Transcription and translation topic 2.7

Question 1

Transcription

Answer 1

Transcription is the synthesis of an RNA sequence from a DNA template
* This process occurs within the nucleus of a cell

Question 2

Transcription is mediated by the enzyme RNA polymerase, which:

Answer 2

• Separates the DNA strands (breaks H bonds between base pairs)
• Covalently joins free complementary RNA nucleotides together

Question 3

Three main types of RNA may be produced:

Answer 3

• mRNA – Transcript used to make protein
• tRNA – Transfers amino acid to ribosome
• rRNA – Catalytic component of ribosome

Question 4

Genetic Code

Answer 4

The genetic code is the set of rules by which information encoded in
mRNA sequences is converted into a polypeptide sequence

Question 5

Codons

Answer 5

Triplets of bases which correspond to a particular amino acid

Question 6

The order of the codons determines the amino acid sequence for a protein

Answer 6

• A coding sequence always begins with a start codon (AUG)
• A coding sequence is terminated with a stop codon

Question 7

The genetic code has two key features:

Answer 7

• Universality – All organisms use the same genetic code
• Degeneracy – Multiple codons may code for the same amino acid

Question 8

Translation

Answer 8

Translation is the process of polypeptide synthesis by the ribosome

Question 9

Translation process

Answer 9

• Messenger RNA (mRNA) is transported to the ribosome
• A ribosome reads an mRNA sequence in base triplets called codons
• Each codon codes for a specific amino acid (as per the genetic code)
• Amino acids are transported to ribosomes by transfer RNA (tRNA)
• Each tRNA aligns opposite a codon via a complementary anticodon
• The ribosome moves along the mRNA sequence (5’ → 3’) and joins
  amino acids together with peptide bonds (condensation reaction)
• The synthesis of a polypeptide is initiated at a start codon (AUG)
  and is completed when the ribosome reaches a STOP codon

Question 10

A gene is a sequence of DNA which encodes a polypeptide sequence

Answer 10

• One gene = one polypeptide (proteins may have multiple polypeptides)

Question 11

There are exceptions to this fundamental gene protein relationship

Answer 11

• Genes may be alternatively spliced (one gene = many polypeptides)
• Genes encoding tRNA or rRNA are transcribed but not translated
• Genes may be mutated to alter the original polypeptide product