How Proteins Are Made

Mrs. Wolfe

DNA: instructions for making proteins

• Proteins are built by the cell according to your DNA

• What kinds of proteins are in your body?– Eye color, hormones,

enzymes, poisons??!!

• Where do we get these proteins?

Proteins are made on the ribosomes!

• Protein Synthesis - – Protein production

• How do the ribosomes know which proteins to make, how to make them, how many of them to make…etc.?

• DNA!!

There are TWO steps to Protein Synthesis

• 1. Transcription– DNA is rewritten as RNA

• 2. Translation– RNA is translated into amino acids

• Amino acids are linked together into a single protein.

Step 1.

Step 2.

Transcription

• DNA is transcribed, or rewritten, into RNA.

• DNA cannot leave the nucleus to travel to the ribosomes. So, it is rewritten as mRNA, which is small enough to leave the nucleus.

• mRNA = messenger RNA• RNA – ribonucleic acid

• **RNA differs from DNA in three ways:

• 1. Single-stranded• 2. Sugar is ribose (not

deoxyribose)• 3. Thymine Uracil (U)

Try base pairing with RNA!

• DNA strand : T G C A T C A G A

• RNA strand: A C G U A G U C U

Transcription cont’d.

• DNA mRNA• RNA polymerase binds to

“promotor” region• Promotor = start signal• RNA polymerase unwinds and

creates a complementary strand to one side of the DNA

• The complementary strand is created of the “coding” region until the “termination” region is reached.

mRNA follows the base pairing rules, except:

DNA base Adenine pairs with mRNA base Uracil!!

Remember, mRNA has the base Uracil instead of Thymine. So, Uracil pairs with Adenine.

Don’t confuse the RNA polymerase of transcription with the DNA polymerase of translation.

Transcription

• View the animation:

• http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf

The mRNA now leaves the nucleus

• It travels to the ribosome, where TRANSLATION OCCURS!

• Translation – the mRNA code is translated into amino acids which are hooked together to make a protein.

The mRNA code is TRANSLATED into amino acids!

• Actually, every three bases code for an amino acid.

• Codon – three nucleotide sequence of mRNA that codes for an amino acid.

• DNA: • ATCGAT• mRNA:• UAGCUA• Codons:• UAG = 1 codon• CUA = 1 codon

Codons code for amino acids

• Scientists have created a chart that shows for what each sequence of bases codes.

Let’s try some. (Page 209)Use the Codon Chart on the next slide.

• Codons

• 1. UUA

• 2. GGU

• 3. AUG

• 4. UGA

• 5. UAA

• 6. UAG

Codon Chart

Phe = phenylalanineLeu = leucineIle = isoleucineMet = methionineVal = valine

Ser = serinePro = prolineThr = threonineAla = alanineTyr = tyrosine

His = histidineGln = glutamineAsn = asparagineLys = lysineAsp = aspartate

Glu = glutamateCys = cysteineTrp = tryptophanArg = arginineGly = glycine

Let’s try some. (Page 209)Use the Codon Chart on the next slide.

• Codons

• 1. UUA - leucine

• 2. GGU - glycine

• 3. AUG - methionine (Start Codon)

• 4. UGA - stop codon

• 5. UAA - stop codon

• 6. UAG - stop codon

Practice Problem

• What is the amino acid sequence for the following DNA strand?

• T A C – A G A – T T A – G G G – A T T

• Start – serine – asparagine – proline – stop

A closer look at the mRNA strand.

• The codon that will start translation is AUG.

• Several codons that will end translation are UGA, UAA, UAG.

Translation cont’d.

• So, the mRNA arrives at the ribosome. • A tRNA (transfer RNA) matches the

corresponding up to each mRNA codon.• Anticodon – a three-nucleotide sequence of tRNA

that is complementary to an mRNA codon.• The tRNA is carrying an amino acid specific to its

anticodon. • This occurs at the P site on a ribosome.

A PE

Translation cont’d.

• The tRNA moves to the P site, where its amino acid detaches and is added to a string of amino acids

• A Peptide bond is formed between the amino acids, to hook them together like beads on a string.

• Finally, the tRNA moves to the E site, where it exits the ribosome.

• Meanwhile, another tRNA moves in to the vacant A site and its anticodon matches to the next mRNA codon. The process continues until the entire mRNA strand is translated into a string of amino acids.

A PE

• The first mRNA codon to be translated is the start codon.

• Translation continues until a stop codon is reached.

• The string of amino acids, held together by peptide bonds, is called a protein.

Translation

• View the animation:

• http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf

Don’t confuse Replication with Protein Synthesis!

DNA Replication Review Tips:

• Visit:

• http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dna-rna2.swf– See if you can identify helicase, DNA

polymerase, and the leading and lagging strand.

Transcription Review Tips:

• Visit:

• http://www.ncc.gmu.edu/dna/mRNAanim.htm

• OR

• http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html

Translation Review Tips:

• Visit:

• http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html

• OR

• http://www.cat.cc.md.us/biotutorials/protsyn/translat.html

Transcription and Translation Review Tips:

• Visit:

• http://www.csam.montclair.edu/%7Esmalley/transcription.mov

• OR

• http://www.so.wustl.edu/science_outreach/curriculum/genetics/pdfs/ModGen_1D_SP.pdf