3.5: Translation and Transcription

Question 1

Compare the structure of DNA and RNA:

Answer 1

DNA and RNA both consist of nucleotides which contain a sugar, a base and a phosphate group. However there are a few differences. Firstly, DNA is composed of a double strand forming a helix whereas RNA is only composed of one strand. Also the sugar in DNA is deoxyribose whereas in RNA it is ribose. Finally, both DNA and RNA have the bases adenine, guanine and cytosine. However DNA also contains thymine which is replaced by uracil in RNA.

Question 2

Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase:

Answer 2

Transcription is the process by which an RNA sequence is produced from a DNA template:

• RNA polymerase separates the DNA strands and synthesises a complementary RNA copy from one of the DNA strands
• It does this by covalently bonding ribonucleoside triphosphates that align opposite their exposed complementary partner (using the energy from the cleavage of the additional phosphate groups to join them together)
• Once the RNA sequence has been synthesised, RNA polymerase will detach from the DNA molecule and the double helix will reform
• The sequence of DNA that is transcribed into RNA is called a gene
• Transcription occurs in the nucleus (where the DNA is) and, once made, the mRNA moves to the cytoplasm (where translation can occur)

Three main types of RNA are predominantly made:
+ Messenger RNA (mRNA): A transcript copy of a gene used to encode a polypeptide
+ Transfer RNA (tRNA): A clover leaf shaped sequence that carries an amino acid
+ Ribosomal RNA (rRNA): A primary component of ribosomes

Question 3

Describe the genetic code in terms of codons comprised of triplets of bases:

Answer 3

The genetic code is the set of rules by which information encoded in mRNA sequences is converted into proteins (amino acid sequences) by living cells.

• Codons are a triplet of bases which encodes a particular amino acid
• As there are four bases, there are 64 different codon combinations (4 x 4 x 4 = 64)
• The order of the codons determines the amino acid sequence for a protein
• The coding region always starts with a START codon (AUG) and terminates with a STOP codon

+ It is universal
+ It is degenrate

Question 4

Explain the process of translation, leading to polypeptide formation:

Answer 4

Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain.

• Ribosomes bind to mRNA in the cell’s cytoplasm and move along the mRNA molecule in a 5’ - 3’ direction until it reaches a start codon (AUG)
• Anticodons on tRNA molecules align opposite appropriate codons according to complementary base pairing (e.g. UAC will align with AUG)
• Each tRNA molecule carries a specific amino acid (according to the genetic code)
• Ribosomes catalyse the formation of peptide bonds between adjacent amino acids (via a condensation reaction)
• The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon, at this point translation stops and the polypeptide chain is released.

Question 5

Explain the relationship between one gene and one polypeptide:

Answer 5

A polypeptide is formed by amino acids liking together through peptide bonds. There are 20 different amino acids so a wide range of polypeptides are possible. Genes store the information required for making polypeptides. The information is stored in a coded form by the use of triplets of bases which form codons. The sequence of bases in a gene codes for the sequence of amino acids in a polypeptide. The information in the genes is decoded during transcription and translation leading to protein synthesis.