4.2 DNA and protein synthesis Flashcards
What is the basic process of how proteins are manufactured by the DNA in the cell’s nucleus?
- 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
What is transcription?
The process of making pre-mRNA using part of the DNA as a template
What is the process of transcription?
- An enzyme acts on a specific region of the DNA causing the 2 strands to separate and expose the nucleotide bases in that region
- The nucleotide bases on one of the 2 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 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 thymine joins to adenine. The exception is adenine, which links to uracil rather than thymine
- As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands re-join 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
What is the splicing of pre-mRNA?
- In prokaryotic cells, transcription results directly in the production of mRNA from DNA. In eukaryotic cells transcription results in the production of pre-mRNA, which is then spliced to form mRNA
- 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
- As most prokaryotic cells do not have introns, splicing of their DNA is unnecessary
- 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 the next stage of the process: translation
What is the process of synthesising a polypeptide? - pairing
- A ribosome becomes attached to the starting codon (AUG) at one end of the mRNA molecule
- The tRNA molecule with the complementary anticodon sequence moves to the ribosome and pairs with the codon on the mRNA. This tRNA carries a specific amino acid
- A tRNA molecule with a complementary anticodon pairs with the next codon on the mRNA. This tRNA molecule carries another amino acid
- 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
What is the process of synthesising a polypeptide? - joining
- The 2 amino acids 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 in the sequence on the mRNA, thereby linking the amino acids on the second and third tRNA molecules
- As this happens, the first tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell
What is the process of synthesising a polypeptide? - ending
- 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 assembles 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
Explain the assembling of a protein.
- 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
Describe the role of tRNA in the process of translation
tRNA molecule attaches an amino acid at one end and has a sequence of 3 bases (anticodon) at the other end
tRNA molecule is transferred to a ribosome on an mRNA molecule
Anticodon on tRNA pairs with the complimentary codon sequence on mRNA
Further tRNA molecules with amino acids attached line up along the mRNA in the sequence determined by the mRNA bases
The amino acids are joined by peptide bonds
tRNA helps to ensure the correct sequence of amino acids in the polypeptide
Name the organelle involved in translation
Ribosomes
State which other enzyme is involved in transcription and describe its role
DNA helicase acts on a specific region of the DNA molecule to break the hydrogen bonds between bases
This causes the 2 strands to separate and expose the nucleotide bases in that region
Describe the role of RNA polymerase in transcription
Enzyme RNA polymerase adds the nucleotides one at a time by forming phosphodiester bonds to build a strand of pre mRNA until it reaches a stop codon
What is the process of transcription?
• Hydrogen bonds between DNA bases break
• Only one DNA strand acts as a template
• Free RNA nucleotides align by complementary base pairing
• In RNA Uracil base pairs with adenine on DNA
• RNA polymerase joins adjacent RNA nucleotides
• By phosphodiester bonds between adjacent nucleotides
• Pre mRNA is spliced to form mRNA
What is the process of translation?
• mRNA attaches to ribosomes
• tRNA anticodons bind to complementary mRNA codons
• tRNA brings a specific amino acid
• Amino acids join by peptide bonds
• Amino acids join together with the use of ATP
• tRNA released after amino acid joined to polypeptide
• The ribosome moves along the mRNA to form the polypeptide
