3.4 Topic 4 - 3.4.2 DNA and protein synthesis

Question 1

What is the basic process of protein Synthesis?

Answer 1

• DNA provides the instructions in the form of a long sequence of bases.
• A complementary section of part of this sequence is made in the form of a molecule called pre-mRNA – a process called transcription.
• The pre-mRNA is spliced to form mRNA.
• The mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide – a process called translation.

Question 2

What is the 1st process of protein synthesis called?

Answer 2

Transcription.

Question 3

What is the 2nd process of protein synthesis called?

Answer 3

Translation.

Question 4

What is the point of transcription?

Answer 4

Making pre-mRNA using part of the DNA as a template.

Question 5

What is the process of transcription?

Answer 5

• An enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region.
• The nucleotide bases on one of the two DNA strands, known as the template strand, pair with their complementary nucleotides from the pool which is present in the nucleus. The enzyme RNA polymerase then moves along the strand and joins the nucleotides together to form a pre mRNA molecule.
• In this way an exposed guanine base on the DNA binds to the cytosine base of a free nucleotide. Similarly, cytosine links to guanine and adenine links to uracil.
• As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind it. As a result, only about 12 base pairs on the DNA are exposed at any one time.
• When the RNA polymerase reaches a particular sequence of bases on the DNA that it recognises as a ‘stop’ triplet code, it detaches, and the production of pre-mRNA is then complete.
• Splicing then occurs in eukaryotic cells not prokaryotic cells.

Question 6

What is the process of splicing during transcription? (only in prokaryotic cells)

Answer 6

The DNA of a gene eukaryotic cells is made up of sections called exons that code for proteins and sections called introns that do not. These intervening introns would prevent the synthesis of a polypeptide. In the pre-mRNA of eukaryotic cells. The base sequences corresponding to the introns are removed and the functional exons are joined together during a process called splicing.

The mRNA molecules are too large to diffuse out of the nucleus and so, once they have been spliced, they leave via a nuclear pore. Outside the nucleus, the mRNA is attracted to the ribosomes to which it becomes attached, ready for translation.

Question 7

What is another name for protein synthesis?

Answer 7

Polypeptide synthesis.

Question 8

What is the process of translation?

Answer 8

• A ribosome becomes attached to the starting codon (AUG) at one end of the mRNA molecule.
• The tRNA molecule with the complementary anticodon sequence
  (UAC) moves to the ribosome and pairs up with the codon on the mRNA. This tRNA carries a specific amino acid (methionine).
• A tRNA molecule with a complementary anticodon (UGC) pairs with the next codon on the mRNA (ACG). This tRNA molecule carries another amino acid (threonine).
• The ribosome moves along the mRNA, bringing together two tRNA molecules at any one time, each pairing up with the corresponding two codons on the mRNA.
• The two amino acids (methonine and threonine) on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide the required energy.
• The ribosome moves on to the third codon (GAU) in the sequence on the mRNA, thereby linking the amino acids (threonine and aspartic acid) on the second and third tRNA molecules.
• As this happens, the first tRNA is released from its amino acid (methionine) and is free to collect another amino acid (methionine) from the amino acid pool in the cell.
• The process continues in this way, with up to 15 amino acids being added each second, until a polypeptide chain is built up.
• Up to 50 ribosomes can pass immediately behind the first, so that many identical polypeptides can be assembled simultaneously.
• The synthesis of a polypeptide continues until a ribosome reaches a stop codon. At this point, the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide chain is complete.

Question 9

What is a summary of translation?

Answer 9

The DNA sequence of triplets that make up a gene determine the sequence of codons on mRNA. The sequence of codons on mRNA determine the order in which the tRNA molecules line up.

Question 10

What are the two roles of ATP in translation?

Answer 10

1. To provide energy to attach amino acids to tRNA.
2. To attach amino acids together.

Question 11

… determine the sequence of amino acids in the polypeptide. in this way genes precisely determine which proteins a cell manufactures. As many of these proteins are enzymes, genes effectively control the activities of cells.

Answer 11

Genes.

Question 12

What is a single polypeptide sometimes called?

Answer 12

A functional protein.

Question 13

A number of polypeptides are linked together to give a …

Answer 13

… functional protein (quaternary structure).

Question 14

What happens to the polypeptide after translation?

Answer 14

• The polypeptide is coiled or folded, producing its secondary structure.
• The secondary structure is folded, producing the tertiary structure.
• Different polypeptide chains, along with any non-protein groups, are linked to form the quaternary structure.