4.2b Polypeptide Synthesis

Describe transcription.

. DNA helicase separates the double helix into two strands. . One of these strands acts as a template strand. . The bases on this template strand are exposed. . Complementary free nucleotides come to join with the template bases. . U replaces T to join with A instead. . RNA polymerase joins these added free nucleotides together to make the backbone and form the pre-mRNA molecule. . When the stop code of the gene is reached, the pre-mRNA molecule leaves. . The DNA strands join back up again.

Briefly describe the four stages of polypeptide synthesis.

. The genetic code provided by the chemical base sequence in DNA provides the information. . The relevant gene is transcribed onto a messenger RNA molecule in transcription and this leaves the nucleus. . The pre-mRNA is spliced to make mature mRNA. . The amino acids are assembled by tRNA in a process called translation.

Describe the roles of ATP in translation.

ATP is needed in bonding the amino acid to the tRNA molecule. It is also needed in forming the peptide bonds between amino acids.

What happens to the DNA whilst and after transcription occurs?

After RNA polymerase has moved past the DNA template strand, they join back up together so that few DNA nucleotides are ever exposed. The strands twist back together into the double helix, unchanged by the process.

What is needed to form the bonds between the amino acids?

An enzyme and the energy which is released by the hydrolysis of ATP.

Name the two enzymes involved in transcription, and what they do.

DNA helicase - this separates the DNA double helix into two strands so that one can act as a template strand. RNA polymerase - this joins the free nucleotides together with a ribose-phosphate backbone to make the mRNA molecule which then detaches and leaves.

How does the codon bind with the anticodon?

Hydrogen bonds form between the chemical bases. The purine A forms 2 bonds with the pyrimidine U;the purine G forms three bases with the pyrimidine C.

What does transcription produce in: eukaryotic cells? prokaryotic cells?

In eukaryotic cells, pre-mRNA is produced. In prokaryotic cells there is little junk DNA so mRNA is produced.

Where are polypeptide chains modified after translation?

In organelles such as the endoplasmic reticulum and, mainly, the Golgi apparatus.

Describe the problem with pre-mRNA.

It contains introns - these are non-coding sections in between the exons (coding sections). With the introns, the mRNA cannot code for a functional protein so these must be removed.

Describe a change that is often made to a polypeptide chain after translation.

Methionine, the start amino acid, is often removed as it isn't needed in the final protein.

How many molecules of tRNA are bonded to the mRNA at once?

Only two. Once a ribosome has moved past a codon, the tRNA molecule detaches and goes to bring more amino acids.

How are the amino acids joined together?

Peptide bonds form between the N in the amino group of one amino acid and the C on the acid group of the other amino acid.

When does transcription know when a full molecule has been made?

RNA polymerase stops transcription when it reaches the triplets which signify the "stop code" on the DNA template strand.

How does splicing happen and what does it produce?

Splicing is done by an enzyme called the spliceosome. It produces mature mRNA, which is made totally of exons and will code for a functional protein.

What happens to pre-mRNA before it is used?

The introns are removed in a process called splicing.

What must happen to polypeptide chains to produce complex proteins like antibodies?

The polypeptide chain is only the primary structure. To form the secondary structure, this is folded into simple 3D shapes like alpha helices and beta pleated sheets held togetehr by H bonds. To make the tertiary structure, the 2ndary structure is folded into highly specific 3D shapes and held together by disulfide bridges. To make the quaternary structure found in antibodies, different polypeptide chains are twisted together, sometimes with prosthetic groups.

What is transcription?

The process where the gene for a particular protein is copied out to make a messenger molecule called mRNA.

How is the ribosome structured to allow translation?

The ribosome has two sub-units, one larger and one smaller. The mRNA molecule runs between these sub-units.

Describe translation.

The ribosome starts at the first codon on the mRNA which is always AUG. The tRNA with the complementary anticodon moves to the ribosome, carrying the amino acid. The codon binds with the anticodon to hold the tRNA there. The ribosome moves to the second codon and the second amino acid is brought next to the first. The two amino acids bond with a peptide bond. The ribosome then moves along again to bring the third amino acid, which bonds with the second amino acid.

What is translation?

Translation is the assembly of amino acids in the right order by tRNA molecules and ribosomes, according to the sequence of codons on the mRNA molecule.

Why is a lot more translation done than transcription?

Up to 50 ribosomes can move along an mRNA molecule at one time, each producing an identical polypeptide chain. This means a lot of polypeptides can be made form one mRNA molecule so less transcription needs to be done.

When does translation end?

When the ribosome reaches the "stop codon" on the mRNA it stops. The tRNA, ribosome and mRNA all separate to leave the amino acid chain - the polypeptide.

What three things does translation involve?

tRNA molecules, ribosomes and mRNA molecules.