’ to its 3’ end.

3. Termination: RNA polymerase lets go of the DNA and releases the mRNA when it gets to the terminator sequence at the end of the gene.

Let’s review transcription…

1. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html# (Click on Protein Synthesis Link)

2. http://www.stolaf.edu/people/giannini/biological%20anamations.html

3. http://www.biostudio.com/d_%20Transcription.htm

4. http://www.dnalc.org/resources/3d/12-transcription-basic.html