PROTEIN SYNTHESIS

Higher Level

Learning OutcomesAt the end of this topic you should be able

to

1. Outline the steps in protein synthesis

2.Know that DNA contains the code for protein

3.Know that enzymes unwind the DNA

4.Know this code is transcribed to mRNA

5.Know the role of RNA polymerase

6.Know the role of a codon

Learning Outcomes(cont.)

At the end of this topics you should be able to

7.Know the code is translated on the ribosome8.Know the ribosome is composed of subunits9. Describe how amino acids are assembled in

the correct order determined by the codons on mRNA to synthesise the protein

10. Know that a stop codon on mRNA signals the release of the protein

Lesson Objectives (cont.)At the end of this lesson you should be able

to

11. Know that the protein folds into its functional shape

12. Give the location of protein synthesis

13. Understand the molecular involvement of DNA, mRNA, tRNA, rRNA and amino acids

in the process of protein synthesis

For Protein Synthesis You need:

1. A supply of amino acids – cytoplasm

2. Instructions as how to join the amino acids together – genetic code

3. An assembly line – ribosomes

4. A messenger to carry information from DNA to ribosomes

Protein Synthesis - Steps

3 stages:

1. Transcription

2. Translation

• Remember: DNA RNA Protein

RNA is composed of 3 parts

1. Ribose: smaller sugar than deoxyribose of DNA

2. Phosphate

3. 4 Nitrogenous Bases A,G,U,C

RNA is single stranded and thus smaller & able to leave the nucleus of the cell

DNA RNA Protein

Nuclearmembrane

Transcription

RNA Processing

Translation

DNA

mRNA

Ribosome

Protein

Eukaryotic Cell

Transcription Translation

Transcription

Translation

DNA

mRNA

Ribosome

Protein

Prokaryotic Cell – No nucleus

DNA RNA Protein

Learning Check• What is RNA composed of?

• How does RNA (ribonucleic acid) differ from DNA (deoxyribonucleic acid)?

• What are the three stages in Protein synthesis?

1. Transcription

Nuclearmembrane

Transcription

RNA Processing

Translation

DNA

mRNA

Ribosome

Protein

Eukaryotic Cell

1. Transcription

• The transfer of information in the nucleus from a DNA molecule to an RNA molecule.

• Only 1 DNA strand serves as the template

• When complete, mRNA molecule is released into the cytoplasm

Video Clip available in extra material folder

Transcription

• Takes places in the nucleus of the cellThe process by which the information from DNA is transferred to RNA. DNA uncoils and unzips.

• The exposed DNA bases are matched up with RNA bases in the nucleus to form mRNA.

1. Transcription

DNA

mRNA

RNA Polymerase

Enzyme

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

This is a molecule of messenger RNA.

It was made in the nucleus by transcription from a DNA molecule.

mRNA molecule

codon

Learning Check

• What type of RNA molecule is responsible for taking the DNA copy from the nucleus into the cytoplasm

• What parts of the cell do you find RNA in?

• Can you outline the stages in transcription?

mRNA

• Takes place in the nucleus of the cellThe process by which the information from DNA is transferred to RNA. DNA uncoils and unzips.

• The exposed DNA bases are matched up with RNA bases in the nucleus to form mRNA.

Types of RNA

• Three types of RNA:

A. messenger RNA (mRNA)

B. transfer RNA (tRNA)

C. ribosomal RNA (rRNA)

• Remember: all produced in the nucleus!

A. Messenger RNA (mRNA)

• Carries the information for a specific protein.

• Made up of 500 to 1000 nucleotides long.

• Made up of codons (sequence of three bases)

• Each codon is specific for one amino acid.

A. Messenger RNA (mRNA)

methionine glycine serine isoleucine glycine alanine stopcodon

protein

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

startcodon

Primary 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

B. Transfer RNA (tRNA)• Made up of 75 to 80 nucleotides long.

• Picks up the appropriate amino acid floating in the cytoplasm

• Transports amino acids to the mRNA.

• Has anticodons that are complementary to mRNA codons.

• Recognizes the appropriate codons on the mRNA and bonds to them with H-bonds.

C. Ribosomal RNA (rRNA)

• Made up of rRNA is 100 to 3000 nucleotides long.

• Important structural component of a ribosome.

• Associates with proteins to form ribosomes.

Ribosomes

• Large and small subunits.

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

• Both units come together and help bind the mRNA and tRNA.

Ribosomes

Largesubunit

Small subunit

mRNA

A U G C U A C U U C G

Learning Check

• What are the three types of RNA?

• Where is each type produced?

• What is the function of each type of RNA?

3. Translation

Nuclearmembrane

Transcription

RNA Processing

Translation

DNA

Pre-mRNA

mRNA

Ribosome

Protein

Eukaryotic Cell

3. Translation

• Synthesis of proteins in the cytoplasm

• Involves the following:

1. mRNA (codons)

2. tRNA (anticodons)

3. rRNA

4. ribosomes

5. amino acids

Translation• In the cytoplasm, translation occurs.

The mRNA binds to a ribosome.

• The strand of mRNA is pulled through the ribosome three bases at a time, in triplets.

• Each of these triplets on the mRNA strand is called a codon.

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

This is a molecule of messenger RNA.

It was made in the nucleus by transcription from a DNA molecule.

mRNA molecule

codon

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

A ribosome on the rough endoplasmic reticulum attaches to

the mRNA molecule.

ribosome

It brings an amino acid to the first three bases (codon) on the mRNA.

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

Amino acid

tRNA molecule

anticodon

U A C

A transfer RNA molecule arrives.

The three unpaired bases (anticodon) on the tRNA link up with the codon.

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

Another tRNA molecule comes into place, bringing a second amino acid.

U A C C C G

Its anticodon links up with the second codon on the mRNA.

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

Another tRNA molecule brings the next amino acid into place.

C C G

A A U

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

A peptide bond joins the second and third amino acids to form a polypeptide chain.

C C G C C G

The polypeptide chain gets longer.

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

A C G

The process continues.

This continues until a termination (stop) codon is reached.

The polypeptide is then complete.

tRNA Transfer RNA (tRNA), reads the strand

of mRNA and translates it into a strand of amino acids.

A molecule of tRNA has at one end a set of three bases that will complement the mRNA strand; this is called the anticodon.

tRNA• If the 3 base anticodon of the tRNA

complements the 3 base codon of the mRNA, they briefly combine.

• The amino acid is left behind when the tRNA leaves.

• As each codon is read, the next tRNA brings in a new amino acid and the polypeptide (protein) chain grows.

• This requires enzymes and ATP.

End Product

• 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

Functional Protein

• The protein now has to undergo folding and the addition of bonds

• Folding allows the Protein to reach its 3D (Tertiary Shape) which influences its Function.

Learning Check

The anticodon UAC belongs to a tRNA that recognizes and binds to a particular amino acid.

What would be the DNA base code for this amino acid?

Answer:

• tRNA - UAC (anticodon)• mRNA - AUG (codon)• DNA - TAC

What have you learned?Can you ………………

1. Outline the steps in protein synthesis

2.Understand that a strand of DNA is copied by transcription

3.Understand the role of mRNA

4.Know the function of a ribosome in protein synthesis

5.Understand the process of translation that leads to the formation of a new protein

6.Know that the shape of a protein determines its function

End