Translation

What I need to know:-

1. What translation is 2. The role of tRNA3. Know about anticodons and codons4. The function of start/stop codons5. One gene can produce many proteins as a result of RNA

splicing and post translation modification.

Protein synthesis is split into 2 processes:

1. TRANSCRIPTION – DNA into mRNA

2. TRANSLATION – mRNA into protein

Translation•Once the code has been transcribed to form mRNA, the mRNA then gets transported out of the nucleus (via nuclear pores) and into the cytoplasm, where it attaches to a ribosome.

• Each triplet of bases on mRNA is called a CODON

•The code on the mRNA is then translated and used to form a polypeptide chain.

tRNA

• tRNA is a single strand of nucleotides that fold back on itself to make a 3D shape

• Only one triplet of bases will be exposed that will complement the codon on mRNA

• At the other end, each tRNA carries a specific amino acid

H Bonds

A 2nd type of RNA is found in the cytoplasm – tRNA (transfer RNA). Each tRNA molecule has a triplet of bases called an ANTICODON.

Ribosomes

• Small spherical structure

• Contains enzymes

needed for protein synthesis

• Has 1 binding site for mRNA and 3 for tRNA

• Site P holds the tRNA carrying the growing polypeptide chain

• Site A holds the tRNA carrying the next amino acid to be joined on the growing chain

• Site E discharges a tRNA from the ribosome once its amino acid has become part of the polypeptide chain

• mRNA has start and stop codons.

• Translation starts when the ribosome binds to the 5’ end of the mRNA. This is the start codon (AUG – which codes for methionine)

• The initial tRNA carrying the amino acid methionine attaches by forming hydrogen bonds between its anticodon UAC and the start codon

• The next tRNA with the correct anticodon bonds to its codon adjacent to the first

• An enzyme within the ribosome now brings about the formation of a peptide bond between the two adjacent amino acids, and the first tRNA disconnects from its amino acid

Translation Stages

• The ribosome moves along one codon, moving the initial tRNA along where it is discharged from the ribosome

• The vacated site A becomes occupied by the next tRNA.

• This process is repeated until a stop codon on the mRNA is reached.

• A release factor then binds to the stop codon and frees the polypeptide from the ribosome.

• http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html

• http://www.educationscotland.gov.uk/highersciences/humanbiology/animations/translation.asp

Polyribosome

• One mRNA can be used to make many copies of the polypeptide by several ribosomes attaching to the mRNA and carrying out multiple translations at the same time.

Alternative RNA splicing

• One gene can code for several different proteins

• ..because a primary transcript (mRNA) has the potential to produce several different mRNA molecules to code for different polypeptides.

• Depending on the circumstances, alternative segments of RNA may be treated as exons and introns.

• .so when a transcript of mRNA is being spliced, there can be many molecules of mRNA made from that primary transcript.

Eg. One gene, two antibodies

Antibodies are Y-shaped proteins. P and Q are both coded for by the same gene, but P has a membrane - anchoring unit and Q does not. This is because the mRNA for P had the necessary exon but the mRNA for Q did not.

Post-translation modifications• Once translation is complete, further modifications may be

required to enable a protein to perform its specific function, eg;

1. Cleavage – some molecules are made in an inactive form and have to have sections removed by enzymes before they can become active

Eg. Insulin is a single chain in its inactive form but is cut by protease enzymes to remove a section and leave two chains held together by sulphur bridges

2. Molecular Addition• Proteins can have additions of carbohydrates or phosphate groups

• Example – mucus is a glycoprotein (protein with carbohydrate).• Example – regulatory proteins require a phosphate group to make

them functional (phosphorylation turns enzymes on )

Now attempt the following questions1. How many tRNA binding sites are present on a ribosome?2. What type of bond forms between adjacent amino acids

attached to a tRNA molecule?3. Copy and complete the table

Stage of synthesis Site in cell

Formation of primary transcript of mRNAModification of primary transcript of mRNACollection of amino acid by tRNA

Formation of codon-anticodon links

Glossary • ALTERNATIVE SPLICING – a method of using one mRNA

strand to give many proteins, by altering which regions are seen as introns and exons

• ANTICODON - a sequence of three nucleotides forming a unit of genetic code in a transfer RNA molecule, corresponding to a complementary codon in messenger RNA

• POLYRIBOSOME - a cluster of ribosomes held together by a strand of messenger RNA which each is translating

• tRNA - RNA molecules present in the cell that attach the correct amino acid to the protein chain that is being synthesized at the ribosome of the cell