Protein synthesis

DNA and GenesDNA and Genes• DNA contains genes, sequences of

nucleotide bases• These genes code for polypeptides

(proteins)

1

Protein Synthesis (Gene Expression)

Proteins (Review)•Proteins make up all living materials

• Proteins are composed of amino acids – there are 20 different amino acids

• Different proteins are made by combining these 20 amino acids in different combinations

• Amino acid chains are called polypeptides.

• Proteins are manufactured (made) by the ribosomes

•Function of proteins:1.Help fight disease

2.Build new body tissue

3.Enzymes used for digestion and other chemical reactions are proteins (Enzymes speed up the rate of a reaction)

4. Component of all cell membranes

Making a Protein—Transcription•First Step: Copying of genetic information from DNA to RNA called Transcription

Why? DNA has the genetic code for the protein that needs to be made, but proteins are made by the ribosomes—ribosomes are outside the nucleus in the cytoplasm.

DNA is too large to leave the nucleus (double stranded), but RNA can leave the nucleus (single stranded).

The enzyme RNA polymerase binds to DNA and part of DNA temporarily unzips and is used as a template to assemble complementary nucleotides into messenger RNA (mRNA). mRNA copies the template strand

• mRNA then goes through the pores of the nucleus with the DNA code and attaches to the ribosome.

Making a Protein—Translation•Second Step: Decoding of mRNA into a protein/polypeptide chain is called Translation.•Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome.

These amino acids come from the food we eat. Proteins we eat are broken down into individual amino acids and then simply rearranged into new proteins according to the needs and directions of our DNA.

•A series of three adjacent bases in an mRNA molecule codes for a specific amino acid—called a codon.

•A triplet of nucleotides in tRNA that is complementary to the codon in mRNA—called an anticodon.

•Each tRNA codes for a different amino acid.

Amino acid

Anticodon

12copyright cmassengale

• mRNA carrying the DNA instructions and tRNA carrying amino acids meet in the ribosomes.

• Amino acids are joined together to make a protein.

Polypeptide = Protein

15

RibosomesRibosomes

• Made of a large and small subunit

• Composed of rRNA (40%) and proteins (60%)

• Have two sites for tRNA attachment --- P and A

copyright cmassengale

16

Step 1- InitiationStep 1- Initiation

• mRNA transcript start codon AUG attaches to the small ribosomal subunit

• Small subunit attaches to large ribosomal subunit

mRNA transcript

copyright cmassengale

17

RibosomesRibosomes

PSite

ASite

Largesubunit

Small subunit

mRNAmRNA

A U G C U A C U U C G

copyright cmassengale

Step 2 - Elongation• As ribosome moves, two tRNA with their

amino acids move into site A and P of the ribosome

• Peptide bonds join the amino acids

18copyright cmassengale

19

InitiationInitiation

mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa2

A U

A

1-tRNA

U A C

aa1

anticodon

hydrogenbonds codon

copyright cmassengale

20

mRNAmRNA

A U G C U A C U U C G

1-tRNA 2-tRNA

U A C G

aa1 aa2

A UA

anticodon

hydrogenbonds codon

peptide bond

3-tRNA

G A A

aa3

ElongationElongation

copyright cmassengale

21

mRNAmRNA

A U G C U A C U U C G

1-tRNA

2-tRNA

U A C

G

aa1

aa2

A UA

peptide bond

3-tRNA

G A A

aa3

Ribosomes move over one codon

(leaves)

copyright cmassengale

22

mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa1

aa2

A UA

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

copyright cmassengale

23

mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa1aa2

A U

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

(leaves)

Ribosomes move over one codoncopyright cmassengale

24

mRNAmRNA

G C U A C U U C G

aa1aa2

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

U G A

5-tRNA

aa5

copyright cmassengale

25

mRNAmRNA

G C U A C U U C G

aa1aa2

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

U G A

5-tRNA

aa5

Ribosomes move over one codoncopyright cmassengale

26

mRNAmRNA

A C A U G U

aa1

aa2

U

primaryprimarystructurestructureof a proteinof a protein

aa3

200-tRNA

aa4

U A G

aa5

C U

aa200

aa199

terminatorterminator or stopor stop codoncodon

TerminationTermination

copyright cmassengale

27

End Product –The Protein!End Product –The Protein!• The end products of protein synthesis

is a primary structure of a protein• A sequence of amino acid bonded

together by peptide bonds

aa1

aa2 aa3 aa4aa5

aa200

aa199

copyright cmassengale

28

Messenger RNA (mRNA)Messenger RNA (mRNA)

methionine glycine serine isoleucine glycine alanine stopcodon

proteinprotein

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

startcodon

Primary structure of a proteinPrimary structure of a protein

aa1 aa2 aa3 aa4 aa5 aa6

peptide bonds

codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1

copyright cmassengale

Use one of the codon charts on the next page to find the amino acid sequence coded for by the following mRNA strands.

CAC/CCA/UGG/UGA

___________/___________/___________/____________

AUG/AAC/GAC/UAA

___________/___________/___________/____________

GGUGUG ACC ACU

UAC UUG CUG AUU

CAC/CCA/UGG/UGA

___________/___________/___________/____________Histidine1st

Bas

e

2nd Base

3rd Base

Proline Tryptophan Stop

AUG/AAC/GAC/UAA

___________/___________/___________/____________Methionine Asparagine Aspartic Acid Stop

Protein Synthesis

Protein Synthesis videos

• http://www.youtube.com/watch?v=983lhh20rGY

• http://www.youtube.com/watch?v=erOP76_qLWA

Movie about translation at bottom of webpage. Click on hyperlink in picture above.