Protein Synthesis - Translation

Protein Synthesis - Translation

3.4.2 - DNA and Protein Synthesis

Multiple Choice

Through the process of transcription, DNA makes a complimentary strand of:

• DNA

• mRNA

• tRNA

• CNA

Multiple Choice

Which enzyme unzips the section of DNA to be transcribed?

• DNA helicase

• RNA helicase

• DNA polymerase

• RNA polymerase

Multiple Choice

Which enzyme connects the RNA nucleotides with phosphodiester bonds?

• DNA polymerase

• DNA helicase

• RNA polymerase

• RNA helicase

Multiple Choice

Which strand of DNA is known as the template strand?

• Sense

• Antisense

Initiating Translation

The second stage of protein synthesis is translation

This involves the transfer of genetic information from mRNA to amino acids

These amino acids join together to make a protein, which in turns folds up into its secondary and tertiary shapes

To the ribosome...

After the mRNA enters the cytoplasm, through the nuclear pore, it associates with a ribosome.

Ribosomes are made of two subunits and the mRNA binds to the smaller subunit at the start codon – always AUG 

transfer RNA (tRNA)

Once the mRNA is bound to the ribosome, it can then be read by tRNA molecules

tRNAs are small, single stranded, clover leaf shaped molecules

They are attached to an amino acid that matches the sequence of bases on its anti-codon (opposite side)

The first tRNA molecule with the complementary anticodon (UAC)

then binds to the start codon on the mRNA by hydrogen bonding

Starting codon

The first tRNA molecule will always carry the amino acid methionine, since its codon is AUG

AUG is always a starting codon

Therefore, all polypeptides will have a starting amino acid of methionine, though in come cases it is clipped off later

Ribosome

The large subunit of the ribosome then binds and translation can begin

From this point on we can actually start making the polypeptide chain

Multiple Choice

What does the t stand for in tRNA?

• transport

• translation

• transfer

• transcription

Multiple Choice

What is the base sequence for a start codon?

• AAG

• UUG

• UAG

• AUG

Multiple Choice

What part of the tRNA molecule binds to the mRNA strand?

• anticodon

• codon

• amino acid

• anticordon

Translation

Once the ribosome is fully assembled, a second tRNA molecule with a complementary anticodon is able to bind to the mRNA

The second tRNA carries the specific amino acid coded for by the mRNA codon

Translation

A peptide bond is formed between the two amino acids, forming a dipeptide, and releasing the methionine from the first tRNA

To form this peptide bond, ATP must be used

The first tRNA is then free to move away and pick up another amino acid

Translation

The ribosome then moves along the mRNA strand by exactly three base pairs (one codon)

The first tRNA is released

The second tRNA is still attached, waiting for the third tRNA to move into place

Translation

A third tRNA, with a complementary anticodon, and carrying a specific amino acid, is now able to bind to the next mRNA codon

It forms another peptide bond between its amino acid and the previous amino acid

Translation

Once a ribosome has moved far enough along the mRNA strand, there is enough space for a second ribosome to attach at the start codon as produce a second polypeptide

Each mRNA molecule is read many times to produce lots of copies of the same protein

Translation

This results in many identical polypeptides being synthesised simultaneously from the same mRNA strand

Multiple Choice

What type of bond forms between two amino acids?

• phosphodiester

• ester

• peptide

• hydrogen

Ending Translation

There are no tRNA molecules with complementary anticodons for the stop codons.

There are three types of stop codon: UGA, UAG and UAA

Therefore, once a stop codon is reached, no tRNAs can bind and the synthesised polypeptide is released from the last tRNA molecule

Ending Translation

Both ribosome subunits separate from the mRNA strand and are quickly reused for another round of translation, maybe even going back to the start codon to move along it again

Polypeptide Structure

Meanwhile, the polypeptide coils and folds into a specific tertiary structure, determined by the amino acid sequence

The polypeptide may also bind to other polypeptides and/or prosthetic groups to form a protein with a quaternary structure

Multiple Choice

What level of structure of a protein is determined by the sequence of amino acids?

• Primary

• Secondary

• Tertiary

• Quaternary

Multiple Choice

What level of protein structure involves the interaction of two or more polypeptide chains?

• Primary

• Secondary

• Tertiary

• Quaternary

Multiple Choice

In what level of protein structure might you find alpha helices or beta pleated sheets?

• Primary

• Secondary

• Tertiary

• Quaternary